The longest-lasting 2023 western North American heat wave was fueled by the record-warm Atlantic Ocean

Domain: earth_science
Published: 2025-07-16
Editorial outcome: major_revision

Abstract
According to the World Meteorological Organization, 2023 was ranked as the second warmest year in the global surface temperature record since 1850, setting warm surface temperature records over more than 20% of the global land surface. In particular, the southwestern United States (US) and Northern Mexico experienced their longest stretch of record-breaking heat wave, affecting over 100 million people, causing over 200 deaths, and $14.5 billion in economic loss. Here we show that the 2023 heat wave event was linked to a strong anticyclonic blocking pattern that persisted for more than six weeks across the western US. Regression analysis and atmospheric model simulations suggest that the anticyclonic pattern was ultimately forced by the extremely warm sea surface temperature in the Atlantic. The combination of a warm Atlantic and a developing Pacific El Niño significantly amplified regional heat waves, doubling their number, tripling their days, and increasing their duration by about 50%.

Introduction
The year 2023 was the second warmest on record, only surpassed by 2024 (Monthly Global Climate Report for Annual 2024). Of particular notoriety was the boreal summer months (i.e., June-August), which were characterized as the hottest summer with more than 20% of the land surface setting extreme warm records and multiple heatwave events over most continents, with several concurrent events simultaneously affecting multiple regions (Lembo et al.). The year 2023 also produced the highest number of heat-related deaths in the United States (U.S.) in the 21st century, with 2325 deaths in association with severe heat waves. One of these severe heatwave events occurred over the southwestern U.S. and Mexico, which extended from mid-June to early August, affected over 100 million people, and was responsible for 303 death that occurred in a span of just two weeks in Maricopa County, Arizona (https://www.maricopa.gov/1858/Heat-Surveillance). The compounded effect of extreme heat and drought was responsible for $14.5 billion in economic loss (https://www.ncei.noaa.gov/news/national-climate-202312), making this event the costliest weather and climate disaster of 2023 in North America.

Excessive heat puts significant stress on human health, resulting in increased morbidity and mortality, with some of the most notorious events being responsible for hundreds and even thousands of deaths in the most extreme cases; for example, the 1980 US heat wave, the 1995 event in Chicago, Illinois, the 2003 European heat wave, the 2010 Russian event, the 2011 event over the Great Plains of the US, the 2021 northwestern North American event, the 2023 South American heat wave. In fact, extreme heat is the leading cause of weather-related mortality in the US, topping other more notorious weather hazards, like tornadoes and hurricanes (US, https://www.weather.gov/hazstat/). Heat waves are also the leading cause of natural hazards-related deaths in Australia, accounting for more than 55% of the reported fatalities. Moreover, these extreme heatwave events have been observed more frequently in many regions, with a significant increase in the number and severity of heat waves in recent decades as a result of rising surface temperatures. However, the effects of increasing temperatures on heat extremes go beyond changes in the mean climate and include shifts in the extremes as well, where the duration and frequency are expected to increase this century. All of these effects are further exacerbated by a projected increased exposure to heat extremes due to population growth, increased urbanization, agricultural loss, and aridification.

In addition to the longer-term trends, understanding shorter term weather and climate variability is essential for improving heat wave predictions and future projections. Heat waves are often linked to large-amplitude atmospheric circulation patterns driven by quasi-stationary and propagating Rossby waves and their interaction with the overall synoptic flow, topography, and land-sea contrast. These interactions result in persistent anticyclonic flow and blocking events, leading to flow stagnation and prolonged periods of clear sky, enhanced incoming solar radiation, drought conditions, and reduced soil moisture, all of which further exacerbates surface warming. While these blocking patterns are part of the atmospheric synoptic circulation, significant effort has been undertaken to further understand longer-term drivers of these patterns, with the aim to improve their predictions beyond the weather forecast range. Slower-acting coupled atmospheric-land-ocean processes are often attributed to heat waves. For example, sub-seasonal variations in midlatitude atmospheric circulation patterns have been shown to precede heat waves over the US by 15–20 days. Enhanced convective activity from the East Asian Monsoon was found to force a mid-latitude wavetrain across the Pacific, leading to enhanced blocking pattern, which promotes the occurrence of US Great Plains heat waves. Others have shown that persistent midlatitude circulation patterns forced by tropical sea surface temperature (SST) anomalies are modulators for drought and extreme heat over the western US. A recent work found that boreal summer tropical Atlantic SST anomalies modulate heat wave occurrences over North America. In that work, it was found that a warmer tropical Atlantic enhances atmospheric convection over the Caribbean Sea and produces a Gill-type atmospheric response. This, in turn, produces an anticyclonic Rossby wave source over the Great Plains, thus enhancing subsidence and significant surface warming, leading to heat domes. The aforementioned works, and many others, have provided a better understanding of the coupled climate system as it pertains to heat extremes. This collectively suggests that the inherently longer timescales of oceanic and land process variabilities could aid in extending the prediction of high-impact extreme events beyond the weather timescales. Although current coupled models tend to underestimate regional terrestrial temperature variability, decreasing prediction skill at longer lead-times.

Besides the multiple land temperature records that were set in 2023, global oceans also experienced record warm SST. Of special notoriety were the record SSTs over the tropical Atlantic, which were up to one degree Celsius warmer than climatology. These warm oceanic temperatures were not just confined to the surface, as oceanic heat content was at record levels in 2023. The North Atlantic has experienced a warm period since around 1995, owing to its relation to the positive phase of the Atlantic Multidecadal Variability. In addition, an increase in the energy imbalance from an upsurge in greenhouse gasses has contributed to a steady rise in ocean surface temperatures and heat content, among other effects. Furthermore, the 2020 emission regulations from the International Maritime Organization aimed at reducing ship sulfate aerosol emissions may have contributed to the recent warm surface temperatures over the North Atlantic. Warm North Atlantic SSTs have been shown to modulate local and remote atmospheric circulation, with significant influences on precipitation, tropical cyclone activity, as well as extreme land surface temperatures, and SSTs over the Pacific Ocean. The tropical Pacific was also characterized by warm SSTs associated with a growing El Niño (i.e., the positive phase of the El Niño Southern Oscillation, ENSO). ENSO is one of the dominant modes of interannual climate variability, which has been shown to affect extreme surface temperatures through modulation of atmospheric circulation, including the occurrence of heat waves over North America.

This work suggests a physical link between the co-occurrence of the long-duration (i.e., several weeks) extreme heat over the southwest US and Mexico and the record warm North Atlantic SSTs and a growing El Niño in the Pacific. Thus, we hypothesize a physical connection that the extremely warm 2023 interbasin Pacific-Atlantic SSTs were responsible for the persistence of the longest-lasting heat wave in the region. For this, we use observational records and general circulation model experiments to show that the growth and persistence of this heat wave event were supported by remote forcing from the record warm SSTs in the Atlantic, a growing El Niño event in the Pacific, and the interbasin synergy effect of Pacific-Atlantic forcing. This interbasin synergy is in reference to the constructive interaction between the warm Atlantic and ENSO in modulating the heat wave occurrence and not to the active debate about tropical Pacific/Atlantic interaction conundrum.

Results
Observed evolution of the heat wave
The maximum and minimum near-surface temperatures along with their climatological mean and 95th percentiles are shown in Fig. 1a for the summer of 2023 for Phoenix, Arizona (see Methods for definition of climatology). Note that the conditions were near their climatological mean for most of June. However, starting around 1 July, both maximum and minimum temperatures were at or exceeded their 95th percentile threshold (T95) for several days. The brunt of the heatwave was experienced from 13 July to 30 July, with 18 consecutive days of maximum temperatures exceeding the T95 by as much as 4.5°C. Beyond that, four other shorter-duration extreme heat periods were experienced up to 10 September, where the temperatures reverted to their climatological mean. Besides being the longest-lasting heatwave event in the region, the 13–30 July 2023 event was also responsible for the warmest minimum temperature and tied for the third warmest maximum temperature on record (Fig. 1b, maximum temperatures of 50°C and 49.4°C were recorder in 1990 and 1995). While the southwest US and Mexico are notorious for extreme, persistent heat, previous events were less severe, with the previous record for Phoenix of consecutive days above T95 being 8 days, less than half the duration of the 2023 event. During the peak of the event, warm temperature anomalies >5°C affected most of the region (Fig. 1c). Similar conditions were observed in other metropolitan areas, such as Las Vegas, Nevada, Albuquerque, New Mexico, El Paso, Texas, and San Antonio, Texas (Supplementary Fig. 1).

The spatiotemporal evolution of the maximum surface temperature anomaly during the heat wave is shown in Supplementary Fig. 2. Note that significant positive temperature anomalies were already present over most of Mexico as early as mid-June, coinciding with negative anomalies over the western US. The warm anomalies subsided by early July, but then reappeared around the second week of July, engulfing most of the western US in the second half of the month. The event then shifted back south in early August and finally progressed eastward over the Great Plains by mid-August, where several record temperatures were broken in other regions (e.g., New Orleans experienced a maximum temperature of 40.5°C on 27 August).

Heat Budget
The physical processes responsible for the growth and long persistence of the temperature anomalies over the southwest US are investigated next. A key question here is, why did the extreme heat last over twice as long as other previous events? Fig. 2 describes the temporal evolution, averaged every 10 days for easier illustration, of several relevant dynamical and thermodynamic variables from June-August and averaged from 30°–35°N to 115°–110°W, representing a box approximately encompassing the state of Arizona. A negative 200hPa geopotential height anomaly was present during early June along with a negative 850hPa temperature and 2-m temperature anomalies, which rapidly degraded and turned positive. These anomalies reached their maximum around mid-July, coinciding with the peak of the heat wave, and remained positive for the rest of the summer. The energy budget (see Methods and Eq. 4) can be used to discern the driving process that led to the rapid growth of the positive low-level temperature anomaly. For example, for most of June, there were significant positive net shortwave, longwave radiations, and sensible heat fluxes at the surface (values >20 Wm2). This coincided with enhanced vertically integrated (975–800hPa, see Methods) heating rates (heating >2°C day−1) due to vertical advection and to enhanced surface heating. By early July, most of the strong heating anomalies weakened, but remained slightly positive. However, significant surface sensible heat flux (>20 Wm2) and vertical diffusive heating rates (heating >2°C day−1) supported the continuation of the T anomalies throughout July, along with longwave radiation heating during August. Of note is that surface latent heat flux was negative throughout the summer, enhancing the surface warming through radiative and sensible heating via an increase in the Bowen ratio, which links water and energy balances of the climate system. A Bowen ratio increase, often present during mega heatwaves, suggests a reduction in the evaporation/evapotranspiration and an increase in the sensible heating and thus increase in near surface temperatures.

In contrast, adiabatic heating anomaly was fairly negative throughout the summer, lessening the impact of the remaining heating terms and thus dampening the temperature tendency. In addition, heating due to horizontal advection played a relatively small role in the evolution of the temperature anomaly, this is expected due to the broad temperature anomaly (Fig. 1c) and stagnant flow pattern (Fig. 3) associated with this heat wave. Overall, the low-level temperatures warmed significantly during June to mid-July, and then remained elevated for most of the summer, aided by significant surface heat fluxes and vertical heating rates (i.e., vertical advective, diffusive, and longwave heating).

We now turn our attention to the spatial distribution of the circulation anomalies during the peak of the heat wave. The analysis uses potential vorticity (PV) and circulation on constant potential temperature (i.e., isentropic surface), which poses several advantages over pressure surfaces in that PV can be used as a parcel tracer and is conserved for frictionless adiabatic motion. Figure 3a depicts the 18 July 2023 potential vorticity and wind at the 350K isentropic level, as well as the location of the dynamical tropopause (thick black contour), defined to be the 2 PVU (2 × 10−6 Km2 kg−1 s−1) iso-surface, which serves as the boundary of tropospheric and stratospheric air. Note that the flow is anticyclonic over the southwestern U.S. and northern Mexico, with significant fluid trapping (thick arrows), suggesting a blocking pattern (see Methods for definition on trapped flow). In fact, the upstream and downstream troughs, along with the anticyclone, are manifestations of a classic “omega” blocking pattern, which is well known to persist for significant periods of time and is responsible for stagnant flow patterns. Regions under anticyclonic trapping often experience significant surface shortwave heating due to clear skies (Supplementary Fig. 3). These conditions contribute to extreme surface warming for prolonged periods, with little ventilation due to the trapped air mass and flow stagnation. While atmospheric blocking is a manifestation of planetary waves, the aforementioned anticyclonic trapping persisted for several weeks, as shown by a time-averaged flow (Supplementary Fig. 4).

A latitude-vertical cross-section along 112°W, which is approximately through the center of the anticyclone, shows the anomalous circulation features during the peak of the heat wave (Fig. 3b). Note that there is a deep upper-level (700–100hPa) anticyclonic circulation centered around 35°N with easterlies (westerlies) around 25°N (45°N). The 2 PVU contour shows a dome-like feature, with PV decreasing northward of 25°N (i.e., a meridional PV inversion) with a subsequent poleward increase. In addition, there is a significant downward (upward) bulging of the potential temperature surfaces at lower (upper) levels, indicating low to mid-level heating. This is shown by the potential temperature anomalies (colors in Fig. 3b), indicating potential temperature anomalies around 8°C near the surface and similar amplitude negative anomalies in the stratosphere. In relation to the strong mid-level anticyclone, the air mass from 25°N to 50°N is significantly trapped (hatching in Fig. 3b, see Eq. 5 for definition) for most of the tropospheric depth, providing little to no ventilation and thus exacerbating the lower-level warm anomalies. In the presence of a trapped flow, most of the ventilation comes from vertical advection from the boundary layer up, which is the case here as shown in the heat budget analysis (Fig. 2). The vertical profile over Tucson, Arizona on 18 July 2023 (Fig. 3c) shows a well-mixed lower troposphere, with a deep dry layer of constant potential temperature (i.e., red line and potential temperature lines are parallel) and mixing ratios (i.e., blue line and dashed black lines are parallel) from the surface up to about 650hPa, indicating strong vertical turbulent heat and moisture fluxes associated with enhanced surface heating from prolonged clear sky conditions. The profile also shows a marked decrease in dew point temperature above 500hPa, indicative of subsidence associated with the high-pressure dome as well as weak wind speed and weak vertical wind shear (hodograph in the top right of Fig. 3c).

A weak North American monsoon in 2023
The southwest U.S. and northern Mexico are regions where most of the precipitation and thus soil moisture is obtained during the summer monsoon (i.e., North American monsoon, NAM, see Methods for definition). In addition, soil moisture and surface air temperature are strongly correlated through longwave radiation, sensible, and latent heating through land-atmospheric coupling, thus a moisture deficit could exacerbate warm surface temperatures and heat wave occurrence. Thus, it is important to assess the state of the NAM, which is the dominant source of precipitation over the region. Note that 2023 was one of the weakest NAM years on record, with precipitation anomalies of −1.4 mm/day (Fig. 4a). A correlation analysis between the NAM index and maximum July temperatures over the study region for the 1979–2022 period reveals a negative correlation r = −0.58 (p < 0.01) and a spatial pattern similar to the 2023 temperature anomalies over the southwest U.S. and Mexico (Supplementary Fig. 5).

It is worth noting that a significant portion of the moisture and thus precipitation that feeds into the NAM region originates from two sources: (1) a Pacific source, mostly through the Sea of Cortez, and (2) an Atlantic source via the Caribbean and Great Plains low-level jet system. Previous works have shown that warmer tropical Atlantic SSTs weaken the western edge of the North Atlantic Subtropical High, weakening the Caribbean low-level jet, thus modulating moisture transport and precipitation over North America. As shown in Fig. 4b, the climatological (vertically-integrated) moisture transport into the NAM region originates from the Pacific and Atlantic sectors, converging over the NAM region and thus producing precipitation there. The anomalous moisture transport shown in Fig. 4c represents the sum of the flux of mean moisture by anomalous wind (v'q), flux of anomalous moisture by mean wind (v q'), and flux of anomalous moisture by anomalous wind (v'q'), where bar (primes) represents climatology (anomaly) respectively, where v is the horizontal wind and q depicts specific humidity. Note that this circulation was disrupted and reversed in 2023 (Fig. 4c), with northerly moisture flux anomalies over the Sea of Cortez and a generally westerly moisture flux anomaly from the Pacific into the Atlantic, all of which resulted in moisture flux divergence over the NAM, reducing the monsoon to one of its weakest on record (Fig. 4a).

To further assess the origin of the reduced NAM and drought conditions, Fig. 4d shows a zonal-vertical cross-section of meridional moisture flux (vq) along 28°N from 120°W to 100°W. Similarly, Fig. 4e shows a meridional-vertical cross-section of the zonal moisture flux along 110°W from 15°N to 40°N. These cross-sections are specifically chosen to depict the main core region of the moisture fluxes that feed the NAM. Note that there are two main cores of climatological northward moisture transport (Fig. 4d): (1) around 110°W at about 925hPa and (2) east of 105°W at higher elevations around 850hPa. The former is associated with a Pacific moisture source while the latter is linked to an Atlantic moisture source. In 2023, significant negative meridional moisture flux anomalies were present over both moisture flux regions that feed the NAM (Fig. 4d) with significant southward moisture transport and divergence south of 30°N (Fig. 4e). Further analysis on moisture transport anomalies was performed by separating changes in circulation (v) and moisture (q). It was found that circulation anomalies were primarily responsible for the 2023 reduction in NAM moisture sources and precipitation (Supplementary Fig. 6 and 7). While the reduction (expansion) of the Atlantic (Pacific) subtropical high during the summer of 2023 appeared to be responsible for the reduction in NAM precipitation, other factors like enhanced atmospheric stability from increasing temperatures could have reduced the available surface moisture, inhibiting precipitation and diminishing the NAM. The increased atmospheric stability is exacerbated under uniform SST warming, and could have been at play in 2023, given the warm interbasin SSTs.

Large scale characteristics
Here, we investigate whether any large-scale climate features were responsible for the persistent heat wave. For example, Fig. 5 shows the SST, mean sea level pressure, and precipitation anomalies for June and July 2023 over the Pacific-Atlantic sector. In the tropical Pacific, the positive SST anomalies are indicative of the growing El Niño event, which attained a moderate amplitude by the end of 2023. Further north, the extratropical Pacific SSTs depict a negative anomaly near the west coast of Mexico and California and a positive anomaly along 40°N over the North Pacific current. Over the Atlantic sector, record-breaking warm SST anomalies were observed for the eastern portion of the basin in June, which then migrated westward, encompassing the entire basin with record SSTs in July. The North Atlantic experienced record warm SSTs in 2023, with temperatures greater than 1°C above the 1981–2010 climatology. Sea level pressure anomalies in June experienced a see-saw pattern with positive anomalies over the Pacific and negative anomalies over the Atlantic, indicating a strengthening of the Pacific subtropical high and a weakening of the Atlantic subtropical high (Fig. 5c). In July, the mean sea level pressure anomalies relaxed over the Pacific and weakened but remained negative over the Atlantic basin. Of interest is the low-pressure anomaly over the southwestern U.S. and Mexico in July (Fig. 5d), indicating the presence of a surface heat low. Precipitation anomalies (Fig. 5e, f) indicate a classical El Niño pattern with a southward shift of the Intertropical Convergence Zone (ITCZ) in the Pacific as well as a reduction in precipitation over the North American Monsoon. Negative precipitation anomalies are also evident over the heat wave region. As depicted in the heat budget analysis (Fig. 2), the surface energy balance showed significant anomalous positive surface sensible heating and negative latent heating during most of the 2023 summer.

