Northeast Pacific warm blobs sustained via extratropical atmospheric teleconnections

Domain: earth_science
Published: 2024-04-02
Editorial outcome: minor_revision

Abstract
Large-scale marine heatwaves in the Northeast Pacific (NEP), identified here and previously as ‘warm blobs’, have devastating impacts on regional ecosystems. An anomalous atmospheric ridge over the NEP is known to be crucial for maintaining these warm blobs, also causing abnormally cold temperatures over North America during the cold season. Previous studies linked this ridge to teleconnections from tropical sea surface temperature anomalies. However, it was unclear whether teleconnections from the extratropics could also contribute to the ridge. Here we show that planetary wave trains, triggered by increased rainfall and latent heat release over the Mediterranean Sea accompanied by decreased rainfall over the North Atlantic, can transport wave energy to the NEP, guided by the westerly jet, and induce a quasi-barotropic ridge there. Our findings provide insights into extratropical teleconnections sustaining the NEP ridge, offering a source of potential predictability for the warm blobs and temperature fluctuations over North America.

Introduction
Large-scale marine heatwaves in the NEP region, identified here and previously as ‘warm blobs’, have attracted wide attention since the long-lived, so-called ‘Blob’ event of 2013–2016 and another event in 2019–2020. Since the probability of such marine heatwaves is expected to increase with global warming, it is critical to understand their driving mechanisms to better understand warm blob potential predictability. Both atmospheric (e.g., anomalous atmospheric pressure ridge and clouds) and oceanic (e.g., salinity-related mixed layer depth variation) processes have been suggested to play important roles in the warm blob formation.

Among these key processes, a strong and persistent higher-than-normal sea level pressure (SLP) over the NEP is thought to be critical by modulating surface latent heat flux as well as wind-driven ocean currents and mixing in winter. A number of studies suggest that atmospheric teleconnections originating from the tropics can trigger this anomalous pressure ridge over the NEP. For example, Wang et al. proposed a cross-Pacific pathway of Rossby wave energy at 850 hPa during winter 2013/14 in response to anomalous convection over the Philippine Sea, which contributed to the anomalous ridge over the Gulf of Alaska. Shi et al. discussed potential teleconnections to the NEP triggered by different types of El Niño events. However, unlike these important processes originating in the tropics, possible contributions from midlatitudes to the anomalous ridge have received much less attention.

Rossby wave trains in the mid-latitudes, accompanied by jet stream variations, play essential roles in generating surface air temperature variations in boreal autumn and winter. Moreover, the importance of Rossby wave-induced effects has increased during the past two decades. Here we will demonstrate the importance of extratropical wave trains, specifically those originating from the North Atlantic (NATL) and Mediterranean Sea regions, on the long-lasting ridge over the NEP, thus providing insights into the role of remote teleconnections in maintaining the NEP ridge.

Results
Using data for 13 warm blob events that peak during the boreal cold season (from November to March; Supplementary Table 1 and Fig. 1), we firstly consider the composite evolution of geopotential height anomalies at 500 hPa from September to the following February (Fig. 1). An anomalous ridge over the NEP emerges in September (Fig. 1a), somewhat weakens and shifts westward in October (Fig. 1b), reaches its maximum intensity in November (Fig. 1c), and weakens and moves northward in the following months (Fig. 1d–f). This ridge has a quasi-barotropic structure and extends from the surface to the upper troposphere (Supplementary Fig. 2). Aside from the ridge, a trough centered north of the Great Lakes deepens (Fig. 1 and Supplementary Fig. 2), entraining cold air over North America. In November, prominent geopotential height anomalies are also found over the eastern NATL, featured by a meridional dipole (Supplementary Fig. 2). This atmospheric pattern may be related to the North Atlantic Oscillation (NAO), with 9 out of 13 warm blob events (~70%) occurring during the positive NAO phase. On the planetary scale, such ridge and trough structures resemble the tropical Northern Hemisphere pattern in winter (Fig. 1d, e), which is potentially important for the warm blobs. To elucidate the intensification of the ridge during the lifetime of the warm blobs, we mainly focus on the results in November hereafter due to the strongest intensity of the anomalous ridge (Fig. 1c).

