Midlatitude mesoscale thermal Air-sea interaction enhanced by greenhouse warming

Domain: earth_science
Published: 2024-09-04
Editorial outcome: major_revision

Abstract
The influence of greenhouse warming on mesoscale air-sea interactions, crucial for modulating ocean circulation and climate variability, remains largely unexplored due to the limited resolution of current climate models. Additionally, there is a lack of theoretical frameworks for assessing changes in mesoscale coupling due to warming. Here, we address these gaps by analyzing eddy-resolving high-resolution climate simulations and observations, focusing on the mesoscale thermal interaction dominated by mesoscale sea surface temperature (SST) and latent heat flux (LHF) coupling in winter. Our findings reveal a consistent increase in mesoscale SST-LHF coupling in the major western boundary current regions under warming, characterized by a heightened nonlinearity between warm and cold eddies and a more pronounced enhancement in the northern hemisphere. To understand the dynamics, we develop a theoretical framework that links mesoscale thermal coupling changes to large-scale factors, which indicates that the projected changes are collectively determined by historical background wind, SST, and the rate of SST warming. Among these factors, the large-scale SST and its warming rate are the primary drivers of hemispheric asymmetry in mesoscale coupling intensification. This study introduces a simplified approach for assessing the projected mesoscale thermal coupling changes in a warming world.

Introduction
Mesoscale air-sea interactions, predominantly active in the Western Boundary Current (WBC) regions in the midlatitudes, play a critical role in modulating extratropical weather and climate systems1–5. The interaction between mesoscale oceanic eddies and the atmosphere actively impacts precipitation, storms, large-scale atmospheric circulations, and provides feedback to the ocean, influencing oceanic circulations and climate variability6–15. A key aspect of this interaction is the coupling between sea surface temperature and turbulent heat flux (SST-THF), hereafter denoted as thermal coupling. In terms of the atmosphere, thermal coupling acts as an energy source, which is crucial for the genesis and development of weather systems and deep troposphere response16–21. In terms of the ocean, thermal coupling serves as an energy sink, which is the key to dissipating oceanic eddy energy and driving oceanic circulation response11,22.

How greenhouse warming will impact mesoscale oceanic eddies remains uncertain, let alone mesoscale air-sea coupling. Satellite observations reveal a rise in eddy activity in the WBC regions during recent decades, while climate models project heterogeneous eddy variations in different WBC regions under warming23,24. Additionally, the theoretical framework for predicting the mesoscale thermal coupling change in response to greenhouse warming is currently absent. Thermodynamically, the SST warming and the associated water vapor increase governed by the Clausius–Clapeyron (C-C) relation, appear to strengthen the thermal air-sea coupling (primarily through the enhancement of latent heat flux) in a warming climate. Dynamically, the non-uniform warming between the upper and lower troposphere under anthropogenic forcing tends to increase atmospheric stability, inhibiting the vertical momentum transfer and thereby suppressing the surface wind and thermal air-sea coupling. This is further complicated by significant uncertainties in atmospheric circulation responses under climate change, introducing additional perturbations to SST-THF coupling. It is noteworthy that the aforementioned oceanic and atmospheric changes discussed within a conventional large-scale framework may not necessarily manifest at mesoscales. Physical processes governing the response of mesoscale thermal coupling to greenhouse warming are multifaceted, making it challenging to pinpoint the ultimate dominant factor.

Utilizing an unprecedented set of high-resolution Community Earth System Model (referred to as CESM-HR, Methods) with ~0.25° atmosphere and ~0.1° ocean components that can explicitly resolve the mesoscale oceanic eddies and their coupling with the atmosphere, we investigated the impact of greenhouse warming on mesoscale thermal coupling in eddy-rich WBC regions. We then constructed a theoretical framework for estimating the mesoscale thermal coupling change in response to greenhouse warming through the decomposition of contributing factors. We further assessed the robustness of the findings by extending the analyses to High-Resolution Model Intercomparison Project (HighResMIP, Methods) models.

Results
Mesoscale thermal coupling during the observational period
We first evaluated the model’s capability in representing the mesoscale thermal coupling in the CESM-HR simulations against observational data (Methods), in four major WBC regions, i.e., the Kuroshio Extension (KE), the Gulf Stream Extension (GS), the Agulhas Return Current (ARC) and the Brazil-Malvinas Confluence Region (BMC), by detecting eddies using sea surface height anomalies and constructing composite characteristics of eddies, such as the averaged number, amplitude and size, as detailed in Tab. S1. It also successfully reproduces the eddy-induced turbulent heat fluxes (including both sensible and latent components) along with their seasonality, for both anticyclonic warm and cyclonic cold eddies (Fig. S1). It is noted that the coupling strength peaks during the winter month. Furthermore, the eddy-induced sensible heat flux (SHF) is approximately an order of magnitude smaller than the latent heat flux (LHF) in both historical and future simulations, and the difference is subtle (Fig. S2). Thus, the mesoscale thermal coupling in the WBC regions and its response to global warming is predominantly influenced by LHF. Consequently, our subsequent analyses will focus on the mesoscale SST-LHF coupling in the hemispheric winter season—DJF for the Northern Hemisphere (NH) and JJA for the Southern Hemisphere (SH).

To assess the potential impact of anthropogenic warming on mesoscale thermal coupling, we analyzed the decadal trend of mesoscale SST-LHF coupling in the observational periods based on high-pass spatial filtering fields (Methods). A significant intensification of mesoscale SST-LHF coupling at an approximate rate of 1.5 W/m2/°C per decade is detected in the WBCs over the past four decades, with the most pronounced increase observed in the GS and KE regions of the NH (Fig. 1a). The CESM-HR successfully simulates the enhanced mesoscale thermal coupling in the WBCs with a magnitude slightly higher than that recorded in the observations (Fig. 1b). The model also captures the north-south hemispheric asymmetry of the changes. The alignment between model simulated and observational trends in the past lends credence to the use of CESM-HR for investigating the mesoscale coupling response under climate change.

Mesoscale thermal coupling under greenhouse warming
A linear regression between mesoscale SST and LHF based on eddy composites averaged across four WBC regions, yields an estimated coupling coefficient of approximately 29 W/m2/°C for both anticyclonic warm and cyclonic cold eddies in historical simulations (1956–2005, Fig. 1c, d). The nonlinearity between warm and cold eddies appears to be minimal during the historical period. Under the high greenhouse forcing scenario of Representative Concentration Pathway 8.5, future simulations (2063-2100) project an approximate 14% enhancement in mesoscale SST-LHF coupling to 33 W/m2/°C (Fig. 1d). A detailed examination of the four WBCs reveals a consistent increase in coupling strength by 6% to 18% in future simulations compared to historical simulations (Fig. 1e). Importantly, future simulations project a greater increase of mesoscale SST-LHF coupling strength for warm eddies than for cold eddies, indicating heightened nonlinearity in a warming climate. The increase in nonlinearity is expected to amplify net heat flux from mesoscale oceanic eddies13,25, thereby impacting future weather and climate systems more significantly. Additionally, the projected increase in mesoscale SST-LHF coupling in the GS and KE is nearly twice that of the ARC and BMC, pointing to a more substantial intensification in the NH than in the SH. Combined with the findings in the observational periods, the results indicate that the hemisphere discrepancy in mesoscale SST-LHF coupling is likely to not only persist but also to become more pronounced with ongoing climate warming.