Given that both the Atlantic and Pacific basins show remarkable SST anomalies (Fig. 5), it is worth investigating whether these SST anomalies have any connection to the temperature in the southwest U.S. and Mexico. For this, a partial least-square regression analysis is performed using a tropical North Atlantic SST index (TNA, area average over 5°N–25°N and 20°W–80°W) and a tropical Pacific SST index (Niño3, area average over 5°N–5°S and 150°W–90°W), shown by green boxes in Fig. 5a. Although these two SST indices have a temporal correlation of 0.16, and thus are not significantly correlated, we carry out a partial regression analysis to adequately extract their independent linear relationship with respect to 200hPa stream function, 200hPa temperature, and 2-m air temperature anomalies (Fig. 6). Note that a combined effect of warm TNA and positive Niño3 (i.e., El Niño) is associated with 200hPa anticyclonic circulation anomalies throughout the tropics along with 200hPa warm temperature anomalies that extend over the southern U.S. (Fig. 6c) and corresponding warm 2-m maximum air temperatures (Fig. 6d) that mimic the anomalies associated with the 2023 heat wave event (Fig. 6a, b). Separating the relative contribution of each basin, it is noted that the Atlantic SST anomalies are playing the dominant role given by the regression of TNA-only shown in Fig. 6e, f. Note that the upper-level response is the formation of an anticyclone and warm temperature anomalies over Mexico (Fig. 6e). This is associated with a Gill-type atmospheric response in association with the enhanced diabatic heating over the tropical Atlantic and Caribbean linked to the warm SST anomalies over the TNA. The 2-m air temperature (Fig. 6f) also shows enhanced warming over Mexico and the southern U.S., similar to the conditions associated with warm TNA and positive Niño3 (Fig. 6d). In contrast, the role of the Pacific SSTs is small, as shown in Fig. 6g, h. Thus, while there was a developing El Niño in the boreal summer of 2023, El Niño events tend to produce tropospheric warming throughout the tropics via a fast equatorial Kelvin wave and off-equatorial anticyclonic anomalies, similar to Fig. 6e. However, ENSO response to the northern hemisphere is strongest later in the seasonal cycle. Thus, the warm Pacific SSTs appear to have played a minimal role, at least in a linear sense, since extratropical ENSO teleconnections are known to peak in the boreal winter and are relatively weaker in other seasons. However, non-linear interactions (i.e., synergy between the warm Atlantic and a growing El Niño) could also be playing an important role. This synergy may not be readily extracted from the simple linear regression presented; thus, a dedicated sensitivity experiment is carried out next.

Model simulation
The previous sections discussed the coincident climate events (i.e., uniform SST warming from the extreme warm SSTs in the Atlantic, a growing El Niño in the Pacific, and a record low NAM) which could have been responsible for the occurrence and extended duration of the southwest US and Northern Mexico heat wave in 2023. While attributing a single cause to heat waves is difficult given their synoptic nature, previous works have shown that large-scale climate variations can modulate their occurrence. For instance, precipitation and thus soil moisture are strongly coupled to surface temperatures. It is also known that remote SST anomalies can modulate US climate through circulation changes on interannual timescales and even on decadal timescales. Therefore, we isolate the effect that the record warm Atlantic SSTs and the growing El Niño had on the longest-lasting southwest US heat wave event of 2023. For this, we perform sensitivity experiments using an atmospheric general circulation model (AGCM, see Methods) by prescribing the observed 2023 SST anomalies over the Atlantic and Pacific basins and climatology elsewhere (Supplementary Fig. 8). Analysis of these AGCM experiments is presented here in terms of the differences between the prescribed global 2023 SST experiment (GBL23) minus the climatology experiment (CTL) (see Methods section). Analysis from the ensemble mean of the 100 ensemble simulations shows enhanced surface net shortwave radiation over northern Mexico and the southwestern US as well as enhanced surface net longwave radiation, enhanced sensible heat flux, and reduced latent heat flux (Supplementary Fig. 9). These surface energy fluxes are consistent with enhanced warming and reduced moisture over the analysis region and consistent with the anomalies observed in 2023.

The large-scale circulation changes associated with 2023 SSTAs are shown by the difference between GBL23 minus CTL experiment for June-July-August (JJA) 200hPa temperature and 200hPa stream function (Fig. 7a). Note the anomalous heat dome structure and associated anticyclonic circulation pattern over Mexico and the southwestern US, which is remarkably consistent with the observed partial regression pattern (Fig. 6c). Here, the model responds to the 2023 SST forcing by producing an anticyclonic circulation anomaly over Mexico (positive contours), leading to enhanced near surface temperatures for most of the region (Fig. 7b). While the AGCM response is very similar to that of the observed analysis (Fig. 6), it is worth separating the relative contributions of Atlantic and Pacific SST anomalies. For this, we look at the SST sensitivity experiments with Atlantic-only (ATL23) and Pacific-only (PAC23) prescribed 2023 SSTs. Consistent with the regression analysis, the Atlantic SST forcing appears to play a more dominant role by creating an upper-level anticyclonic circulation anomaly (Fig. 7c) and enhanced near-surface warm temperatures (Fig. 7d) comparable in amplitude to that of the GBL23 experiment. In contrast, the PAC23 experiment shows relatively weak homogeneous upper tropospheric warming and anticyclonic circulation at low latitudes (Fig. 7e), a feature typical of a developing El Niño event. Thus, the surface temperature signal over the southwestern U.S. and Mexico is very small in comparison (Fig. 7f).

The sources for these teleconnections are analyzed via Rossby wave sources (RWS, see Methods). Note that there are a few regions of anticyclonic RWS in the GBL23 experiment (Fig. 8a) over the equatorial Pacific and Atlantic, near the location of the ITCZ, and also over the Greater Antilles. These are consistent with the warm 2023 SST forcing prescribed in the GBL23 experiment, which led to upper-level 200hPa divergence (Fig. 8c) and 200hPa anticyclonic circulation consistent with a Gill-type response (Fig. 8b). Further decomposition of the RWS into its component demonstrates that mean vortex stretching by the anomalous divergent flow (Fig. 8e) was the main driver of the anticyclonic RWS and thus the anticyclonic circulation over Mexico.

The individual roles of each ocean basin are investigated using the ATL23 and PAC23 sensitivity experiments. For the Atlantic-only forcing (Supplementary Fig. 10), a similar pattern emerges where the upper-level divergent flow being responsible for the anticyclonic RWS over the Atlantic basin, which leads to a downstream (westward) intensification of the upper-level anticyclone over Mexico and the eastern Pacific between 20 and 40°N. In contrast, the Pacific-only SST sensitivity experiment (Supplementary Fig. 11) yields anticyclonic RWS over the ITCZ, in association with the developing El Niño, and consistent with upper-level divergence over the heating region. However, this signal is smaller than those from the ATL23 forcing. The larger RWS from the ATL23 compared to the PAC23 sensitivity experiment is probably due to the fact that SSTs in the Atlantic were at their peak while the El Niño in the Pacific was still in the growing phase (Supplementary Fig. 8).

As discussed earlier, non-linear interactions (i.e., synergy between the warm Atlantic and a growing El Niño) could also be playing an important role in exacerbating extreme heat in the region. This interaction can be readily extracted from the AGCM experiments by isolating the interbasin synergy following Eq. 7 (see Methods) and is shown in Fig. 7g, h. Note that the synergy component mostly shows upper tropospheric warming and relatively warm near surface temperatures over the southwest US, while troughing and cooler temperatures upstream and downstream of the heat wave region (Fig. 7h). Besides changes in large-scale circulation patterns, the occurrence of heat wave events is also investigated within the AGCM experiments. For this, daily outputs of maximum temperature from the model experiment are used to compute changes in several heat wave characteristics. Here, the daily T95 percentile maximum temperature is computed from a 300-year control simulation. Then, heat wave amplitude, number of heat wave events, number of heat wave days, and the duration of each event are computed relative to the control T95 (Fig. 9). For all four indices, the difference between the GBL23 and CTL experiment is normalized by the expected occurrences in the CTL experiment, e.g., (GBL23 − CTL)/CTL. The relative changes between the GBL23 relative to the CTL experiment show an amplitude increase on the order of 10% (mostly over Mexico, where amplitude is measured as the maximum 2-meter temperature anomaly averaged over the duration of the event). The number of heat wave events more than doubled (e.g., greater than 100% increase), the number heat wave days tripled, and a >20% increase in the duration of heat waves occurred over the southwestern US and Mexico in the GBL23 sensitivity experiment. Thus, the sensitivity experiment further validates the role of the 2023 SSTs in modulating the occurrence and persistence of heat wave events over the study region.

A composite analysis for several heat wave metrics is shown in Fig. 10 for the grid point closest to Phoenix, Arizona from the AGCM experiment. For instance, the CTL (GBL23) case produced 77 (184) heat wave events. This is a 138% increase over the expected climatology, of which 75% is attributed to Atlantic SSTs, 31% to the Pacific, and 32% to non-linear or synergic interbasin interactions (Fig. 10a). Similarly, a total of 307 (853) heat wave days were produced by the CTL (GBL23) experiment (Fig. 10b), a 177% increase above climatology of which 89%, 27%, and 61% are attributed to the Atlantic-only, Pacific-only, and interbasin SST synergy respectively. The average event duration also increased by around 18% from the climatological 3.9 days to 4.6 days (Fig. 10c), of which 10% was due to the Atlantic-only and the remaining 8% from the interbasin synergy, and no contribution from the Pacific-only case. The longest-lasting heat wave in the CTL (GBL23) experiment was 9 (14) days, all while the GBL23 experiment experienced several double-digit event durations (Supplementary Table 1). Besides more and longer-lasting events, the GBL23 experiment also produced larger amplitude events. Looking at the return period in years of a very high threshold (a high threshold is chosen here as a temperature similar to the 2023 heat wave event) and modeled by a Pareto distribution, a 45°C daily maximum temperature is observed once every 45 (14) years in the CTL (GBL23) experiment (Fig. 10d). This represents a 222% increase in frequency above climatology (e.g., frequency is inversely proportional to return period), where 137%, 60%, and 25% were due to the Atlantic-only, Pacific-only, and interbasin SST synergy respectively.

The JJA daily maximum temperature is also significantly higher in the GBL23 experiment (ensemble mean of 40.2 ± 0.31°C) compared to the CTL experiment (ensemble mean of 39.1 ± 0.30°C), see Supplementary Table 1. This difference of over 1°C is significant against the background weather noise, which is taken from the ensemble spreads of the 100 realizations from each experiment. Similarly, the minimum temperature is also higher in the GBL23 experiment (26.8 ± 0.34°C) compared to the CTL case (25.8 ± 0.31°C). A daily minimum temperature excess over a very high threshold (here 31°C) is observed every 30 (11) years in the CTL (GBL23) experiments (Supplementary Table 1).

Methods
Observational data
Atmospheric variables (e.g., vertical profiles of temperature, moisture, geopotential height, wind, and heating rates) are obtained from monthly and daily means from the Japanese 55-year Reanalysis (JRA55) for the period of 1955–2023 with a 55 km spatial resolution, and 60 vertical levels up to 0.1 hPa. Data interpolated to 1.25° × 1.25° spatial resolution and 37 vertical levels from 1000 hPa to 1 hPa were used. Heating rates are provided directly by the JRA55 reanalysis, which includes net atmospheric radiation (short and longwave radiation), vertical diffusion heating rate, which represents the energy transfer without phase change; latent heat from moist processes, which includes the large-scale condensation heating rate and the convective heating rate. Daily maximum and minimum temperatures are obtained from meteorological observations for several stations along the southwestern U.S. from the NOAA/National Center for Environmental Information from 1950 to 2023. Observed SSTs are obtained from the Hadley Centre HadSSTv2 product at a 1° horizontal resolution for the period of 1900–2023. Anomalies are defined relative to the climatological period from 1979 to 2022.

Heat wave definition
We assess heat waves by the Excess Heat Factor (EHF) used in operational forecasts as well as in research studies. The EHF index (Eq. 1) is defined by combining two excess heat indices, namely an acclimatization index (Eq. 2), which measures the current 3-day temperature anomaly relative to the previous 30-day period; and a significant exceedance index, which measures the excess 3-day temperature over a high threshold, taken here as the 95th percentile temperature for that given day (Eq. 3). Here, Ti corresponds to the daily maximum surface temperature for day i and T95 is the 95th percentile temperature for that given day. A positive EHF characterizes heat wave conditions that persist for a minimum of three days.

EHF = max(1, EHI(accl.) × EHI(sig.))

EHI(accl.) = (Ti + Ti-1 + Ti-2)/3 - (Ti-3 + ... + Ti-32)/30

EHI(sig.) = (Ti + Ti-1 + Ti-2)/3 - T95

Heat budget
To isolate the physical processes responsible for the development and long persistence of the heat wave, a heat budget analysis is performed (Eq. 4) for the temporal evolution of a temperature anomaly T due to horizontal advection (~u·∇T), vertical advection (ω∂T/∂p), adiabatic heating (ωRT/Cp), and diabatic heating (Qnet). This last term consists of shortwave, longwave, diffusive, and latent heating. In Eq. 4, ~u and ω are the horizontal and pressure velocities, R = 287 J Kg−1 K−1 is the ideal gas constant for dry air, and Cp = 1,004 J Kg−1 K−1 is the specific heat of dry air at constant pressure. Equation 4 is vertically integrated from pressure level 975–800 hPa, chosen here to be representative of low-level temperature variations. The integral is mass-weighted and normalized by the total integration thickness ΔP to preserve the original units of °C/day of the heating rates and for easier comparison.

1/ΔP ∫ 975 800 ∂T/∂t dp = 1/ΔP ∫ 975 800 [−~u·∇T − ω∂T/∂p + ωRT/Cp + Qnet] dp

Trapped flow characteristics
Flow characteristics associated with blocking pattern and heat wave dome are assessed by the instantaneous local Lyapunov exponent (Eq. 5).

λ+ = 1/2 [D + (E2 + ζ2)1/2]

Where, λ+ is the positive Lyapunov exponent, which is equivalent to the dilation rate. Here, D = ∂u/∂x + ∂u/∂y is the horizontal divergence, E = (Esh2 + Est2)1/2 is the deformation, Est = ∂u/∂x − ∂v/∂y is the stretching deformation and Esh = ∂u/∂y − ∂v/∂x is the shear deformation, and ζ = ∂v/∂x − ∂u/∂y is the relative vorticity. The imaginary component of λ+ (i.e., where ζ = ∂v/∂x − ∂u/∂y) is used to represent regions of fluid trapping, or where the flow is dominated by vorticity. If ζ < 0, then the trapped flow is anticyclonic in nature, thus leading to significant air mass modification due to reduction in ventilation from adjacent air masses, and reinforcing the heat dome and surface warming.

North American Monsoon (NAM) definition
The NAM index follows the concept of the global monsoon, defined by area-averaging precipitation over western North America with the constraints that the June-July-August-September minus December-January-February-March precipitation range is greater than 2 mm/day and the local summer precipitation exceeds 55% of the total annual precipitation.

Rossby wave source
Rossby wave source is defined by Eq. 6, where the anomalous RWS is defined by the anomalous vorticity advection by the mean divergent wind, mean vorticity advection by the anomalous divergent winds, anomalous vortex stretching, and mean vortex stretching, respectively. Here, u is the divergent wind (i.e., irrotational component of the flow), ζ is the relative vorticity, and f is the Coriolis parameter. Overbar denotes the mean climatology, and primes represent the deviation from climatology.

RWS = −∇·(u ζ) = −u'·∇ζ − u·∇ζ' − f∇·u' − (ζ' + f)∇·u

Model experiments
To isolate the effect of significant tropical SST warming, several atmospheric general circulation model (AGCM) experiments are performed. We prescribe SST anomalies to the Community Atmosphere Model version 6 (CAM6) coupled to the Community Land Model version 5 (CLM5), which are part of the Community Earth System Model version 2 (CESM2). First, the control case is integrated by prescribing the SST climatology globally based on the 1979-2022 observing period, which is referred to here as the CTL case and is run for 300 years. Then, a model experiment is conducted where SST is prescribed based on the observed 2023 SST only over the Atlantic and Pacific and is held to climatology elsewhere, referred here onward as the GBL23 case (see Supplementary Fig. 8 for prescribed SST anomalies). For the GBL23 case, the last 100 years of the CTL are used as initial conditions to create 100 GBL23 ensembles. That is, the experiments are integrated for one year starting on every 1 January from the CTL (e.g., branch simulation), thus creating ensembles with initial conditions that are one year apart from each other. This approach guarantees more than sufficient separation among ensembles as it pertains to weather noise (i.e., the atmospheric initial state is completely different and independent among ensembles) while each ensemble is being forced by the same underlying SST anomalies.

To isolate the relative influences of the Atlantic and Pacific SST anomalies on atmospheric circulation, two additional model experiments are performed where the Atlantic-only and Pacific-only 2023 SST anomalies are prescribed, these experiments are referred to as ATL23 and PAC23, respectively. All experiments were integrated under a year-2000 atmosphere composition (i.e., CESM2 component set F2000). The experiments are evaluated with respect to their differences relative to the CTL runs, thus, any contrast between the two experiments is attributed to SST anomalies in 2023, whereas the ensemble spread of each model run is used as uncertainty estimation (i.e., weather noise) through a bootstrapping technique. It is worth mentioning that besides the modulating effects of circulation, heat waves are also influenced by complex land-atmosphere interactions/feedbacks that may not be well resolved in CESM2. In addition, significant topographic features are present in the study area, which have been shown to play an important role in surface temperature variations, but may not be well-resolved at these horizontal resolutions. Moreover, complex air-sea interactions are also missing from these AGCM simulations as the atmospheric model is forced by SSTs with no ocean thermodynamic nor dynamical feedback.

Atlantic-Pacific synergy (non-linear interactions)
The synergy between the Atlantic and Pacific SST sensitivity is extracted from the AGCM experiments following Eq. 7. This is possible because the GBL23 experiment comprises the total linear plus non-linear SST sensitivities, whereas the targeted basin experiments (i.e., ATL23 and PAC23) isolate interbasin interactions. See Supplementary Information for more details on the definition of interbasin synergy and derivation of Eq. 7.

Interbasin Synergy = GBL23 − ATL23 − PAC23 + CTL

Statistical significance—bootstrapping technique
A bootstrapping method is used to determine confidence intervals by subsampling the dataset. All analyses presented are obtained by randomly selecting r samples out of n observations with replacement (Eq. 8). This is done 500 times in order to build a significant distribution of composites and assign 95th percentile confidence levels.

(n r) = n! / r!(n − r)! = possible combinations

Data availability
Hadley Centre HadSSTv2 product for the period of 1900–2023 was obtained from https://www.metoffice.gov.uk/hadobs/hadisst/ (Rayner et al.). The Japanese 55-year Reanalysis was obtained from https://rda.ucar.edu/datasets/ds628.0/ (JRA55, Kobayashi et al.) for the period of 1955–2023. Daily maximum temperatures are obtained for meteorological observations for Phoenix, Arizona from the NOAA/National Climate for Environmental Information from 1950–2023 https://www.ncdc.noaa.gov/cdo-web/search?datasetid=GHCND. Source data are provided with this paper.