To explore the potential relationship between the anomalous ridge and midlatitude wave trains, we analyze meridional wind anomalies (shading in Fig. 2a) at 300 hPa in November for warm blobs peaking during the cold season. Two prominent wave trains appear to originate over the NATL, possibly associated with the NAO. One propagates along the subtropical jet stream with a larger wavenumber and another is along the subpolar jet stream with a smaller wavenumber. The jet streams act as waveguides for the transportation of energy downstream. The two wave trains converge near Japan, intensifying the wave amplitude and further propagating towards the NEP. Rossby wave energy, indicated by the wave activity flux (WAF; “Methods”), greatly intensifies over the Mediterranean Sea (vectors in Fig. 2a), implying a potential Rossby wave source (RWS) in this region. Although the two wave trains somewhat weaken over the Eurasian continent, they converge and strengthen again near Japan and travel northeastward to the NEP, which appears to be important for the development and persistence of the anomalous ridge above the warm blobs (Fig. 1 and Supplementary Fig. 2).

For comparison, meridional wind anomalies and WAF are also shown for October (Supplementary Fig. 3a). Although the wave train structure is clear, it is much weaker compared to that in November (Fig. 2a). Consistently, the WAF is much weaker, especially over the Mediterranean Sea and Europe (Supplementary Fig. 3a). Note that the NATL and Mediterranean regions originated circumnavigating wave trains were not reported in Wang et al., but rather they emphasized the wave origin near Japan. Although similar wave trains have been documented in previous studies, none of these focus on the wave propagation towards the NEP. Nevertheless, a recent study implied that the subtropical wave train is closely connected to intense Rossby wave activity over the Mediterranean region and anomalous warming over the NEP.

Rossby waves are usually triggered when strong convection (or rainfall) generates anomalous condensation (latent) heating, inducing anomalous flow divergence and vorticity in the upper troposphere, which often occurs over tropical oceans and the NATL. East of the NATL, Rossby wave trains often bifurcate into two branches over the Mediterranean, acting as a pivotal region for Rossby wave propagation and generation. The reasons for this wave train bifurcation are: (1) the wave train near the Mediterranean is located outside of the area with strong absolute vorticity gradient so that it is partially reflected, and (2) the wave train is sensitive to the origin of the Rossby wave.

To uncover the origin of the Rossby wave train that contributes to the NEP warm blobs, we calculate the RWS (“Methods”; shading in Fig. 2b) at 300 hPa in November for warm blobs peaking in the cold season. A large area with a positive RWS is identified over the mid-latitude eastern NATL while negative RWS values are detected over the Mediterranean region. The two RWS areas correspond to anomalous convergence and divergence in the upper troposphere, respectively (not shown). We mainly focus here on these two RWS regions due to their stronger WAF. In addition, a band of positive RWS is found from southern China to south of Japan, which is consistent with the WAF trajectory along the subtropical jet (Fig. 2a). Further north, another area with a negative RWS is also significant over northern China (Fig. 2b). According to previous studies, anomalous diabatic heating sustained by rainfall/convection is essential for exciting atmospheric Rossby waves. Hence, we show a composite apparent heat source (“Methods”) at 300 hPa for warm blobs peaking during the cold season (Fig. 3b). Prominent heating and cooling anomalies are found over the Mediterranean and NATL regions, respectively, consistent with the rainfall anomalies (Fig. 3a). However, the two rainfall and RWS centers over China and surrounding seas are not robust in terms of anomalous heating (Fig. 3b).

Further, to verify the roles of the Mediterranean and NATL regions in triggering the wave trains that contribute to the long-lasting anomalous ridge over the NEP, we impose anomalous heating/cooling in November within an atmospheric linear baroclinic model (LBM; “Methods”). Background atmospheric conditions in November given in the LBM are presented in Supplementary Fig. 6. The vertical structure of the apparent heat source over the Mediterranean region (black box in Fig. 3) exhibits anomalous heating induced by the positive rainfall anomalies throughout the whole troposphere, with a maximum magnitude at 300 hPa (Fig. 4a). After the simulation reaches a quasi-equilibrium state, a wave train originating from the Mediterranean region is identified in the extratropics with the WAF propagating eastward to the NEP (Fig. 4b). Over the NEP (purple box in Fig. 4b), an anomalous ridge is evident at 500 hPa (Fig. 4b). At the surface, the anomalous high-pressure system shifts a bit northwestward (Fig. 4c), with easterly anomalies blowing against climatological midlatitude southwesterlies (Supplementary Fig. 6c), which are favorable for inducing the NEP warm blobs by reducing latent heat loss from the ocean to the atmosphere.

