Shoreward shift of oceanic mesoscale activity over the last three decades

Domain: earth_science
Published: 2025-11-24
Editorial outcome: major_revision

Abstract
Oceanic mesoscale activity critically mediates the cross-shelf exchange of heat, nutrients, carbon, and pollutants. While its meridional shifts under global warming are well studied, zonal migration remains unknown. Here, based on multi-source observation and reanalysis datasets, we find that zonal centroids of both global mesoscale eddy kinetic energy (EKE) and global sea surface temperature gradient magnitudes have shifted shoreward at approximately 35.4±22.5 and 34.4±20.3 km per decade respectively over the last three decades (1993-2022). Energy budget analysis indicated that a shoreward shift of wind power and baroclinic energy conversion under global warming primarily has occurred in parallel with the EKE onshore migration. These shifts are reshaping coastal-open ocean exchanges, and thereby are essential for predicting coastal ecosystem resilience under climate change.

Introduction
Oceanic mesoscale activity, characterized by horizontal scales ranging from tens to hundreds of kilometers, accounts for almost 90% of the global ocean kinetic energy1, serving as principal regulators in the global energy budget and biogeochemical cycling2–4. In coastal transition zones, mesoscale fronts and eddies act as critical interfacial mediators, facilitating cross-shelf transport of momentum, heat, dissolved carbon, nutrients, and pollutants1,5–12. These processes exert cascading effects on environments and ecosystems13, as well as resource sustainability14,15 in coastal regions. Rapid ocean warming observed over the past few decades16, however, can alter these fundamental processes. Therefore, elucidating the zonal evolution of global mesoscale activity under global warming emerges as a scientific imperative for refining climate prediction, advancing sustainable ocean governance frameworks, and informing evidence-based coastal adaptation strategies.

Recent studies have investigated changes in global oceanic mesoscale activity via observations, reanalysis, and numerical simulation, but mainly focused on the meridional15,17–24. The poleward shift of oceanic mesoscale activity in the western boundary currents of the Southern Hemisphere15,19, as well as the increase of mesoscale activity near both poles are observed20,23,24. Based on eddy kinetic energy (EKE) and sea surface temperature gradient magnitudes (|∇SST|), J. Martínez-Moreno et al.18 found that oceanic mesoscale eddies in eddy-rich regions, such as the western boundary currents and their extensions, have exhibited increased variability over the past several decades. From long-term observations, Beal and Elipot25 found that the increased mesoscale activity broadens the Agulhas Current via enhanced lateral viscous effects. Furthermore, recent observation25 and numerical26 studies indicated that the western boundary currents migrated shoreward over the past decades, implying a possible shoreward shift of oceanic mesoscale activity. However, little is known about the zonal evolution of mesoscale activity in global oceans under current and future warming.

To investigate the zonal evolution of ocean mesoscale activity, we examine EKE and |∇SST| from multiple data sources (see “Methods” for details), including one eddy-resolving reanalysis product (GLORYS), one eddy-resolving ocean hindcast (OFES), three satellite-derived datasets (AVISO, CMEMS and OISST), as well as two eddy-resolving climate models (FGOALS and HadGEM3) from Climate Model Intercomparison Project phase 627 (CMIP6) models. A tracer-weighted average operator is applied to quantify the zonal distribution of EKE and |∇SST|, named as “zonal centroid” (enlightened by Yang et al.28, see “Methods” for details). The zonal centroids of EKE and |∇SST| are then calculated for 10 key regions29, including five western regions and five eastern regions (Fig. 1a and Table S1). The shift trends of zonal centroids are obtained by linear regression, and the significance is tested via the Student’s t-test. Significant shoreward intensification of oceanic mesoscale activity is found in most of the regions. The energy analysis reveals that both surface wind power and baroclinic energy conversion have also shifted onshore. Furthermore, we compute the zonal centroids of EKE and |∇SST| under the high-emission RCP8.5 scenario (2015–2050) from CMIP6 models to discuss the zonal evolution of oceanic mesoscale activity in response to global warming.

Results
Shoreward shift of oceanic mesoscale activity in past decades
Figure 1a shows the global trends of sea surface EKE from 1993 to 2022, derived from GLORYS. In the western boundary current regions, the EKE shows increased trends in the polar flank and decreased trends in the equatorial flank, implying the poleward shifts of EKE, consistent with previous studies15,17,19,28,30,31. In terms of the zonal distribution, the trend of EKE generally increases near the coasts, such as the China marginal seas, the California coast, and so on. Except for the western South Atlantic (WSA), all regions exhibit shoreward intensification of EKE (Fig. 1b–k). The global averaged zonal shift speed of sea surface EKE from GLORYS is 41.7±14.4 km per decade (Fig. 1l). Notably, we select a wide region from the coast to the middle of the open ocean to calculate the zonal centroids of EKE. In this way, the overall changes in mesoscale activity distribution across the ocean basin over the recent three decades are quantified. To reveal changes of mesoscale activity from near-shelf to coast, we also select the nearshore region and calculate the shifts of the associated zonal centroids (Fig. S1). The shoreward shifts of EKE appear in most of the nearshore regions, consistent with the results obtained from the large domain (Fig. 1).

|∇SST|, a well-established proxy for oceanic mesoscale activity, also exhibit pronounced onshore intensification patterns. As shown in Fig. 2 and Fig. S2, the 1993–2022 period shows marked amplification of |∇SST| signatures within coastal regions. This spatial coherence aligns with observed asymmetries in regional SST warming, i.e., faster warming is observed near the coasts compared to adjacent open oceans (Fig. S3). The concurrent intensification of nearshore |∇SST| (Fig. 2a) and EKE centroid migration (Fig. 1a) suggests both momentum and temperature processes drive the coastal mesoscale activity over the recent three decades.

Additionally, Figs. 1a and 2a show that approximately 40% and 30% of the grid points exhibit significant local trends in EKE and |∇SST|, respectively. However, these significant temporal trends in EKE and |∇SST| are distinct from the significance of the zonal centroid shifts. The former are local metrics, while the latter evaluates large-scale spatial redistribution. Consequently, the local trends cannot be used to directly determine the significance of changes in the zonal centroids.

Coherent shoreward shift of zonal centroids of EKE and |∇SST| appears in the global average across all observational, reanalysis and model hindcast products (Fig. 3a, b), with a multi-dataset mean displacement rate of 35.4±22.5 km per decade and 34.4±20.3 km per decade over the past three decades, respectively. Notably, reanalysis systems (GLORYS), model hindcast (OFES) and satellite-derived datasets demonstrate convergent trends (~35 km per decade).

Regionally, western basin regions exhibit robust patterns, showing statistically significant EKE/|∇SST| centroid shoreward shifts in the analysis (Fig. 3a, b). For EKE, the WNA, WNP, WSP, and WSI regions demonstrate cross-dataset consistency, whereas the WSA region presents paradoxical behavior, its zonal EKE centroid demonstrates an offshore displacement in all datasets except CMEMS. The offshore shift in WSA is potentially linked to South Atlantic SST front intensification and Brazil Current frontal variability32,33. For |∇SST|, shoreward shifts appear in all regions and all datasets, except OFES in the WNA. Eastern basin regions display greater dynamical complexity. The zonal centroids of EKE in all regions display shoreward shift trends in most of the datasets (Fig. 3a), while |∇SST| exhibits inter-dataset spread (Fig. 3b). The results from GLORYS and OFES are not all consistent with those from OISST, likely reflecting deficiencies in models. The offshore trends of |∇SST| in the eastern basin regions primarily resulted from the enhancement of SST in open oceans, evident in SST trends (Fig. S3). In contrast, the shoreward trends of |∇SST| in OISST are apparent in the nearshore regions (Fig. S2), consistent with the same regionalsignificant shoreward EKE centroid shifts (Fig. S1).

Overall, there is a consistent shoreward shift of the zonal centroids of EKE and |∇SST| in approximately 77% regions, suggesting the redistribution of oceanic mesoscale activities over the past several decades. Observations from satellite observations are generally consistent with the results of GLORYS reanalysis, as well as the model hindcast (OFES).

Primary drivers
To investigate the potential dynamics driving the shoreward shift of the oceanic mesoscale activity over the past three decades, we analyze global trends of primary EKE source terms, including surface wind power (WP), baroclinic energy conversion (BC), and barotropic energy conversion (BT), derived from GLORYS34,35 (see “Methods” for details). The trend of these terms is spatially heterogeneous and obvious near the shore (Fig. 4a, c, e), implying the drastic changes of EKE over the recent decades. Note that surface wind power primarily acts as eddy-killing effects in regions with strong currents, while the wind work can be positive in coastal regions where the winds can drive small-scale currents36,37. The global mean distribution of WP38 shows that the mean WP is primarily negative along major ocean currents, while positive near the coasts. The zonal centroids of these energy terms are calculated to provide an integrated metric for evaluating whether the net energy forcing is shifting onshore. Since these energy terms contribute to the temporal evolution of EKE, the qualitative direction of these centroid movements indicates an onshore or offshore shifting in energy forcing. Figure 4b, d demonstrate significant shoreward shifts in the zonal centroids of WP (0.36±0.28 km per decade) and BC (0.07±0.04 km per decade), whereas the zonal centroid of BT (0.21±0.32 km per decade) exhibits an insignificant offshore trend (Fig. 4f). The shifting directions of WP and BC are consistent with that of oceanic mesoscale activity, suggesting both WP and BC contribute to the onshore migration of EKE. Furthermore, the spatial trend distribution of the shoreward shift of WP is more consistent with that of EKE and |∇SST|, implying that the WP is more responsible for the onshore shift of mesoscale activity.