Code availability
Details on installing and running CESM2 can be found at https://github.com/ESCOMP/CESM. The data in this study were analyzed using the publicly available Gridded Analysis and Display System (GrADS; http://cola.gmu.edu/grads/).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The manuscript\n\"The longest-lasting 2023 western North American heat wave was fueled by the record-warm Atlantic Ocean\" by Hosmay\nLopez, Sang-Ki Lee, Robert West, Dongmin Kim, Liwei Jia\nfeatures an in-depth investigation of the causes and processes related to the outstanding heatwave that affected\nsouthwestern US and Mexico between June and July 2023. By analysing in-situ measurements, remote sensing, gridded\nobservations and reanalyses, the authors argue that the event in the region of interest, and particularly in the area of\nPhoenix, was characterized by abnormal positive anomaly of sensible, diffusive, and radiative surface fluxes towards the\nsurface. They also found that an anticyclonic pattern, suppressing advective fluxes and enhancing local adiabatic heating,\nwere responsible for the long heat streak across much of July. The North American Monsoon, and moisture fluxes entering\nthe area from the oceans, were anomalously weak. Regression analyses evidenced that the role of North Atlantic SSTs was\nmore releant than that associated with subtropical Pacific SSTs. This was confirmed by sensitivity analyses using a General\nCirculation Model in AMIP settings, forcing the atmosphere with prescribed SSTs representative either of the overall SST\nanomalies in Summer 2023, or of the anomalies over the Atlantic and over the Pacific during the same period. The\ninterbasin feedback is claimed to play a substantial role in the build-up of the anomaly, therefore the authors stress that the\naction of both basins has to be taken into account when attempting to attribute the North American heatwaves to\nanthropoegenic climate change enhanced by oceanic natural variability.\nOverall, I think that the manuscript has scientific merits, and is worth publication. However, there are several substantial\naspects that should be carefully taken into account before the manuscript can be accepted.\nMy main concern is that in the section involving observations authors fall a bit short in justifying the correlations of different\naspects of the circulation with process understanding. Another aspect that should be clarified is the attribution of extreme\nevents to the role of Atlantic-Pacific-synergy of both SSTs. Therefore, I recommend the manuscript for major revision.\nSpecific comments are detailed below.\n- l. 27: 2024 surpassed 2023 as warmest year on record. Please mention it.\n- l. 36: Is there any reason why Phoenix would be an exceptional spot where to focus the analysis? The rational of the work\nwould be certainly improved by providing a justification for this;\n- ll. 43-45: I see that all these examples mention Northern Hemisphere continents, but outstanding heatwaves also affected\nthe Southern Hemisphere. Perhaps worth mentioning it.\n- ll. 53-54: it might be worth mentioning that the effects of heatwaves are also exarcebated by increasing urbanization,\nagricultural loss, increased aridification;\n- l. 75: furnished -> provided\n- l. 77: I may agree that the oceanic timescales have timescales that are longer than those pertaining on local heat extremes,\nbut I am not sure this applies to land process variabilities as well (e.g. Hebert et al. 2022);\n- ll. 82-83: Perhaps it is worth mentioning Kuhlbrodt et al. 2023 on the role of ocean heat content for North Atlantic 2023\nextreme;\n- l. 97: it is surprising that the authors seem to be unaware of the active debate on the interbasin Atlantic-Pacific\nteleconnections, especially in the Tropics, that has developed in years. I am not in the position to provide a conclusive\nliterature survey, but I may suggest that the authors have a look at the debate around the Jiang et al. 2023 paper (e.g. Richter\net al. 2024) and provide some references in the manuscript.\n- l. 101: related to my previous comment, it could be better emphasized that the findings that are actually contributing\nsubstantially to the topic regard the inter-basin synergy. So far, it seems to the reader that is just one of the many aspects\ntouched here in the context of the 2023 heatwave;\n- l. 104: it shall be mentioned explicitly here what is the reference period for the computation of the climatology;\n- l. 109: I struggle a bit to read Figure 1. I wonder why the authors prefer showing absolute temperatures instead of\nanomalies, that would much better emphasize the T95 exceedances;\n- l. 129: Figure 2 is very chaotic in my opinion, and I think it shall be improved by showing actual time series, instead of\nboxes with 10-days averages. If the daily spread is too high, a 10-days moving average could be superimposed on the\nactual time series. Also, I think that it is very misleading to position color bars next to x-axes, whose labels are only found at\nthe bottom of the figure;\n- l. 140: Figure 3c may be obvious to a reader with some sort of meteorology background, but it is not at all for many readers\nof Nature Communications. Therefore, the skew-T should be carefully explained (perhaps in the Methods?) making a bit\nmore explicit what the authors discuss, particularly at lines 184-188 and in Figure 3c description;\n- l. 142: it is nowhere explained what the authors mean by \"diffusive heating\" and how this is computed;\n- l. 146: similarly to my comments above: it is not worth mentioning that the Bowen ratio increases if it is not explained what\nthis means in terms of process understanding;\n- l. 159: wouldn't it be clearer to state that the dynamical tropopause crosses the 2 PVU isosurface where the thick solid line\nis?\n- l. 165: not clear why I would infer from Figure 2 that there is shortwave surface heating in clear sky, i.e. why I would have to\nassume clear sky;\n- l. 168: it is textbook literature that atmospheric blocking is among the most prominent features of extratropical low frequency\nvariability (e.g Rex et al. 1950), then some sort of agreement shall be reached on what the authors refer to as \"synoptic\",\nbecause blocking is not proper of synoptic timescales (although it might be to some extent of synoptic spatial scales,\nalthough I would rather refer to it as a manifestation of planetary waves; cfr. Stoll et al. 2023 and references therein)\n- l. 198: what is the exact definition of \"western North America\" here?\n- l. 202: this is another example of one of my main concerns about (this section of) the manuscript. Besides showing that the\nNorth American Monsoon is weak in 2023, claiming that some sort of correlation is present between moisture fluxes enter\nthe region and negative latent heat fluxes locally (in Phoenix, at least), what do we learn about the relation between\nexceptional heatwaves in the region and the monsoon? Could it be just a chance? What happened in 1982 (the weakest\nNAM on records, according to the time series in Figure 4a)? Is the anomaly significant wrt. NAM climatology?\n- l. 211: related to my previous comment; authors are comparing the NAM activity along the whole season, but the heatwave\npeaked during 14 days in July and short heatwave periods emerged during August and September (possibly not related to\nthe major one). It would perhaps be more informative if the evolution of daily moisture transports could be tracked during the\nseason;\n- l. 254: how relevant is negative anomaly (shown in mm/day) compared to the precipitation that usually occurs during July in\nthe region (i.e. 22 mm, according to NOAA Weather data)? I would assume that precipitation in July would be very\nintermittent in this region, and having -6 mm/day during one specifc month of a specific year would not mean much;\n- ll. 257-258: this could be already stated at l. 146;\n- ll. 262-263: for sake of clarity, these boxes could be highlighted on Figure 5;\n- l. 267: this should be compared with the available literature mentioned above;\n- ll. 318-319: the role of topography seems to be crucial here. Can the authors comment on this?\n- ll. 348-350: I find this argument a bit illogic. I see no reason why the sum of percentages from ATL and PAC sensitivity\nexperiments would result in percentage increase of GLB23 minus the residual (\"synergic interbasin interactions\"). It might be\ninformative to obtain composite mean maps for the three experiments, as in Figure 7, then subtract from the complete SST\nprescribed field to have an idea of the role of interbasin interactions. I do not think though, that the same can be done to infer\nthe relative role of the three components for the overall GLB23 increase in extremes. The role of Atlantic and Pacific\nalone could determine a growth in the occurrence of the chosen events that would be larger than the combined effects, i.e. the\ninterbasin interaction role would be effectively to reduce the role of the individual basins. This would not be captured if using\nthe described approach, and it would be particularly relevant when dealing with extreme events. Please clarify this;\n- l. 375: once again, it is not clear what you mean by \"western North America\", and I do not think that your arguments are\nequally valid for the whole of western North America, e.g. including Canada and Alaska;\n- ll. 412-419: this paragraph does not seem to belong to conclusions, as it would be better fitting into\nIntroduction;\n- ll. 426-427: is there a specific reason why authors decided to use this Reanalysis product instead of others? What is the\nspatial resolution of this product?\n- ll. 451-452: the choice of 850hPa temperature seems a bit arbitrary, 2-metres temperature would have been a more valid\nalternative for near-surface temperature;\n- ll. 472-473: although I can understand that for the sake of comparison with Reanalyses, it was necessary to choose a\nreference period that was also encompassed by JRA55, it is a bit odd that a climatology for a period characterized by a\nstrong positive trend and so close to the 2023 event has been considered. Perhaps a previous period (same length, still\nencompassing the 1958-present JRA55 coverage) would have been a more appropriate choice;\n- l. 477: how is this ensemble created? What initial conditions are chosen, and how distant from each other? This should be\nmade explicit;\n- ll. 480-481: same as in my previous comment. There are several bootstrapping techniques that could be used for the\nscope. For the sake of reproducibility, the technique shall be described here;\n- ll. 483-484: it is not clear if ATL23 and PAC23 are ensembles as well;\nReferences\n- Hebert et al. 2022: https://www.nature.com/articles/s41561-022-01056-4\n- Jiang et al. 2022: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2023GL103777\n- Kuhlbrodt et al. 2023: https://journals.ametsoc.org/view/journals/bams/105/3/BAMS-D-23-0209.1.xml\n- Rex et al. 1950: https://www.tandfonline.com/doi/abs/10.3402/tellusa.v2i4.8603\n- Richter et al. 2023: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2024GL111563\n- Stoll et al. 2023: https://wcd.copernicus.org/articles/4/1/2023/", "reviewer_2": "Summary:\nOverall, this is an interesting and well-written paper. Lopez et al. investigate the extreme and persistent heatwave that\nimpacted the southwestern U.S. and northern Mexico in the summer of 2023. Using observational data, regression analysis,\nand atmospheric model simulations, the study finds that warm Atlantic SSTs, combined with a developing El Niño in the\nPacific, significantly contributed to the increase in heatwave frequency, duration, and intensity. The authors employ heat\nbudget analysis to identify key thermodynamic and radiative processes that sustained extreme surface temperatures. The\nstudy concludes that the interbasin synergy between the Atlantic and Pacific basins exacerbated the conditions that led to\nthe record-breaking heatwave. The findings suggest that continued ocean warming may lead to an increased likelihood of\nprolonged and extreme heat events in the future. I do not have any substantial comments about the analysis or the main\nconclusions, which seem solid and well-supported. I recommend some minor revisions before this manuscript can be\npublished, please see my comments below.\nMain comments:\n1. Some of the narrative and factual statements in the abstract and introduction can use finetuning, and I have left a number\nof specific line comments with my suggestions for changes.\n2. The multitude of different time periods used for baselines seems unnecessary. For example, depending on the variable,\nthe authors use 1979-2022 (SST and vertically integrated moisture transport), 1982-2023 (Phoenix temperatures), 1979-\n2023 (heat budget analysis and NAM), and 1960-2023 (SST regression). Could the authors find one or maybe at most two\ncommon baselines to use for all the analyses?\nSpecific comments:\nLine 14: WMO 2023 reference not in citations.\nLine 18: Hazard >> disaster\nLine 18: Was this the costliest natural disaster of 2023 in North America? Perhaps specify this. Globally, I believe the Turkey\nearthquake in February was more costly.\nLine 32-33: Is there a citation for “200 deaths”? I believe the death toll in Maricopa County alone was more than 200.\nLine 35: Similar to my earlier comment, it should be specified that this was the costliest natural disaster in North America, not\nglobally.\nLine 36-37: I think the more relevant statistic to highlight is the record streak of days above 110F in Phoenix (31 straight),\nwhich broke the previous record of 18 straight days. Days >40°C may not be the most relevant statistic for Phoenix where the\naverage high temperature in July is 41.4°C, meaning that 40°C is a cool day. The record warm Tmin of 97°F is interesting\nand can be kept in the text.\nLine 38-40: This sentence, as currently worded, is inaccurate and should probably be deleted. The heat wave that produced\nthe all-time record in New Orleans was a completely separate synoptic event that occurred on August 27 in a different region\nof the country. Similarly, the San Juan record resulted from a different tropical heatwave that was not part of the western\nNorth American heatwave that the authors are studying.\nLine 60: delete “the”\nLine 73: Might be good to add a quick sentence that explains how a Gill-type response can produce blocking over the Great\nPlains. I am assuming that the broad readership of Nature Communications will not be familiar with this.\nLine 83-84: I would delete this sentence, since the AMO itself has been called into question as a “real” mode of internal\nclimate variability.\nLine 79-93: The effect of the 2020 international shipping sulfate reductions over the N. Atlantic might need to be mentioned\nin this paragraph, since they likely played a role in the warm SST during 2023.\nFigure 1b: It might be good to adjust the x-axis to make the highest Tmax more visible. For example, Phoenix hit 122F (50C)\nin 1990 and 121F (49.4C) in 1995 but these temperatures are not visible in the histogram, making it seem like 119F (48.3C)\nin 2023 was the highest tmax between 1982-2023.\nLine 113: This is incorrect, the 2023 maximum of 119F/48.3C was tied for the 3rd warmest maximum in Phoenix after 122F\nin 1990 and 121F in 1995 (see my comment above).\nLine 117: “engulfed” >> “affected”\nLine 124-125: Why highlight this particular Tmax on Aug 23 in New Orleans? They were even hotter on Aug 27th, when their\nall-time record high of 105F was reached. Perhaps this should be highlighted instead.\nFigure 4a: It is surprising that 2020 doesn’t appear as a very strong negative anomaly. This was a well-known failed\nmonsoon season in the SW US, e.g.: Hoell et al., 2022: Record low North American monsoon rainfall in 2020 reignites\ndrought over the American southwest. The authors may want to double check their monsoon index calculations, and if found\nto be correct, perhaps provide the appropriate caveats as to why their index does not capture certain anomalous years.\nLine 197-201: Related to the above comment, the methodological details of the monsoon index calculations should be fully\nexplained in the methods section.\nLine 246: Please check the “(negative)”, it doesn’t seem to match the rest of the sentence.\nLine 261: I am confused by “partial regression”. Do the authors mean partial least squares regression?\nLine 346: I may have missed it, but I don’t recall seeing the extreme value analysis described in the methods?\nLine 387: “persistent heat wave” >> “persistence of the heat wave”\nLine 416-419: The statement about the earliest 110°F on record in June 2024 is only true for Las Vegas, not Phoenix.\nLine 429: “Climate” >> “Centers”\nLine 501: Delete “the”", "reviewer_3": "\"The longest-lasting 2023 western North American heat wave was fueled by the record-warm Atlantic Ocean\" by Lopez et al.\nSummary:\nThis study investigates the causes of the longest-lasting heat wave in western North America in 2023 through observational\nanalysis and atmospheric general circulation model (GCM) experiments. The key conclusion is that the 2023 heatwave was\nsignificantly driven by record-warm Atlantic sea surface temperature (SST) and growing El Niño in the Pacific. The authors\nconducted a comprehensive analysis of the roles of both the atmosphere and the ocean in driving the heat wave, using\nenergy budget analysis, 3-dimensional atmospheric circulation and moisture transport, and the impact of SST over both the\nAtlantic and Pacific Oceans. I believe this manuscript is well-written and makes a valuable scientific contribution to\nunderstanding the dynamic and thermodynamic changes that triggered this heatwave event. However, I found several\nissues that need to be addressed to improve the manuscript. For publication, please revise the manuscript to satisfactorily\naddress the specific comments provided below.\nSpecific Comments:\n1. The authors concluded that the exceptionally warm Atlantic Ocean and the growing El Niño in the Pacific caused changes\nin circulation, leading to an anticyclonic blocking pattern during the summer of 2023. They supported their findings with\nevidence of interbasin synergy effect captured in model experiments. However, it remains unclear about how the Pacific\nSST interacts with the Atlantic in this synergy effect to create the persistent blocking pattern. The dynamic mechanism needs\nto be addressed as to how the El Niño-driven SST in the Pacific is linked to the ongoing atmospheric blocking over the\nwestern U.S.\n2. Potential driver of the Atlantic warming in the discussion section: I’m still not convinced about the possible impact of El\nNiño on the Atlantic SST increase (e.g., L404-406), as warmer-than-average Atlantic SST was not frequently observed\nduring the previous El Niño developing years. Could you provide a more compelling explanation of the El Niño-driven\nincrease in Atlantic SST?\n3. This study shows many results for Phoenix, Arizona (e.g., Figures 1, 3b,c, and 9). Extremely high temperatures were,\nhowever, observed in other regions as well, including Texas, northern Mexico, and Louisiana. I recommend that the authors\nprovide evidence showing that these other areas share quite similar characteristics to Phoenix, or at least discuss these\nregions in greater detail.\n4. Equation 4 for budget analysis: I believe vertical integration over a range of pressure levels would be more preferable than\nconsidering a single level (850hPa). This would be specifically true for vertical advection.\n5. Advection in budget analysis: Could you elaborate which factor, the wind versus temperature gradient, and mean versus\nanomaly, has a greater influence on determining the advection in the budget analysis?\n6. The authors mention that they present the PV pattern in Figure 3 during the peak of the heatwave. However, it seems this\nis based on the heatwave that occurred in Arizona. I am wondering how the large-scale atmospheric condition shown in\nFigure 3 could support the development of heatwave condition over E Mexico, E Texas and Louisiana, where northerly\nwinds are prevalent. Is the PV pattern here explaining the large-scale condition for the heatwave over Arizona only?\n7. Following the comment above (#6), the timing of the heatwave peak seems to vary across different regions, such as\nArizona, Texas, Louisiana, NW Mexico, and E Mexico, as seen in Supplementary Figure 1. The PV and circulation pattern\nduring the peak (June, July, or August) of the heatwave could differ between regions. The authors may need to address this\nissue.\n8. Anomalous moisture transport: Figure caption 4 states this field is computed from departures from climatology. Does this\nmean that the departure of wind and moisture from climatology is used to compute the anomalous moisture transport and its\nconvergence? If so, I believe this approach may be incorrect. Moisture transport is determined by four components: 1)\nadvection of mean moisture by mean wind, 2) advection of mean moisture by anomalous wind, 3) advection of anomalous\nmoisture by mean wind, and 4) advection of anomalous moisture by anomalous wind. The fourth term may be an order of\nmagnitude smaller than the other terms. I recommend that authors clarify how the Figure 4c pattern was calculated.\n9. The moisture transport field suggests that the heat transport from the Atlantic might also be lower than climatology,\npotentially due to the prevailing wind direction, which is not southerly. Could you offer some discussion on this?\n10. L249: Is it “significantly” negative? The negative anomaly observed in June weakens in July. Additionally, could you\nexplain how this negative anomaly in the Atlantic is related to the anticyclonic circulation linked to atmospheric blocking in\nthe upper troposphere? Are they connected, or is the blocking a consequence of upper-level dynamics with no influence\nfrom the surface level shown in Figure 5?