In terms of the detailed evolution, after imposing heating over the Mediterranean region, positive geopotential height anomalies appear over the NEP approximately 16 days after the heating and then develop and become stable in the following days (Supplementary Fig. 7). Moreover, Rossby wave energy can disperse further downstream from the anomalous ridge, amplifying the trough over North America and leading to a displacement of the polar vortex. In addition, when the anomalous heating is centered at 700 hPa, the positive geopotential height anomalies are weaker and shift westward (Supplementary Fig. 8). Hence, the LBM experiment reproduces the anomalous ridge reasonably well over the NEP forced by anomalous heating over the Mediterranean region.

Further examining the role of the NATL forcing, we impose cold anomalies over the NATL (Fig. 4d). This results in positive geopotential height anomalies at 500 hPa over the NEP (Fig. 4e), which resemble the observations very well (Fig. 1c). However, surface westerly wind anomalies associated with a cyclonic circulation anomaly in the study area are not favorable for the warm blobs (Fig. 4f). The difference between circulation anomalies at 500 hPa and those at the surface may arise from the baroclinic response of the LBM.

On the other hand, we also notice that the WAF converges near East Asia (Fig. 2a), which might further boost the wave train. To test this possibility, we impose anomalous cooling/heating according to the structure of the apparent heat source over this region (Supplementary Figs. 10 and 11). Although a weak anomalous ridge emerges in the heating experiment (Supplementary Fig. 11), the heating (Fig. 3b) in these two regions is scattered and not robust. In this study, we have mainly explored the roles of the Mediterranean and NATL regions, which appear to be the origin of extratropical teleconnections, rather than a midway booster, that sustain the anomalous ridge.

Next, to confirm the driving role of the Mediterranean and NATL regions in exciting the wave trains and anomalous ridge over the NEP, we conduct atmospheric model experiments using the sophisticated CAM5 (i.e., version 5.0 of the Community Atmosphere Model; “Methods”). Results show positive geopotential height anomalies over the NEP and subpolar North America in October (Fig. 5b) and November (Fig. 5c) when the anomalous warming pattern is imposed in the Mediterranean region (Fig. 5a). The location of the modeled anomalous ridge is shifted eastward compared to the observations (Fig. 1). When the sea surface temperature (SST) warming pattern associated with the largest enhanced rainfall in the Mediterranean region is prescribed, the anomalous ridge over the NEP is stronger, with a similar location to the observations (left column of Supplementary Fig. 12), suggesting that the Mediterranean region can play an important role in driving this anomalous ridge. When composite SST anomalies during the warm blobs are superimposed onto the climatology in the NATL (Fig. 5d), the anomalous ridge is robust in November (Fig. 5f). In the Exp_NAtl_NAO experiment (right column of Supplementary Fig. 12), tripole-like SST anomalies with a broad cooling in the eastern NATL are prescribed, associated with the positive phase of the NAO. A prominent anomalous ridge over the NEP and polar regions is found in October and November, resembling the observations (Fig. 1). Thus, the effects of both the Mediterranean and NATL regions on the NEP anomalous ridge are captured by the CAM5 simulations.

Methods
Observational and reanalysis datasets
For SST, we use monthly data from the National Oceanic and Atmospheric Administration (NOAA) Extended Reconstructed SST version 5 (ERSST v5), gridded at 2°×2°. For precipitation, wind, SLP, temperature, and geopotential height, we use the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 reanalysis. The horizontal resolution of ERA5 data is 0.25°×0.25°. The data prior to 1959 are from an ERA5 preliminary version. We also use velocity potential, divergence, and relative vorticity data from the Japanese 55-year Reanalysis (JRA-55) at 2.5°×2.5° resolution. We analyze the period from 1951 to 2021 in this study. Anomalies of variables are calculated relativeto a baseline climatology from 1979 to 2021.

Composite analysis of the NEP warm blobs
The study area of the warm blobs is 40°N–50°N, 160°W–135°W. Following the definition from previous studies, a warm blob event is identified when normalized monthly SST anomalies averaged over the study area (i.e., the blob index) are greater than 1.0 for no fewer than 5 months with at most 1-month interruption. Accordingly, 13 warm blob events (Supplementary Table 1) peaking during the cold season (from November to March) are selected to perform our composite analysis. The blob index is calculated after removing linear trends; such an approach is widely used, although the SST variations do have nonlinear features. Peak intensity is given by the blob index when reaching the maximum. The return period is computed according to refs. 7,53. Note that due to the period of the JRA-55 dataset, we use 11 warm blob events after 1958 when calculating the RWS.