Decomposition of contributing factors
The LHF is proportional to both the surface wind and the humidity difference at the air-sea interface according to the Bulk formula26. To estimate the mesoscale SST-LHF coupling associated with oceanic eddies, we decompose the relevant fields into large-scale background and mesoscale components in line with previous studies27,28. The mesoscale SST-LHF coupling coefficient can be quantified using the following relationship (see detailed derivation in Methods):

d/dS QL0 = ρa Λv Ce U10 d/dS qs0 - d/dS qa0 + d/dS U10 (qs0 - qa0) (1)

Here, the prime (′) represents mesoscale anomalies defined by the high-pass spatial filtering, and the overbar (‾) denotes large-scale background excluding the mesoscale signal. The mesoscale SST-LHF coupling coefficient d/dS QL0 is determined by two components: the thermodynamic adjustment to mesoscale SST multiplied by the large-scale wind (U10 d/dS qs0 - d/dS qa0, hereafter denoted as mesoscale thermodynamic adjustment term), and the dynamic adjustment to mesoscale SST multiplied by the large-scale humidity difference (d/dS U10 (qs0 - qa0), hereafter denoted as mesoscale dynamic adjustment term). Evaluation of historical and future simulations in the WBC regions reveals an increase in both terms due to greenhouse forcing (bars with black borders in Fig. 2a–d). Particularly, the enhancement of the thermodynamic adjustment term is about 3 to 5 times that of the dynamic adjustment term across all four WBCs (Fig. 2a–d), indicating the dominant contribution of the thermodynamic adjustment term to mesoscale SST-LHF coupling modulation. Although the dynamic adjustment term is substantially weaker than the thermodynamic adjustment, it is notably stronger in the NH compared to the SH. A decomposition of the dynamic adjustment term shows that the response of large-scale air-sea humidity difference under warming generally surpasses the mesoscale wind response to oceanic eddies across the four WBCs (Fig. S3). However, the mesoscale wind response to eddies is more pronounced in the KE and GS regions and minimal in the ARC and BMC regions, contributing to the stronger mesoscale dynamic adjustment in the NH. The intensified mesoscale wind response to eddies in the KE and GS may be associated with the vigorous western boundary currents, stronger oceanic eddy activities8, and the frequent passage of synoptic weather systems that collocate with the KE and GS. These factors lead to an unstable planetary boundary layer and enhanced downward momentum transfer as discussed in previous studies2,29.

A further decomposition of the thermodynamic adjustment term reveals that the large-scale wind change between historical and future simulations is negligible (bars with magenta borders in Fig. 2a–d), and the surface air temperature response is subtle. Specifically, the mesoscale moisture adjustment is the primary driver for the strengthened mesoscale SST-LHF coupling under climate change.

The above analyses indicate that changes in mesoscale SST-LHF coupling due to warming can be effectively estimated via the mesoscale moisture adjustment process. Nonetheless, this estimation still relies on the availability of both mesoscale and large-scale fields from historical and future simulations. To circumvent this, we apply a Taylor series expansion to the mesoscale moisture derivative (see detailed derivation in Methods). The resultant expression provides a simplified approach to assess mesoscale SST-LHF coupling change using large-scale fields:

d/dS QL0 (F-P) = (ρa Λv Ce) (P) U10 (P) d2/dT2 qs |T=SST(P) * ΔSST (2)

Here, ‘F’ represents future values and ‘P’ represents historical values in the past. It is evident that changes in mesoscale SST-LHF coupling are collectively affected by the large-scale wind and the curvature of the C-C scaling from the historical simulations, along with the projected warming of large-scale SST. Note that the curvature of the C-C scaling is inherently linked with the background SST, with higher temperature corresponding to a more pronounced moisture-temperature sensitivity. The relationship suggests that projections of future mesoscale SST-LHF coupling are significantly influenced by the historical large-scale oceanic and atmospheric conditions. Given that the large-scale wind and the curvature of the C-C scaling stay positive, it can be inferred that the direction of mesoscale SST-LHF coupling is exclusively determined by the sign of projected SST changes, leading to consistent intensification in line with the warming of underlying SST.

We then assessed the regional variations in mesoscale SST-LHF coupling responses among the four WBC regions using Eq. (2), with emphasis on the north-south hemisphere asymmetry. The simplified relationship efficiently captures the more pronounced increase in coupling strength within the KE and GS regions compared to the ARC and BMC regions (Fig. 3a), consistent with the projected intensification of mesoscale SST-LHF coupling between future and historical simulations. Detailed examination of contributing factors in CESM-HR demonstrates consistently higher values for large-scale wind and SST in the KE and GS regions in the NH from historical simulations, alongside a more rapid SST warming rate (Fig. 3c), all of which jointly contribute to the enhanced augmentation in mesoscale coupling strength in the NH under climate change. The historical large-scale wind in the KE is approximately 30% stronger than that in the ARC and BMC, and the background SST in the GS is 7°C higher than in the BMC, corroborated by observations (Fig. 3d). Additionally, the accelerated SST warming trend in the NH than in the SH is also confirmed by the observational data (Fig. 3d), underscoring the robustness of the hemisphere asymmetry under warming. Further decomposition of the three factors indicates their respective contribution to the overall hemispheric discrepancy (Fig. 3b), with the large-scale SST warming rate being the dominant factor, followed by the background SST.

Validations in HighResMIP
We extend the analyses to HighResMIP simulations (Methods) and to different warming periods (2030–2050), aiming to verify the robustness of the findings. All models examined show an increase in mesoscale SST-LHF coupling in response to greenhouse warming across four WBCs by the year 2050 (Tab. 1). A more pronounced intensification of this coupling is projected in the NH compared to the SH, in line with CESM-HR results (Fig. 1e), albeit with a generally lower magnitude of change. The magnitude discrepancy is due to the HighResMIP projections here terminating in 2050, whereas CESM-HR projections shown in Fig. 1 extend to a later period of the century (2100).

The effectiveness of the proposed theoretical framework for estimating changes in mesoscale SST-LHF coupling due to greenhouse warming was also evaluated across different models. A comparison between the coupling strength changes estimated by the theoretical framework and those projected by CESM-HR reveals a significant linear relationship across four WBCs (Fig. S4). The estimated and actual projected changes in mesoscale SST-LHF coupling are highly correlated, with a correlation coefficient of around 0.7 in the KE and GS, and 0.5 in the ARC and BMC (Tab. 1). Similar linear correlations, ranging generally from 0.5 to 0.8 (Tab. 1), are found between estimated and projected mesoscale coupling strength changes in HighResMIP models, confirming the broad applicability of the simplified framework across diverse climate models.

Methods
CESM-HR, HighResMIP simulations and observations
We utilized the high-resolution Community Earth System Model (CESM-HR) simulations with ~0.25° atmosphere and ~0.1° ocean components developed by the National Center for Atmosphere Research (NCAR)32. The simulations include a 500-year preindustrial control simulation and a 250-year historical and future simulation from 1850 to 2100. Historical radiative forcing is applied from 1850 to 2005 while the Representative Concentration Pathway 8.5 (RCP8.5, a high greenhouse gas emission) warming forcing is switched from 2006 onwards. The longest available periods with high-frequency (daily) output were chosen to assess the mesoscale thermal coupling response to greenhouse warming in CESM-HR: a historical period from 1956 to 2005 (referred to as HIS) and a future period from 2063 to 2100 (referred to as RCP).

Five HighResMIP simulations with relatively high oceanic resolution were selected: CMCC-CM2-VHR4 (0.25° atmosphere and 0.25° ocean), HadGEM3-GC31-HH (0.5° atmosphere and 0.08° ocean), EC-Earth3P-HR (0.5° atmosphere and 0.25° ocean), CNRM-CM6-1-HR (1° atmosphere and 0.25° ocean), MPI-ESM1-2-XR (0.5° atmosphere and 0.5° ocean). We note that only one model (HadGEM3-GC31-HH) has comparable oceanic resolutions with CESM-HR, yet its atmospheric resolution is coarser. All these selected models include a 100-year historical and future (under RCP8.5 scenario) simulation from 1950 to 2050. For a consistent comparison between CESM-HR and HighResMIP simulations, 1950-1969 for historical and 2031-2050 for future projections were selected for the corroborative analysis shown in Table 1.