\n11. The authors present Figure 5 to explore large-scale climate factors that may drive heatwave conditions. However, in\nterms of causality, it is not clear whether the precipitation over U.S. land is a cause or an effect of the heatwave. This needs\nto be explicitly addressed in the manuscript.\n12. Is the upper-level anticyclonic circulation pattern in Figure 6a (contoured) similar to the observed pattern in 2023? If so, is\nthis pattern generally pre-dominant throughout the summer?\n13. Why does the Atlantic-only experiment produce warmer than average temperature along the western coast of Mexico,\nwith no notable warming over the central to eastern side of Mexico (Figure 7)?\n14. While the amplitude change of heatwave events in the AGCM experiments shows the highest values over Mexico, the\nnumber of events, heatwave days, and duration in Figure 8 show the greatest increase in the far inland areas like Colorado,\nas well as western Mexico and the southern coastal region. Have you considered why the largest values appear in the far\ninland area? Could the large-scale atmospheric pattern in the AGCM experiment offer any insights to help explain this?"}, "author_response": "Please extend our gratitude to the three anonymous reviewers for their constructive\ncomments/suggestions that have led to a significant improvement to our manuscript.\nThe following overarching changes have been made to the manuscript in order to address the reviewers’\ncomments/suggestions:\n● Added more up-to-date statistics and chronological description of the heat wave, including the\nimpact of the heat wave in several cities besides Phoenix, Arizona.\n● More in-depth analysis on the physical mechanisms driving the heat wave, including clarifying\ndynamic mechanisms and equations\n● Added more context in relation to existing literature\nIn what follows, we respond to each comment of all reviewers. Our responses are in regular font and the\noriginal reviewers’ comments are in blue italic font. Each comment is addressed in the revised manuscript.\nWe point to specific line numbers where the changes were made, these line numbers refer to the lines of\nthe tracked-changes version of the text, A copy of the revised manuscript is provided with tracked changes\nrevisions.\nReviewer #1 (Remarks to the Author):\nThe manuscript\n“The longest-lasting 2023 western North American heat wave was fueled by the record-warm Atlantic\nOcean” by Hosmay Lopez, Sang-Ki Lee, Robert West, Dongmin Kim, Liwei Jia\nfeatures an in-depth investigation of the causes and processes related to the outstanding heatwave that\naffected southwestern US and Mexico between June and July 2023. By analysing in-situ measurements,\nremote sensing, gridded observations and reanalyses, the authors argue that the event in the region of\ninterest, and particularly in the area of Phoenix, was characterized by abnormal positive anomaly of\nsensible, diffusive, and radiative surface fluxes towards the surface. They also found that an anticyclonic\npattern, suppressing advective fluxes and enhancing local adiabatic heating, were responsible for the\nlong heat streak across much of July. The North American Monsoon, and moisture fluxes entering the\narea from the oceans, were anomalously weak. Regression analyses evidenced that the role of North\nAtlantic SSTs was more releant than that associated with subtropical Pacific SSTs. This was confirmed by\nsensitivity analyses using a General Circulation Model in AMIP settings, forcing the atmosphere with\nprescribed SSTs representative either of the overall SST anomalies in Summer 2023, or of the anomalies\nover the Atlantic and over the Pacific during the same period. The interbasin feedback is claimed to play\na substantial role in the build-up of the anomaly, therefore the authors stress that the action of both\nbasins has to be taken into account when attempting to attribute the North American heatwaves to\nanthropogenic climate change enhanced by oceanic natural variability.\nOverall, I think that the manuscript has scientific merits, and is worth publication. However, there are\nseveral substantial aspects that should be carefully taken into account before the manuscript can be\naccepted.\nMy main concern is that in the section involving observations authors fall a bit short in justifying the\ncorrelations of different aspects of the circulation with process understanding. Another aspect that should\nbe clarified is the attribution of extreme events to the role of Atlantic-Pacific-synergy of both SSTs.\nTherefore, I recommend the manuscript for major revision.\nResponse: We are very grateful for your constructive comments/suggestions and hope to have\nsuccessfully addressed all your comments. Please find our replies to your specific comments below in\nregular fonts and your original comments in blue italic fonts. We point to specific line numbers where the\nchanges were made, these line numbers refer to the lines on the tracked-changes version of the paper.\nSpecific comments are detailed below.\n- l. 27: 2024 surpassed 2023 as warmest year on record. Please mention it.\nResponse: Agree. We did not add this fact in the original version as the final report for 2024 was not\nreleased by the time we submitted the paper for peer-review (November 2024). The discussion section\nnow talks about 2024 extreme temperatures and we also have acknowledged this fact in the introduction\nof the revised manuscript as follows (Lines 28 – 30):\n“The year 2023 was the second warmest on records1,2, only surpassed by 2024 (Monthly Global Climate\nReport for Annual 2024). Of particular notoriety was the 2023 boreal summer months (i.e., June-August),\nwhich were characterized by more than 20% of the land surface setting new extreme warm records and\nmultiple heatwave events over most continents3.”\nNOAA National Centers for Environmental Information, Monthly Global Climate Report for Annual\n2024, published online January 2025, https://www.ncei.noaa.gov/access/monitoring/monthly-\nreport/global/202413. DOI: https://www.ncei.noaa.gov/access/metadata/landing-\npage/bin/iso?id=gov.noaa.ncdc:C00672\n- l. 36: Is there any reason why Phoenix would be an exceptional spot where to focus the analysis? The\nrational of the work would be certainly improved by providing a justification for this;\nResponse: Agree. We used Phoenix as an example of a major metropolitan area impacted. In addition,\nPhoenix also provides one of the longest reliable and continuous observational records in the area of\ninterest. With that said, the focus of our paper is on the heat wave event as a whole, which was fairly\nlarge-scale and exerted severe impacts throughout the southwestern U.S. and Mexico.\nWe have now added more discussion on the effects of the 2023 heat wave on other important population\ncenters. For example, we added a new supplementary figure (Supplementary Fig. 1) and discussion (Lines\n142 – 144) in the revised paper showing the temperature evolution during the summer of 2023 for Las\nVegas, Nevada, Albuquerque, New Mexico, El Paso, Texas, and San Antonio, Texas. Note that all these\ncities were impacted by the heat wave with both maximum and minimum temperatures exceeding their\ncorresponding 95 percentile for multiple days. Also note that the analysis only includes cities within the\ncontinental U.S. as long-term continuous and reliable data are not available for northern Mexico.\n- ll. 43-45: I see that all these examples mention Northern Hemisphere continents, but outstanding\nheatwaves also affected the Southern Hemisphere. Perhaps worth mentioning it.\nResponse: Agree. We have added examples of Southern Hemisphere heat waves, which 2023 was an\nexceptional year for global impacts. For example, South America experienced major heat waves, with\nrecord-breaking temperatures (Perkins-Kirkpatrick et al. 2023). Heat waves are also the leading cause of\nnatural hazards related deaths in Australia, accounting for more than 55% of the reported fatalities\n(Coates et al. 2014). See Lines 54 - 58 of the revised text.\nCoates, L., Haynes, K., O’brien, J., McAneney, J., & De Oliveira, F. D. (2014). Exploring 167 years of\nvulnerability: An examination of extreme heat events in Australia 1844–2010. Environmental\nscience & policy, 42, 33-44.\nPerkins-Kirkpatrick, S., Barriopedro, D., Jha, R., Wang, L., Mondal, A., Libonati, R., & Kornhuber, K.\n(2024). Extreme terrestrial heat in 2023. Nature Reviews Earth & Environment, 5(4), 244-246\n- ll. 53-54: it might be worth mentioning that the effects of heatwaves are also exacerbated by increasing\nurbanization, agricultural loss, increased aridification;\nResponse: Great point, we added the role of increased urbanization and land use changes and the\nfollowing references to the revised text (Lines 64 - 65).\nFischer, E. M., Oleson, K. W., & Lawrence, D. M. (2012). Contrasting urban and rural heat stress\nresponses to climate change. Geophysical research letters, 39(3).\nMishra, V., Ganguly, A. R., Nijssen, B., & Lettenmaier, D. P. (2015). Changes in observed climate\nextremes in global urban areas. Environmental Research Letters, 10(2), 024005.\nOverpeck, J. T., & Udall, B. (2020). Climate change and the aridification of North America. Proceedings\nof the national academy of sciences, 117(22), 11856-11858.\n- l. 75: furnished -> provided\nResponse: Done.\n- l. 77: I may agree that the oceanic timescales have timescales that are longer than those pertaining on\nlocal heat extremes, but I am not sure this applies to land process variabilities as well (e.g. Hebert et al.\n2022);\nResponse: The intention of this sentence was to onto potential oceanic (mostly SST) predictors for heat\nwaves at the seasonal timescales. This stems from the fact that, at seasonal and longer timescales, the\npredicted portion of a climate variable comes mostly from oceanic processes (modes of variability) (Fig. 2\nin Mariotti et al. 2018). For example, seasonal outlooks like temperature and precipitation over the U.S.,\nbasin wide tropical cyclone activity, etc. draw their predictors primarily from the global SST state.\nThe study of Hebert et al. 2022 reports on the longer term (years to centuries) variability over regional\nterrestrial domains and how it may be underrepresented in future projections from climate models,\nespecially for regions with significant maritime influences. That is, these regions would experience even\nlarger natural variability than what is currently not well represented in GCM projections, which is shown\nto be more relevant at timescales longer than multi-decadal. Their finding has great implications as this\nenhanced natural variability would cause unpredictable climate shifts as opposed to a more homogeneous\nprojected warming.\nHowever, this is somewhat unrelated to our argument stated in the sentence as we are not referring to\nfuture projected warming versus natural factors, instead we are interested in the predictable aspects of\nnatural variability at the interannual timescales, which mostly comes from ocean variations. With that\nsaid, we have added a brief discussion and the reference to Hebert et al. 2022 in the revised text (Lines 93\n- 95).\nMariotti, A., Ruti, P. M., & Rixen, M. (2018). Progress in subseasonal to seasonal prediction through a\njoint weather and climate community effort. Npj Climate and Atmospheric Science, 1(1), 4.\n- ll. 82-83: Perhaps it is worth mentioning Kuhlbrodt et al. 2023 on the role of ocean heat content for\nNorth Atlantic 2023 extreme;\nResponse: We added the reference Kuhlbrodt et al. 2023 to the revised text (Line 97).\n- l. 97: it is surprising that the authors seem to be unaware of the active debate on the interbasin Atlantic-\nPacific teleconnections, especially in the Tropics, that has developed in years. I am not in the position to\nprovide a conclusive literature survey, but I may suggest that the authors have a look at the debate\naround the Jiang et al. 2023 paper (e.g. Richter et al. 2024) and provide some references in the\nmanuscript.\nResponse: We are aware of the current debate regarding the Atlantic-Pacific Interbasin interactions. Some\nof the co-authors here in our paper have recent publications regarding the pantropical interactions\nbetween the three (Indian, Pacific, and Atlantic) basins (Lee et al. 2023) and how the Pacific-Atlantic\ninterannual variability could explain extreme events like major hurricanes (Kim et al. 2023).\nThe papers of Jiang et al. 2023 and references therein and the comments from Richter et al. 2024 are\ncentered around the equatorial Pacific-Atlantic interactions, mostly with regards to how El Niño-Southern\nOscillation (ENSO) influences and is influenced by its Atlantic “little cousin”, Atlantic Niño. However,\nour paper instead focuses on the impact of tropical Atlantic and Pacific warm sea surface temperatures on\nNorth American heat waves. Thus, the physical mechanisms driving these SSTs anomalies, including the\ntropical Pacific/Atlantic interaction conundrum, are beyond the scope of our work. We made this\ndistinction in the revised paper and added Jiang et al. 2023 and Richter et al. 2024 (Lines 121 - 124).\nLee, S. K., Lopez, H., Tuchen, F. P., Kim, D., Foltz, G. R., & Wittenberg, A. T. (2023). On the genesis of\nthe 2021 Atlantic Niño. Geophysical Research Letters, 50(16), e2023GL104452.\nKim, D., Lee, S. K., Lopez, H., Foltz, G. R., Wen, C., West, R., & Dunion, J. (2023). Increase in Cape\nVerde hurricanes during Atlantic Niño. Nature Communications, 14(1), 3704.\n- l. 101: related to my previous comment, it could be better emphasized that the findings that are actually\ncontributing substantially to the topic regard the inter-basin synergy. So far, it seems to the reader that is\njust one of the many aspects touched here in the context of the 2023 heatwave;\nResponse: As per our previous reply, our focus is not on explaining the tropical dynamics of SST\nvariations. Instead, our focus in this paper is with regards to remote impacts. What we refer to as the\n“Interbasin synergy effect of Pacific-Atlantic forming” is in reference to the role of SSTs on heat waves.\nWe politely disagree with the reviewer in that we believe that performing a literature review on ENSO-\nAtlantic Niño interactions will cause some distraction and steer the focus of this paper away from the\n2023 heat wave event.\nWith that said, we agree that more clarification needs to be made with regard to the “interbasin synergy\neffect”. For that, we added a statement clarifying that this interbasin synergy is referring to the\nconstructive interaction between the warm Atlantic and ENSO in modulating the heat wave occurrence\nrather than alluding to the tropical Pacific/Atlantic interaction conundrum (i.e., Jiang et al. 2023; Richter\net al. 2024). See Lines 121 - 124 of the revised text.\n- l. 104: it shall be mentioned explicitly here what is the reference period for the computation of the\nclimatology;\nResponse: The reference period for the computation of climatology is the 1979 – 2022 period as this is the\noverlapping period for all observational and reanalysis datasets used in the paper. We have clarified this\nin the revised text’s Method section.\n- l. 109: I struggle a bit to read Figure 1. I wonder why the authors prefer showing absolute temperatures\ninstead of anomalies, that would much better emphasize the T95 exceedances;\nResponse: One of the reasons to show the absolute temperature this way is that it is more efficient when\nshowing both minimum and maximum temperatures in fewer numbers of figure panels. If we choose to\nshow anomalies instead, then separate panels for a) and b) will be needed for minimum and maximum\ntemperatures as these will need to be centered by a zero mean.\nHowever, we agree that some of the details are difficult to depict. Therefore, we have modified Fig. 1 to\nmore easily depict the exceedance over the T95. For example, panel a) shows the mean temperature\n(thick-dashed line), T5 to T95 range (filled color range), and the actual absolute temperature thick line.\nThe exceedances over the T95 are now highlighted by the red shading which now includes red-dot\nhatching, signifying when the absolute temperature overshoots the T95 threshold. See Fig. R1 below\nwhich is now the new version of Fig. 1.\nFigure R1. Heat wave event of 2023. A) Seasonal evolution of maximum (red) and minimum (blue)\ntemperature for Phoenix, Arizona for the year 2023 from 1 June to 20 September. The long-term daily mean\nis shown by the dashed line whereas the 5th and 95th percentiles are shown by the shading region. Excess\nabove the 95th percentile is shown by red shading plus hatching for both maximum and minimum\ntemperatures. B) Observed histogram of maximum (red) and minimum (blue) temperature for Phoenix,\nArizona for the period 1 June to 31 August from 1955-2023. The extremely warm temperatures of 19 July\n2023, during the peak amplitude of the heat wave, are shown for reference. c) Five-day averaged surface\ntemperature anomaly centered on 19 July 2023.\n- l. 129: Figure 2 is very chaotic in my opinion, and I think it shall be improved by showing actual time\nseries, instead of boxes with 10-days averages. If the daily spread is too high, a 10-days moving average\ncould be superimposed on the actual time series. Also, I think that it is very misleading to position color\nbars next to x-axes, whose labels are only found at the bottom of the figure;\nResponse: We politely disagree. The reason such an approach was chosen is twofold. (1) There is a lot of\ninformation to convey in Fig. 2 (14 timeseries in total), so interpretation of the signal and the relative\nstrength of each entry is difficult, requiring multi-panels due to different units (see Fig. R2 below). (2)\nThe second issue is that given the large number of timeseries shown, using a color-blind neutral scheme\nfor each of the 14 lines is extremely difficult, as some of the co-authors themselves are color blind. The\noriginal approach of boxes sharing the same color palette and color bar (blue -> red) among all variables\nis a fix to these two issues. It is also easier to interpret a very busy figure at a glance, given the\nhomogeneity in color scheme and signal amplitude.\nAlso note that due to another reviewer’s suggestion, we added an additional entry for the 2-meter air\ntemperature anomaly. Please note that we also added x-axis labels to all sub-panels for easier\nrepresentation of the time axis. See revised Fig. 2.\nFigure R2. Heat budget analysis. Energy budget averaged every 10 days over the southwest U.S. and\nnorthwest Mexico from 6 June to 24 August 2023. A) 200 hPa geopotential height anomaly [gpm], 850\nhPa temperature and 2-meter temperature anomalies [°C]. b) Vertically integrated anomalous heating\nrates from 975 – 800 hPa layer [°C day-1], and c) surface heat fluxes (net surface shortwave and longwave\nradiation, sensible and latent heat fluxes) [Wm-2]. Daily anomalies are derived from the long-term\nmonthly mean for the 1979-2023 period.\n- l. 140: Figure 3c may be obvious to a reader with some sort of meteorology background, but it is not at\nall for many readers of Nature Communications. Therefore, the skew-T should be carefully explained\n(perhaps in the Methods?) making a bit more explicit what the authors discuss, particularly at lines 184-\n188 and in Figure 3c description;\nResponse: Agree. We added a more detailed explanation of the skew-T plot and what each line represents\nfor the untrained reader (Lines 220 - 221). Also note that this reference to the skew-T plot and Fig. 3 in\ngeneral was premature, so we deleted the clause and introduced Fig. 3 in more detail later in the revised\npaper, which should aid the reader in interpreting the diagram.\n- l. 142: it is nowhere explained what the authors mean by “diffusive heating” and how this is computed;\nResponse: Diffusive heating rate is a variable provided directly by the Japanese reanalysis JRA55, which\nentails vertical diffusive heating (K / day). This is a motivation for using the JRA55 analysis as it\nprovides various diabatic heating rate fields at pressure levels, which other reanalysis products do not\noffer.\nThe total diabatic heating rate (i.e., the Q term in the thermodynamic energy equation eq. 4) consists of\nnet\nthe net atmospheric radiation (short and longwave), vertical diffusion heating rate, and latent heat from\nmoist processes (including the large-scale condensation heating rate and convective heating rate).The\nvertical diffusive heating (i.e., sensible heating from the surface up into the air column) and adiabatic\nheating (due to subsidence) have been shown to play dominant roles in exacerbating surface warming\nduring heat waves (Zhou et al. 2024). The JRA55 heating rates have been extensively used in other\nstudies of atmospheric dynamics (Ma and Franzke 2021; Shi et al. 2020; Lopez et al. 2022).\nWe added this explanation to the Method section (Lines 496 - 500). However, more details on the\ndefinition of the heating rates can be found at: https://www.jma.go.jp/jma/jma-eng/jma-\ncenter/nwp/outline2013-nwp/index.htm\nLopez, Hosmay, Dongmin Kim, Robert West, and Ben Kirtman. “Modulation of North American heat\nwaves by the tropical Atlantic warm pool.” Journal of Geophysical Research: Atmospheres 127,\nno. 21 (2022): e2022JD037705.\nMa, Qiyun, and Christian LE Franzke. “The role of transient eddies and diabatic heating in the\nmaintenance of European heat waves: a nonlinear quasi-stationary wave perspective.” Climate\nDynamics 56 (2021): 2983-3002.\nShi, Ning, SuolangTajie, Pinyu Tian, Yicheng Wang, and Xiaoqiong Wang. “Contrasting relationship\nbetween wintertime blocking highs over Europe–Siberia and temperature anomalies in the\nYangtze River basin.” Monthly Weather Review 148, no. 7 (2020): 2953-2970.