The shoreward shift of WP is primarily attributed to intensified wind stress near coastal boundaries (Fig. S4). One possibility for intensified wind stress near coastal boundaries is amplified land-ocean thermal contrast under global warming35. The shoreward shift of BC maybe associated with enhanced open-ocean stratification, primarily induced by temperature changes under global warming in the subtropics and mid-latitudes39,40. Zonal variations in mesoscale activity may also be linked to the shoreward shift of the western boundary currents25,26,41.

Furthermore, the increased nearshore |∇SST| may also be linked to the intensification of alongshore winds. According to the established hypothesis of Bukun42, intensified alongshore winds enhance upwelling in eastern boundary current systems, which inhibits nearshore SST rise and amplifies the cross-shelf gradient. In western boundary regions, which lack major upwelling systems, intensified alongshore winds may enhance mixing in the upper ocean, thereby suppressing the SST increasing rate and also strengthening the SST gradient. Concurrently, the enhanced stirring of mesoscale processes can also increase the nearshore SST gradient. Additionally, under global warming, enhanced ocean stratification can lead to onshore intensification of western boundary currents39, and thereby contribute to the |∇SST| shoreward shift. A more targeted analysis is required to quantitatively disentangle the relative contributions of these mechanisms to this trend.

Implications from high-resolution CMIP6 projections
To investigate zonal shift trends of oceanic mesoscale activity under global warming, we analyze two high-resolution simulations (HadGEM3 and FGOALS) from the CMIP6 HighResMIP future experiments under the RCP8.5 scenario (from 2015 to 2050). RCP8.5, representing the worst-case climate pathway in IPCC projections, projects 4.3–4.8°C global warming by 2100 under unabated fossil fuel dependence.

HadGEM3 and FGOALS exhibit divergent zonal shift trends for EKE and |∇SST| (Fig. 3c). HadGEM3-historical simulates a significant shoreward EKE displacement, while FGOALS-historical shows an insignificant offshore trend. Although the future shift trend of EKE zonal centriod aligns with that of |∇SST|, the inter-model discrepancy reveals substantial uncertainties in projecting future mesoscale activity. These uncertainties emphasize the need to prioritize coastal monitoring. Furthermore, accurately simulating the zonal shift of mesoscale activities is critical for reducing air-sea flux biases and improving ocean circulation representation, an ongoing challenge for climate model development. Our findings provide a quantitative metric for evaluating eddy-resolving climate models.

Methods
Datasets
For satellite observations, we use the daily Optimum Interpolation SST (OISST) sea surface temperature data from the National Oceanic and Atmospheric Administration, sea water velocity from the daily Global Ocean Gridded L4 Sea Surface Heights And Derived Variables Nrt of the Copernicus Marine Environment Monitoring Service (CMEMS), and the monthly Archiving, Validation and Interpretation of Satellite Oceanographic data (AVISO), with 0.25° horizontal resolution.

For eddy-resolving reanalysis product and ocean model simulation, we use the daily sea water velocity and SST from Global Ocean Physics Reanalysis (GLORYS) of CMEMS (1/12° horizontal resolution) and the monthly Ocean General Circulation Model for the Earth Simulator (OFES) developed at the Japan Agency for Marine-Earth Science and Technology (0.1° horizontal resolution). GLORYS is widely used in the study of oceanic mesoscale dynamics and has been proven to do well in regions such as the NW Atlantic shelf49. The time range of the above datasets is from 1993 to 2022.

For climate model simulations, we use the monthly sea water velocity and SST of the historical and future experiment from CMIP6 High Resolution Model Intercomparison Project (HighResMIP), including two models (10 km horizontal resolution), namely FGOALS-f3-H (FGOALS) and HadGEM3-GC31-HH (HadGEM3). Normally, the future experiment under the RCP 8.5 scenario and the historical experiment are on the 2015–2050 and 1950–2014 period, respectively. Due to the lack of data, however, only the historical experiments of HadGEM3 from 1955 to 2014 are obtained.

The wind stress data with 0.25° horizontal resolution is obtained directly from the monthly Ocean Reanalysis System 5 (ORAS5) prepared by the European Center for Medium-Range Weather Forecasts (ECMWF) ocean analysis-reanalysis system.

Zonal centroid calculation
The zonal centroid defined in this study serves as a metric to quantify the overall distribution of oceanic mesoscale activity across the entire region. It is essentially the regional average of the λ-direction arc length relative to a certain reference longitude (for ESA it is 180° and for other regions it is 0°), which is weighted by the variable field F(λ,φ) and the spherical area element dS at (λ,φ). The unweighted arc length at the location (λ,φ) relative to the reference longitude is:

l(λ,φ) = (Rcosφ) * (λ - λ_ref) * (π / 180) = (πR / 180) * λ * cosφ (1)

dS at (λ,φ) is:

dS = (Rdφ) * (Rcosφdλ) = R^2 cosφ dλ dφ (2)

Therefore, the average of the weighted arc length over the whole region, namely the zonal centroid, is:

x_bar = (Integral(l * F * dS)) / (Integral(F * dS)) = (πR / 180) * (Integral(λ * F * cosφ * dλ * dφ)) / (Integral(F * cosφ * dλ * dφ)) (3)

where R is the radius of the Earth, R is 6371 km. λ and φ are longitude and latitude, respectively. F = F(λ,φ) is the variable field under latitude-longitude coordinates. In this study, F can be the field of variables. dλ and dφ are the elements of λ and φ in the spherical coordinate, respectively. Herein, dλ and dφ are the horizontal spatial resolutions of F in the zonal and meridional directions, respectively.

The calculation proceeds as follows. We first calculate annual mean fields from quality-controlled monthly data. For each year, we then compute the zonal centroid from these annual fields, creating a centroid time series. The shift speed is the linear trend of this time series, with a positive value indicating a shoreward shift. Finally, the global average shift speed is the area-weighted average of the regional speeds.

EKE calculation
EKE is defined as the kinetic energy of oceanic mesoscale activities. The calculation formula of EKE is as follows:

EKE = 1/2 * (u'^2 + v'^2) (4)

where u' and v' are the zonal and meridional components of perturbation sea surface velocity, respectively, which are obtained by applying 150 km spatial high-pass filtering to the GLORYS, AVISO, CMEMS, OFES and CMIP6 data at their respective minimum temporal resolutions. The overbar denotes a spatial average over 150 km. Additionally, we tested several cutoff wavelengths of filtering, including 50 km, 100 km, and 200 km. The results are consistent with each other. 150 km is applied for all spatial filtering in the study.

|∇SST| calculation
|∇SST| is defined as the magnitude of ∇SST,

|∇SST| = sqrt((∂SST'/∂x)^2 + (∂SST'/∂y)^2) (5)

where SST' is the mesoscale sea surface temperature field, which is obtained by applying a 150 km high-pass filtering to OISST, GLORYS, OFES and CMIP6 sea surface temperature data at their minimum temporal resolution.

EKE source terms
We calculate the global trends of the primary source terms responsible for the temporal evolution of EKE, including surface wind power (WP), baroclinic energy conversion rates (BC), and barotropic energy conversion rates (BT) from monthly GLORYS34,35 (see “Methods” for details).

WP = 1/ρ_0 * τ_s * u' (6)

BT = - (u'u' * ∂u_bar/∂x + u'v' * ∂u_bar/∂y + u'v' * ∂v_bar/∂x + v'v' * ∂v_bar/∂y) (7)

BC = - (g/ρ_0) * (u'ρ' * ∂ρ_bar/∂x + v'ρ' * ∂ρ_bar/∂y) (8)

where τ_s is the surface wind stress from ORAS5 and u' is the horizontal current at the sea surface. u and v are the components of the seawater velocity. Here, g is the acceleration of gravity, ρ_0 = 1025 kg/m^3 is the reference density. The overbar (prime) represents the low-pass (high-pass) filter.