Calculation of WAF
To analyze the propagation of Rossby wave energy in the Northern Hemisphere, the horizontal WAF is computed following Takaya and Nakamura:

W = Pcos\phi \times \begin{pmatrix} \frac{U}{2|U|} \left[ \frac{\partial \Psi'}{\partial \lambda} \frac{\partial \Psi'}{\partial \lambda} - \Psi' \frac{\partial^2 \Psi'}{\partial \lambda^2} \right] + \frac{V}{2|U|} \left[ \frac{\partial \Psi'}{\partial \lambda} \frac{\partial \Psi'}{\partial \phi} - \Psi' \frac{\partial^2 \Psi'}{\partial \lambda \partial \phi} \right] \\ \frac{U}{2|U|} \left[ \frac{\partial \Psi'}{\partial \lambda} \frac{\partial \Psi'}{\partial \phi} - \Psi' \frac{\partial^2 \Psi'}{\partial \lambda \partial \phi} \right] + \frac{V}{2|U|} \left[ \frac{\partial \Psi'}{\partial \phi} \frac{\partial \Psi'}{\partial \phi} - \Psi' \frac{\partial^2 \Psi'}{\partial \phi^2} \right] \end{pmatrix}

where W (unit: m2 s−2) denotes the horizontal WAF; P is pressure/ (1000hPa); U (unit: m s−1) represents the basic flow with the zonal component U and meridional component V; \Psi (= \Phi/f) is the stream function; a is the radius of the Earth; \phi and \lambda denote latitude and longitude, respectively; \Phi is geopotential height, and f = 2\Omega sin\phi is the Coriolis parameter; where \Omega is the rotation rate of the Earth. Primes indicate anomalies relative to climatology.

Calculation of RWS
To locate the regions that can trigger Rossby waves in the upper troposphere, the Rossby wave source (RWS) is calculated at 300 hPa following Sardeshmukh and Hoskins:

RWS = -\nabla_H \cdot (u_\chi (f + \zeta))

where u = (u, v) denotes the horizontal wind velocity vector and subscript \chi indicates its divergent component. \zeta and f are relative vorticity and planetary vorticity, respectively; \nabla_H is the horizontal gradient operator. Primes indicate anomalies.

Calculation of the apparent heat source
To isolate the anomalous heating and cooling pattern associated with precipitation anomalies, we calculate the atmospheric diabatic heating (q) as below based on data from the ERA5 dataset:

q = \frac{\partial T}{\partial t} + V \cdot \nabla_H T + \omega \left( \frac{\partial T}{\partial p} - \frac{RT}{c_p p} \right)

where T is air temperature; and \omega is the vertical velocity in pressure coordinates. Static stability is expressed as \sigma = (RT/c_p p) – (\partial T/\partial p), where c_p is the specific heat capacity of air at constant pressure; R denotes the specific gas constant, and p is pressure. V = (u, v) is the horizontal wind velocity vector, and q includes latent, radiative, and sensible heat fluxes, as well as subgrid-scale heat flux convergence.

Experimental setup in the LBM
The atmospheric linear baroclinic model (LBM) is widely used to explore linear atmospheric dynamics. The LBM is based on a linearized version of the primitive equations. Given a basic state X, a steady response X follows an equation written in a matrix form as

L X = F

where F indicates a forcing vector, and L is a linear dynamical operator related to the perturbed primitive equations. In this study, the LBM is utilized to examine whether prescribing heating/cooling over the Mediterranean and NATL can drive a Rossby wave train that sustains an atmospheric ridge over the NEP in accordance with the observations. The model is run at T42L20, with a nominal horizontal resolution of 2.8°×2.8° and 20 vertical sigma levels. The model consists of basic equations linearized with respect to the atmospheric mean state in November derived from the NCEP reanalysis. Dissipation in the model includes (1) a bi-harmonic horizontal diffusion with a damping timescale of 1 day for the shortest waves, (2) a weak vertical diffusion (a damping timescale of 1000 days) to remove noise arising from finite differencing, (3) Newtonian damping, and (4) Rayleigh friction represented by a linear drag, with a timescale of 1 day applied only to the lower boundary layers and the uppermost two levels. The simulations are run for 20 days to reach a quasi-equilibrium atmospheric state. Output variables, including geopotential height, winds, and sea level pressure, are averaged over days 16–18 of the simulations, which yields a steady response to the prescribed heating/cooling. Note that the imposed heating/cooling in the Mediterranean and NATL experiments is multiplied by a factor of two to obtain a robust response as we calculate mean values over these regions.