\nZhou, Jie, Haipeng Yu, Peiqiang Xu, Wen Zhao, Bofei Zhang, Yu Ren, Shanling Cheng, Yongqi Gong,\nand Yaoxian Yang. “Extreme heat event over Northwest China driven by Silk Road Pattern\nteleconnection and its possible mechanism.” Atmospheric Research 297 (2024): 107090.\n- l. 146: similarly to my comments above: it is not worth mentioning that the Bowen ratio increases if it is\nnot explained what this means in terms of process understanding;\nResponse: Agree.\nThe Bowen ratio is used to link water and energy balances of the climate system. A Bowen ratio increase,\noften present during mega heat waves (Miralles et al. 2014), suggests a reduction in the\nevaporation/evapotranspiration and an increase in the sensible heating and thus increase in near surface\ntemperatures. We added more explanation and reference to the revised text (Lines 175 - 177).\nMiralles, D. G., Teuling, A. J., Van Heerwaarden, C. C., & Vilà-Guerau de Arellano, J. (2014). Mega-\nheatwave temperatures due to combined soil desiccation and atmospheric heat accumulation. Nature\ngeoscience, 7(5), 345-349.\n- l. 159: wouldn’t it be clearer to state that the dynamical tropopause crosses the 2 PVU isosurface where\nthe thick solid line is?\nResponse: The dynamical tropopause is defined here as the 2 PVU isosurface. We clarified this in the\nrevised text (Line 193).\n- l. 165: not clear why I would infer from Figure 2 that there is shortwave surface heating in clear sky,\ni.e. why I would have to assume clear sky;\nResponse: Agree. While the skew-T diagram (Fig. 3c) shows a very dry vertical profile, i.e., a well-\nseparated air temperature (red) and dew point temperature (blue), it is not obvious from Fig. 2 that there\nare clear sky conditions. To show this, we have computed the cloud radiative effect from the reanalysis\nduring the middle of the heat wave (Fig. R3 below). This effect is easily extracted by taking the difference\nbetween the clear-sky minus the all-sky radiative fluxes at the top of the atmosphere (TOA) following\nKobayashi et al. 2015. Note the fairly large swath area of no clouds over the southwestern U.S. (our study\nregion), which is evident by the near-zero net cloud radiative effects, including shortwave (Fig. R3b) and\nlongwave (Fig. R3c) effects. We also obtained a similar pattern using direct retrieval of radiative effects\nfrom the Cloud and the Earth’s Radiative Energy System (CERES) observations.\nWe added Fig. R3 to the revised manuscript as Supplementary Fig. 4.\nKobayashi, S., Ota, Y., Harada, Y., Ebita, A., Moriya, M., Onoda, H., ... & Takahashi, K. (2015). The\nJRA-55 reanalysis: General specifications and basic characteristics. Journal of the\nMeteorological Society of Japan. Ser. II, 93(1), 5-48.\nFigure R3. Cloud radiative effects computed from the differences between clear-sky minus all-sky\nradiative fluxes at the top of the atmosphere (TOA) for a) net fluxes, b) shortwave reflected fluxes, and c)\noutgoing longwave fluxes. These values represent the five-day average centered on 19 July 2023,\nduring the middle of the heat wave. Regions with dark blue represent little or no clouds.\n- l. 168: it is textbook literature that atmospheric blocking is among the most prominent features of\nextratropical low frequency variability (e.g Rex et al. 1950), then some sort of agreement shall be\nreached on what the authors refer to as “synoptic”, because blocking is not proper of synoptic timescales\n(although it might be to some extent of synoptic spatial scales, although I would rather refer to it as a\nmanifestation of planetary waves; cfr. Stoll et al. 2023 and references therein)\nResponse: Agree. Atmospheric blocking events, such as “omega”, “high-over-low”, and “Rex” blocks,\nare often semi-persistent due to their underlying dynamical and thermodynamical forcing. Several of\nthese blocks have been observed to persist for weeks (e.g., the Russian heatwave and the event studied in\nthis work). Thus, we changed the language to reflect that these are manifestations of planetary waves and\nadded Stoll et al. 2023 to the reference (Line 203).\n- l. 198: What is the exact definition of “western North America” here?\nResponse: Here we geographically define western North America as the region enclosed by 15N-50N\nand 125W-90W. However, with regards to the exact boundaries of what constitutes the North American\nMonsoon (NAM), we follow the concept of the global monsoon (Wang and Ding 2008; Lee and Wang\n2014). This index is defined as where the local summer (May – September) precipitation minus the winter\n(November–March ) precipitation exceeds 2.5 mm per day and the local summer precipitation exceeds\n55% of total annual precipitation (Lee and Wang 2014). We applied these constraints over the\ngeographical box defined above, which is shown in Fig. R4, depicting the location of the NAM index by\nthe red stipples, which is the location meeting the NAM definition constraints.\nWe have clarified this in the revised text and added the red stipples to the monsoon associated\nprecipitation plot (Fig. 4) and the definition of the NAM index in the Method section (Lines 546 - 550).\nWang, B., Ding, Q., & Liu, J. (2011). Concept of global monsoon. In The global monsoon system:\nresearch and forecast (pp. 3-14).\nLee, J. Y., & Wang, B. (2014). Future change of global monsoon in the CMIP5. Climate Dynamics, 42,\n101-119.\nFigure R4. June – September (JJAS) precipitation anomaly during 2023. The red stipples indicate the\nNorth American monsoon region defined as where the local summer (May – September) precipitation\nminus the winter (November–March) precipitation exceeds 2.5 mm per day and the local summer\nprecipitation exceeds 55% of total annual precipitation. Dataset from the JRA55 reanalysis.\n- l. 202: this is another example of one of my main concerns about (this section of) the manuscript.\nBesides showing that the North American Monsoon is weak in 2023, claiming that some sort of\ncorrelation is present between moisture fluxes enter the region and negative latent heat fluxes locally (in\nPhoenix, at least), what do we learn about the relation between exceptional heatwaves in the region and\nthe monsoon? Could it be just a chance? What happened in 1982 (the weakest NAM on records,\naccording to the time series in Figure 4a)? Is the anomaly significant wrt. NAM climatology?\nResponse: We agree that more quantification of the relationship between the NAM precipitation and\nsurface temperatures needs to be shown. For this we computed the NAM index (Fig. 5a) correlation with\nmaximum July temperatures over the study region for the 1979-2022 period (see Fig. R5b below). Note\nthat while this correlation does not include the year 2023 (done to prevent aliasing the relationship with\n2023), the spatial pattern is very similar to the 2023 temperature anomalies over the southwest U.S. and\nMexico (Fig. R5a) which shows warmer temperatures when the NAM is negative (note that the\ncorrelation colorbar in Fig. R5b was reversed intentionally to depict the negative correlations and for\neasier comparison between panels).\nWith regards to the year 1982, while this was the weakest NAM based on our definition, the temperature\nanomalies over the study region were just slightly above climatology (Fig. R5c). So, the year 1982 was an\noutlier in the NAM – maximum temperature relationship. In contrast, the year 2023 is much closer to the\nregression line (Fig. R5c) and thus fits the NAM – maximum temperature negative relationship.\nWe this analysis to the revised manuscript (Lines 238 - 241) and added Fig. R5 as supplementary Fig. 5.\nFigure R5. A) July 2023 maximum 2-meter temperature anomalies. B) Correlation of maximum 2-meter\ntemperatures and the North American Monsoon (NAM) index for the 1979 – 2022 period (the year 2023\nwas excluded from the correlation analysis intentionally to avoid aliasing the results). Note that the\ncolorbar is reversed for easier comparison and reflection of the negative correlations. C) Scatter plot of\nthe NAM index versus 2-meter air temperatures averaged over the southwestern U.S. and northern\nMexico (black box in panel b). The best-linear fit is shown by the dashed line along with the years 1982\n(blue) and 2023 (red). The correlation between the NAM and index and 2-m temperatures is r = -0.58, p-\nvalue < 0.01. See Methods for definition on the NAM index.\nIs the anomaly significant wrt. NAM climatology?\nResponse: The anomalous NAM index has a mean = 0.00 mm/day and an interannual standard deviation\nsigma = 0.61 mm/day. Using a student-T test, with sample size N = 45 (years), the confidence interval is\n±0.25 at a 99% significance level from a student-T test. Thus, any NAM anomalies larger than 0.25 are\nsignificantly away from zero. We added the significance test (see Fig. R6) to the revised manuscript (Fig.\n4a).\nFigure R6. Time series of the North American monsoon index for June-August. Years of significant\nNAM index anomalies are highlighted by color-filled bars. Statistical significance is based on the 99-\npercentile significance level based on a Student’s-t test.\n- l. 211: related to my previous comment; authors are comparing the NAM activity along the whole\nseason, but the heatwave peaked during 14 days in July and short heatwave periods emerged during\nAugust and September (possibly not related to the major one). It would perhaps be more informative if\nthe evolution of daily moisture transports could be tracked during the season;\nResponse: Please note that the main scope of this paper is not to describe the synoptic evolution of the\nheat wave, rather than investigating why the extreme heat was semi-persistent throughout the summer.\nThat is, we are interested in the longer-term aspects of the event in association with forcing from large-\nscale interannual variability (e.g., ENSO, Atlantic SSTs, N. American monsoon). The evolution of daily\nmoisture transports would be extremely noisy and driven by high-frequency weather patterns outside of\nthe paper’s main scope. With that said, we agree that it would be informative to discuss the daily\nevolution of the NAM, which nicely depicts the weak rainfall and late onset in 2023 relative to\nclimatology (Fig. R7).\nThe daily evolution of precipitation is divided into regional indices (a northern and a southern index),\nfollowing Duan et al. 2024, as these are very distinct in their rainfall behavior. As shown in Fig. R7, the\nprecipitation stays below climatology until the middle of August 2023 for the northern region for most of\nthe summer. For the region over Mexico, the precipitation does not reach its climatological value until 9\nJuly, well past the historical onset which occurs around 29 June. However, it then decreases well below\nclimatology for most of the remaining active monsoon season.\nDuan, S., Ullrich, P., & Boos, W. R. (2024). Meteorological drivers of North American monsoon\nextreme precipitation events. Journal of Geophysical Research: Atmospheres, 129(13),\ne2023JD040535.\nFigure R7. Daily evolution of the North American Monsoon (NAM) indices for a) northern domain (blue\nregion in the inserted map) and b) southern domain (red region in the inserted map).\n- l. 254: how relevant is the negative anomaly (shown in mm/day) compared to the precipitation that\nusually occurs during July in the region (i.e. 22 mm, according to NOAA Weather data)?\nResponse: Very relevant and significant above the 95-percentile confidence level based on a Student’s t-\ntest (see Fig. R8 below). For example, the mean climatological July precipitation in the southwestern U.S.\nand northern Mexico peaks at around 10 mm per day (Fig. R8b). Thus, anomalies of around 5 mm per day\n(Fig. R8d) are fairly large in comparison. Please note that the units of precipitation used in our work are\nmm per day, so this is a precipitation rate instead of an aggregated/integrated rainfall amount, which we\nbelieve it is what the reviewer is alluding to. Also note that these precipitation rates are taken from\nmonthly mean data. In addition, Fig. R8 below is a zoomed in version from Fig. 5e and f of the paper\nWith that said, we added significant tests to the analysis of Fig. 5 in the revised text and explicitly\nclarified that the rainfall units here are rainfall rates rather than total precipitation.\nI would assume that precipitation in July would be very intermittent in this region, and having -6 mm/day\nduring one specific month of a specific year would not mean much;\nResponse: We politely disagree with the reviewer. A -6 mm/day monthly mean precipitation rate anomaly\nis fairly significant for this region (Fig. R8), which was also shown to be the second driest NAM index in\nthe 1979 – 2023 period (Fig. 5). Please note that this is a region with monthly mean climatological\nrainfall rates of around 10 mm/day. Even regions of significant rainfall amounts (Indian Monsoon region,\nBay of Bengal, and tropical Pacific Ocean) only experience around 25 mm/day monthly mean\nprecipitation rates in July\n(see: https://gpm.nasa.gov/data/imerg/precipitation-climatology#monthlyprecipitationclimatology)\nAlso note that we are assessing monthly mean rainfall statistics and not daily precipitation and that a\nstandard unit convention for rainfall rate is mm/day.\nFigure R8. a and b) June and July precipitation rate climatology respectively. Panels c) and d) show the\nprecipitation rate anomalies for June and July 2023 respectively. Black stipples in panels c and d) indicate\nstatistical significance at a 95% confidence level based on a Student’s t-test. The precipitation rate is\nmeasured in mm per day.\n- ll. 257-258: this could be already stated at l. 146;\nResponse: Agree, we removed this clause as it is already mentioned.\n- ll. 262-263: for sake of clarity, these boxes could be highlighted on Figure 5;\nResponse: Done\n- l. 267: this should be compared with the available literature mentioned above;\nResponse: This is the first time, to the best of our knowledge, that the combined effect of Atlantic and\nPacific SST anomalies on atmospheric circulation is investigated. However, we added references to the\nrevised text describing their individual roles in generating these upper-level flow anomalies (Lines 324 –\n327). For example, an El Niño event produces a tropospheric warming throughout the tropics via a fast\nequatorial Kelvin wave and off-equatorial anticyclonic anomalies, similar to Fig. 6e (Trenberth et al.\n1998, Lee et al. 2018; Lopez et al. 2019). However, ENSO response to the northern hemisphere is\nstrongest later in the seasonal cycle (Trenberth et al. 1998). In contrast, the tropical Atlantic warm SSTs\nhave been shown to produce a Gill-response over Central America, similar to Fig. 6c (Fig. 8 of Wang et\nal. 2008).\nTrenberth, K. E., Branstator, G. W., Karoly, D., Kumar, A., Lau, N. C., & Ropelewski, C. (1998). Progress\nduring TOGA in understanding and modeling global teleconnections associated with tropical sea\nsurface temperatures. Journal of Geophysical Research: Oceans, 103(C7), 14291-14324.\nLee, S. K., Lopez, H., Chung, E. S., DiNezio, P., Yeh, S. W., & Wittenberg, A. T. (2018). On the fragile\nrelationship between El Niño and California rainfall. Geophysical Research Letters, 45(2), 907-\n915.\nLopez, H., & Kirtman, B. P. (2019). ENSO influence over the Pacific North American sector: Uncertainty\ndue to atmospheric internal variability. Climate dynamics, 52(9), 6149-6172\nWang, C., Lee, S. K., & Enfield, D. B. (2008). Climate response to anomalously large and small Atlantic\nwarm pools during the summer. Journal of Climate, 21(11), 2437-2450.\n- ll. 318-319: the role of topography seems to be crucial here. Can the authors comment on this?\nResponse: Topography plays a major role in the generation of Rossby waves via vortex stretching\n(Sardeshmukh and Hoskins 1988). We computed the Rossby wave source (RWS, see equation 6 in the\nrevised text Method section), streamfunction and velocity potential to show a more detailed description of\nthe dynamics involved.\nThe RWS is computed for each SST sensitivity experiment. For example, note that there are a few regions\nof anticyclonic RWS in the GBL23 experiment (Fig. R13a) over the equatorial Pacific and Atlantic, near\nthe location of the ITCZ, and also over the Greater Antilles. These are consistent with the warm 2023 SST\nforcing prescribed in the GBL23 experiment, which led to upper level 200 hPa divergence (Fig. R13c)\nand 200 hPa anticyclonic circulation consistent with a Gill-type response (Fig. R13b). Further\ndecomposition of the RWS into its component demonstrates that mean vortex stretching by the\nanomalous divergent flow (Fig. R13e) was the main driver of the anticyclonic RWS and thus the\nanticyclonic circulation over Mexico.\nAs the reviewer suggested, topographic features induced cyclonic RWS over western Mexico and\nsouthwestern U.S. (Fig. R13), mostly driven by mean vorticity advection and mean vortex stretching\nsource terms (see Methods for decomposition of the RWS).\nIn addition, this region is home to fairly tall topographic features, like the Sierra Madre, which steers low-\nlevel moisture modulating the North American Monsoon (Duan et al. 2024). Topography is also crucial in\nthis region where convection typically develops in the boreal summer over high-elevation terrains (King\nand Balling 1994) and has been shown to play a major role in steering low-level flow, which feeds the\nmonsoon (Collier and Zhang 2007; Pascale et al. 2016)\nWe added this discussion and the analysis on the RWS (Lines 371 - 389) and added Fig. R13 to the\nrevised manuscript as Fig. 8. We also added Figs. R14 and R15 to the supplementary materials\n(supplementary figs. 10 and 11).\nCollier, J. C., & Zhang, G. J. (2007). Effects of increased horizontal resolution on simulation of the North\nAmerican monsoon in the NCAR CAM3: An evaluation based on surface, satellite, and reanalysis\ndata. Journal of climate, 20(9), 1843-1861.\nDuan, S., Ullrich, P., & Boos, W. R. (2024). Meteorological drivers of North American monsoon extreme\nprecipitation events. Journal of Geophysical Research: Atmospheres, 129(13), e2023JD040535.\nKing, T. S., & Balling Jr, R. C. (1994). Diurnal variations in Arizona monsoon lightning data. Monthly\nWeather Review, 122(7), 1659-1664.\nPascale, S., Bordoni, S., Kapnick, S. B., Vecchi, G. A., Jia, L., Delworth, T. L., ... & Anderson, W.\n(2016). The impact of horizontal resolution on North American monsoon Gulf of California moisture\nsurges in a suite of coupled global climate models. Journal of Climate, 29(21), 7911-7936.\nSardeshmukh, P. D., & Hoskins, B. J. (1988). The generation of global rotational flow by steady idealized\ntropical divergence. Journal of Atmospheric Sciences, 45(7), 1228-1251.\n- ll. 348-350: I find this argument a bit illogic. I see no reason why the sum of percentages from ATL and\nPAC sensitivity experiments would result in percentage increase of GLB23 minus the residual (“synergic\ninterbasin interactions”). It might be informative to obtain composite mean maps for the three\nexperiments, as in Figure 7, then subtract from the complete SST prescribed field to have an idea of the\nrole of interbasin interactions. I do not think though, that the same can be done to infer the relative role\nof the three components for the overall GLB23 increase in extremes. The role of Atlantic and Pacific\nalone could determine a growth in the occurrence of the chosen events that would be larger than the\ncombined effects, i.e. the interbasin interaction role would be effectively to reduce the role of the\nindividual basins. This would not be captured if using the described approach, and it would be\nparticularly relevant when dealing with extreme events. Please clarify this;\nResponse: This is a very important comment that needs further clarification and a more detailed\ndescription of the approach which was not adequately provided in the original text. Therefore, we address\neach concern separately in the discussion below, which has been added to the Supplementary Information\nof the revised paper.\nI see no reason why the sum of percentages from ATL and PAC sensitivity experiments would result in\npercentage increase of GLB23 minus the residual (“synergic interbasin interactions”)\nResponse: We first explain how equation 6 of the paper was derived. Using the AGCM sea surface\ntemperature anomaly (SSTA) sensitivity experiments, any variable (e.g., heat wave days, number, surface\ntemperature) response to the prescribed forcing can be extracted as follows:\nThe climatological occurrence of any event (heat wave number, days) is first extracted from the control\n(CTL) simulation (equation R1). Note that we obtained this climatology from the 300 year-long CTL\nsimulation so as to get an accurate expected value in the absence of any SSTA forcing. Then, the\noccurrences of such events in the SSTA sensitivity experiments are thus governed by eq. R2 for the\nAtlantic and eq. R3 for the Pacific experiments. For the case of both Atlantic and Pacific SSTA forcing\n(i.e., the GBL23 experiment), a linear combination of ATL23 and PAC23 is not sufficient as pointed out\nin the paper and by the reviewer, given that there could be interbasin interactions between the ATL and\nPAC SST forcing, i.e., the Synergy term in eq. R4.\n𝐶𝑇𝐿 = 𝐶𝑙𝑖𝑚𝑎𝑡𝑜𝑙𝑜𝑔𝑖𝑐𝑎𝑙 𝑒𝑥𝑝𝑒𝑐𝑡𝑒𝑑 𝑜𝑐𝑐𝑢𝑟𝑟𝑒𝑛𝑐𝑒𝑠 (𝑒𝑞.𝑅1)\n𝐴𝑇𝐿23 = 𝐶𝑇𝐿 +𝐴𝑡𝑙𝑎𝑛𝑡𝑖𝑐 𝑆𝑆𝑇𝐴 𝑠𝑒𝑛𝑠𝑖𝑡𝑖𝑣𝑖𝑡𝑦 (𝑒𝑞.𝑅2)\n𝑃𝐴𝐶23 = 𝐶𝑇𝐿 +𝑃𝑎𝑐𝑖𝑓𝑖𝑐 𝑆𝑆𝑇𝐴 𝑠𝑒𝑛𝑠𝑖𝑡𝑖𝑣𝑖𝑡𝑦 (𝑒𝑞.𝑅3)\n𝐺𝐵𝐿23 = 𝐶𝑇𝐿 +𝐴𝑡𝑙𝑎𝑛𝑡𝑖𝑐 𝑆𝑆𝑇𝐴 +𝑃𝑎𝑐𝑖𝑓𝑖𝑐 𝑆𝑆𝑇𝐴 𝑠𝑒𝑛𝑠𝑖𝑡𝑖𝑣𝑖𝑡𝑖𝑒𝑠 +𝑆𝑦𝑛𝑒𝑟𝑔𝑦 (𝑒𝑞.𝑅4)\nSubtracting eq. R2 and R3 from R4, we obtain the interbasin synergy component:\n𝐼𝑛𝑡𝑒𝑟𝑏𝑎𝑠𝑖𝑛 𝑆𝑦𝑛𝑒𝑟𝑔𝑦 = 𝐺𝐵𝐿23−𝐴𝑇𝐿23−𝑃𝐴𝐶23+𝐶𝑇𝐿 (𝑒𝑞.𝑅5)\nThus, the sum from ATL and PAC sensitivity experiments would result in the GLB23 minus the residual\n(i.e., synergic interbasin interactions) plus a climatology, which is present in all experiments.