Significance test
In this study, a unary linear regression model x_bar = β_0 + β_1 * T is used to analyze the linear trend of zonal centroids, where the superscript (hat) represent the estimated values. β_1 is the estimate of the slope, β_0 is the estimate of the intercept, x_bar and T represent the zonal centroid and time, respectively. The significance test for the linear trend (namely the slope β_1) used in this study is based on the Student’s t-test, where the used t-statistic is:

t = β_1 / σ_hat_β_1 (9)

σ_hat_β_1 = sqrt(σ^2 / (n * (T_i - T_bar)^2)). x_bar and T_bar represent the estimated value of zonal centroid and time, respectively. n is the length of the time series. σ_hat is the unbiased estimate of the residual standard deviation. If,

|t| > t_α(n-2) (10)

Then the trend is significant, else the trend is not significant. In this paper, α = 0.05, 0.1. To eliminate heteroscedasticity and autocorrelation in each time series, we implemented the Newey-West heteroscedasticity and autocorrelation consistent (HAC) covariance matrix estimator to correct standard errors and adjust confidence intervals.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This paper reports a shoreward shift of global oceanic mesoscale activity observed over the recent decades and projected in a GHG warming scenario. They quantify the results using an oceanic reanalysis (GLORYS12V1) and a satellite observation (OISST v2), and four model projections of CMIP6 HighResMIP. They discuss the possible reasons responsible for the shoreward shift. Based on the CMIP6 projections, they suggest the shoreward shift of the mesoscale activity may increase with global warming. The findings are novel and valuable for climate and other communities. The method and results are clearly presented, and the paper is well written. However, I would like the authors clarify some more issues about the sensitivity of the results to the data and methods:\n\nMajor issues:\n1. GLORYS12V1 is an oceanic reanalysis assimilating along-track satellite altimetry observations of SSH. I suggest the authors use Variance (SSH_eddy) or EKE from satellite altimetry (AVISO), as a ground truth, to validate the reanalysis results.\n2. The paper is kind of descriptive in explaining the mechanism of the shoreward shift of eddies. To make the results explainable, reasonable, and robust, I suggest the authors include one or two eddy-resolving oceanic model simulations to confirm the observed results. There are several eddy-resolving oceanic model hindcasts from 1950s-present. They have dynamical advantages to investigate the oceanic eddies in response to the long-term change of surface winds and radiative fluxes (atmospheric forcing, including recent GHG radiative fluxes).\n3. Before discussing the projection of CMIP6 models under GHG warming scenario, the authors need to check the performance of the CMIP6 historical runs on the observed shoreward shift of eddies over recent decades. Based on the comparison between historical runs and observations, and projections, the authors can discuss the relative roles of internal vs. external forcings on the shoreward shift.\nBased on the present paper, the shoreward shift in the projections (~3km/dec) is much weaker than the recent observations (~40km/dec). If the simulated results in historical runs are comparable with the observations, does it suggest that the recently observed shoreward shift might be dominated by other reasons, such as internal variability, but not the GHG warming. Please discuss this issue.\n\nSpecific comments:\nL95: Fig. S2?\nL124: Can you show any evidence from observations or just list some literatures for this hypothesis? By the way, Fig. S2 can suggest the increase in the alongshore wind in the eastern boundary from the SST perspective. But I would like to see some direct evidence.\nL129-130: The authors need to notice and clarify that the warming is faster near the western boundary (Fig. S2) than the open oceans.\nL131: how is the sandwich-like structure? Please specify it.\nL121-138: I feel this part is a little confusing. Do you want to propose that the shoreward shift in the west part of oceans is due to broadening of western boundary currents, while the shoreward shift in the east part is due to the increase in the alongshore winds, stronger upwelling and hence the intensified baroclinic instability?\nL139: Please add some comparisons and discussions of the historical runs.\nL154: Fig. 4k?\nL194: The captions in Fig. S4 are too simple, which makes readers confusing and the figure useless in the statement. I cannot draw a conclusion that the shoreward shift occurs in deeper layers from the figure. Need to inform readers the data origin.\nL209: Where do you use CESM simulations? You mean CMIP6 models?\nL210: I am wondering why using ORAS5 in the paper? For the broadening index of the western boundary currents? GLORYS has 3-D currents. Please inform data origins in some figures if it is not based on the major data of the paper.\nL287: Are the results sensitive to the cutoff wavelength of filtering?\nL289: For the proxy of mesoscale activity based on SST, why not using variance of 150km high-pass filtering SST? |grad(SST)| is not only related to mesoscale activity, but also largescale fronts, upwelling, and air-sea heat fluxes.", "reviewer_2": "Title: Shoreward Shift of Oceanic Mesoscale Activity in a Warming World\nShuyi Zhou, Yihang Zhang, Hong Li, Lingxiao Liu, Enhui Liao, Fanghua Xu\n\nSummary\nThe manuscript describes shifts in global surface eddy kinetic energy and sea surface temperature (SST) gradients towards the coastlines during the past 30 years, as found in ocean satellite SST from Optimum Interpolation Sea Surface Temperature Version 2.1 or OISST (0.25°x0.25 ° resolution) and Global Ocean Physics Reanalysis or GLORYS (1/12°x1/12° resolution). Further the authors find some evidence of the same result in four climate models (all 0.25°x0.25° horizontal resolution) that are part of the CMIP6 ensemble, when run at different future carbon dioxide scenarios. The authors also suggest they detect broadening of the width of western boundary currents, with a figure showing this in the supplemental figures.\n\nRecommendation\nI find the authors’ ideas intriguing but 1) do not see enough evidence in the paper to conclude that in a warming world the eddy field will shift closer to shore or broaden. 2) I also find that the analysis of the variation of SST over the past 30 years could be improved. I think the strongest analysis presented in the paper is of GLORYS, an ocean reanalysis. My conclusion is that the paper would benefit from having a more limited scope (focusing on time up to the present) and being published in a journal other than Nature Communications.\n\nMajor Comments\nRegarding 1), Hewitt et al. (2020) describe current work on parameterizing the ocean mesoscale in earth system models. The 0.25° horizontal resolution models are “eddy present;” however challenges remain near high latitudes where the Rossby radius is smaller (See Chelton et al. (1998) for an estimate of the first Baroclinic Rossby radius globally) and the eddies in the models may have characteristics different than “eddy rich” models (1/10° models). Climate models, especially the ensemble projects like CMIP6 are designed to estimate the mean climate in the future well. The climate models, in their current setup, were not designed to study the mesoscale variability. In the paper, the authors find some evidence of shoreward movement, but not in the western or eastern north Atlantic, the western north Pacific, or the western South Indian. I do not think climate models, as currently set up, should be used to ask questions about shifting locations of mesoscale variability – it is not the right tool.\n\nRegarding point 2) The authors have defined eddy kinetic energy by separating the small and large scale through a 150-km high pass filter. However, they use results of SST gradients without discussing scale separation. If this is on purpose, the authors are also tracking the movements of currents like the western boundary currents. If authors are trying to complete a complementary analysis to their EKE calculation, the authors should also filter the SST field to isolate the mesoscale eddies, as done in Martinez-Moreno et al. (2021). In general, the methods lack description of how the trends in EKE and grad(SST) were calculated. Is the average (in time) over each year calculated first for EKE and grad(SST)? How are the trend lines subsequently presented as “zeroed” such that they pass through “zero shift” approximately halfway through the time series? For the statistics, the sample number should be reported for the trend significance tests. In addition, the confidence interval on the estimate of the linear trend should be reported in the text. For example, “47 km +/- (confidence interval shift) in the zonal centroid.”\n\nThe GLORYS dataset has been shown to do well in the NW Atlantic shelf (Castillo-Trujillo et al. 2023). Other comparisons of GLORYS to observational data including in-situ and satellite data exist for other regions. This suggests that GLORYS could be accurately representing the surface ocean for the past 30 years. GLORYS uses a numerical model on a 1/12° horizontal grid which allows it to resolve more eddies. I think it is reasonable to present the results of the EKE calculated in GLORYS as representing what may have occurred over the past 30 years. As Todd et al. (2023) measured a shoreward movement of the Gulf Stream, it seems possible that shift in the mesoscale EKE has occurred. Have the Western Boundary currents globally all shifted poleward and shoreward in GLORYS? More work on understanding why the currents and the mesoscale variability have shifted would make the paper more novel.