Daily sea surface height (SSH) derived from Copernicus Marine Environment Monitoring Service (CMEMS) during 2003–2007 was used to identify eddies in observations (Tab. S1). Concurrently, SST obtained from the National Oceanic and Atmospheric Administration daily Optimum Interpolation SST (NOAA-OISST) and heat fluxes derived from Japanese Ocean Flux Data Sets with Use of Remote Sensing Observations version 3 (J-OFURO3) during the same period were employed to construct observational eddy composites (Fig. S1). ERA5 reanalysis (the fifth generation European Centre for Medium-Range Weather Forecasts atmospheric reanalysis33) with an extended temporal span from 1979 to 2022 was utilized to examine the decadal trend of mesoscale coupling strength (Fig. 1a).

Eddy detection, Eddy composites and high-pass filtering
In both CMEMS observations and CESM-HR simulations, mesoscale eddies were detected using daily SSH anomalies derived by applying high-pass spatial filtering (20° longitude x 10° latitude) to remove the large-scale signal, following previous studies34,35. Cyclonic (anticyclonic) eddies are classified by closed contours of SSH anomalies that include a single minimum (maximum), with an SSH anomaly increment (decrement) of 0.05 cm between successive contours. The edge of an eddy is delineated by the outmost closed contour of SSH anomalies. The radius of an eddy corresponds to the radius of a circle with an equivalent area to that enclosed by the outmost contour. The amplitude of an eddy is defined by the SSH anomaly difference between the eddy’s peak and its defined edge. Only eddies with a radius ranging from 70 to 200 km and an amplitude exceeding 3 cm are included in the analysis.

Eddy composites were constructed by aligning associated variables to the reference coordinate of the eddy core. The variables were normalized by the individual eddy radius and oriented to the prevailing direction of the large-scale background surface wind, in line with previous research9. Variables within twice the eddy radius were included for composite analysis.

To isolate mesoscale SST and LHF, we also applied a high-pass spatial filtering (30° longitude x 10° latitude, similar to a 5°x5° box car average3,11) in ERA5, CESM-HR and HighResMIP, and examined the spatial distribution of their coupling strength (Fig. 1a, b and Fig. S4). The coupling strength at each grid point was computed using the linear regression coefficient between high-pass filtered monthly SST and LHF. It yields a coupling coefficient comparable to that obtained when the filter is first applied to daily data, which is then aggregated into a monthly mean before calculating the coefficient. To highlight the regions with pronounced mesoscale SST-LHF coupling, mesoscale signals where the SST anomaly fell below 0.4°C in ERA5, below 0.6°C in CESM-HR, and coupling strength below 20 W/m2/°C in CESM-HR were excluded when computing the decadal trend in coupling strength (Fig. 1a, b).

Decomposition of mesoscale SST-LHF coupling
According to Bulk formula26, the latent heat flux QL is determined by the equation:

QL = ρa Λv Ce U10 (qs - qa) (3)

Here, ρa is the surface air density, Λv is the latent heat of vaporization, and Ce is the transfer coefficients for evaporation. U10 is the 10m wind speed, qs is the saturated specific humidity at the ocean surface, and qa is air specific humidity at 2m.

Following previous studies27,28, Eq. (3) can be decomposed into large-scale background and mesoscale components. The mesoscale component of latent heat flux is estimated as follows:

QL0 = ρa Λv Ce [U10 (qs0 - qa0) + U0 (qs - qa)] (4)

Here, the prime (′) represents mesoscale anomalies defined by the high-pass spatial filtering, and the overbar (‾) denotes large-scale background excluding the mesoscale signal.

By differentiating with respect to SST, the mesoscale SST-LHF coupling is expressed as:

d/dS QL0 = ρa Λv Ce [U10 d/dS qs0 - d/dS qa0 + d/dS U10 (qs0 - qa0)] (5)

Based on calculations, the second term d/dS U10 (qs0 - qa0) on the right-hand side of Eq. (5) is an order of magnitude smaller than the first term U10 d/dS qs0 - d/dS qa0. Furthermore, d/dS qa0 is an order of magnitude smaller than d/dS qs0. Disregarding the relatively smaller terms and assuming the changes in ρa Λv Ce is minimal under global warming (with a relative change of approximately 2%, which is negligible compared to the 17% mesoscale moisture adjustment), the mesoscale SST-LHF coupling changes is predominately influenced by U10 d/dS qs0, which can be represented as:

d/dS QL0 (F-P) = (ρa Λv Ce) (P) [U10 (F) d/dS qs0 (F) - U10 (P) d/dS qs0 (P)] (6)

Where ‘F’ represents future values and ‘P’ represents historical values in the past. As the large-scale wind, U10(F) and U10(P), experience minimal changes (ranging between -0.4% and -3.5% as shown in Fig. 2) in the WBCs, Eq. (6) can be further simplified as:

d/dS QL0 (F-P) = (ρa Λv Ce) (P) U10 (P) [d/dS qs0 (F) - d/dS qs0 (P)] (7)

Applying a Taylor expansion, the right-hand side term can be approximated as:

d/dS qs0 (F) - d/dS qs0 (P) = d2qs/dT2 |T=SST(P) * ΔSST (8)

Substituting (8) into (7) yields:

d/dS QL0 (F-P) = (ρa Λv Ce) (P) U10 (P) d2qs/dT2 |T=SST(P) * ΔSST (9)