\nThe percentages shown in Fig. 10 (of revised text) are relative to the CTL simulation i.e., an\nincrease/decrease relative to the expected climatology. For each of the sensitivity experiments (ATL23,\nPAC23, and GBL23), these could be larger or smaller than the CTL. In our case they are all larger for the\nstudy region, indicating that SSTAs were responsible for an increase in heat waves relative to\nclimatology. This increase is quantified as follows:\nAs an example, for the heat wave number (Fig. 10a): There were 77 events in the CTL case and 184\nevents in the GBL23 experiment, for a total increase of 138%. The entire azimuthal range (angular\ndistance of 360 degrees) of the donut plot represents the GBL23 (e.g., CTL + ATL23 + PAC23 +\nSynergy) with the individual components scaled azimuthally by their respective fractions. These were\ncomputed from the ATL23 experiment which produced 135 heat waves and from the PAC23 which\nproduced 100 heat wave events, an increase of 75% and 31% from the climatology of 77 heat wave\nevents respectively. This leaves 32% unaccounted for in the 138% increase seen in the GBL23 case, thus\nit is attributed to the synergy component.\nIt is worth noting that all experiments (GBL23, ATL23, and PAC23) contain a climatological component\nwhich is the same as the CTL simulation (i.e., same model physics, biases, etc.). These values are the sum\n(composite) from multi-ensemble simulations (100 ensembles in total with integration length of 1 year)\nmimicking 100 years for each experiment case. Thus, 77 (184) heat wave events consist of less than one\n(almost two) events per year in the CTL (GBL23) case.\nThe approach used here to extract changes in the occurrence of an event is widely used in the attribution\nanalysis (Pearl 2000; Hannart and Naveau 2018; Lopez et al. 2024). For example, when employing causal\ncounterfactual theory to assess the probability that an event E would not have occurred in the absence of a\ncause I, where event E is defined as a heat wave and C is defined as a warm Atlantic and/or Pacific SST\nanomalies. Probability of necessary causation (PN) can thus be constructed to measure the fraction of\nextreme events attributed to C, defined as (eq. R6):\n𝑃\n0\n𝑃𝑁(𝐶 → 𝐸)= 𝑚𝑎𝑥{1− ,0} (𝑒𝑞.𝑅6)\n𝑃\n1\nwhere P is the probability of an event occurring in the counterfactual world (e.g., our control simulation\n𝑅𝑒𝑙𝑎𝑡𝑖𝑣𝑒 𝑐ℎ𝑎𝑛𝑔𝑒 = (𝐸𝑥𝑝𝑒𝑟𝑖𝑚𝑒𝑛𝑡 − 𝐶𝑇𝐿)/𝐶𝑇𝐿 (𝑒𝑞.𝑅7)\nWe added the above explanation of the synergy component along with equations R1, R2, R3, R4, and R5\nto the Supplementary Information of the revised paper.\nThe role of Atlantic and Pacific alone could determine a growth in the occurrence of the chosen events\nthat would be larger than the combined effects, i.e. the interbasin interaction role would be effectively to\nreduce the role of the individual basins\nAgree. The role of the combined Atlantic and Pacific alone (i.e., ATL23 and PAC23) could be larger than\nthe GBL23, thus suggesting that the synergy interaction would have to be negative (see equations R4 and\nR6). Note that we do not make any assumptions when extracting the synergy effect. In fact, this effect is\nnon-existent with regards to the duration of the heat waves (Fig. 10c) but positive for all other indices\ne.g., heat wave number (Fig. 10a), heat wave days (Fig. 10b), and frequency (Fig. 10d). With that said, we\nacknowledge that Fig. 10 should not be generalized for other regions where CTL could produce more\nevents than the GBL23 experiments (i.e., less heat waves due to the SST sensitivity). However, Fig. 10 is\na concise representation of the fractional increase in heat wave characteristics due to specific SST\nanomalies prescribed in the AGCM experiment.\nPearl, J. (2000). Models, reasoning and inference. Cambridge, UK: Cambridge University Press, 19(2),\n3.\nHannart, A., & Naveau, P. (2018). Probabilities of causation of climate changes. Journal of\nClimate, 31(14), 5507-5524.\nLopez, H., Lee, S. K., West, R., Kim, D., Foltz, G. R., Alaka Jr, G. J., & Murakami, H. (2024). Projected\nincrease in the frequency of extremely active Atlantic hurricane seasons. Science Advances,\n10(46), eadq7856.\nIt might be informative to obtain composite mean maps for the three experiments, as in Figure 7, then\nsubtract from the complete SST prescribed field to have an idea of the role of interbasin interactions.\nPlease note that this is already done by the composite maps shown in Fig. 7g,h. The role of the interbasin\nsynergy interaction is to exacerbate the warm surface temperatures over northern Mexico and the\nsouthwestern U.S. This warmer temperature is surrounded by cooler temperatures over the Pacific coast\nand the eastern U.S. (Fig. 7h).\n- l. 375: once again, it is not clear what you mean by “western North America”, and I do not think that\nyour arguments are equally valid for the whole of western North America, e.g. including Canada and\nAlaska;\nResponse: We use the term “western North America” loosely in the paper, which we agree could bring\nsome confusion as we are definitely not including Canada and Alaska here. We now clarified in the\nrevised text that the focus is on the 2023 heatwave that impacted the southwestern U.S. and northern\nMexico. Any geographical index (e.g., area averaged variable) is explicitly defined in the revised text\nmethod section and figure captions.\n- ll. 412-419: this paragraph does not seem to belong to conclusions, as it would be better fitting into\nIntroduction;\nResponse: We agree with the reviewer that this paragraph reads more like introductory material.\nHowever, since the focus of the paper and the special call targets extremes during 2023, we felt that\nmentioning extreme conditions that occurred in 2024 would steer the reader away from the paper’s focus.\nWith that said, we remove the paragraph to make the paper more focused on 2023.\n- ll. 426-427: is there a specific reason why authors decided to use this Reanalysis product instead of\nothers? What is the spatial resolution of this product?\nResponse: As we mentioned in an earlier comment, the motivation for using the JRA55 analysis is that it\nprovides various diabatic heating rate fields at pressure levels as diagnostic variables, which other\nreanalysis products do not offer. These heating rates are used to diagnose the thermodynamics of the heat\nwave event.\nThe JRA55 spans the period from 1958 to present with a 55 km spatial resolution, and 60 vertical levels\nup to 0.1 hPa. Data interpolated to 1.25° × 1.25° spatial resolution and 37 vertical levels from 1,000 hPa\nto 1 hPa are available to download. We clarified this in the Method section (Lines 492 - 500).\n- ll. 451-452: the choice of 850hPa temperature seems a bit arbitrary, 2-metres temperature would have\nbeen a more valid alternative for near-surface temperature;\nResponse: Please note that we chose to show the temperature anomaly at the 850hPa level to be consistent\nwith the pressure level at which the heat budget analysis is performed. Quantifying the heat budget near\nthe surface (2-meters) would result in too much emphasis on the surface net heat fluxes, obscuring the\nrole of advective and diffusive heating terms.\nWith that said, we agree that the 2-meter air temperatures are more relevant for characterizing impacts of\nextreme heat. Thus, we have modified Fig. 2 to include both the 2-meter and the 850hPa temperatures\nnext to each other. Note that their anomalies are fairly similar partly due to the dominance of vertical\nadvective and diffusive heating and a lack of horizontal temperature advection, features common under\nstagnant weather i.e., heat dome conditions. The 850ha temperature anomalies persist a bit longer into\nAugust 2023 probably due to longwave heating at 850hPa and reduced surface shortwave heating (Fig. 2).\n- ll. 472-473: although I can understand that for the sake of comparison with Reanalyses, it was\nnecessary to choose a reference period that was also encompassed by JRA55, it is a bit odd that a\nclimatology for a period characterized by a strong positive trend and so close to the 2023 event has been\nconsidered. Perhaps a previous period (same length, still encompassing the 1958-present JRA55\ncoverage) would have been a more appropriate choice;\nResponse: The SST sensitivity experiment is designed this way to explicitly isolate the exceptional SST\nanomalies of 2023 relative to the recent climatological period (i.e., 1979 – 2022). Note that the goal is for\nevaluating interannual variability changes in SSTs and its influence on atmospheric circulation and\nheatwaves. Extending the climatological period back to 1958 would complicate the interpretation of the\nmodel runs when comparing 2023 SST-forced runs versus the climatology run. This is because the\nobserved long-term trend will also need to be considered as well as the roles of multidecadal climate\nvariations (e.g., the Pacific decadal oscillation, Atlantic multidecadal variability) along with external\nforcing from natural and anthropogenic causes.\nWith that said, we repeated the model experiment (GBL23) but prescribed the 2023 SST anomalies\nrelative to an extended climatology period (1958 – 2022). We compared this extended climatology run to\nthe original GBL23 run (Fig. R9). Besides some small changes, the overall atmospheric response is very\nsimilar between the two SST sensitivity experiments. Some minor changes are expected as the longer\nclimatological period further exacerbates the model response to the 2023 SST forcing i.e., from interbasin\nwarming (warmer SST anomalies), due to the global SSTs experiencing a positive trend. For example, the\nextended climatology experiment produced more heat wave events, days, and larger amplitude than the\nGBL23 experiment (Table R1).\nAnother motivation for using the more recent period for climatology is due to the quality of observational\nrecords before the satellite era, especially for remote regions of the world oceans. The effect of the longer\nSST trends is excluded from the revised text; however, we are investigating this in a separate study.\nTable R1. Similar to Supplementary Table 1 and showing the changes in heat wave characteristics from\nthe CTL (climatology), GLB23 (2023 SST anomaly relative to 1979 – 2022 period), and GBL23-\nExtended climatology (SST anomalies relative to the 1958 – 2022 period) AGCM experiments for JJA\nmaximum for the grid-point closest to Phoenix, Arizona.\nHeat wave Characteristics\nExperiment Number Days Duration (longest) Amplitude\n(strongest)\nCTL 77 307 3.9 (9) days 5.1 (7.7) °C\nGBL23 184 853 4.6 (14) days 5.3 (8.3) °C\nGBL23-Extended Climo 219 1074 4.9 (15) days 5.5 (10.4) °C\nFigure R9. SST sensitivity from an atmospheric model experiment. Composite difference of simulated a)\n200 hPa temperature (color) and 200 hPa streamfunction (black contour, 106 s-1) and b) 2-meter air\ntemperature from the AGCM experiment with prescribed 2023 global SSTs (GLB23). c) and d) are the\nsame as a) and b) but for the AGCM experiments with 2023 SST forcing relative to an extended\nclimatology (1958 – 2022). The composite differences are with respect to the control experiment (CTL)\nfor JJA.\n- l. 477: how is this ensemble created? What initial conditions are chosen, and how distant from each\nother? This should be made explicit;\nResponse: Agree. The ensemble runs are created by first integrating the model without any SST anomaly\nforcing, just prescribing global climatology for the 1979 – 2022 period; this is referred to here as the\ncontrol (CTL) run with an integration length of 300 years. The last 100 years of the CTL are used as\ninitial conditions for the ensembles for the GBL23, ATL, and PAC experiments. That is, the experiments\nare integrated for one year starting on every 1 January from the CTL (e.g., branch simulation), thus\ncreating ensembles with initial conditions that are one year apart from each other. This approach\nguarantees more than sufficient separation among ensembles as it pertains to weather noise (i.e., the\natmospheric initial state is completely different and independent among ensembles) while each ensemble\nis being forced by the same underlying SST anomalies (i.e., GBL23, PAC, ATL). We have clarified this\nin the revised text’s Method section (Lines 568 - 574).\n- ll. 480-481: same as in my previous comment. There are several bootstrapping techniques that could be\nused for the scope. For the sake of reproducibility, the technique shall be described here;\nResponse: For statistical significance, we used a Monte Carlo bootstrapping method to determine\nconfidence intervals by subsampling the dataset. All analyses presented are obtained by randomly\nselecting 𝑟 samples out of 𝑛 observations with replacement (eq. R8). This is done 500 times in order to\nbuild a significant distribution of composites and assign 95th percentile confidence levels. This is added in\nthe Method section of the revised manuscript (Lines 601 - 606).\n𝑛!\n(𝑛 𝑟 ) = = 𝑝𝑜𝑠𝑠𝑖𝑏𝑙𝑒 𝑐𝑜𝑚𝑏𝑖𝑛𝑎𝑡𝑖𝑜𝑛𝑠 (𝑒𝑞.𝑅8)\n𝑟!(𝑛−𝑟)!\n- ll. 483-484: it is not clear if ATL23 and PAC23 are ensembles as well;\nResponse: All model experiments were performed as ensemble simulations. We have clarified this in the\nrevised manuscript Method section ((Lines 568 - 574).\nReferences\n- Hebert et al. 2022: https://www.nature.com/articles/s41561-022-01056-4\n- Jiang et al. 2022: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2023GL103777\n- Kuhlbrodt et al. 2023: https://journals.ametsoc.org/view/journals/bams/105/3/BAMS-D-23-0209.1.xml\n- Rex et al. 1950: https://www.tandfonline.com/doi/abs/10.3402/tellusa.v2i4.8603\n- Richter et al. 2023: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2024GL111563\n- Stoll et al. 2023: https://wcd.copernicus.org/articles/4/1/2023/\nThese references were added to the revised text.\n________________________________________________________________________\nReviewer #2 (Remarks to the Author):\nSummary:\nOverall, this is an interesting and well-written paper. Lopez et al. investigate the extreme and persistent\nheatwave that impacted the southwestern U.S. and northern Mexico in the summer of 2023. Using\nobservational data, regression analysis, and atmospheric model simulations, the study finds that warm\nAtlantic SSTs, combined with a developing El Niño in the Pacific, significantly contributed to the increase\nin heatwave frequency, duration, and intensity. The authors employ heat budget analysis to identify key\nthermodynamic and radiative processes that sustained extreme surface temperatures. The study\nconcludes that the interbasin synergy between the Atlantic and Pacific basins exacerbated the conditions\nthat led to the record-breaking heatwave. The findings suggest that continued ocean warming may lead to\nan increased likelihood of prolonged and extreme heat events in the future. I do not have any substantial\ncomments about the analysis or the main conclusions, which seem solid and well-supported. I recommend\nsome minor revisions before this manuscript can be published, please see my comments below.\nResponse: We are very grateful for your constructive comments/suggestions and hope to have\nsuccessfully addressed all your comments. Please find our replies to your specific comments below in\nregular fonts and your original comments in blue italic fonts. We point to specific line numbers where the\nchanges were made, these line numbers refer to the lines on the tracked-changes version of the text\nMain comments:\n1. Some of the narrative and factual statements in the abstract and introduction can use finetuning, and I\nhave left a number of specific line comments with my suggestions for changes.\nResponse: We have implemented the suggested changes in the revised manuscript.\n2. The multitude of different time periods used for baselines seems unnecessary. For example, depending\non the variable, the authors use 1979-2022 (SST and vertically integrated moisture transport), 1982-2023\n(Phoenix temperatures), 1979-2023 (heat budget analysis and NAM), and 1960-2023 (SST regression).\nCould the authors find one or maybe at most two common baselines to use for all the analyses?\nResponse: Agree. We have now synthesized all station data baseline periods for 1955 - 2023 to match the\nJapanese 55-year reanalysis data. Note that the 1982-2023 Phoenix temperatures were a typo in the\noriginal figure 1 caption, the histogram represents the 1955 - 2023 period. This is fixed in the revised text.\nFor the reanalysis data (SSTs, 3-dimensional atmospheric variables, and monsoon analysis), we now use\nthe 1979 - 2022 baseline period to define climatology.\nAlso note that for the SST sensitivity atmospheric model experiments, we intentionally used the 1979 -\n2022 period for the climatological run so that to avoid aliasing the prescribed sensitivity runs based on the\nobserved 2023 SSTs.\nWe clarified this in the Method section of the revised manuscript (Lines 504 - 505).\nSpecific comments:\nLine 14: WMO 2023 reference not in citations.\nResponse: This is in reference to the State of the Climate in 2023, which is listed as reference 1 and now\nupdated to the State of the Climate in 2024. We added the appropriate reference to the abstract (Line 14).\nBlunden, J., & Boyer, T. (2024). State of the Climate in 2023. Bulletin of the American Meteorological\nSociety, 105(8), S1-S484.\nLine 18: Hazard >> disaster\nResponse: Changed.\nLine 18: Was this the costliest natural disaster of 2023 in North America? Perhaps specify this. Globally,\nI believe the Turkey earthquake in February was more costly.\nResponse: Agree, this is confined to just North America. We specify this in the revised text (Line 18).\nLine 32-33: Is there a citation for “200 deaths”? I believe the death toll in Maricopa County alone was\nmore than 200.\nResponse: We have updated the death toll number based on more current studies and government official\nreports. Below is the modification to the revised manuscript (Lines 33 – 38).\nThe year 2023 produced the highest number of heat-related deaths in the United States in the 21st century,\nwith 2325 deaths (Howard et al. 2024). One of these severe heat wave events occurred over the\nsouthwestern United States (US) and Mexico, which extended from mid-June to early August, affected\nover 100 million people, and was responsible for 303 deaths that occurred in a span of just two weeks in\nMaricopa County, Arizona (https://www.maricopa.gov/1858/Heat-Surveillance).\nHoward, J. T., Androne, N., Alcover, K. C., & Santos-Lozada, A. R. (2024). Trends of heat-related deaths\nin the US, 1999-2023. JAMA, 332(14), 1203-1204.\nLine 35: Similar to my earlier comment, it should be specified that this was the costliest natural disaster\nin North America, not globally.\nResponse: We specified in the revised text that this figure is only for North America (Line 43).\nLine 36-37: I think the more relevant statistic to highlight is the record streak of days above 110F in\nPhoenix (31 straight), which broke the previous record of 18 straight days. Days >40°C may not be the\nmost relevant statistic for Phoenix where the average high temperature in July is 41.4°C, meaning that\n40°C is a cool day. The record warm Tmin of 97°F is interesting and can be kept in the text.\nResponse: Great point, 40°C is not that impressive of a temperature for the area. We have now changed it\nto stress the consecutive days of 43.3°C (110°F), which occurred from 30 June - 30 July of 2023 in the\nrevised text (Lines 44– 45).\nLine 38-40: This sentence, as currently worded, is inaccurate and should probably be deleted. The heat\nwave that produced the all-time record in New Orleans was a completely separate synoptic event that\noccurred on August 27 in a different region of the country. Similarly, the San Juan record resulted from a\ndifferent tropical heatwave that was not part of the western North American heatwave that the authors\nare studying.\nResponse: Agree, the original intention was to stress on the anomalous warm summer of 2023, but these\nspecific events were not related to the heat wave being studied in this paper. We have removed this\nsentence from the revised text.\nLine 60: delete “the”\nResponse: Done\nLine 73: Might be good to add a quick sentence that explains how a Gill-type response can produce\nblocking over the Great Plains. I am assuming that the broad readership of Nature Communications will\nnot be familiar with this.\nResponse: Agree. We have now added some references on the Gill-response (Lines 83 – 86).\n“A recent work found that boreal summer tropical Atlantic SST anomalies modulate heat wave\noccurrences over North America (Lopez et al. 2022). In that work, it was found that a warmer tropical\nAtlantic enhances atmospheric convection over the Caribbean Sea and produces a Gill-type atmospheric\nresponse (Gill et al. 1980). This in turn produces an anticyclonic Rossby wave source over the Great\nPlains, thus enhancing subsidence and significant surface warming, leading to heat domes.”\nNote also that we added an additional figure and analysis describing the large-scale atmospheric response\nto prescribed SST forcing via Rossby wave sources (See Fig. 8 of the revised manuscript).\nLine 83-84: I would delete this sentence, since the AMO itself has been called into question as a “real”\nmode of internal climate variability.\nResponse: We agree, the current literature consensus calls for reframing the formerly known AMO into\nAtlantic Multi-decadal variability (AMV) as to not make any assumptions on whether its multi-year\nvariations are deemed as an internal oscillation. We have updated the revised text and replaced AMO with\nAMV (Line 101).\nLine 79-93: The effect of the 2020 international shipping sulfate reductions over the N. Atlantic might\nneed to be mentioned in this paragraph, since they likely played a role in the warm SST during 2023.\nResponse: We added the following to the revised manuscript (Lines 104 - 106).\n“Furthermore, the 2020 emission regulations from the International Maritime Organization aimed at\nreducing ship sulfate aerosol emissions may have contributed to the recent warm surface temperatures\nover the North Atlantic (Gettelman et al. 2024, Manshausen et al. 2023).”