\n\nOverall, with only the GLORYS result one in which I feel confident, I suggest rejection for the paper and revision and resubmission in another Nature journal rather than Nature Communications.\n\nMinor Comments\nLine 33: “known as the ‘frontline of climate change’” should be removed because it is vague in a scientific context (here in reference to eddies)\nLine 46: remove that the eddies “have become richer” over recent decades. Instead state that the eddy variability is higher.\nLine 293-296: Since you have described the zonal centroid as x-bar, keep similar notation here and describe “x” as the zonal centroid, rather than labeling “y” the zonal centroid and “x” as time. Use another variable name for time.\nLine 300-301: describe what y-hat_l and x-hat_l are in the equations as well as what n is.\nFigures 1,2, and S1: Describe in the caption that “V=#” on subplots “b-i” are describing the shift speed in km per decade.\nExtended Data Fig. S4: a colorbar, x-axis labels, and y-axis labels are needed for each of the subplots. In addition, red and black vertical temperature gradients are not readily apparent. Perhaps the figure and figure caption here are mismatched?\n\nReferences:\nHelene T Hewitt, Malcolm Roberts, Pierre Mathiot, Arne Biastoch, Ed Blackley, Eric P Chassignet, Baylor Fox-Kemper, Pat Hyder, David P Marshall, Ekaterina Popova, Anne-Marie Treguier, Laure Zanna, Andrew Yool, Yongqiang Yu, Rebecca Beadling, Mike Bell, Till Kuhlbrodt, Thomas Arsouze, Alessio Bellucci, Fred Castruccio, Bolan Gan, Dian Pustrasahan, Christopher D Roberts, Luke Van Roekel, and Qiuying Zhiang. Resolving and parameterising the ocean mesoscale in earth system models. Current Climate Change Reports, September 2020. URL: https://doi.org/10.1007/s40641-020-00164-w\nChelton, D. B., DeSzoeke, R. A., Schlax, M. G., El Naggar, K., & Siwertz, N. (1998). Geographical variability of the first baroclinic Rossby radius of deformation. Journal of Physical Oceanography, 28(3), 433-460.\nCastillo-Trujillo, A. C., Kwon, Y. O., Fratantoni, P., Chen, K., Seo, H., Alexander, M. A., & Saba, V. S. (2023). An evaluation of eight global ocean reanalyses for the Northeast US continental shelf. Progress in Oceanography, 219, 103126."}, "author_response": "Point-to-point responses to the reviewers’ comments\nOn\n“Shoreward Shift of Oceanic Mesoscale Activity in a Warming World”\nWe wish to thank the Editor and two anonymous reviewers for their valuable time,\ncareful evaluation, and constructive comments. We have taken all comments seriously\nand have modified the manuscript accordingly. Following are our point-to-point\nresponses, where the comments are in black and our responses are in blue.\nReviewer #1 (Remarks to the Author):\nMajor issues:\n1. GLORYS12V1 is an oceanic reanalysis assimilating along-track satellite altimetry\nobservations of SSH. I suggest the authors use Variance (SSH_eddy) or EKE from\nsatellite altimetry (AVISO), as a ground truth, to validate the reanalysis results.\nResponse: As suggested, we checked EKE from satellite altimeter data. From Figure\nR1 (AVISO) and Figure R2 (CMEMS), it is evident that pronounced shoreward shifts\nof EKE centroids exist in most regions, consistent with the results from GLORYS12V1\n(Fig. 1 in the text). The global averaged shoreward shift speeds are 33.1±15.1 km per\ndecade and 37.0± .. km per decade from AVISO and CMEMS respectively, also\nconsistent with the value ( 2.7 ±1 .1 km per decade) estimated from GLORYS12V1.\nOverall, the reanalysis data GLORYS12V1 compares well with the satellite altimeter\ndata. The Figs. R1 and R2 are added to the extended data.\nFigure R1. Global trends of EKE and their zonal centroid shifts from 1993 to 2022 from\nAVISO. a, Distribution of surface EKE trends. b-k, Time-series of zonal centroid\nanomalies of EKE (grey) in west boundary regions (b-f), east boundary regions (g-k),\nand the global ocean (l), superimposed with the linear trends. Grey shadings are the 95%\nconfidence intervals for the linear fit based on the Student’s t-test. The figure labeled\nwith * and ** indicates significance at the 90% and 95% confidence level, respectively.\n‘V=’ on subplots ‘b-l’ describes the shift speed in km per decade.\nFigure R2. Same as Fig. R1 but for CMEMS.\n2. The paper is kind of descriptive in explaining the mechanism of the shoreward shift\nof eddies. To make the results explainable, reasonable, and robust, I suggest the authors\ninclude one or two eddy-resolving oceanic model simulations to confirm the observed\nresults. There are several eddy-resolving oceanic model hindcasts from 1950s-present.\nThey have dynamical advantages to investigate the oceanic eddies in response to the\nlong-term change of surface winds and radiative fluxes (atmospheric forcing, including\nrecent GHG radiative fluxes).\nResponse: As suggested, an ocean hindcast (OGCM for the Earth Simulator, OFES)\ndata at a horizontal simulation of 0.1 degrees (https://www.jamstec.go.jp/ofes/ofes.html)\nis analyzed to further validate our findings. Figure R3 shows shoreward shift trends of\nEKE centroids in most regions from OFES simulation. The global average trend is\n35.3±15.3 km per decade, consistent with our previous findings, thus reinforcing the\nrobustness of our conclusion. Figure R3 is added to the extended data as well.\nWe agree that the eddy-resolving oceanic models have dynamic advantages to\ninvestigating the oceanic eddies in response to long-term changes of surface winds and\nradiative fluxes. However, the results might rely on a single model performance and be\nbiased. To better understand the underlying processes accounting for the shoreward\nshifts (i.e. intensification) of the oceanic mesoscale activity, we calculate the global\ntrends of the primary source terms responsible for EKE intensification, including\nsurface wind power (WP), baroclinic energy conversion rates (BC), and barotropic\nenergy conversion rates (BT) from GLORYS (Figure R9, Wang et al. 2022 and Wang\net al. 202 ). Calculation details can be referred to the updated method section. Figure\nR9 shows the global distribution of the rates of changes in surface WP, BC, and BT.\nClearly, the zonal centroids of WP and BC have a significant shoreward shift trend (Fig.\n9b and d), while the BT shows an insignificant tendency of offshore shift (Fig. 9f). Both\nsurface WP and BC contribute to the EKE onshore migration. The changes of surface\nWP is primarily caused by enhanced wind stress near boundaries (as shown in Fig. R6),\nwhile the enhanced BC is associated with onshore intensification of western boundary\ncurrents and the enhanced stratification in the open ocean under global warming, as\ndescribed in Yang et al. (2025). Furthermore, the shoreward shift of WP is about .3\ntimes larger than that of BC, indicating that changes in wind stress are dominant for\nchanges in oceanic mesoscale activity. Discussion about the underlying processes\naccounting for the global oceanic mesoscale activity changes is added in the text.\nFigure R3. Same as Fig. R1 but for OFES.\n3. Before discussing the projection of CMIP6 models under GHG warming scenario,\nthe authors need to check the performance of the CMIP6 historical runs on the observed\nshoreward shift of eddies over recent decades. Based on the comparison between\nhistorical runs and observations, and projections, the authors can discuss the relative\nroles of internal vs. external forcings on the shoreward shift.\nResponse: This is a great point and we agree. As suggested, the historical runs from\nCMIP6 HadGEM3 and FGOALS data with 10 km horizontal resolution are checked.\nThe HadGEM3 dataset spans a 65-year period from 1950 to 201 (Figure R ), and the\nFGOALS spans from 195 to 201 (Figure R5). The shoreward shifts of the EKE\ncentroids appear in most regions, generally consistent with satellite altimeter and ocean\nreanalysis. It is noteworthy that the global averaged shift speed of EKE centroids from\nCMIP6 historical runs are markedly smaller than those from the observational and\nreanalysis datasets, implying that the slow shift speed of |∇SST| in CMIP6 projections\nmay be caused by discrepancies of CMIP6 models. The discrepancies may come from\nunresolved processes in climate models. The climate-coupled climate models need to\nbe further improved. Even so, the comparison between historical runs and observations,\nand projections indicates the continuous shoreward shift in the warming world,\nimplying the importance of external forcings.\nFurthermore, the mechanism analysis (Fig. R9) reveals that enhanced wind stress and\nenhanced baroclinic energy conversion contribute to the EKE shoreward shifts. These\nenhancements are mainly induced by global warming, as the energy redistribution\n(Wang et al., 202 ) and the enhanced stratification (Yang et al., 2025). Text is revised\nto add the comparison and clarify relative roles of internal vs. external forcings on the\nshoreward shift.\nFigure R . Same as Fig. R1 but for FGOALS.\nFigure R5. Same as Fig. R1 but for HadGEM3.\nBased on the present paper, the shoreward shift in the projections (~3km/dec) is much\nweaker than the recent observations (~ 0km/dec). If the simulated results in historical\nruns are comparable with the observations, does it suggest that the recently observed\nshoreward shift might be dominated by other reasons, such as internal variability, but\nnot the GHG warming. Please discuss this issue.