Where d2qs/dT2 is determined by C-C scaling and exhibits an exponential increase with temperature. Note that the composite coefficient (ρa Λv Ce) is retained to align the estimated coupling strength changes with magnitude analogous to the actual projections.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Overall comment\nIn this study, Zhang et al. explored the connection between mesoscale SST and LHF during winter by analyzing eddy-resolving high-resolution climate simulations and observational data. They employed eddy-resolving, high-resolution coupled simulations using CESM-HR, along with five HighResMIP simulations to validate the robustness of their findings. Additionally, they developed a theoretical framework linking mesoscale SST-LHF coupling to background conditions such as large-scale wind speed and SST warming. The authors noted that mesoscale SST-LHF coupling in major western boundary current regions intensifies under changing climates, characterized by heightened nonlinearity between warm and cold eddies. Their results highlight a more pronounced enhancement in SST-LHF coupling in the northern hemisphere, primarily attributed to hemispheric asymmetry in large-scale SST and its warming rate.\nOverall, this study presents intriguing insights that contribute to our comprehension of climate change and midlatitude air-sea interactions. However, a significant revision to the manuscript is necessary, particularly to elucidate the relationship between the potential value of this study and its focus. Below are specific comments and questions.\nSpecific comments (not necessarily in order of significance)\n1. In this study, the authors focus particularly on the mesoscale SST-LHF coupling. Although I understand that the mesoscale SST-LHF coupling is more nonlinear than the SST-SHF coupling as shown in the manuscript, which results from the nonlinearity of the Clausius-Clapeyron relation, I still find an inconsistency between the authors’ focus and the value proposed by the authors. As the authors mentioned in the manuscript, a key of midlatitude air-sea interactions is the coupling between SST and THF. There is mounting evidence that the SST-THF coupling plays a crucial role in forming and maintaining weather systems (e.g., atmospheric fronts, cyclones), background atmospheric circulation (e.g., jet streams and storm tracks), and ocean circulation. SHF and LHF (or similarly, heating rates in the atmosphere related to vertical mixing and precipitation) have been shown to play different roles in maintaining the baroclinicity and storm track activity (e.g., Hoskins and Valdes 1990; Haualand and Spengler 2021; Okajima et al. 2022). Therefore, the discussion should predominantly revolve around mesoscale SST-THF coupling, with mesoscale SST-LHF coupling employed to interpret SST-THF coupling results.\n2. The rationale behind selecting specific periods for CESM-HR simulations (1956-1975 & 2063-2082) to show future changes is unclear. Extending the period for as long as output data is available would enhance the robustness of the results.\n3. There's uncertainty regarding whether the change in Ce_bar (transfer coefficient) under global warming is minimal (L310). Given its dependence on wind speed, the authors should provide justification or refer to previous studies demonstrating this. Additionally, it is unclear if U_10(F) and U_10(C) experience minimal changes in the WBCs or not (L314). Previous studies pointed out that the midlatitude jet streams are projected to significantly shift poleward under global warming (e.g., Oudar et al., 2020).\n4. In Fig. 3, the change between the historical and future periods in the CESM-HR simulations is compared with the trend over the last 40 years derived from the observations. The change in SST in the CESM-HR, especially its interhemispheric asymmetry, is much larger than the observational trends, owing probably to this difference. I am thus wondering if trends calculated similarly based on the historical periods in the CESM-HR simulations are consistent with the observations. Recent studies suggest that the interhemispheric asymmetry in SST trends is likely influenced largely not only by greenhouse warming but also by aerosol forcing (e.g., Deser et al. 2020).\n5. In the conclusions, the authors argue that “our findings suggest an increasingly important role of mesoscale oceanic processes in shaping weather and climate systems in a warming climate, especially in the NH”. I am afraid, however, that further clarification is needed to articulate how the findings support this assertion.\n6. The authors simply touch on the relative magnitude of the differences of dqs’/dSST’ and dqa’/dSST’ between the future and historical periods (in LL138-139). While this is likely a primary factor for the importance of background SST warming for the strengthened mesoscale SST-LHF coupling under global warming, its reason remains unknown in this study. The authors might want to discuss this to obtain a dynamical understanding. I speculate that it is determined by how the atmosphere responds to mesoscale SST and how it changes, and I am wondering if there are previous studies or existing theoretical frameworks about that.\n7. I do not understand what is shown in Fig. 3b. The panel requires clarification regarding \"north-south hemispheric asymmetry,\" units, and the definition of \"hemispheric mean.\"\n8. Improving the readability of x-axis labels in figures is recommended.\nReferences\nDeser, C., Phillips, A. S., Simpson, I. R., Rosenbloom, N., Coleman, D., Lehner, F., ... & Stevenson, S. (2020). Isolating the evolving contributions of anthropogenic aerosols and greenhouse gases: A new CESM1 large ensemble community resource. Journal of climate, 33(18), 7835-7858.\nHaualand, K. F., & Spengler, T. (2020). Direct and indirect effects of surface fluxes on moist baroclinic development in an idealized framework. Journal of the Atmospheric Sciences, 77(9), 3211-3225.\nHoskins, B. J., & Valdes, P. J. (1990). On the existence of storm-tracks. Journal of Atmospheric Sciences, 47(15), 1854-1864.\nOkajima, S., Nakamura, H., & Kaspi, Y. (2022). Energetics of transient eddies related to the midwinter minimum of the North Pacific storm-track activity. Journal of climate, 35(4), 1137-1156.\nOudar, T., Cattiaux, J., & Douville, H. (2020). Drivers of the northern extratropical eddy driven jet change in CMIP5 and CMIP6 models. Geophysical Research Letters, 47(8), e2019GL086695.", "reviewer_2": "This is a clear & concise paper that demonstrates the increasing strength of ocean-eddy - atmosphere interaction with climate change, and attributes this change primarily to the impact of sea-surface temperature (SST) on the saturation specific humidity, a direct consequence of the Clausius-Clapeyron equation. Because these results stem from fundamental thermodynamics, we can be confident that they are robust.\nI recommend publication.\nA few comments/suggestions:\n1) In figures 2a and 2b the second column, resulting from the change in wind speed with eddy SST is not negligible(roughly 1/3 of the first column). The authors should say something about this: what is the (possible) physical mechanism; why is this effect much stronger for the KE & GS than for the other regions?\n2) Also in figure 2, the column labels, using mathematical expressions, are hard to read - I suggest replacing them with larger labels using words or abbreviations.\n3) While recognizing that space is limited, the reader is left wondering about the impacts of increased coupling strength on the actual fluxes. Perhaps a figure/table could be added showing this (i.e. change in actual latent heat flux over these regions). Increasing coupling strength need not lead to increased fluxes if the eddies weaken - do they?\n4) Similarly, increased coupling strength implies increased damping of eddies by air-sea interaction. What happens the eddy energy?"}, "author_response": "Reply to reviewer #1\nWe would like to thank the reviewer for the invaluable comments. We have\ncarefully considered each of your comments (listed as bold and Italic) and revised the\nmanuscript accordingly. Below are point-to-point replies to your comments.