\nGettelman, A., Christensen, M. W., Diamond, M. S., Gryspeerdt, E., Manshausen, P., Stier, P., ... &\nYuan, T. (2024). Has reducing ship emissions brought forward global warming? Geophysical Research\nLetters, 51(15), e2024GL109077.\nManshausen, P., Watson-Parris, D., Christensen, M. W., Jalkanen, J. P., & Stier, P. (2023). Rapid\nsaturation of cloud water adjustments to shipping emissions. Atmospheric Chemistry and Physics,\n23(19), 12545-12555.\nFigure 1b: It might be good to adjust the x-axis to make the highest Tmax more visible. For example,\nPhoenix hit 122F (50C) in 1990 and 121F (49.4C) in 1995 but these temperatures are not visible in the\nhistogram, making it seem like 119F (48.3C) in 2023 was the highest tmax between 1982-2023.\nResponse: While we agree that it is worth mentioning that two other days saw higher temperatures,\nrepresenting the 50C that occurred on June 26, 1990 and the 49.4C on July 28, 1995 is very difficult in\nthe histogram given that the x-axis is representing occurrences and these two events are in a bin of their\nown. We tried to increase the bin number (e.g., refining the intervals to 0.5 and even 0.25C) of the\nhistogram, which is the only way of rescaling the x-axis, but but the histogram becomes discontinuous,\nappearing like a \"broken comb\", and does not accurately represent the data\nNote that the number of bins for the test and histogram below were chosen based on Sturge’s rule by\ntaking into account the size of the data (n = 74 years, 1950 - 2023) and the following relation:\nN bins = 1 + log (number of data points)\n- 138).\nLine 113: This is incorrect, the 2023 maximum of 119F/48.3C was tied for the 3rd warmest maximum in\nPhoenix after 122F in 1990 and 121F in 1995 (see my comment above).\nResponse: Good point. We have corrected this mistake in the revised manuscript (Lines 137 - 138).\nLine 117: “engulfed” >> “affected”\nResponse: Done\nLine 124-125: Why highlight this particular Tmax on Aug 23 in New Orleans? They were even hotter on\nAug 27th, when their all-time record high of 105F was reached. Perhaps this should be highlighted\ninstead.\nResponse: Agree, this is corrected in the revised text (Line 152).\nFigure 4a: It is surprising that 2020 doesn’t appear as a very strong negative anomaly. This was a well-\nknown failed monsoon season in the SW US, e.g.: Hoell et al., 2022: Record low North American\nmonsoon rainfall in 2020 reignites drought over the American southwest. The authors may want to\ndouble check their monsoon index calculations, and if found to be correct, perhaps provide the\nappropriate caveats as to why their index does not capture certain anomalous years.\nResponse: Our definition of the North American Monsoon (NAM) follows the concept of the global\nmonsoon (Wang and Ding 2008; Lee and Wang 2014). This index is defined as where the local summer\n(May - September) precipitation minus the winter (November–March ) precipitation exceeds 2.5 mm per\nday and the local summer precipitation exceeds 55% of total annual precipitation (Lee and Wang 2014).\nIn contrast, studies like Hoell et al., 2022 defined the NAM as an area over northwestern Mexico\nextending into the southwestern United States, a region known as an extension of the NAM (Adams and\nComrie 1997; Pascale et al. 2017, 2019).\nThis definition of the NAM represents a more coherent and geographically constrained approach (Wang\nand Ding 2008; Lee and Wang 2014; Wang et al. 2020). For example, Fig. R11 and R12 below shows the\nprecipitation anomaly for the wet season of the NAM during 2020 and 2023 from two datasets. The red\nstipples indicate the regions that meet the NAM criteria in our study. Note that 2020 was very dry mostly\nover the southwestern U.S (i.e., the northern extension of the NAM) and away from the core monsoon\nregion (red stipples). This is in contrast with the year 2023 where the core region of the NAM was much\ndrier, although concentrated mostly over Mexico.\nSince our NAM index is the area-averaged rainfall over the red-stippled region (as defined in Wang and\nDing 2008), our NAM index and that of Hoell et al. 2022 are different, thus showing much drier\nconditions for 2023 relative to 2020 in our case.\nWe also added the red stipples to Fig. 4 of the revised manuscript for identification of the monsoon region\nand also added more details on the monsoon definition in the Methods section (Lines 546 - 550).\nLee, J. Y., & Wang, B. (2014). Future change of global monsoon in the CMIP5. Climate Dynamics, 42,\n101-119.\nWang, B., & Ding, Q. (2008). Global monsoon: Dominant mode of annual variation in the\ntropics. Dynamics of Atmospheres and Oceans, 44(3-4), 165-183.\nWang, B., Biasutti, M., Byrne, M. P., Castro, C., Chang, C. P., Cook, K., ... & Zhou, T. (2021). Monsoons\nclimate change assessment. Bulletin of the American Meteorological Society, 102(1), E1-E19.\nFigure R11. June - September (JJAS) precipitation anomaly for a) 2020 and b) 2023. The red stipples\nindicate the North American monsoon region defined as where the local summer (May - September)\nprecipitation minus the winter (November–March) precipitation exceeds 2.5 mm per day and the local\nsummer precipitation exceeds 55% of total annual precipitation. Dataset from the JRA55 reanalysis.\nFigure R12. Same as Fig. R11 but using the ERA5 reanalysis.\nLine 197-201: Related to the above comment, the methodological details of the monsoon index\ncalculations should be fully explained in the methods section.\nResponse: We added more detailed explanations of the monsoon calculations as described in the replies to\nthe above comments in the Method section of the paper (Lines 546 - 550).\nLine 246: Please check the “(negative)”, it doesn’t seem to match the rest of the sentence.\nResponse: Thanks for catching this grammatical error, we have deleted the word “negative” in\nparenthesis.\nLine 261: I am confused by “partial regression”. Do the authors mean partial least squares regression?\nResponse: Correct, the analysis looks at partial least squares regression to linearly extract independent\nrelationships. This is clarified in the revised text (Line 304).\nLine 346: I may have missed it, but I don’t recall seeing the extreme value analysis described in the\nmethods?\nResponse: Extreme value analysis refers here as the analysis on several heat wave metrics (i.e.,\noccurrence, duration, amplitude) where the heat waves are defined by the excess heat factor (see Method\nsection and equations 1, 2, and 3). However, we agree that the term extreme value analysis is misleading\nas it is often referred to as a specific branch of statistics dealing with the tail of distributions (e.g., extreme\nvalue theory, Coles 1999).\nThus, we have modified the text to reflect this change (Line 412).\nColes, S. (1999). Extreme value theory and applications. Notes from a Course on EVT and Applications\nPresented at the 44th Reunio Annual da RBRAS e 8th SEAGRO.\nLine 387: “persistent heat wave” >> “persistence of the heat wave”\nResponse: Changed.\nLine 416-419: The statement about the earliest 110°F on record in June 2024 is only true for Las Vegas,\nnot Phoenix.\nResponse: We deleted this text as it relates to anomalous 2024 conditions, which may have distracted the\nreaders from the main scope of the study.\nLine 429: “Climate” >> “Centers”\nResponse: Fixed\nLine 501: Delete “the”\nResponse: Done\n_________________________________________________________________________\nReviewer #3 (Remarks to the Author):\n\"The longest-lasting 2023 western North American heat wave was fueled by the record-warm Atlantic\nOcean\" by Lopez et al.\nSummary:\nThis study investigates the causes of the longest-lasting heat wave in western North America in 2023\nthrough observational analysis and atmospheric general circulation model (GCM) experiments. The key\nconclusion is that the 2023 heatwave was significantly driven by record-warm Atlantic sea surface\ntemperature (SST) and growing El Niño in the Pacific. The authors conducted a comprehensive analysis\nof the roles of both the atmosphere and the ocean in driving the heat wave, using energy budget analysis,\n3-dimensional atmospheric circulation and moisture transport, and the impact of SST over both the\nAtlantic and Pacific Oceans. I believe this manuscript is well-written and makes a valuable scientific\ncontribution to understanding the dynamic and thermodynamic changes that triggered this heatwave\nevent. However, I found several issues that need to be addressed to improve the manuscript. For\npublication, please revise the manuscript to satisfactorily address the specific comments provided below.\nResponse: We are very grateful for your constructive comments/suggestions and hope to have\nsuccessfully addressed all your comments. Please find our replies to your specific comments below in\nregular fonts and your original comments in blue italic fonts. We point to specific line numbers where the\nchanges were made, these line numbers refer to the lines on the tracked-changes version of the text\nSpecific Comments:\n1. The authors concluded that the exceptionally warm Atlantic Ocean and the growing El Niño in the\nPacific caused changes in circulation, leading to an anticyclonic blocking pattern during the summer of\n2023. They supported their findings with evidence of interbasin synergy effect captured in model\nexperiments. However, it remains unclear about how the Pacific SST interacts with the Atlantic in this\nsynergy effect to create the persistent blocking pattern. The dynamic mechanism needs to be addressed as\nto how the El Niño-driven SST in the Pacific is linked to the ongoing atmospheric blocking over the\nwestern U.S.\nResponse: We made further analysis on the mechanisms involved in the relationship between the Pacific-\nAtlantic interbasin SST forcing and associated atmospheric response, which led to the blocking pattern\nover the heat wave region. Since both the Atlantic and Pacific SST forcing are tropical heating anomalies,\nthey could force remote teleconnections. These teleconnections are analyzed via Rossby wave sources\ndescribed in equation R9 (Sardeshmukh and Hoskins (1988) and Kirtman et al. (2001)), where the\nanomalous RWS is defined by the anomalous vorticity advection by the mean divergent wind, mean\nvorticity advection by the anomalous divergent winds, anomalous vortex stretching, and mean vortex\nstretching, respectively. Here, 𝑢 is the divergent wind (i.e., irrotational component of the flow), 𝜁 is the\n𝑖\nrelative vorticity, and 𝑓 is the Coriolis parameter. Overbar denotes the mean climatology and primes\nrepresent the deviation from climatology.\n𝑅𝑊𝑆 = −𝛻(𝑢 𝜁)′ = −𝑢̅ ⋅𝛻𝜁′−𝑢′⋅𝛻(𝜁̅+𝑓)−𝜁′𝛻⋅𝑢̅ −(𝜁̅+𝑓)𝛻⋅𝑢′ (𝑒𝑞.𝑅9)\n𝑖 𝑖 𝑖 𝑖\nThe RWS is computed for each SST sensitivity experiment. For example, note that there are a few regions\nof anticyclonic RWS in the GBL23 experiment (Fig. R13a) over the equatorial Pacific and Atlantic, near\nthe location of the ITCZ, and also over the Greater Antilles. These are consistent with the warm 2023 SST\nforcing prescribed in the GBL23 experiment, which led to upper level 200 hPa divergence (Fig. R13c)\nand 200 hPa anticyclonic circulation consistent with a Gill-type response (Fig. R13b). Further\ndecomposition of the RWS into its component demonstrates that mean vortex stretching by the\nanomalous divergent flow (Fig. R13e) was the main driver of the anticyclonic RWS and thus the\nanticyclonic circulation over Mexico.\nThe individual roles of each ocean basin are investigated using the ATL23 and PAC23 sensitivity\nexperiments. For the Atlantic-only forcing (Fig. R14), a similar pattern emerges where the upper-level\ndivergent flow being responsible for the anticyclonic RWS over the Atlantic basin, which leads to a\ndownstream (westward) intensification of the upper-level anticyclone over Mexico and the eastern Pacific\nbetween 20 - 40°N. In contrast, the Pacific-only SST sensitivity experiment (Fig. R15) yields anticyclonic\nRWS over the ITCZ, in association with the developing El Niño, and consistent with upper-level\ndivergence over the heating region (Trenberth et al. 1998). However, this signal is smaller than those\nfrom the ATL23 forcing. The larger RWS from the ATL23 compared to the PAC23 sensitivity\nexperiment is probably due to the fact that SSTs in the Atlantic were at their peak while the El Niño in the\nPacific was still in the growing phase (Supplementary Fig. 3).\nWe added this discussion to the revised text (Lines 371 - 389) and figure R13 as Fig. 8 of the revised text.\nWe also added figures R14 and 15 to the supplementary Figs. 10 and 11. The RWS analysis and equation\nR9 were added to the Method section (Lines 762 – 769).\nKirtman, B. P., Paolino, D. A., Kinter III, J. L., & Straus, D. M. (2001). Impact of tropical subseasonal\nSST variability on seasonal mean climate simulations. Monthly weather review, 129(4), 853-868.\nSardeshmukh, P. D., & Hoskins, B. J. (1988). The generation of global rotational flow by steady\nidealized tropical divergence. Journal of Atmospheric Sciences, 45(7), 1228-1251.\nTrenberth, K. E., Branstator, G. W., Karoly, D., Kumar, A., Lau, N. C., & Ropelewski, C. (1998). Progress\nduring TOGA in understanding and modeling global teleconnections associated with tropical sea\nsurface temperatures. Journal of Geophysical Research: Oceans, 103(C7), 14291-14324.\nFigure R13. SST sensitivity from an atmospheric model experiment for the Rossby wave source (RWS)\ncomponents and circulation. Composite difference of simulated a) 200 hPa RWS, b) 200 hPa\nstreamfunction (contour, 106 s-1) and anomalous vorticity advection (color), c) 200hPa velocity potential\n(contour, 106 s-1) and mean vorticity advection (color), d) 200 hPa rotational wind (vector, ms-1) and\nanomalous vortex stretching (color), and d) 200hPa divergent wind component (vector, ms-1) and mean\nvortex stretching. The units for the RWS terms are 10-11 s-2, see Methods for definition. Composites are\nfrom the AGCM experiment with prescribed 2023 global SSTs (GLB23). The composite differences are\nwith respect to the control experiment (CTL) for June-July-August.\nFigure R14. Same as R13 but for the ATL23 model experiment which compares the sensitivity to Atlantic\nSST forcing.\nFigure R15. Same as R13 but for the PAC23 model experiment which compares the sensitivity to Pacific\nSST forcing.\n2. Potential driver of the Atlantic warming in the discussion section: I’m still not convinced about the\npossible impact of El Niño on the Atlantic SST increase (e.g., L404-406), as warmer-than-average\nAtlantic SST was not frequently observed during the previous El Niño developing years. Could you\nprovide a more compelling explanation of the El Niño-driven increase in Atlantic SST?\nResponse: Please note that we are not arguing for an El Niño event being the cause of the exceptional\nwarm Atlantic SSTs, just pointing out that it could be one of the contributors as described in Cheng et al.\n2019 and in Li et al. 2024. For example, during El Niño events, the tropical troposphere is generally\nwarmer globally, which modulates surface winds and surface heat fluxes that eventually shoals the\ntropical ocean mixed layer, making it more stable, which leads to surface warming.\nHowever, we agree that El Niño alone could not cause such extreme north Atlantic SST anomalies\nobserved in 2023 (which were exceptionally warm to the extent of 3 standard deviations above the mean\nclimatology). Other factors that could play more relevant roles are the Atlantic Ocean internal variability\n(i.e., Atlantic Multi-decadal variability or AMV) and external forcing from natural or anthropogenic\neffects like aerosols (Booth et al. 2012; Bellucci et al. 2017). For example, recent work found that\nemission regulations from the International Maritime Organization aimed at reducing ship sulfate aerosol\nemissions may have contributed to the recent warm surface temperatures over the North Atlantic\n(Gettelman et al. 2024, Manshausen et al. 2023).\nWe updated the discussion and added more references on the 2023 warming of the North Atlantic in the\nrevised text (Lines 96 - 113 and 462 - 478).\nBellucci, A., Mariotti, A., & Gualdi, S. (2017). The role of forcings in the twentieth-century North Atlantic\nmultidecadal variability: The 1940–75 North Atlantic cooling case study. Journal of Climate, 30(18),\n7317-7337.\nBooth, B. B., Dunstone, N. J., Halloran, P. R., Andrews, T., & Bellouin, N. (2012). Aerosols implicated as\na prime driver of twentieth-century North Atlantic climate variability. Nature, 484(7393), 228-232.\nCheng, L., Trenberth, K. E., Fasullo, J. T., Mayer, M., Balmaseda, M., & Zhu, J. (2019). Evolution of ocean\nheat content related to ENSO. Journal of Climate, 32(12), 3529-3556.\nGettelman, A., Christensen, M. W., Diamond, M. S., Gryspeerdt, E., Manshausen, P., Stier, P., ... &\nYuan, T. (2024). Has reducing ship emissions brought forward global warming?. Geophysical\nResearch Letters, 51(15), e2024GL109077.\nLi, K., Zheng, F., Zhu, J., & Zeng, Q. C. (2024). El Niño and the AMO sparked the astonishingly large\nmargin of warming in the global mean surface temperature in 2023. Adv. Atmos. Sci. 41, 1017–1022.\nManshausen, P., Watson-Parris, D., Christensen, M. W., Jalkanen, J. P., & Stier, P. (2023). Rapid\nsaturation of cloud water adjustments to shipping emissions. Atmospheric Chemistry and Physics,\n23(19), 12545-12555.\n3. This study shows many results for Phoenix, Arizona (e.g., Figures 1, 3b, c, and 9). Extremely high\ntemperatures were, however, observed in other regions as well, including Texas, northern Mexico, and\nLouisiana. I recommend that the authors provide evidence showing that these other areas share quite\nsimilar characteristics to Phoenix, or at least discuss these regions in greater detail.\nResponse: Agree. The spatial coverage of the heat waves was fairly large, which encompassed multiple\nstates and most of northern Mexico. Our choice of showing results from just Phoenix was arbitrary but\npartially motivated by this population center being one of the largest impacted by the event. With that\nsaid, we have now added more discussion on the effects of the 2023 heat wave on other important\npopulation centers. For example, fig. R16 below is similar to fig. 1a of the paper and shows the\ntemperature evolution during the summer of 2023 for a) Las Vegas, Nevada, b) Albuquerque, New\nMexico, c) El Paso, Texas, and d) San Antonio, Texas. Note that all these cities were impacted by the heat\nwave (supplementary fig. 1) with both maximum and minimum temperatures exceeding their\ncorresponding 95 percentile for multiple days.\nAlso note that the analysis only includes cities within the continental U.S. as long-term continuous and\nreliable data are not available for northern Mexico.\nWe added Fig. R16 as a supplementary Fig. 1 and associated discussion in the revised text (Lines 140 –\n144)\nFigure R16. Heat wave event of 2023. Seasonal evolution of maximum (red) and minimum (blue)\ntemperature for the year 2023 from 1 June to 20 September for a) Las Vegas, Nevada, b) Albuquerque,\nNew Mexico, c) El Paso, Texas, and d) San Antonio, Texas. The long-term daily mean is shown by the\ndashed line whereas the 5th and 95th percentiles are shown by the shading region. Excess above the 95th\npercentile is shown by red shading for both maximum and minimum temperatures. The observed\nhistogram of maximum (red) and minimum (blue) temperature for the period 1 June to 31 August from\n1955-2023 is also shown on the right with bin size of 1°C. The extremely warm temperatures for the 2023\nsummer are shown for reference.\n4. Equation 4 for budget analysis: I believe vertical integration over a range of pressure levels would be\nmore preferable than considering a single level (850hPa). This would be specifically true for vertical\nadvection.\nResponse: We were following the approach of Wei et al. 2023 by computing the heating rates at a\npressure level relatively low enough to represent near-surface conditions. However, we agree that\nintegrating over a range of pressures levels close to the surface would be preferable, so we have\nrecomputed the energy budget integrated from 975 to 800 hPa (equation R10), which includes the\nfollowing pressure levels: 975, 950, 925, 900, 875, 850, 825, and 800 hPa. The integral is mass-weighted\nand normalized by the total integration thickness 𝛥𝑃 to preserve the original units of °C/day of the\nheating rates and for easier comparison with the original units.\n1 800𝜕𝑇 1 800 𝜕𝑇 𝑅𝑇\n∫ 𝑑𝑝 = ∫ [−𝑢⃗ ∙𝛻𝑇−𝜔( − )+𝑄 ]𝑑𝑝 (𝑒𝑞.𝑅10)\n∆𝑃 𝜕𝑡 ∆𝑃 𝜕𝑝 𝐶 𝑃 𝑛𝑒𝑡\n975 975 𝑝\nNote that the overall results and the relative contributions of each of the heating terms do not change (Fig.\nR17), and there is only a small increase in the role of the horizontal advection term leading up to the heat\nwave (June) and later in the period (second half of August).\nWe have replaced Fig. 2 with Fig. R17 below and updated equation 4 in the revised text (Lines 525 - 533)\nto represent the vertical integration. Note that we have also added a new sub-panel showing the 2-meter\nair temperature anomalies as requested by another reviewer.\nWei, J., Wang, B., Luo, J. J., Li, C., & Yuan, C. (2023). Synoptic characteristics of heatwave events in\nAustralia during austral summer of 1950/1951–2019/2020. International Journal of Climatology, 43(12),\n5662-5680.\nFigure R17. Heat budget analysis. Energy budget averaged every 10 days over the southwest U.S. and\nnorthwest Mexico from 6 June to 24 August 2023. Each row represents (from the top): 200 hPa\ngeopotential height anomaly [gpm], 850 hPa temperature and 2-meter temperature anomalies [°C],\nvertically integrated anomalous heating rates from 975 - 800 hPa [°C day-1], and surface heat fluxes (net\nsurface shortwave and longwave radiation, sensible and latent heat fluxes) [Wm-2]. Daily anomalies are\nderived from the long-term monthly mean for the 1979-2022 period.