\nResponse: The comparison shows the shoreward shift speeds in the CMIP6 historical\nruns are much smaller than those from observation data and reanalysis data. The small\nshift speeds in coupled climate simulations may be due to discrepancies of CMIP6\nmodels. Even so, the consistent shoreward shift trends in historical runs and future\nprojections suggest the importance of GHG warming. Discussion is added in the text\naccordingly.\nSpecific comments:\n. L95: Fig. S2?\nResponse: Thanks. Yes, Fig. S2. Text is revised accordingly.\n5. L12 : Can you show any evidence from observations or just list some literatures for\nthis hypothesis? By the way, Fig. S2 can suggest the increase in the alongshore wind in\nthe eastern boundary from the SST perspective. But I would like to see some direct\nevidence.\nResponse: Thanks. To show direct evidence, we plot the trend of wind stress from 1993\nto 2022 from ORAS5 dataset (Fig. R6). It is clear that the wind stress is enhanced along\nmost of the eastern and western boundaries, primarily resulted from global warming\n(Yang et al., 2020; Chen et al., 2025).\nFigure R6. Global trends of wind stress from 1993 to 2022.\n6. L129-130: The authors need to notice and clarify that the warming is faster near the\nwestern boundary (Fig. S2) than the open oceans.\nResponse: Thanks for your suggestion. We have clarified it in the revised text.\n7. L131: how is the sandwich-like structure? Please specify it.\nResponse: Thanks. The sandwich-like structure means that the |∇SST| has a negative\ntrend near the current axis region with two positive trends in two flanks of the negative\ntrend in the western boundary current regions, such as the Kuroshio Current region and\nthe Gulf Stream region. Text is revised to clarify.\n.. L121-13.: I feel this part is a little confusing. Do you want to propose that the\nshoreward shift in the west part of oceans is due to broadening of western boundary\ncurrents, while the shoreward shift in the east part is due to the increase in the\nalongshore winds, stronger upwelling and hence the intensified baroclinic instability?\nResponse: Yes, we suggest here that the shoreward shift of EKE in the eastern boundary\nregions is mainly due to the increase in the alongshore winds, stronger upwelling and\nhence the intensified baroclinic instability. In the western boundary regions, the causes\nare likely to be more complex, including an increase in alongshore winds, expansion\nand shoreward shift of the western boundary currents. A recent study by Yang et al.\n(2025) suggests that enhanced ocean stratification under global warming leads to\nonshore intensification of subtropical western boundary currents, thereby promoting a\nshoreward shift in ocean mesoscale activity. We have rephrased this part in the revised\nmanuscript.\n9. L139: Please add some comparisons and discussions of the historical runs.\nResponse: Thanks. Text is revised as suggested.\n10. L15 : Fig. k?\nResponse: Thanks for your comment. Yes, it is Fig. k, and the text is revised\naccordingly.\n11. L19 : The captions in Fig. S are too simple, which makes readers confused and\nthe figure useless in the statement. I cannot draw a conclusion that the shoreward shift\noccurs in deeper layers from the figure. Need to inform readers the data origin.\nResponse: Thanks. The GLORYS data is used. We agree that the figure is confusing.\nSince it is not useful, the figure is deleted in the revision.\n12. L209: Where do you use CESM simulations? You mean CMIP6 models?\nResponse: Yes. The text is revised to clarify.\n13. L210: I am wondering why using ORAS5 in the paper? For the broadening index\nof the western boundary currents? GLORYS has 3-D currents. Please inform data\norigins in some figures if it is not based on the major data of the paper.\nResponse: ORAS5 data is used since it spans a longer time period than GLORYS. In\nthe revision, the broadening indexes are calculated for both datasets. The regions for\nboundary current calculation are shown in Fig. R7a, following Yang et al. (2025).\nFigure R7b-k shows broadening of most western and eastern boundary currents in both\ndatasets, with the broadening being more pronounced in ORAS5. This widespread\nwidening suggests an impact on the redistribution of upper-ocean mesoscale activity.\nText is revised accordingly, and the data origins are informed in the revised caption.\nFigure R7. The regions for boundary current calculation (a), and the time series and\nlinear trends of the broadening index from ORAS5 (blue) during 195.-2022 and\nGLORYS (red) during 1993-2022 in all the five WBC regions (b-f), namely the\nKuroshio Current region (KC, 119°E-12.°E, 23°N-31°N), the Gulf Stream region (GS,\n72°W-.1°W, 31°N-39°N), the Agulhas Current region (AC, 2 °E-35°E, 26°S-36°S),\nthe Eastern Australia Current region (EAC, 1 6°E-157°E, 25°S-3 °S), the Brazil\nCurrent region (BC, 39°W-50°W, 20°S-30°S) and all the five EBC regions (g-k),\nnamely the California Current region (Ca, enclosed by (105°W, 20°N) (120°W, 20°N)\n(135°W, 0°N) and (120°W, 0°N)), the Canary Current region (Cn, 5°W-20°W, 20°N-\n50°N), the Western Australia Current region (WAC, 100°E-120°E, 10°S-35°S),\nthe Humboldt Current region (Hu, 70°W-90°W, 20°S-50°S) and the Benguela Current\nregion (Be, 5°E-20°E, 10°S-35°S). The first and the second parentheses in the title of\neach subplot show the confidence level of the linear trends of ORAS5 and GLORYS\ndata, respectively (based on the student’s t test). * and ** represents confidence level\nover 90% and 95%, respectively. ‘-’ represents confidence level lower than 90%.\n1 . L2.7: Are the results sensitive to the cutoff wavelength of filtering?\nResponse: To check, we tested several cutoff wavelengths of filtering, including 50 km,\n100 km, 200 km. The results are consistent with each other, with the global averaged\nspeeds of EKE zonal centroids 7, .1, and 1.9 km per decade, respectively. Thus,\nthe results are not sensitive to the cutoff wavelength. The selection of the cutoff\nwavelength is explained in the method.\n15. L2.9: For the proxy of mesoscale activity based on SST, why not using variance of\n150km high-pass filtering SST? |∇SST| is not only related to mesoscale activity, but\nalso largescale fronts, upwelling, and air-sea heat fluxes.\nResponse: As suggested, we have recalculated the ∇SST with 150km high-pass filtering.\nText and Fig. 2 are revised accordingly.\nReferences:\nChen, W. B. (2025). Gradual strengthening of global oceanic surface winds: Correlations with\nsea surface temperature and implications for wind power extraction. Heliyon, 11( ).\nWang, Q., & Tang, Y. (2022). The interannual variability of eddy kinetic energy in the Kuroshio\nlarge meander region and its relationship to the Kuroshio latitudinal position at 1 0\nE. Journal of Geophysical Research: Oceans, 127(2), e2021JC017915.\nWang, S., Jing, Z., Wu, L., Sun, S., Chen, Z., Ma, X., & Gan, B. (202 ). A more quiescent deep\nocean under global warming. Nature Climate Change, 14(9), 961-967.\nYang, H., Guo, H., Chen, Z., Cai, W., Wu, L., Cai, J., ... & Wang, Y. (2025). Onshore\nintensification of subtropical western boundary currents in a warming climate. Nature\nClimate Change, 1-7.\nYang, H., Lohmann, G., Krebs‐Kanzow, U., Ionita, M., Shi, X., Sidorenko, D., ... & Gowan,\nE. J. (2020). Poleward shift of the major ocean gyres detected in a warming\nclimate. Geophysical Research Letters, 47(5), e2019GL0.5.6.. \nReviewer #2 (Remarks to the Author):\nMajor Comments\nRegarding 1), Hewitt et al. (2020) describe current work on parameterizing the ocean\nmesoscale in earth system models. The 0.25° horizontal resolution models are “eddy\npresent;” however challenges remain near high latitudes where the Rossby radius is\nsmaller (See Chelton et al. (199.) for an estimate of the first Baroclinic Rossby radius\nglobally) and the eddies in the models may have characteristics different than “eddy\nrich” models (1/10° models). Climate models, especially the ensemble projects like\nCMIP6 are designed to estimate the mean climate in the future well. The climate models,\nin their current setup, were not designed to study the mesoscale variability. In the paper,\nthe authors find some evidence of shoreward movement, but not in the western or\neastern north Atlantic, the western north Pacific, or the western South Indian. I do not\nthink climate models, as currently set up, should be used to ask questions about shifting\nlocations of mesoscale variability – it is not the right tool.\nResponse: Thank you for point this out and we agree.\nTo check the performance of the CMIP6 simulation, we analyze two high-resolution\nsimulations from the CMIP6 HighResMIP experiments with 1/10° horizontal resolution,\nnamely FGOALS-f3-H (FGOALS) and HadGEM3-GC31-HH (HadGEM3). The\nhistorical experiments (from 1950 to 201 ) and future experiments under RCP..5\nscenario (from 2015 to 2050) are analyzed.\nFor the historical experiments, both HadGEM3 and FGOALS data reveal that the\nglobal-averaged zonal centroids of EKE and |∇SST| are shifting shoreward (Fig. R.).\nThe shift speed of the zonal centroid of EKE outpaces that of |∇SST|. However, both\nspeeds are systematically lower than those of the observation data and the reanalysis\ndata, probably due to the limited spatiotemporal representation of mesoscale activities\nin climate models.