\nOverall comment：\nIn this study, Zhang et al. explored the connection between mesoscale SST and\nLHF during winter by analyzing eddy-resolving high-resolution climate simulations\nand observational data. They employed eddy-resolving, high-resolution coupled\nsimulations using CESM-HR, along with five HighResMIP simulations to validate\nthe robustness of their findings. Additionally, they developed a theoretical framework\nlinking mesoscale SST-LHF coupling to background conditions such as large-scale\nwind speed and SST warming. The authors noted that mesoscale SST-LHF coupling\nin major western boundary current regions intensifies under changing climates,\ncharacterized by heightened nonlinearity between warm and cold eddies. Their\nresults highlight a more pronounced enhancement in SST-LHF coupling in the\nnorthern hemisphere, primarily attributed to hemispheric asymmetry in large-scale\nSST and its warming rate.\nOverall, this study presents intriguing insights that contribute to our\ncomprehension of climate change and midlatitude air-sea interactions. However, a\nsignificant revision to the manuscript is necessary, particularly to elucidate the\nrelationship between the potential value of this study and its focus. Below are specific\ncomments and questions.\nSpecific comments (not necessarily in order of significance)\n1. In this study, the authors focus particularly on the mesoscale SST-LHF\ncoupling. Although I understand that the mesoscale SST-LHF coupling is more\nnonlinear than the SST-SHF coupling as shown in the manuscript, which results\nfrom the nonlinearity of the Clausius-Clapeyron relation, I still find an inconsistency\nbetween the authors’ focus and the value proposed by the authors. As the authors\nmentioned in the manuscript, a key of midlatitude air-sea interactions is the coupling\nbetween SST and THF. There is mounting evidence that the SST-THF coupling plays\na crucial role in forming and maintaining weather systems (e.g., atmospheric fronts,\ncyclones), background atmospheric circulation (e.g., jet streams and storm tracks),\nand ocean circulation. SHF and LHF (or similarly, heating rates in the atmosphere\nrelated to vertical mixing and precipitation) have been shown to play different roles\nin maintaining the baroclinicity and storm track activity (e.g., Hoskins and Valdes\n1990; Haualand and Spengler 2021; Okajima et al. 2022). Therefore, the discussion\nshould predominantly revolve around mesoscale SST-THF coupling, with mesoscale\nSST-LHF coupling employed to interpret SST-THF coupling results.\nWe agree that the discussion should predominantly revolve around mesoscale\nSST-THF coupling, using mesoscale SST-LHF coupling to interpret SST-THF coupling\nresults. We conducted a comparative analysis of eddy-induced sensible heat flux (SHF)\nand latent heat flux (LHF), along with their changes under global warming within the\nfocused study area of western boundary current regions. As shown in Fig. R1 a,b (also\nin Fig. 1 and S2), SHF values are observed to be approximately half that of LHF values.\nMoreover, while mesoscale SST-LHF exhibits significant enhancement under warming,\nmesoscale SST-SHF shows a subtle change and even a slight decrease in response to\nglobal warming, between 1/3 to 1/5 of the magnitude of LHF changes (Fig. R1c &Fig.\nS2). Therefore, our study emphasizes that mesoscale SST-THF coupling and its\nresponse to global warming is predominantly influenced by LHF. Consequently, we\nfocused our analyses on the mesoscale SST-LHF coupling to interpret the implications\nfor SST-THF coupling as discussed in the paper. This has now been further clarified in\nthe revised manuscript: “Furthermore, the eddy-induced sensible heat flux (SHF) is\napproximately half that of latent heat flux (LHF) and the differences between historical\nand future simulations are subtle (Fig. S2). Thus, the mesoscale thermal coupling in the\nWBC regions and its response to global warming is predominantly influenced by LHF.\nConsequently, our subsequent analyses will focus on the mesoscale SST-LHF\ncoupling…” (Lines 85-88, please refer to line numbers in the tracked manuscript).\nAdditionally, we have revised the abstract and the conclusions to better emphasize\nthe implifications of mesoscale thermal coupling, predominately interpreted through\nmesoscale SST-LHF coupling. The revised abstract reads: “This study …, focusing on\nthe mesoscale thermal interaction dominated by mesoscale sea surface temperature\n(SST) and latent heat flux (LHF) coupling in winter” (Lines 15-16). In the conclusion\nand discussion part, the revised text reads: “we found a ubiquitous intensification of\nmesoscale thermal coupling in WBC regions by the end of the 21st century under the\nRCP8.5 warming scenario. The intensification is primarily driven by mesoscale SST-\nLHF coupling…” (Lines 229-231).\nFig. R1 (a) Composite of SST (color) and LHF (contours) anomalies associated with\nmesoscale oceanic eddies during historical periods(1956-2005) in CESM-HR. (b) Same\nas (a), but for SHF anomalies. Both panels show winter season mean anomalies in warm\nminus cold eddy composites within four WBC regions. (c) Differences in mesoscale\nSST-LHF (solid bars) and mesoscale SST-SHF (slashed bars) coupling strength\nbetween future (2063-2100) and historical periods (1956-2005) in CESM-HR for warm\n(red bars) and cold (blue bars) eddies in the KE,GS,ARC and BMC regions.\n2. The rationale behind selecting specific periods for CESM-HR simulations\n(1956-1975 & 2063-2082) to show future changes is unclear. Extending the period\nfor as long as output data is available would enhance the robustness of the results.\nTo enhance the robustness of the results, we have extended the CESM-HR\nsimulation periods to encompass the longest available spans with high-frequency\noutput data, from 1956 to 2005 for historical simulations and from 2063 to 2100 for\nfuture simulations. This has been introduced and revised in Methods (Lines 284-287).\nAll related figures (Fig.1-3, Tab S1 and figures in the supplementary materials) and\ndescriptions in the manuscript have been updated accordingly. The extended periods\nhave also revealed consistent increase in mesoscale SST-LHF coupling and similar\nhemisphere asymmetry in response to global warming except that the enhancement is\nmost pronounced in the Kuroshio Extension region (Fig. 1e, Fig. 2-3), whereas the\nprevious 20-year analysis indicated the strongest enhancement in the Gulf Stream\nregion. The overall consistency between the 50-year analysis and the previous 20-year\nanalysis supports the robustness of our main findings. Detailed findings are updated in\nthe revised manuscript (e.g., Lines 106-114).\n3. There's uncertainty regarding whether the change in Ce_bar (transfer\ncoefficient) under global warming is minimal (L310). Given its dependence on wind\nspeed, the authors should provide justification or refer to previous studies\ndemonstrating this. Additionally, it is unclear if U_10(F) and U_10(C) experience\nminimal changes in the WBCs or not (L314). Previous studies pointed out that the\nmidlatitude jet streams are projected to significantly shift poleward under global\nwarming (e.g., Oudar et al., 2020).\nWe diagnosed the time-averaged transfer coefficient (𝐶\"\") in the WBC regions for\n!\nhistorical and future simulations using the equation 𝐶\"\" = \n\"\"\"\"\"\"\"\"\"\"\n!\n\"\"\"\"\"\"\"\" , where 𝜌\n! *\n#\"$#%$%('&('\")\n(1.22kgm-3) and Λ (2.5x106Jkg-1) are considered constants (following Hausmann et\n+\nal. 2017). The fractional differences in \"𝐶\"\" between the historical and future\n!\nsimulations are shown in Fig. R2 (green bars). The changes in \"𝐶\"\" under global\n!\nwarming are approximately 2% for the Kuroshio Extension (KE), 4% for the Gulf\nStream Extension (GS), 3% for the Agulhas Return Current (ARC) and 0.76% for the\nBrazil-Malvinas Confluence Region (BMC), with an averaged value of ~2%. These\nchanges are relatively minimal, especially when compared to the mesoscale moisture\nadjustment term (\n,'& '\n-\n,'\" '\n),\n,--.' ,--.'\nwhich shows enhancements of 25%, 20%, 15%, and\n,--.' ,--.'\n8.5% (Fig. R2, purple bars), respectively, with an averaged value of ~17%. In the\nrevision, the related description has been modified for clarity: “assuming the changes\nin 𝜌̅ Λ( 𝐶̅ are minimal under global warming (with a relative change of\n* + !\napproximately 2%, which is negligible compared to the 17% mesoscale moisture\nadjustment)” (Lines 361-362).