Experimental setup in CAM5
CAM5 model was developed by NCAR. Five experiments were conducted: a control experiment (CTRL), two Mediterranean experiments (Exp_Med and Exp_Med_rain), and two NATL experiments (Exp_NAtl and Exp_NAtl_NAO; Supplementary Table 2). We use the f19_g16 model configuration (i.e., 2.5°×1.9° atmospheric resolution) for the experiments. The prescribed SST anomalies are computed from the merged products of HadISST version 1 and NOAA OISST version 2. The CTRL run is forced by monthly climatological SST globally over 1981–2010. For the Exp_Med run, composite SST anomalies during the warm blobs are superimposed onto climatological SST in the Mediterranean region (Fig. 5a). For the Exp_Med_rain run, SST anomalies averaged in 2013 and 2019 (when the Mediterranean has the largest enhanced rainfall) are superimposed onto climatological SST in the Mediterranean region (Supplementary Fig. 10a). For the Exp_NAtl run, composite SST anomalies during the warm blobs are superimposed onto climatological SST in the NATL (Fig. 5d). For the Exp_NAtl_NAO run, SST anomalies in 1993 with the largest NAO are superimposed onto climatological SST in the NATL (Supplementary Fig. 10d). For these sensitivity experiments, SST climatology is prescribed in the regions outside of the above forcing area. Each experiment is initialized from January 1, 1979 and integrated for 25 years, with the last 20 years averaged for our analyses.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The study titled \"Teleconnection from heating by Mediterranean rainfall sustains the atmospheric ridge over the Northeast Pacific warm blob\" utilized observations/reanalysis datasets and linear baroclinic model simulations to examine the teleconnection effects on the Northeast Pacific warm blob. The findings are intriguing and may contribute to our comprehension of the mechanisms responsible for the formation of the Northeast Pacific blob. However, upon reviewing the study, I have identified a few concerns that could introduce ambiguity in presenting the results and understanding the underlying mechanism. Please find below my major and minor comments.\nMajor comments:\n1. The definition of blob. The definition of the blob used in this study could benefit from improvement or further clarification. Firstly, while the study area is limited to 40°N-50°N and 160°W-135°W, the evolution of the blob, such as the 2014-2015 event, extends beyond the defined box of interest. Warm sea water can deform and extend towards coastal regions outside the designated area. Additionally, the vertical structure of the blob has been shown to be somewhat significant in generating extreme heating. Therefore, it raises the question of whether there are more suitable three-dimensional tracking algorithms available to identify each individual blob event and better characterize its spatial evolution.\nSecondly, regarding the intensity of the blob, the authors considered normalized monthly sea surface temperature (SST) anomalies with box-averaged values greater than 1.0. It would be helpful to know the standard deviation for each identified blob event in this study. Are the most recent events characterized by significantly larger standard deviation values compared to other events? If so, it suggests that the formation mechanisms and preconditions for strong blobs may differ considerably from those of weaker blobs, and perhaps being modified by global warming. Furthermore, it would be informative to understand where these blobs lie within the full distribution of SST anomalies in the specified regions and their return period. Providing these basic statistical analyses could enhance the illustration of extreme events in this region.\nOverall, addressing these points would contribute to a clearer understanding of the spatial and intensity characteristics of the blob events under investigation in this study.\n2. Atmospheric teleconnection in different month. The atmospheric teleconnection is derived from the analysis of winter blobs, which occur between November and March (Extended Data Table 1), as well as the preceding months. The authors' perspective is that this teleconnection is a result of stationary Rossby wave trains (Figures 2 and 4). In light of this viewpoint, it becomes important to carefully evaluate the impact of different background winds during various months on the atmospheric teleconnection. Background fields play a crucial role in the generation and guidance of Rossby waves, consequently influencing the establishment of distinct teleconnection patterns (e.g., Hoskins and Woollings 2015). Following the logic of examining prior months with lead times of 3, 2, 1, and zero, there is a concern that the teleconnection pattern observed between November and February may differ significantly from the pattern observed between August and November. This disparity could also lead to inconsistencies in the mean state of the atmosphere, as utilized in the LBM simulations, which will be further discussed in the subsequent comment.