\n5. Advection in budget analysis: Could you elaborate which factor, the wind versus temperature gradient,\nand mean versus anomaly, has a greater influence on determining the advection in the budget analysis?\nResponse: We decomposed the advection term and found that the anomalous advection by the anomalous\nwind contributed the most to the total advection.\nThe relative contribution of wind (temperature gradient) was assessed by first computing the root-mean-\nsquare amplitude of the temperature gradient (winds) shown in brackets (e.g., time-invariant vector),\nthen multiplied by the wind (temperature gradient) anomalies shown by primes. Overall, the wind anomalies\nproved to be more important than the anomalous temperature gradient (Fig. R18).\nWith that said, note that heating due to advection is very small during the peak of the event (July), this is\nexpected due to the broad temperature anomaly (Fig. 1c) and stagnant flow pattern (Fig. 3)\nassociated with this heat wave.\nPlease note that we did not include Fig. R18 in the revised text due to the relatively small contribution of\nthe horizontal advection to the heat wave and the already large number of figures presented. However, we\nmention the relative contributions of the advection terms in the revised text (Lines 180 - 181).\nFigure R18. Decomposition of the total anomalous temperature advection averaged every 10 days over\nthe southwest U.S. and northwest Mexico from 6 June to 24 August 2023. Each row represents (from the\ntop): anomalous temperature advection, anomalous temperature advection by the mean wind, mean\ntemperature advection by the anomalous wind, anomalous temperature advection by the anomalous wind,\nthe role of circulation-only, and the role of temperature gradient only in the horizontal advection. All\nvalues were vertically integrated from 975 - 800 hPa [°C day-1]. Daily anomalies, denoted by primes, are\nderived from the long-term monthly mean climatology for the 1979-2022 period, denoted by the overbar.\nThe entries within brackets are defined by the root mean square amplitude from June-August, effectively\nholding their contribution to the advection constant.\n6. The authors mention that they present the PV pattern in Figure 3 during the peak of the heatwave.\nHowever, it seems this is based on the heatwave that occurred in Arizona. I am wondering how the large-\nscale atmospheric condition shown in Figure 3 could support the development of heatwave conditions\nover E Mexico, E Texas and Louisiana, where northerly winds are prevalent. Is the PV pattern here\nexplaining the large-scale condition for the heatwave over Arizona only?\nResponse: The PV analysis shown in Fig. 3a and b) encompasses the broad scale of the heat wave at its\npeak. However, this heat wave was not confined to Arizona, we picked this specific longitude cross-\nsection to represent the upper-level dynamics. However, similar large-scale atmospheric conditions were\npresent during the early (Fig. R19) and late stages (Fig. R20) of the heat wave. For example, a deep anti-\ncyclonic blocking pattern and warm near-surface potential temperature anomalies were also present. This\nis consistent with the semi-persistent anticyclone and long-duration of the heat wave (Supplementary Fig.\n4).\nFigure R19. Similar to figure 3 of the manuscript. a) Potential vorticity and wind at the 340K isentropic\nlevel during the maximum amplitude of the heat wave on 20 June 2023. Thick vectors depict anti-cyclonic\nfluid trapping, a proxy for heat dome and air flow stagnation. The thick black line indicates the location of\nthe dynamical tropopause. The blue star represents the location of San Antonio, Texas. b) Latitude-vertical\ncross-section along 100ºW on 20 June 2023 of anomalous potential temperature (color) and potential\ntemperature (magenta 10K intervals). Also shown are zonal wind anomalies (light black contours at 3 m/s\nintervals), the tropopause level as measured by the 2 PVU (thick black line), and anti-cyclonic fluid trapping\n(circle hatching at 10-5 s-1).\nFigure R20. Same as figure R12 but for 06 August 2023. The cross section is shown along 100ºW ,\nnear\nEl Paso, Texas.\n7. Following the comment above (#6), the timing of the heatwave peak seems to vary across different\nregions, such as Arizona, Texas, Louisiana, NW Mexico, and E Mexico, as seen in Supplementary Figure\n1. The PV and circulation pattern during the peak (June, July, or August) of the heatwave could differ\nbetween regions. The authors may need to address this issue.\nResponse: Please note that the purpose of Fig. 2 is just a descriptive snapshot to gain some insight of the\nupper-level dynamics involved. We agree that these patterns are time dependent and evolving during the\nevent. As such, and as our reply to your previous comment, we have also analyzed these PV and\ncirculation patterns for different regions and months. These are included for 20 June 2023 (Fig. R19) and\nand warm near-surface potential temperature anomalies) are still present in these different cross-sections\nat different times.\n8. Anomalous moisture transport: Figure caption 4 states this field is computed from departures from\nclimatology. Does this mean that the departure of wind and moisture from climatology is used to compute\nthe anomalous moisture transport and its convergence? If so, I believe this approach may be incorrect.\nMoisture transport is determined by four components: 1) advection of mean moisture by mean wind, 2)\nadvection of mean moisture by anomalous wind, 3) advection of anomalous moisture by mean wind, and\n4) advection of anomalous moisture by anomalous wind. The fourth term may be an order of magnitude\nsmaller than the other terms. I recommend that authors clarify how the Figure 4c pattern was calculated.\nResponse: The anomalous moisture transport shown in Fig. 4 represents the sum of the last three terms\ndescribed by the reviewer, which are: advection of mean moisture by anomalous wind, advection of\nanomalous moisture by mean wind, and advection of anomalous moisture by anomalous wind. This is\nclarified in the revised text (Lines 250 - 253). Note that anomalous is referred to here as not including the\nmean moisture advection by the mean wind, because we are interested in the 2023 anomalous conditions.\nThis is readily shown in equation (eq. R11 and R12) below where moisture flux is the product of the\nhorizontal wind u and specific humidity q and the overbar denotes climatology and primes denotes\ndeviation from climatology:\n𝑢𝑞 = (𝑢̅ +𝑢′)(𝑞̅+𝑞′) = 𝑢̅𝑞̅+𝑢′𝑞̅+𝑢̅𝑞′+𝑢′𝑞′ 𝑒𝑞.𝑅11\nThus, what is shown in figure 4 is:\nAnomalies = (𝑢𝑞)′ = 𝑢𝑞−𝑢̅𝑞̅ = 𝑢′𝑞̅+𝑢̅𝑞′+𝑢′𝑞′ 𝑒𝑞.𝑅12\nAs the reviewer mentioned, the last term is an order of magnitude smaller. And the moisture transport is\ndominated by the circulation anomalies (𝑢′𝑞̅). This is shown in Figs. R21 and R22 below and clarified in\nthe revised text, which includes a supplementary figure showing the decomposition of the fluxes\n(Supplementary Figs. 6 and 7).\nFigure R21. Decomposition of the anomalous June-August 2023 vertically integrated moisture transport\n(vector, Kg m-1 s-1) and its divergence (color, Kg m-2 s-1), where negative values indicate convergence. a)\nAdvection of mean moisture by anomalous wind, b) advection of anomalous moisture by the mean wind,\nand c) advection of anomalous moisture by anomalous wind. The overbars denote climatology and primes\ndenote deviation from climatology computed from the departure from the 1979-2022 climatology.\nFigure R22. (left-column) Zonal-vertical cross-section along 28°N of the 2023 moisture transport\nanomaly (color). (Right column) is the same but for the meridional cross-section along 110°W. The\ntransport is decomposed into its components such as (a and b) advection of mean moisture by anomalous\nwind, (c and d) advection of anomalous moisture by the mean wind, and (e and f) advection of anomalous\nmoisture by anomalous wind. The overbars denote climatology and primes denote deviation from\nclimatology computed from the departure from the 1979-2022 climatology.\n9. The moisture transport field suggests that the heat transport from the Atlantic might also be lower than\nclimatology, potentially due to the prevailing wind direction, which is not southerly. Could you offer some\ndiscussion on this?\nResponse: Indeed, the low-level (surface to 600hPa) vertically integrated heat transport anomaly from the\nAtlantic was reduced in 2023 (Fig. R23). Note that we used equation R12 above for the computation of\nthe anomalous heat transport replacing specific humidity with temperature instead.\nFigure R23. a) Climatological vertically integrated moisture transport (vector, K Kg m-1 s-1) and its\ndivergence (color, K Kg m-2 s-1), where negative values indicate convergence. b) Same as a) but for the\n2023 anomalies computed from the departure from the 1979-2022 climatology. Vertical integration is\nfrom the surface to 600hPa.\n10. L249: Is it “significantly” negative? The negative anomaly observed in June weakens in July.\nAdditionally, could you explain how this negative anomaly in the Atlantic is related to the anticyclonic\ncirculation linked to atmospheric blocking in the upper troposphere? Are they connected, or is the\nblocking a consequence of upper-level dynamics with no influence from the surface level shown in Figure\n5?\nResponse: Agree, the sea level pressure anomaly is much weaker in July over the Atlantic as well\ncompared to the June anomalies. We have softened the language here to reflect this weakening.\nAs per the role of the negative low level pressure anomaly on the upper-level flow, please see our reply to\nyour Comment 1 and the analysis of atmospheric circulation and the Rossby wave source (equation R9).\nIn summary, the warm Atlantic SSTs forced Gill response by creating lower (upper) level convergence\n(divergence) shown in Fig. R24, which through vortex stretching led to anti-cyclonic Rossby wave\nsource, leading to the atmospheric blocking in the upper troposphere. This mechanism of Atlantic SST\nforcing is described in Wang et al. 2008.\nWang, C., Lee, S. K., & Enfield, D. B. (2008). Climate response to anomalously large and small Atlantic\nwarm pools during the summer. Journal of Climate, 21(11), 2437-2450.\nFigure R24. a) 200 hPa streamfunction (black contour, 106 s-1) and velocity potential (color, 106 s-1)\nanomalies from the AGCM experiment with prescribed 2023 Atlantic SSTs (GLB23). The anomalies are\ndefined with respect to the control experiment (CTL) for June-July-August.\n11. The authors present Figure 5 to explore large-scale climate factors that may drive heatwave\nconditions. However, in terms of causality, it is not clear whether the precipitation over U.S. land is a\ncause or an effect of the heatwave. This needs to be explicitly addressed in the manuscript.\nResponse: We added more explanation on the physical processes involved in the persistence of the heat\nwave in the revised text. For example, the upper troposphere analysis on circulation (rotational and\ndivergent flow, as well as the Rossby wave source RWS analysis) describes how the large-scale SST\nforcing was linked to the reduced monsoon and heat wave occurrence in the region of interest. See Fig. 8\nand associated discussion (Lines 371 - 389).\nWe also added more analysis showing the historical negative correlation between an inactive monsoon\nand warm 2-meter temperatures over the southwestern U.S. (discussion in Lines 238 - 241and\nSupplementary Fig. 5).\n12. Is the upper-level anticyclonic circulation pattern in Figure 6a (contoured) similar to the observed\npattern in 2023? If so, is this pattern generally pre-dominant throughout the summer?\nResponse: Yes, the upper-level circulation pattern from Fig. 6a is very similar to the observed 2023\nanomalous pattern as shown in Fig. R25 below. Of course, the anomalous 2023 pattern is a bit noisier as\nit is just one realization instead of the regression pattern shown in Fig. 6a. Even the surface temperature\nanomalies are extremely similar (Figs. R25b and d). We added the comparison between the observed\n2023 anomalies and those extracted from the regression analysis to the revised Fig. 6.\nIf so, is this pattern generally pre-dominant throughout the summer?\nPlease note that these are regression coefficients computed for the June-August period, which depends on\nthe El Niño conditions in the Pacific (Niño3 index) plus the tropical North Atlantic SST anomalies (TNA\nindex). See description in the main text.\nWe have added panels a and b of Fig. R25 to Fig. 6 of the revised paper as it shows relevant information.\nFigure R25. a) Anomalous 200 hPa temperature (color) and streamfunction (black contour, 106 s-1) for\nJune - August of 2023. b) same as a) but for 2-meter maximum air temperature anomaly. c) Regression\ncoefficient of Niño3 plus TNA SSTs and 200 hPa temperature (color) and 200 hPa streamfunction (black\ncontour, 106 s-1). d) Same as c) but for 2-meter maximum air temperatures. The regression coefficients are\ncomputed for June-August for the 1960-2023 period through partial regression and the units are per\nstandard deviation of the SST anomalies.\n13. Why does the Atlantic-only experiment produce warmer than average temperature along the western\ncoast of Mexico, with no notable warming over the central to eastern side of Mexico (Figure 7)?\nResponse: Please note that while the larger amplitude temperatures are located on the western portion of\nMexico, warm temperatures are found through the country, with the exception of the extreme south, along\nthe Pacific coast (Fig. 7d). In fact, warm temperature anomalies occur all along the U.S. Gulf Coast from\nCentral America to the Florida peninsula, very similar to the GBL23 (all forcing experiment). With that\nsaid, we agree that the ATL23 tends to push the anomalies westward compared to the GBL23 case. This\nis also evident in the circulation anomalies (Fig. 7c) where the upper-level anticyclone is strongest just\nwest of the North American continent (e.g., westerly intensified), compared to the GBL23, which anchors\nthe upper-level cyclone over Mexico. The synergy component appears to be playing an active role in\ndefining the location of the maximum temperature anomalies, by shifting it inland and north into the\nArizona-New Mexico-Texas region (Fig. 7h).\n14. While the amplitude change of heatwave events in the AGCM experiments shows the highest values\nover Mexico, the number of events, heatwave days, and duration in Figure 8 show the greatest increase in\nthe far inland areas like Colorado, as well as western Mexico and the southern coastal region. Have you\nconsidered why the largest values appear in the far inland area? Could the large-scale atmospheric\npattern in the AGCM experiment offer any insights to help explain this?\nResponse: This is a great point. The AGCM response to prescribed SST forcing appears more consistent\nover Mexico, where all four heat wave parameters show consistent increase. In contrast, all parameters\nexcept the amplitude are also enhanced over the U.S, consistent with the enhanced duration of the heat\nwave in 2023.\nWe examined the large-scale circulation pattern (Figs. R13) and could not deduct a clear reason for such.\nIt is worth mentioning that besides the modulating effects of circulation, heat waves are also influenced\nby complex land-atmosphere interactions/feedbacks that may not be well resolved in CESM2. In addition,\nsignificant topographic features are present in this area, which have been shown to play an important role\nin surface temperature variations, but may not be well-resolved at these horizontal resolutions. Moreover,\ncomplex air-sea interactions are also missing from these AGCM simulations as the atmospheric model is\nforced by SSTs with no ocean thermodynamic nor dynamical feedback. Thus, further studies may be\nneeded to investigate these differences.\nAll these caveats and model limitations have been included in the Method section of the revised\nmanuscript (Lines 587 - 593).\nPlease extend our gratitude to the three anonymous reviewers for their constructive\ncomments/suggestions that have led to a significant improvement to our manuscript.\nIn what follows, we respond to each comment of all reviewers. Our responses are in regular font\nand the original reviewers’ comments are in blue italic font. Each comment is addressed in the\nrevised manuscript. We point to specific line numbers where the changes were made, these line\nnumbers refer to the lines of the tracked-changes version of the text, A copy of the revised\nmanuscript is provided with tracked changes revisions.\nReviewer #1 (Remarks to the Author):\nI acknowledge that the manuscript “The longest-lasting 2023 western North American heat wave\nwas fueled by the record-warm Atlantic Ocean” by Hosmay Lopez, Sang-Ki Lee, Robert West,\nDongmin Kim, Liwei Jia featured substantial improvement compared to the previous version.\nParticularly, I appreciated the great deal of additional work that the authors put into the\nmanuscript, thoroughly addressing comments by myself and the other reviewers (and also\nproving that I was incorrect about a few remarks!). I do acknowledge that the amount of\ninformation provided is now fairly larger and the message is more complete and robust, also\ninvolving aspects of the large-scale circulation that were barely touched upon in the previous\nmanuscript. I do have a few minor remaining comments that I want to raise, mainly regarding\nadditional references and better context on a few aspects.\nProvided this, I do believe that the manuscript would be ready for publication.\nResponse: We are very grateful for your constructive comments/suggestions and hope to have\nsuccessfully addressed all your comments. Please find our replies to your specific comments\nbelow in regular fonts and your original comments in blue italic fonts. We point to specific line\nnumbers where the changes were made, these line numbers refer to the lines on the tracked-\nchanges version of the paper.\nMINOR COMMENTS\nll. 29-30: given that the authors acknowledge the spatial compounding nature of the event, it\nwould be nice if the authors could mention already here something about the relevance of what\nthey find for the large-scale atmospheric circulation, especially given that they adopt the\nstrategy of looking into inter-basin synergies and the role of planetary-scale Rossby waves (e.g.\nLembo et al. 2024).\nLembo, V., and Coauthors, 2024: Dynamics, Statistics, and Predictability of Rossby Waves,\nHeat Waves, and Spatially Compounding Extreme Events. Bull. Amer. Meteor. Soc., 105, E2283–\nE2293\nResponse: We added the fact that several heatwave events were concurrent and affected multiple\nregions as a part of the large-scale flow characteristics. We also added the reference to Lembo et\nal. 2024 to the revised text. With that said, mentioning the relevance of our findings this early in\nthe introduction may be a bit premature and steer the focus away from the historical context\nconveyed in the introduction with regards to how anomalous 2023 was.\nll. 59-61: quasi-resonant amplification is not the only possible explanation for large-scale\natmospheric circulation signature on heatwaves (see reference above and literature cited\ntherein);\nResponse: We added more explanation on large-scale atmospheric circulation signature on heat\nwave. For example, the role of quasi-stationary and propagating Rossby waves and their\ninteraction with the overall synoptic flow, topography, and land-sea contrast described in Lembo\net al. 2024 and references therein. We also added Lembo et al. 2024 to the reference list (ref. 4).\nSee lines 61 – 63 of the revised text.\nll.73-74: I think it is very relevant to mention here a recent paper (England et al. 2025) on the\ndrivers of the North Atlantic SST exceptional anomalies;\nEngland, M.H., Li, Z., Huguenin, M.F. et al. Drivers of the extreme North Atlantic marine\nheatwave during 2023. Nature (2025). https://doi.org/10.1038/s41586-025-08903-5\nResponse: We added the England et al. 2025 to the discussion section were we discuss drivers of\nthe extremely warm 2023 North Atlantic SSTs, see lines 443 and 446 (ref. 90)\nll. 107-110: In this respect, it is also worth mentioning that the tropical atmospheric bridge here\nmentioned is linked itself with the extratropical Atlantic variability, as argued for instance in An\net al. 2021;\nAn, X., B. Wu, T. Zhou, and B. Liu, 2021: Atlantic Multide Atlantic Multidecadal Oscillation Drives Interdecadal\nPacific Variability via Tropical Atmospheric Bridge. J. Climate, 34, 5543–5553\nResponse: We added the An et al. 2021 to the reference list (ref. 59) and revised text, see line 98.\nFigure 2: I still think that Figure R2 is actually more informative than the boxes approach (for\ninstance, showing the relative magnitude of anomalies, and some peculiar behavior, such as for\ninstance the one exhibited by latent heat fluxes), but I acknowledge the readability issue and\ntherefore leave it to the authors;\nResponse: Given the readability issue with the spaghetti/line plot, we choose to keep the box plot\nversion instead.\n_____________________________________________________________________\nReviewer #2 (Remarks to the Author):\nI thank the authors for addressing my points. I believe this manuscript is ready to be published\npending the approval of the other reviewers.\nResponse: Thanks a lot for your constructive comments/suggestions, which led to a much-\nimproved manuscript.\n_____________________________________________________________________\nReviewer #3 (Remarks to the Author):\nI appreciate the authors for revising their manuscript. My previous concerns have been\naddressed satisfactorily, and I have no remaining major issues. I recommend accepting the\nrevised version as it stands.\nResponse: Thanks a lot for your constructive comments/suggestions, which led to a much-\nimproved manuscript."}]

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