\nAdditionally, changes in the background large-scale wind term (𝑈\"\")\"\"\"\"\" under global\n/0\nwarming are also minimal (yellow bars in Fig. R2 and also shown in Fig. 2), which\nfurther supports the assumption that both the transfer coefficient and large-scale winds\nexperience only minimal changes under global warming. This has also been clarified\nin the revision: “As the large-scale wind, 𝑈( and 𝑈( , experience minimal\n/0(1) /0(2)\nchanges (ranging between -0.4% and -3.5% as shown in Fig. 2) in the WBCs” (Lines\n367-368).\nFig. R2 (a) Fractional differences in transfer coefficient (𝐶\"\"), large-scale wind (\"U\"\")\n!\nand mesoscale moisture adjustment (MA,\n,'& '\n-\n,'\" '\n)\n,--.' ,--.'\nbetween future and historical\n,--.' ,--.'\nperiods in CESM-HR in the KE region. (b-d) as in a, but for the GS, ARC and BMC,\nrespectively.\n4. In Fig. 3, the change between the historical and future periods in the CESM-\nHR simulations is compared with the trend over the last 40 years derived from the\nobservations. The change in SST in the CESM-HR, especially its interhemispheric\nasymmetry, is much larger than the observational trends, owing probably to this\ndifference. I am thus wondering if trends calculated similarly based on the historical\nperiods in the CESM-HR simulations are consistent with the observations. Recent\nstudies suggest that the interhemispheric asymmetry in SST trends is likely\ninfluenced largely not only by greenhouse warming but also by aerosol forcing (e.g.,\nDeser et al. 2020).\nFollowing your suggestion, we compared the SST trends over the past 40 years\n(1983-2022) as simulated by CESM-HR with observations. During this process, we\nidentified an error in the original Fig. 3d, where the trend displayed was the 40-year\n(1956-1995) SST trend in CESM-HR simulations. We apologize for any confusion this\nmay have caused. This has now been fixed in the revised calculation, and the updated\nresults are now presented in the new Figure 3 and also in Fig. R3 below.\nWhile there are discrepancies in the absolute average SST between CESM-HR\nsimulations and observational data (Fig. R3a) in the four WBC regions, the trends in\nSST are notably similar. Both datasets show pronounced warming in regions such as\nthe KE, GS, and BMC (Fig. R3b, with slashed bars for observations and solid bars for\nCESM-HR). Additionally, the warming trend in the Northern Hemisphere is\nsignificantly more pronounced than that in the Southern Hemisphere. Overall, both the\nwarming trend and the hemispheric asymmetry are well captured by the CESM-HR\nsimulations. The consistency between the observed and simulated SST trends suggests\nthat the warming observed over the last 40 years is predominantly driven by greenhouse\ngases, aligning with the findings of Deser et al. (2020), which suggest that while aerosol\nforcing was a dominant factor before 1970, greenhouse warming has been the primary\ndriver since then.\nThe consistency between observed and simulated SST trend has been further\nemphasized in the revision: “Additionally, the accelerated SST warming trend in the\nNH than in the SH is also confirmed by the observational data (Fig. 3d), underscoring\nthe robustness of the hemisphere asymmetry under warming” (Lines 201-203).\nFig. R3 (a) Time series of observed OISST (solid lines) and CESM simulated SST\n(grey lines) in the KE (green), GS (red), ARC (purple) and BMC (yellow) regions\nduring 1983-2022. (b) The observed (slashed bars) and simulated (solid bars) SST trend\nwithin the four WBCs.\n5. In the conclusions, the authors argue that “our findings suggest an\nincreasingly important role of mesoscale oceanic processes in shaping weather and\nclimate systems in a warming climate, especially in the NH”. I am afraid, however,\nthat further clarification is needed to articulate how the findings support this\nassertion.\nWe acknowledge that our original assertion about the increasingly important role\nof mesoscale oceanic processes in shaping weather and climate systems was speculative\nand not directly supported by the findings presented. To clarify, we have removed this\nstatement from the abstract. Also, in the conclusion, we have revised the sentence:\n\"Considering the recognized importance of mesoscale oceanic processes in influencing\nweather and climate systems, as highlighted in previous studies (Ma et al. 2016; Czaja\net al. 2019; Seo et al. 2023), our results underscore the critical need for climate models\nto incorporate mesoscale air-sea interaction for more reliable climate projections.\"\n(Lines 233-235).\n6. The authors simply touch on the relative magnitude of the differences of\ndqs’/dSST’ and dqa’/dSST’ between the future and historical periods (in LL138-139).\nWhile this is likely a primary factor for the importance of background SST warming\nfor the strengthened mesoscale SST-LHF coupling under global warming, its reason\nremains unknown in this study. The authors might want to discuss this to obtain a\ndynamical understanding. I speculate that it is determined by how the atmosphere\nresponds to mesoscale SST and how it changes, and I am wondering if there are\nprevious studies or existing theoretical frameworks about that.\nThank you for your insightful comments. The saturated specific humidity at the\nocean surface (q ) is primarily related to SST change, while the specific humidity of\n3\nthe air (q ) is more influenced by the air temperature (T ) change. Previous studies\n* *45\n(Moreton et al. 2021; Hausmann et al. 2017) indicate that at mesoscales, there is a\nthermal adjustment of surface air temperature to SST, which is quantified by the\nrelationship: 𝑇 ′ = 𝛿𝑆𝑆𝑇′, where 𝛿 is the atmospheric adjustment parameter\n*45\nestimated to be around 0.3. This suggests that the surface air temperature anomaly is\nabout one-third of the mesoscale SST anomaly. This relationship likely explains why\n,'\" '\nis substantially smaller than\n,'& '\n.\n,--.' ,--.'\nWe have incorporated a discussion on this in the revised manuscript: “Specifically,\n,'& '\nis an order of magnitude greater than\n,'\" '\n, which may be attributed to the\n,--.' ,--.'\natmospheric boundary layer thermal adjustment that results in a reduced surface air\ntemperature anomaly relative to the mesoscale SST anomaly as indicated by Moreton\net al. (2021) and Hausmann et al. (2017)” (Lines 164-167).\n7. I do not understand what is shown in Fig. 3b. The panel requires clarification\nregarding \"north-south hemispheric asymmetry,\" units, and the definition of\n\"hemispheric mean.\"\nWe apologize for the lack of clarity in our initial description of Fig. 3b. The\n\"hemisphere mean\" is defined as the average mesoscale SST-LHF coupling response\nto global warming across the four WBC regions (with unit W/m²/°C). To improve\nclarity, we replaced it with “WBC mean” in the revision. The bars in Fig. 3b show the\ntotal anomalous SST-LHF coupling strength changes in each WBC region relative to\nthis four-WBC mean. A positive anomaly indicates an enhancement in mesoscale SST-\nLHF coupling strength above the mean, while a negative anomaly indicates a coupling\nstrength below mean.\nThe total anomaly in each bar is further decomposted into three contributing\nfactors: 𝑈( (𝑨), ,('& (.) (𝑩) 𝑎𝑛𝑑 𝑥𝑺𝑺𝑻 (𝑪). For example, the anomoulous higher\n/0 ,.( \nmesoscale SST-LHF coupling response in the KE can be represented as\n9 ,\" ! ' : −9 ,\" ! ' : = 𝐴=𝐵\"𝐶̅+𝐵=𝐴̅𝐶̅+𝐶=𝐴̅𝐵\". Here, A, B, and C denotes\n,--.'\n(78)\n,--.'\n(9:2 ;!*<)\nthe respective three terms; the overbar denotes the average of corresponding values\nacross the four WBCs; the prime denotes the deviation from the WBC mean. For\ninstance, 𝐴=𝐵\"𝐶̅ calculates the contribution of the wind speed anomaly (as represented\nby the green bar in Fig. 3b), assuming that B and C in the KE are at the WBC-mean\nlevel, and the actual 𝑈( anomaly in the KE is multiplied by these mean values. Similar\n/0\ncalculations apply to the other two terms, providing a detailed breakdown of the three\nfactors contributing to each region’s total anomaly.\nDetailed description has been included in the figure caption in the revised\nmanuscript (Lines 564-573): “Contributions of 𝑈( (𝐴), ,('& (.) (𝐵) 𝑎𝑛𝑑 𝑥𝑺𝑺𝑻 (𝐶) to\n/0 ,.( \nregional variations in mesoscale SST-LHF coupling response to global warming in the\n,\"' \nfour WBCs. Taking the KE region as an example, the deviation of ! response from\n,--.'\n,\"' \nthe four-WBC mean can be represented as: 9 ! :| =\n,--.' (78( 9:2 ;!*<)\n𝐴=𝐵\"𝐶̅+𝐵=𝐴̅𝐶̅+𝐶=𝐴̅𝐵\". Here, A, B, and C denotes the respective three terms; the overbar\ndenotes the average values across the four WBCs; the prime denotes the deviation from\nthe WBC mean. For instance, 𝐴=𝐵\"𝐶̅ calculates the 𝐴=(𝑈( anomaly) contribution to\n/0\nthe total anomaly, assuming that B and C are at their mean levels, as represented by the\ngreen bar in Fig. 3b.”