\nFurthermore, HadGEM3 and FGOALS diverge significantly in the zonal shift feature\nof EKE and |∇SST| under RCP..5 scenario. The zonal centroid of EKE simulated by\nHadGEM3 has an insignificant offshore shift trend, while FGOALS shows a significant\nshoreward shift trend. Therefore, although the results of historical experiments are\ngenerally consistent with the conclusions of observational and reanalysis data, this\ninter-model discrepancy highlights substantial uncertainties in projecting future zonal\nredistributions of mesoscale activity using current climate models. The discrepancies\nmay come from unresolved processes in climate models. The coupled climate models\nneed to be further improved.\nPrevious CMIP6 analysis is deleted. Discussion about the CMIP6 HighResMIP\nexperiments are added in the text.\nFigure R. Values of the shift speed of zonal centroids of EKE (green bar) and |∇SST|\n(yellow bar) from CMIP6 HighResMIP historical and future experiment.\nRegarding point 2) The authors have defined eddy kinetic energy by separating the\nsmall and large scale through a 150-km high pass filter. However, they use results of\nSST gradients without discussing scale separation. If this is on purpose, the authors are\nalso tracking the movements of currents like the western boundary currents. If authors are trying to complete a complementary analysis to their EKE calculation, the authors should also filter the SST field to isolate the mesoscale eddies, as done in Martinez-\nMoreno et al. (2021). In general, the methods lack description of how the trends in EKE\nand |∇SST| were calculated. Is the average (in time) over each year calculated first for\nEKE and |∇SST|? How are the trend lines subsequently presented as “zeroed” such that\nthey pass through “zero shift” approximately halfway through the time series? For the\nstatistics, the sample number should be reported for the trend significance tests. In\naddition, the confidence interval on the estimate of the linear trend should be reported\nin the text. For example, “ 7 km +/- (confidence interval shift) in the zonal centroid.”\nResponse: As suggested, we filter the SST field through a 150-km high pass filter, and\nrecalculate the |∇SST|. The text and Figure 2 are updated accordingly.\nIn terms of the methods, the average (in time) over each year is calculated first.\nThe description about how the trends in EKE and |∇SST| were calculated is added in\nthe Methods section.\nThe trends of the zonal centroids presented as “zeroed” because we used the\nanomalies of the zonal centroids. Figure caption is updated to clarify.\nThe sample number equals to the length of the time series (30 years), which is\nadded in the text. The confidence interval on the estimate of the linear trend is added in\nthe revised figures.\nThe GLORYS dataset has been shown to do well in the NW Atlantic shelf (Castillo-\nTrujillo et al. 2023). Other comparisons of GLORYS to observational data including\nin-situ and satellite data exist for other regions. This suggests that GLORYS could be\naccurately representing the surface ocean for the past 30 years. GLORYS uses a\nnumerical model on a 1/12° horizontal grid which allows it to resolve more eddies. I\nthink it is reasonable to present the results of the EKE calculated in GLORYS as\nrepresenting what may have occurred over the past 30 years. As Todd et al. (2023)\nmeasured a shoreward movement of the Gulf Stream, it seems possible that shift in the\nmesoscale EKE has occurred. Have the Western Boundary currents globally all shifted\npoleward and shoreward in GLORYS? More work on understanding why the currents\nand the mesoscale variability have shifted would make the paper more novel.\nResponse: A recent study (Yang et al., 2025) proposed that all subtropical western\nboundary currents are strengthening onshore due to enhanced stratification in the open\nocean under global warming. This corroborates our findings, since the onshore\nintensification of the western boundary currents can enhance the associated ocean\nmesoscale activities onshore.\nTo better understand why the mesoscale variability shifts (i.e. intensified)\nshoreward, we calculate the global trends of the primary source terms responsible for\nEKE intensification, including surface wind power (WP, Eq.1), baroclinic energy\nconversion rates (BC, Eq.2), and barotropic energy conversion rates (BT, Eq.3) from\nGLORYS (Wang et al. 2022 and Wang et al. 202 ).\n𝑊𝑃 = 1 𝜏′ ∙𝑢′ (1)\n𝑤 𝑠\n𝜌0\n𝐵𝑇 = −(𝑢′𝑢′𝜕𝑢̅ +𝑢′𝑣′( 𝜕𝑢̅ + 𝜕𝑣̅ )+𝑣′𝑣′𝜕𝑣̅ ) (2)\n𝜕𝑥 𝜕𝑦 𝜕𝑥 𝜕𝑦\n𝐵𝐶 = −\n𝑔 (𝑢′𝜌′𝜕𝜌̅ +𝑣′𝜌′𝜕𝜌̅\n) (3)\n𝜌0 𝑑\n𝑑\n𝜌̅\n𝑧\n𝜕𝑥 𝜕𝑦\nwhere 𝜏 is the surface wind stress and 𝑢 is the horizontal current at the sea surface.\n𝑤 𝑠\nu and v are the component of the sea surface current. Here g is the acceleration of\ngravity, 𝜌 = 1025 kg/𝑚3 is the reference density. The overbar (prime) represents the\n0\nlow-pass (high-pass) filter.\nFigure R9 shows the global distribution of the rates of changes of surface WP, BC\nand BT. The zonal centroids of WP and BC have a significant shoreward shift trend\n(Fig. 9b and d). On the other hand, the BT shows an insignificant tendency of offshore\nshift (Fig. 9f). Both surface WP and BC contribute to the EKE onshore shift. The\nchanges of surface WP is primarily caused by enhanced wind stress near boundaries (as\nshown in Fig. R6), while the enhanced BC is associated with onshore intensification of\nwestern boundary currents and the enhanced stratification in the open ocean, as\ndescribed in Yang et al. (2025). Furthermore, the shoreward shift of WP is about .3\ntimes larger than that of BC, indicating that changes in wind stress are dominant for\nchanges in oceanic mesoscale activity. Discussion about the underlying processes\naccounting for the global oceanic mesoscale activity changes are added in the text.\nFigure R9 Global distribution of trends of surface wind power (WP, a), baroclinic\nconversion rates (BC, c), and barotropic conversion rates (BT, e) derived from\nGLORYS. Global-averaged time-series of zonal centroid anomalies of surface wind\npower (d), baroclinic instability (f) and barotropic instability (h).\nOverall, with only the GLORYS result one in which I feel confident, I suggest rejection\nfor the paper and revision and resubmission in another Nature journal rather than Nature\nCommunications.\nResponse: We thank the reviewer for raising the critical point. To make the results more\nrobust, we added analysis of two satellite datasets (AVISO, Fig. R1 and CMEMS, Fig.\nR2), one eddy-resolving ocean hindcast (OFES, Fig. R3), and two CMIP6 historical\nruns with 10 km horizontal resolution (FGOALS, Fig. R and HadGEM3, Fig R5).\nIncluding GLORYS and OISST, all datasets show a significant shoreward shift trend,\nespecially in the western boundary current regions (Figure R10). The figure is added in\nthe revised text.\nFigure R10. The shift trend and speed of EKE and |∇SST| zonal centroid in different\ndatasets\nMinor Comments\nLine 33: “known as the ‘frontline of climate change’” should be removed because it is\nvague in a scientific context (here in reference to eddies)\nA: Thanks. The sentence is deleted as suggested.\nLine 6: remove that the eddies “have become richer” over recent decades. Instead state\nthat the eddy variability is higher.\nA: Thanks. Text is revised as suggested.\nLine 293-296: Since you have described the zonal centroid as x-bar, keep similar\nnotation here and describe “x” as the zonal centroid, rather than labeling “y” the zonal\ncentroid and “x” as time. Use another variable name for time.\nA: Thanks. The variable name for the zonal centroid is changed to 𝑥̅, and the variable\nname for time is changed to T.\nLine 300-301: describe what y-hat_l and x-hat_l are in the equations as well as what n\nis.\nA: Thanks. The detailed description of these variables are added in the Methods. n\ndenotes the sample number. Here it is the length of the time series. Text is revised\naccordingly to clarify.\nFigures 1,2, and S1: Describe in the caption that “V=#” on subplots “b-i” are describing\nthe shift speed in km per decade.\nA: Thanks. The description is added as suggested in Figures 1 and 2. The original figure\nS1 is removed.\nExtended Data Fig. S : a colorbar, x-axis labels, and y-axis labels are needed for each\nof the subplots. In addition, red and black vertical temperature gradients are not readily\napparent. Perhaps the figure and figure caption here are mismatched?\nA: We apologize. This figure is not clear and misleading, so it is deleted in the revision.\nRemarks on code availability:\nI would recommend more detailed methods section. Code is not essential for\nreproducing the results. Due to the large amount of data files, this manuscript is not\nsuited for install and run type verification based on uploaded codes.\nResponse: Thanks for your suggestion, we have revised the Methods section to make it\nmore detailed.\nReferences:\nChen, W. B. (2025). Gradual strengthening of global oceanic surface winds: Correlations with\nsea surface temperature and implications for wind power extraction. Heliyon, 11( ).\nWang, Q., & Tang, Y. (2022). The interannual variability of eddy kinetic energy in the Kuroshio\nlarge meander region and its relationship to the Kuroshio latitudinal position at 1 0\nE. Journal of Geophysical Research: Oceans, 127(2), e2021JC017915.\nWang, S., Jing, Z., Wu, L., Sun, S., Chen, Z., Ma, X., & Gan, B. (202 ). A more quiescent deep\nocean under global warming. Nature Climate Change, 14(9), 961-967.\nYang, H., Guo, H., Chen, Z., Cai, W., Wu, L., Cai, J., ... & Wang, Y. (2025). Onshore\nintensification of subtropical western boundary currents in a warming climate. Nature\nClimate Change, 1-7.