\nThe issue of causality in the study poses some challenges in interpretation. In Figure 1, the authors aimed to employ composite analysis with a lead month to provide insights into causality. However, two questions arise from this approach. Firstly, it is unclear why the strongest geopotential height anomalies do not occur at the peak time but rather at a lead of 1 month. Secondly, how can the authors be confident that there is not a third-party or confounding factor simultaneously driving both the blob and teleconnection phenomena? Lead-lag composite or correlation analyses alone may not be sufficient to reject such a possibility. Furthermore, Figure 2a presents composite meridional wind anomalies and WAF, but it remains questionable whether the WAF is causing the wind anomalies or vice versa, or by a confounding factor. We could also rationalize that the winds anomalies in the Mediterranean region are a response to upstream WAF emanating from the North Atlantic. The utilization of the Granger causality algorithm (e.g., Silva et al. in 2021) could potentially provide further insights in this regard, although I am not sure the authors have time or efforts to carry out the analysis in this study.\nThe authors' inclusion of LBM simulations serves the purpose of addressing the causality issue.\nHowever, it is important to note that LBM simulations may also have their limitations, as described in the subsequent comment.\n3. The design of IBM simulations. In addition to the concerns regarding the background wind issue mentioned earlier, it is worth noting that the LBM used in this study appears to be a simplified representation of atmospheric dynamics. It is important to acknowledge that there is a significant disparity between the LBM and state-of-the-art global climate models (GCMs), such as CESM2, which are commonly utilized to investigate teleconnections and associated dynamics. Considering this disparity, it would be valuable for the authors to consider conducting GCM simulations to complement and support their findings based on observations and reanalysis data, as well as LBM simulations. By employing a GCM, they could potentially enhance the robustness of their conclusions by capturing a broader range of atmospheric processes and interactions. GCM simulations could provide a more comprehensive and realistic representation of the teleconnection phenomenon under investigation.\nTo further address the causality issue, it could be beneficial to conduct a series of LBM simulations (and potentially GCM simulations) with forcings specifically applied to hotspots in southeast China, the Ural Mountain region, or the North Atlantic. By doing so, the authors can examine whether a similar teleconnection pattern emerges, as indicated by the statistically significant RWS presented in Figure 2b. These targeted simulations would provide an opportunity to assess the robustness of the observed teleconnection\npattern and its relationship to the specified forcings. If inconsistent teleconnection patterns are observed across multiple simulations with different forcings applied to these specific regions, it would lend support to the hypothesis that the Mediterranean regions play a causal role in driving the teleconnections. This approach would strengthen the authors' argument regarding the causality between the identified hotspots and the observed teleconnection pattern.\nMinor comments:\nLine 46: What is salinity variation? Can the authors discuss more about its impact on blob?\nLine 65: Are all blobs peaking in boreal winter? I saw in the Extended Data Table 1 some blobs peak in March and November, which may not be typical boreal winter months: December, January, and February. How about using cold season in the Northern Hemisphere?\nLine 74: do you use TNH abbreviation in the following part of manuscript?\nLines 76-77: do the authors mean wintertime midlatitude wave trains? Or autumn wave trains?\nLines 76-83 and 84-86: Why the wave train, originating in the North Atlantic, does not coincide with the WAF, from Mediterranean region?\nLines 99-100: A recent study (Hong and Hsu 2023) showed that the central Pacific could affect Northeast Pacific. How do the authors reconcile their results and those presented in this study?\nLines 108-110: why the wave trains often bifurcate into two branches over the Mediterranean? Can the authors discuss more?\nLines 136-137: can the authors show their evolution?\nLines 154-155: Does the strongest magnitude happen at 300 hPa because of the vertical resolution? If the vertical resolution increases, could the magnitude shift to, for example, 309 hPa?\nLines 158-159: what are the background mean fields given in the LBM simulation? Can the authors show their spatial maps?\nLine 231: As the authors had calculated three-dimension WAF, I am curious what is the role of vertical component here?\nLine 268: what does the gamma-type mean?\nLine 455, Figure 1L since the authors mentioning TNH pattern, which has anomaly centers in the North Atlantic, can you extend the domain to the North Atlantic?\nExtended Data Table 1: the bottom two events are the same, except “2019” and “2020”. Can the authors clarify?\nReferences\nHong, H.J. and Hsu, H.H., 2023. Remote tropical central Pacific influence on driving sea surface temperature variability in the Northeast Pacific. Environmental Research Letters, 18(4), p.044005.\nHoskins, B. and Woollings, T., 2015. Persistent extratropical regimes and climate extremes. Current Climate Change Reports, 1, pp.115-124.