\n8. Improving the readability of x-axis labels in figures is recommended.\nWe have revised all figures to enhance the readability of axis labels.\nReferences\nCzaja, Arnaud, et al. (2019). Simulating the midlatitude atmospheric circulation: what\nmight we gain from high-resolution modeling of air-sea interactions?. Current\nClimate Change reports 5, 390-406.\nDeser, C., et al. (2020). Isolating the evolving contributions of anthropogenic aerosols\nand greenhouse gases: A new CESM1 large ensemble community resource. Journal\nof climate, 33(18), 7835-7858.\nHausmann, U., Czaja, A., & Marshall, J. (2017). Mechanisms controlling the SST air-\nsea heat flux feedback and its dependence on spatial scale. Climate Dynamics, 48,\n1297-1307.\nMa, Xiaohui, et al. 2016. Western boundary currents regulated by interaction between\nocean eddies and the atmosphere. Nature, 535, 533-537.\nMoreton, S., Ferreira, D., Roberts, M., & Hewitt, H. (2021). Air‐sea turbulent heat\nflux feedback over mesoscale eddies. Geophysical Research Letters, 48(20),\ne2021GL095407.\nSeo, Hyodae, et al. 2023. Ocean Mesoscale and Frontal-Scale Ocean–Atmosphere\nInteractions and Influence on Large-Scale Climate: A Review. Journal of\nClimate 36.7, 1981-2013.\nReply to reviewer #2\nWe would like to thank the reviewer for the invaluable comments. We have\ncarefully considered each of your comments (listed as bold and Italic below) and\nrevised the manuscript accordingly. Below are point-to-point replies to your comments:\nThis is a clear & concise paper that demonstrates the increasing strength of\nocean-eddy - atmosphere interaction with climate change, and attributes this change\nprimarily to the impact of sea-surface temperature (SST) on the saturation specific\nhumidity, a direct consequence of the Clausius-Clapeyron equation. Because these\nresults stem from fundamental thermodynamics, we can be confident that they are\nrobust. I recommend publication.\nA few comments/suggestions::\n1. In figures 2a and 2b the second column, resulting from the change in wind\nspeed with eddy SST is not negligible(roughly 1/3 of the first column). The authors\nshould say something about this: what is the (possible) physical mechanism; why is\nthis effect much stronger for the KE & GS than for the other regions?\nThank you for your insightful comments. Following the suggestion of the first\nreviewer, we have extended our analysis to the longest available periods with high-\nfrequency output, from 1956 to 2005 for historical simulations and from 2063 to 2100\nfor future simulations. Our findings demonstrate that the mesoscale dynamic\n>?'\nadjustment $% (q\" - q\" ) under global warming remains approximately 1/3 of the\n>@@A' @ B\nthermodynamic adjustment term during the extended periods, as shown in revised Fig.\n2.\nAs you pointed out, although the dynamic adjustment term is substantially weaker\nthan the thermodynamic adjustment, it is notably stronger in the NH compared to the\nSH. A further decomposition of the dynamic adjustment term reveals that the response\nof large-scale air-sea humidity differences (q\" - q\" ), which varies from 15% to 25%\n@ B\n>?'\nin the four WBCs, generally outweighs the response of mesoscale wind coupling $%,\n>@@A'\nwhich ranges from -1% to 15% (Fig. R4 below). This is especially true except in the\nGS region, where the two components are comparable. Notably, the mesoscale wind\n>?'\ncoupling response $% is more pronouced in the KE and GS regions, and is minimal\n>@@A'\nin the ARC and BMC regions. This contribute to the stronger mesoscale dynamic\n>?'\nadjustment observed in the NH compared to the SH. The intensified $% response in\n>@@A'\nthe KE and GS may be associated with the vigorous western boundary currents and the\nfrequent passage of synoptic weather systems (Sheldon et al. 2017; Parfitt and Seo\n2018). These lead to an unstable planetary boundary layer, facilitating enhanced\ndownstream momentum transfer. Further in-depth study is required to fully understand\nthe detailed physical mechanisms of the mesoscale dynamic response, which we plan\nto explore in future research.\nThe figure illustrating the decomposition of the dynamic adjustment has been\nadded to the Supplementary (Fig. S3), and the related discussion has been updated in\nthe revised manuscript. It states: “Although the dynamic adjustment term is\nsubstantially weaker than the thermodynamic adjustment, it is notably stronger in the\nNH compared to the SH. A decomposition of the dynamic adjustment term shows that\nthe response of large-scale air-sea humidity difference under warming generally\nsurpasses the mesoscale wind response to oceanic eddies across the four WBCs (Fig.\nS3). However, the mesoscale wind response to eddies is more pronounced in the KE\nand GS regions and minimal in the ARC and BMC regions, contributing to the stronger\nmesoscale dynamic adjustment in the NH. The intensified mesoscale wind response to\neddies in the KE and GS may be associated with the vigorous western boundary\ncurrents and the frequent passage of synoptic weather systems, which lead to an\nunstable planetary boundary layer and enhanced downstream momentum transfer as\ndiscussed in previous studies (Sheldon et al. 2017; Parfitt and Seo 2018).” (Lines 149-\n159, please refer to line numbers in the tracked manuscript).\nFig. R4 Decomposition of mesoscale dynamic adjustment response under greenhouse\n>?'\nwarming. (a) Fractional changes in mesoscale wind coupling $% and large-scale air-\n>@@A'\nsea humidity differences (q\" - q\" ) between future and historical periods in CESM-\n@ B\nHR in the KE region. (b-d) as in a, but for the GS, ARC and BMC, respectively.\n2. Also in figure 2, the column labels, using mathematical expressions, are hard\nto read - I suggest replacing them with larger labels using words or abbreviations.\nThank you for your suggestion. We have replaced the mathematical expressions\nwith words for clearer readability and have updated the figure caption accordingly.\nPlease see the revised Fig. 2.\n3. While recognizing that space is limited, the reader is left wondering about the\nimpacts of increased coupling strength on the actual fluxes. Perhaps a figure/table\ncould be added showing this (i.e. change in actual latent heat flux over these regions).\nIncreasing coupling strength need not lead to increased fluxes if the eddies weaken\n- do they?\nThank you for raising this question. We have analyzed the responses of latent heat\nflux (LHF) and sea surface temperature (SST) associated with oceanic eddies under\nwarming across the four Western Boundary Current (WBC) regions (Fig. R5). The\nresults indicate that while there is a general enhancement in mesoscale SST-LHF\ncoupling across all four WBCs as shown in Fig. 1, the behavior of individual\ncomponents such as eddy-induced SST and LHF varies by region. Specifically, eddy-\ninduced LHF is typically increasing across the WBC regions, except in the GS region\nwhere there is a slight decrease around eddy center. In contrast, eddy-induced SST\nanomalies are decreasing in the KE, GS and ARC regions, and slightly increasing in\nthe BMC region. Overall, the eddy composite analysis reveals that while there is a\ngeneral increase in LHF across the WBCs, the eddies and the associated SST anomalies\nare showing signs of weakening.\nAs you rightly point out, the results suggest that an increase in coupling strength\ndoes not necessarily translate to increased heat fluxes, especially if eddies themselves\nare weakening. This weakening of oceanic eddies is corroborated by our additional\nanalysis of eddy kinetic and potential energy responses, which will be discussed in\nresponse to your subsequent comments 4.\nWe have included composite analyses of eddy-SST and eddy-LHF in the\nSupplementary materials (Fig. S5) and updated the discussion accordingly in the\nrevised manuscript: “It is important to recognize that an increase in mesoscale thermal\ncoupling strength does not necessarily correspond to higher heat fluxes. The eddy\ncomposite analysis reveals a general increase in LHF across the WBCs, yet the\naccompanying eddies and SST anomalies are weakening (Fig. S5).” (Lines 269-272).\nAdditionally, the discussion on reduced eddy activity is further detailed in the response\nto your comment 4.