\nYang, H., Lohmann, G., Krebs‐Kanzow, U., Ionita, M., Shi, X., Sidorenko, D., ... & Gowan,\nE. J. (2020). Poleward shift of the major ocean gyres detected in a warming\nclimate. Geophysical Research Letters, 47(5), e2019GL0.5.6.. \nReferences:\nHelene T Hewitt, Malcolm Roberts, Pierre Mathiot, Arne Biastoch, Ed Blackley, Eric\nP Chassignet, Baylor Fox-Kemper, Pat Hyder, David P Marshall, Ekaterina\nPopova, Anne-Marie Treguier, Laure Zanna, Andrew Yool, Yongqiang Yu,\nRebecca Beadling, Mike Bell, Till Kuhlbrodt, Thomas Arsouze, Alessio Bellucci,\nFred Castruccio, Bolan Gan, Dian Pustrasahan, Christopher D Roberts, Luke Van\nRoekel, and Qiuying Zhiang. Resolving and parameterising the ocean mesoscale\nin earth system models. Current Climate Change Reports, September 2020. URL:\nhttps://doi.org/10.1007/s 06 1-020-0016 -w\nChelton, D. B., DeSzoeke, R. A., Schlax, M. G., El Naggar, K., & Siwertz, N. (199.).\nGeographical variability of the first baroclinic Rossby radius of deformation.\nJournal of Physical Oceanography, 2.(3), 33- 60.\nCastillo-Trujillo, A. C., Kwon, Y. O., Fratantoni, P., Chen, K., Seo, H., Alexander, M.\nA., & Saba, V. S. (2023). An evaluation of eight global ocean reanalyses for the\nNortheast US continental shelf. Progress in Oceanography, 219, 103126."}, {"round": 2, "reviewer_comments": {"reviewer_1": "I am pleased that the authors have included more datasets (reanalysis, observational and numerical), which enhance the result as a robust one. They also added a discussion that key drivers include enhanced coastal wind power and baroclinic energy conversion. While the topic is novel and datasets are enough, major revisions are still required to address critical gaps in definition, interpretation, dynamics, and broader context.\n\nFirst of all, the meaning of zonal centroids of EKE or SST gradient is too extensive or vague and hard to interpret. Because the authors calculated this proxy within a wide region from the coast to the middle of the open ocean, the zonal centroid shift can mean shift from middle to near-shelf (this part is very wide), or from near-shelf to coast. I think the authors should focus on the latter component, as they claim the impacts on coastal ecosystem or cross-shelf exchange. However, when the paper states that the shoreward shift is ~40 km/decade over such a wide ocean basin, I am confused what does it exactly mean.\nTake WSP and WSA in Fig. 1a as an example. The present proxy suggests that the eddy activity moves shoreward in WSP, while moves offshore in WSA. However, in fact, as shown in Fig. 1a, the coastal EKE in WSP has decreased while increased in WSA. It is a conflict with the zonal centroid proxy. So, I suggest the authors minimize the calculated region from the coast to the near shelf, such as 200 km distance away from the 200m isobath. The authors can still calculate the zonal centroid, but I suggest they just calculate the trend of EKE within the region from the coast to the near shelf, which is more straightforward and simpler.\n\nSecond, the calculation is not rigorous from Eq. 4 to 8. The turbulent energy or stress is defined in average temporally or spatially. That is, when two turbulent variables multiply each other (energy or stress), the result should be averaged in space or time. Therefore, EKE should be 0.5*(bar_u’^2+ bar_v’^2), and u’v’ should be bar_u’v’, and tau’u’ should be bar_tau’u’. The bar here denotes 150km spatially low-pass filtered as the authors define. The present spatial patterns in Figs. 2 and 4 are too noisy and lack of statistical significance in space.\n\nLine 350: Sample number used for significance test should be the effective sampling number, but not just the time length.\nFor the EKE source terms, note that GLORYS, as a reanalysis dataset, is not physically-conserving. While Renualt et al. (e.g., 2016) have proved that the wind stress works as a sink of the EKE because of the air-sea interface friction, Fig. 4a shows mostly positive wind work as a source to EKE in the western boundary regions; why?\nRenault, L., Molemaker, M. J., McWilliams, J. C., Shchepetkin, A. F., Lemarié, F., Chelton, D., ... & Hall, A. (2016). Modulation of wind work by oceanic current interaction with the atmosphere. Journal of Physical Oceanography, 46(6), 1685-1704.\nRenault, L., Molemaker, M. J., Gula, J., Masson, S., & McWilliams, J. C. (2016). Control and stabilization of the Gulf Stream by oceanic current interaction with the atmosphere. Journal of Physical Oceanography, 46(11), 3439-3453.\n\nIn Fig. 5, most broadening indices are overestimated in ORAS5. And most trends in GLORYS are not significant. That means the broadening of boundary currents is not robust.\n\nThe authors only use two coupled model (HadGEM3 and FGOALS), which cannot conclude an instructive result. I suggest to remove this part and change the title to “… over the altimeter record” or “the recent three decades”.\n\nAt last, there are many typos throughout the manuscript, such as many redundant spaces in many places (line 18, 99, 146, 176)\nLine 31: ‘thermal energy’, you mean ‘heat’?\nLine 76: I do not see positive trend in the Coral Sea.\nLine 83-84: Cannot see clear trend patterns in western boundary currents.\nLine 85: you mean faster warming in coastal regions compared to adjacent open oceans?\nLine 103: I do not see CMIP6 in the figure. You mean CMEMS?\nLine 111: shoreward\nLine 246: shoreward\nLine 321: Additionally\nLine 328: monthly? Should be daily", "reviewer_2": "[Editorial Note: See end of file]"}, "author_response": "Point-to-point responses to the reviewers’ comments\nOn\n“Shoreward Shift of Oceanic Mesoscale Activity over the Recent Three Decades”\nWe wish to thank the Editor and two anonymous reviewers for their valuable time, careful\nevaluation, and constructive comments. We have taken all comments seriously and revised the\nmanuscript accordingly. Following are our point-to-point responses, where our responses are\nin blue, and the revised text in the manuscript is indicated in blue as well.\nReviewer #1 (Remarks to the Author):\nI am pleased that the authors have included more datasets (reanalysis, observational and numerical),\nwhich enhance the result as a robust one. They also added a discussion that key drivers include\nenhanced coastal wind power and baroclinic energy conversion. While the topic is novel and\ndatasets are enough, major revisions are still required to address critical gaps in definition,\ninterpretation, dynamics, and broader context.\n1. First of all, the meaning of zonal centroids of EKE or SST gradient is too extensive or vague and\nhard to interpret. Because the authors calculated this proxy within a wide region from the coast to\nthe middle of the open ocean, the zonal centroid shift can mean shift from middle to near-shelf (this\npart is very wide), or from near-shelf to coast. I think the authors should focus on the latter component,\nas they claim the impacts on coastal ecosystem or cross-shelf exchange. However, when\nthe paper states that the shoreward shift is ~40 km/decade over such a wide ocean basin, I am\nconfused what does it exactly mean. Take WSP and WSA in Fig. 1a as an example. The present\nproxy suggests that the eddy activity moves shoreward in WSP, while moves offshore in WSA.\nHowever, in fact, as shown in Fig. 1a, the coastal EKE in WSP has decreased while increased in\nWSA. It is a conflict with the zonal centroid proxy. So, I suggest the authors minimize the calculated\nregion from the coast to the near shelf, such as 200 km distance away from the 200m isobath. The\nauthors can still calculate the zonal centroid, but I suggest they just calculate the trend of EKE within\nthe region from the coast to the near shelf, which is more straightforward and simpler.\nA: Thanks for your insightful comments. As suggested, taking EKE as an example, we selected the\nnearshore region (black boxes in Fig. R1a) and calculated the EKE trends as well as the shifts of the\nassociated zonal centroids. The shoreward shifts of EKE appear in most of the nearshore regions\n(Fig. R1b-k), consistent with the results from the original large domain (Fig. 1b-k).\nWe apologize for not providing a clear explanation for choosing a wide basin region. Since the\nwide basins split the ocean basins in the middle, we can quantify the overall distribution of\nmesoscale activities across the entire basin, while selecting a narrow coastal region only reflects the\nEKE variations near the coasts. Thus, we prefer to keep the large domain and add the EKE analysis\nfor the nearshore region in the text. The global-averaged shoreward shift of the zonal centroid with\na ~35 km/decade speed indicates that the overall distribution of mesoscale activities across the entire\nocean basin has moved shoreward with a speed of ~35 km/decade. Text is revised accordingly and\nFigure R1 is added in the supplementary.\nFigure R1. Global trends of global sea surface eddy kinetic energy (EKE) and their zonal\ncentroid shifts from GLORYS for nearshore regions. a, Distribution of global EKE trends. b-\nk, Time-series of zonal centroid anomalies (compared to the mean location over the time period)\nof EKE (grey) in west boundary regions (b-f), east boundary regions (g-k), and the global ocean. l,\nsuperimposed with the linear trends. Grey shadings are the 95% confidence intervals for the linear\nfit based on the Student’s t-test. The figure labeled with * and ** indicates significance at the 90%\nand 95% confidence level, respectively. ‘V=’ on subplots ‘b-l’ describes the shift speed in km per\ndecade.