\nSilva, F.N., Vega-Oliveros, D.A., Yan, X., Flammini, A., Menczer, F., Radicchi, F., Kravitz, B. and Fortunato, S., 2021. Detecting climate teleconnections with Granger causality. Geophysical Research Letters, 48(18), p.e2021GL094707.", "reviewer_2": "This paper looks at the potenfial role of the Mediterranean region in forcing “warm blobs” in the northeast Pacific by inducing an anomalous atmospheric ridge in that region. The authors claim that enhanced rainfall over the Mediterranean drives a wave train in the northern hemisphere that generates this anomalous ridge, which subsequently leads to marine heatwave events in the North Pacific. The manuscript is well wriften, but I am unconvinced regarding the causality based on the evidence that has been presented. My main comments are as follows:\nPrevious studies have linked Northeast Pacific marine heatwaves to the tropics. While the authors reference the link to ENSO and the tropics in the introducfion, as far I can tell it is not accounted for in this study nor has the influence of ENSO or the tropics been removed in any way. Therefore I don’t think it’s possible to rule out the role of the tropics in forcing these extratropical wave trains or at least contribufing towards the marine heatwave/anomalous North Pacific anficyclone. Related to this, the rainfall anomaly plots (Fig 3b and extended data fig 4) show a very limited area. It would be useful to see the rainfall anomalies including the rest of the northern hemisphere (including the tropics), to see any other regions of enhanced or suppressed rainfall, as I find it hard to believe that the tropics is not forcing the extratropics in any way. Many areas within the Mediterranean region don’t actually have significant rainfall anomalies in most months prior, either, even with the fairly generous significance threshold used (extended data fig 4)\nWhile wave acfivity flux can be a useful diagnosfic, it doesn’t give any informafion about causality. The authors state that “Anomalous Rossby wave energy, indicated by wave acfivity flux, flows from the Mediterranean region into the subtropical jet”. While there are wave acfivity flux vectors over the Mediterranean, there are also vectors in most other parts of the extratropics and there is no evidence here to suggest that the waves originate over the Mediterranean itself. The vectors are also scaled in such a way on the plot that they are difficult to disfinguish. I also don’t think that wave acfivity flux is parficularly good at showing propagafion of waves from the tropics to extratropics, which I think is the only evidence the authors use to rule out the role of the tropics. To me, all this panel shows is waves propagafing around the northern hemisphere but with no clear informafion about where they originate or what is forcing them. This is parficularly the case when looking at the monthly figures (extended data fig 2)\nThe Rossby wave source plot (Fig 2b) has many areas of significant source, so why the focus on the small area over the Mediterranean? Many of these areas of RWS will also be generated from tropical rainfall, which doesn’t seem to have been taken into account.\nIt seems to me that what is more likely is that the anomalous ridge over the North Pacific is actually influencing Mediterranean rainfall through perturbing the North Atlanfic jet. Again, this doesn’t seem to have been considered in the present study. This is also made harder to see by the plots being cut off between 90W-0 longitude. Related to this, there seems to be a break in the wave train over central Asia (Fig 2), which would also support the North Pacific influencing Europe, rather than the other way round.\nThe authors state that the LBM experiment “closely reproduces the anomalous ridge over the NEP”, which seems like a bit of a strong statement to me. Yes, there is an anomalous ridge over the North Pacific but it is shifted south somewhat. The wave acfivity flux is also extremely messy which makes it hard to see what’s going on, but this doesn’t seem to correspond very closely to Fig 2a either. I suspect that if a similar heafing was applied elsewhere in the northern hemisphere (e.g. north of India) that a similar paftern would emerge, so I don’t think this provides sufficient evidence of the Mediterranean forcing the North Pacific.", "reviewer_3": "The paper describes an interesfing possible connecfion between anomalous heafing in the Mediterranean region and North Pacific “warm blobs”. The results are potenfially new, but they need to be befter substanfiated and supported by more analysis. The paper is well structured, but a few points need to be clarified before being considered for publicafion.\n1. The discussion in lines 76 – 83 need a befter mofivafion. The wavetrain is idenfified only by two contours and the amplitude over the Mediterranean is minor. The WAF seems to be distributed over a much larger pafter over Europe rather than only over the Med, the RWS has a smaller spafial scale by construcfion and exhibit many other maxima, for instance over North Europe. In summary, it is hard to conclude that the Med plays any special role.\n2. The verfical structure of the heafing computed as a residual is surprising (Fig, 7 Extended data and other). At these lafitudes is mostly shallow heafing (see for instance Ling and Change, 2013), so the profile showing maximum heafing at 300mb is rather surprising.\n3. The results from the linear model also need to be befter jusfified. The model needs to be described befter, especially regarding the dissipafion formulafion used and the verfical structure of the heafing imposed. The crypfic statement at the end of the method secfion (“A gamma-type …) needs to be befter formulated.