\nFig. R5 Composite difference of LHF anomlaies (color shading) associated with\nmesoscale oceanic eddies between future(2063-2100) and historical periods(1956-2005)\nthe KE (a), GS (b), ARC (c), BMC (d) region. Shown are winter season mean anomalies\nin warm minus cold eddy composites within four WBC regions. The white circle\nrepresents one eddy radius and the white dot marks the eddy center. (e-h) as in (a-d),\nbut for eddy-induced SST anomalies.\n4. Similarly, increased coupling strength implies increased damping of eddies\nby air-sea interaction. What happens the eddy energy?\n(C'(D+'()\nWe analyzed the surface eddy kinetic energy (EKE) using and\nE\ndiagnosed the surface eddy potential energy (EPE) using 𝑇’E, where the prime denotes\nthe 30-100 day bandpass filtered field (shown in Figs. R6 and R7). The results indicate\na general weakening of both EKE and EPE across the WBCs, corresponding with the\nweakening of the SST signal as shown in Fig. R5. This reduction in eddy energy is\nconsistent with the enhanced mesoscale thermal coupling, which damps the EPE and\nthereby reduces its conversion to EKE (Ma et al. 2016). Additionally, this weakened\neddy activity is likely linked to increased oceanic stratification under warming\nconditions, which supresses the generation of oceanic eddies. Further in-depth\ninvestigation is required to fully understand the dynamics of eddy activity changes\nunder global warming.\nIn conjunction with the response to your comments 3, a discussion on the changes\nof eddy activity has been included in the revision: “This reduced eddy activity is\npossibly linked to enhanced mesoscale thermal coupling, which dampens eddy\npotential energy (Ma et al. 2016), and to increased oceanic stratification, which inhibit\neddy formation. Further in-depth investigation is required to fully understand the\nchanges in eddy dynamics under global warming.” (Lines 272-275).\n(C'(D+'()\nFig. R6 Winter season mean (DJF) surface EKE of in historical periods\nE\n(contours) and diffenreces(color shading) between future(2063-2100) and historical\nperiods(1956-2005) in the KE (a), GS (b), ARC (c) and BMC (d) region. The eddy field\nis defined by the 30-100 day bandpass filtered field.\nFig. R7 Winter season mean (DJF) surface 𝑇’E in historical periods (contours) and\ndiffenreces(color shading) between future(2063-2100) and historical periods(1956-\n2005) in the KE (a), GS (b), ARC (c) and BMC (d) region. The eddy field is defined\nby the 30-100 day bandpass filtered field.\nReferences\nMa, Xiaohui, et al. (2016). Western boundary currents regulated by interaction\nbetween ocean eddies and the atmosphere. Nature, 535, 533-537.\nParfitt, R., & Seo, H. (2018). A new framework for near‐surface wind convergence\nover the Kuroshio extension and Gulf Stream in wintertime: The role of atmospheric\nfronts. Geophysical Research Letters, 45(18), 9909-9918.\nSheldon, L., Czaja, A., Vannière, B., Morcrette, C., Sohet, B., Casado, M., & Smith,\nD. (2017). A ‘warm path’for Gulf Stream–troposphere interactions. Tellus A:\nDynamic Meteorology and Oceanography, 69(1), 1299397."}, {"round": 2, "reviewer_comments": {"reviewer_1": "I appreciate the authors’ effort toward the improvement of the manuscript, and I found that their arguments have become more convincing. Overall, I find that this study presents interesting materials that provide intriguing insights contributing to our comprehension of climate change and midlatitude air-sea interactions. Therefore, I consider that the manuscript is now ready to be published as an article, besides the minor comments shown below.\nComments:\n1. LL155-159: The argument here does not make sense to me. I think that “downstream” (L158) would be “downward” to mean enhanced vertical mixing contributing to increased surface wind speed over warm ocean eddies. If so, I also wonder if the authors simply mean that the atmospheric synoptic-scale eddy-related activity along the ocean currents is stronger in KE and GS than in ARC and BMC, or rather that the downward momentum transfer associated with synoptic-scale features tends to be more pronounced in these NH frontal regions. If the latter is the case, my understanding is that Sheldon et al. (2017) does not fit well here, whereas Parfitt and Seo (2018) discussed the relationship between surface wind convergence and atmospheric fronts.\n2. L274: inhibit -> inhibits\n3. The caption of Fig. S5: LHF anomlaies -> LHF anomalies", "reviewer_2": "The authors have thoroughly and satisfactorily addressed the comments in my previous review. I recommend publication."}, "author_response": "Reply to reviewer #1\nWe would like to thank the reviewer again for the invaluable comments. We have\ncarefully considered each of your comments (listed as bold and Italic) and revised the\nmanuscript accordingly. Below are point-to-point replies to your comments.\nReviewer #1 (Remarks to the Author)：\nI appreciate the authors’ effort toward the improvement of the manuscript, and I found\nthat their arguments have become more convincing. Overall, I find that this study presents\ninteresting materials that provide intriguing insights contributing to our comprehension of\nclimate change and midlatitude air-sea interactions. Therefore, I consider that the\nmanuscript is now ready to be published as an article, besides the minor comments shown\nbelow.\nComments:\n1. LL155-159: The argument here does not make sense to me. I think that “downstream”\n(L158) would be “downward” to mean enhanced vertical mixing contributing to increased\nsurface wind speed over warm ocean eddies. If so, I also wonder if the authors simply mean\nthat the atmospheric synoptic-scale eddy-related activity along the ocean currents is stronger\nin KE and GS than in ARC and BMC, or rather that the downward momentum transfer\nassociated with synoptic-scale features tends to be more pronounced in these NH frontal\nregions. If the latter is the case, my understanding is that Sheldon et al. (2017) does not fit\nwell here, whereas Parfitt and Seo (2018) discussed the relationship between surface wind\nconvergence and atmospheric fronts.\nYes, the “downstream” should be revised to “downward” momentum transfer. See L157\nin the revised manuscript.\nThe KE and GS regions exhibit stronger mesoscale oceanic eddy activity compared to the\nARC and BMC, as noted by Chelton et al. (2011). Additionally, atmospheric synoptic storm\nactivity is also more vigorous in the KE and GS and tends to align more closely with these\ncurrents than in the ARC and BMC regions. We believe that both these factors—the vigorous\noceanic eddy activity and the frequent passage of synoptic weather systems over the KE and\nGS—contribute to the intensified mesoscale wind response to oceanic eddies in the Northern\nHemisphere (NH).\nFollowing your suggestion, we have removed the reference to Sheldon et al. (2017). The\nfindings from Parfitt and Seo (2018), which discuss the relationship between atmospheric\nfronts and surface wind convergence, are particularly relevant, as their emphasis on the role of\nsynoptic weather systems in establishing mesoscale air-sea coupling strongly supports our\nargument.\nWe have revised the text as well as the references to improve clarity and accuracy: “The\nintensified mesoscale wind response to eddies in the KE and GS may be associated with\nvigorous western boundary currents, stronger oceanic eddy activities8 (Chelton et al. 2011),\nand the frequent passage of synoptic weather systems that collocate with the KE and GS. These\nfactors lead to an unstable planetary boundary layer and enhanced downward momentum\ntransfer, as discussed in previous studies2,29 (Small et al. 2008; Parfitt and Seo 2018).\" (See\nL153-157).\nChelton D. B., et al. Global observations of nonlinear mesoscale eddies. Progress in Oceanography,\n(2011): 91(2), 167-216.\nParfitt, R., & Seo, H. A new framework for near-surface wind convergence over the Kuroshio extension\nand Gulf Stream in wintertime: The role of atmospheric fronts. Geophysical Research Letters,\n(2018): 45(18), 9909-9918.\nSmall, RJ D., et al. Air–sea interaction over ocean fronts and eddies. Dynamics of Atmospheres and\nOceans 45.3-4 (2008): 274-319.\n2. L274: inhibit -> inhibits\nRevised. See L269.\n3. The caption of Fig. S5: LHF anomlaies -> LHF anomalies\nRevised. See Fig. S5 in supplementary information."}]

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