\n4. Assuming that the solufion has converged successfully, the way it develops is suspect. The development of the response in the Pacific can be due to residual amplitude along the equatorial region deriving from the spectral truncafion of the localized anomaly prescribed over the Mediterranean. At T42 resolufion the expansion of the localized heafing over the spectral transform will generate amplitude along the equator that will be picked up by the model in the transform cycle and result in further forcing for the mid-lafitudes. This is parficularly important in the West Pacific because the basic state is very sensifive to forcing there and it can amplify easily. Depending on the dissipafion and cooling imposed this effect may be more or less emphasized, so the modeling choices are very important to clarify."}, "author_response": "REVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nI thank the authors’ efforts answering my questions. For comment 2, could the authors\ncheck the composite atmospheric circulation patterns lagged warm blob events? If the\ncirculation patterns are consistent, then this mechanism should work. I do not have\nfurther comments.\n>>Reply:\nWe very much appreciate the reviewer’s positive comments. We further checked the\ncomposite atmospheric circulation patterns lagged warm blob events. An anomalous\ntrough occurred over the Alaska region, which was statistically significant.\nFig. R1 As in Fig. 1, but for lagging one month of the peak.\nReviewer #2 (Remarks to the Author):\nI am now satisfied that the authors have addressed my concerns.\n>>Reply:\nThanks very much.\nReviewer #3 (Remarks to the Author):\nI understand the hard work the authors did to try to improve the paper, but I feel that\nstill there are issues:\n>>Reply:\nWe greatly appreciate the reviewer’s constructive comments that have helped us\nfurther improve the manuscript. Throughout our responses below, for convenience,\nthe line numbers in the new revised manuscript are referred to as Revision Lines\n(RL).\n1. The calculation of the heating is still not clear as they do not say on which data set\nthe residual were calculated.\n>>Reply:\nThe calculation of the heating is based on data from the ERA5 dataset. We added this\ninformation in RL 249 and 292.\n2. The issue of compensation of heating in the mid latitudes is standard QG dynamics\n(Hoskins and Karoly, 1981).\n>>Reply:\nThank you for reminding us about this paper, which helps understand the QG-related\ncompensation of heating in the mid latitudes. We have now cited this paper in RL 119\nand 194.\n3. Allow me to differ on the convergence rate. Allow me to differ on the convergence\ninterpretation. The near field response is stable, but the far field response is not, and\nthat, of course, is what matters in the paper. I think that it would be much better to\nobtain a direct (i.e. matrix inversion) linear solution, eliminating issues of\nconvergence.\n>>Reply:\nThank you for this suggestion. However, we stress that, according to previous studies\nand our LBM daily outputs (Extended Data Figs. 7 and 9), the response over the\nNortheast Pacific region is generally stable after about 16 days. With respect, we do\nnot see the value or necessity to take this further given the stability of our analysis\nalready undertaken and outlined.\n4. You have to double the amplitude of the heating in the Med and Atlantic\nexperiments. It would be fine in the attempt to understand the signature of possible\neffects, but in terms of assessing the role of this heating in the real atmosphere it sort\nof show that the impact is minor.\n>>Reply:\nWe have doubled the amplitude of the heating to better highlight the signature of the\neffects from the Mediterranean and North Atlantic. However, the response over the\nNortheast Pacific Ocean (NEP) is very similar to that produced without doubling the\nheating, as per our results in the previous version of our manuscript.\nIn addition, we show that the contribution from the tropics may not be large enough in\nNovember, compared to that in December to February. The geopotential height\nresponse over the NEP forced by the tropics is negligible or comparable to the forcing\nfrom the mid-latitudes (Figs. R1–R3).\nFig. R1 As in Extended Data Fig. 7, but imposing anomalous cooling over the\nMaritime Continent.\nFig. R2 As in Extended Data Fig. 7, but imposing anomalous heating over the tropical\nIndian Ocean.\nFig. R3 As in Extended Data Fig. 7, but imposing anomalous heating over the tropical\nwestern Pacific.\nIn conclusion, the paper is vastly improved, but I think that the impact of the\nMediterranean region as a source of anomalous heating for the North Pacific has not\nbeen conclusively demonstrated. It can still be published, removing the statements\nand the conclusions in this direction.\n>>Reply:\nThank you again for your suggestions and recommendations to improve the paper.\nWe believe we have provided a compelling demonstration of important contributions\nfrom anomalous heating in the Mediterranean/NATL regions to the ridge over the\nNEP. However, we respectfully acknowledge and appreciate the reviewer’s concerns,\nand consequently we have now toned down the strength of our statements regarding\nthe role of the Mediterranean."}]

Decision letter