\n2. Second, the calculation is not rigorous from Eq. 4 to 8. The turbulent energy or stress is defined\nin average temporally or spatially. That is, when two turbulent variables multiply each other (energy\nor stress), the result should be averaged in space or time. Therefore, EKE should be 0.5*(bar_u’^2+\nbar_v’^2), and u’v’ should be bar_u’v’, and tau’u’ should be bar_tau’u’. The bar here denotes 150km\nspatially low-pass filtered as the authors define. The present spatial patterns in Figs. 2 and 4 are too\nnoisy and lack of statistical significance in space.\nA: Thanks for your comments. As suggested, we have added a low-pass filtered (150 km, consistent\nwith the spatial high-pass filtering) after two turbulent variables multiply each other. Figure S2\nshows the new EKE trend of GLORYS. It is less noisy and the statistical significance is added in\nFigure 1 and 2. The reason of noisy Figure 4 is that the trend of energy term is spatially\nheterogeneous. The significant change mainly focus on the nearshore, so we do not add the statistical\nsignificance to Figure 4 to ensure the readability of the nearshore area but explained in the text. The\ncorresponding figures and text are revised in the manuscript.\nFigure R2. Global trends of global sea surface eddy kinetic energy (EKE) and their zonal\ncentroid shifts from GLORYS. a, Distribution of global EKE trends. b-k, Time-series of zonal\ncentroid anomalies (compared to the mean location over the time period) of EKE (grey) in west\nboundary regions (b-f), east boundary regions (g-k), and the global ocean. l, superimposed with the\nlinear trends. Grey shadings are the 95% confidence intervals for the linear fit based on the\nStudent’s t-test. The figure labeled with * and ** indicates significance at the 90% and 95% confidence level, respectively. ‘V=’ on subplots ‘b-l’ describes the shift speed in km per decade.\n3. Line 350: Sample number used for significance test should be the effective sampling number, but\nnot just the time length.\nA: Yes, the effective sample number is n-2, but not just the sample number n. The significance\ntesting methodology has been revised accordingly to clarify.\n4. For the EKE source terms, note that GLORYS, as a reanalysis dataset, is not physically-conserving.\nWhile Renualt et al. (e.g., 2016) have proved that the wind stress works as a sink of the\nEKE because of the air-sea interface friction, Fig. 4a shows mostly positive wind work as a source\nto EKE in the western boundary regions; why?\nA: Thanks for the comments. Renualt et al. (2016a) demonstrated that the current feedback has an\neffect on the surface stress, modulating energy transfer from the atmosphere to the ocean. Renualt\net al. (2016b) further indicated that the current feedback can induce eddy killing effect in the Gulf\nStream region. However, it doesn’t mean the wind stress only works as a sink of the EKE. The\nnegative eddy wind work (eddy killing effect) primarily dominates in regions with strong currents\nwhile the eddy wind work can be positive in coastal regions where the winds can drive small scale\ncurrents. For example, Renualt et al. (2016a) estimated that the total eddy wind work is 16.9 ×\n106 m5 s−3 over the whole U.S. West Coast domain during the 1995–99 period, while the energy\nconversion induced by the current feedback is -0.9× 106 m5 s−3.\nWe agree that GLORYS is not physically-conserving due to data assimilation. To check the eddy\nwind work estimated from GLORYS, we compare our estimated global mean distribution of eddy\nwind work (Fig. R3) with the eddy wind power input estimated directly from satellite data in Rai et\nal. (2021, see their Fig. 2a). The patterns are generally consistent, such as negative values along\nmajor ocean currents while positive values near the coasts. However, the eddy wind work near the\nGulf Stream is more positive in our study, likely due to the differences in spatial filtering scales and\ndata sources. Note that 260 km is used for filtering in Rai et al (2021).\nFuthermore, Figure 4a plot the trends of the eddy wind work. Both positive and negative values are\npresent near the western boundary regions, reflecting changes in eddy wind work over the recent\nthree decades. Text is revised accordingly to clarify.\nFigure R3. Global mean distribution of surface wind power from 1993 to 2022 estimated from\nGLORYS and ORAS5.\nReference\nRai S, Hecht M, Maltrud M, et al. (2021). Scale of oceanic eddy killing by wind from global satellite\nobservations. Science Advances, 7(28): eabf4920.\nRenault, L., Molemaker, M. J., McWilliams, J. C., Shchepetkin, A. F., Lemarié, F., Chelton, D., ...\n& Hall, A. (2016a). Modulation of wind work by oceanic current interaction with the\natmosphere. Journal of Physical Oceanography, 46(6), 1685-1704.\nRenault, L., Molemaker, M. J., Gula, J., Masson, S., & McWilliams, J. C. (2016b). Control and\nstabilization of the Gulf Stream by oceanic current interaction with the atmosphere. Journal of\nPhysical Oceanography, 46(11), 3439-3453.\n5. In Fig. 5, most broadening indices are overestimated in ORAS5. And most trends in GLORYS\nare not significant. That means the broadening of boundary currents is not robust.\nA: We agree. This part is removed in the revised text.\n6. The authors only use two coupled model (HadGEM3 and FGOALS), which cannot conclude an\ninstructive result. I suggest to remove this part and change the title to “… over the altimeter record”\nor “the recent three decades”.\nA: Thanks for your comment. As suggested, the title is revised to “…. Over the Recent Three\nDecades”. We only use two coupled model because there are only two eddy-resolving simulation\navailable for downloading in CMIP6. The results are only for discussion in the revised manuscript.\n7. At last, there are many typos throughout the manuscript, such as many redundant spaces in many\nplaces (line 18, 99, 146, 176)\nA: Thanks for your comments. We have removed these redundant spaces.\n8. Line 31: ‘thermal energy’, you mean ‘heat’?\nA: Yes, you are right. We have corrected it. Thanks.\n9. Line 76: I do not see positive trend in the Coral Sea.\nA: Thanks for your comment. It is removed.\n10. Line 83-84: Cannot see clear trend patterns in western boundary currents.\nA: Thanks for your suggestion. The sentence about western boundary current is removed.\n11. Line 85: you mean faster warming in coastal regions compared to adjacent open oceans?\nA: Yes, thanks. The sentence is revised for clarification.\n12. Line 103: I do not see CMIP6 in the figure. You mean CMEMS?\nA: Yes, thanks for your comment. Revised.\n13. Line 111: shoreward\nA: Revised. Thanks.\n14. Line 246: shoreward\nA: Revised. Thanks.\n15. Line 321: Additionally\nA: Revised. Thanks for your comment.\n16. Line 328: monthly? Should be daily\nA: Yes, thanks. Revised.\nReviewer #2 (Remarks to the Author):\n1. Please provide equations or the name the code package used for the high-pass spatial filter.\nA: Thanks. The high-pass spatial filter code has been uploaded in the latest code link\n(https://zenodo.org/records/16741928).\n2. I see the edit about averaging in the Methods section. For better clarity, I suggest the following\ngrammar fixes: “When calculating the time series of the zonal centroids, annual mean fields of EKE\nand |∇SST’| are calculated from the monthly mean data first, and then the zonal centroid of each\nsingle year is calculated. Finally, a 30-year time series is computed.”\nA: Thanks for your suggestion. We have rephrased the sentence as suggested.\n3. The figure caption has not been updated. Please update. I suggest language such as “…anomaly\ncompared to the mean location over the time period” to describe what has been plotted.\nA:Thanks for your comment. All related figure captions have been revised as suggested.\n4. What is used as the confidence interval (90%, 95%)? Add to the caption or the methods. Thank\nthe authors for edits. Now that I understand how the averaging has been done, I see the need to\ninclude the impact of autocorrelation in the significance tests. There are many ways to do this, but\nthe effective degrees of freedom may be lower than 30 due to autocorrelation, which is often\nfound in geophysical time-series.\nA: Thanks for your valuable suggestions. The confidence interval here is obtained by the Ordinary\nLeast Squares. To eliminate the influence of autocorrelation, we use the Ljung-Box Q-test in\nMATLAB to determine whether the original sequence is significantly autocorrelated. If so, the\ninfluence of the autocorrelation would be removed using the Newey-West HAC method and then\nanalyzed. The method is revised to clarify.\n5. I would remove the reasoning in this manuscript that all subtropical western boundary currents\nare strengthening onshore due to enhanced stratification in the open ocean as found by Yang et al.\n2025. While Yang 2025 was recently published, the physical oceanography theory is weak, and the\nmechanisms remain to be explained. For example, to conserve PV, a decrease in oceanic scale\nheight under shallow water PV suggests that a parcel of water should move southward to decrease\nits planetary vorticity (f) or reduce its relative vorticity (zeta). Why should a decrease in oceanic\nscale height cause a current to move closer to shore? I did not find a satisfactory explanation in\nYang 2025.\nA: We agree that the mechanisms for onshore strengthening of all subtropical western boundary\ncurrents remain to be explained. The reasoning part is removed as suggested.\n6. I appreciate the calculations done by the authors in the revision. Please add the shift speed and\nconfidence interval to the figure 4 to be consistent with figures 1 and 2. I would also test for\nsignificance using an effective sample size that considers autocorrelation. I assume the same\nhighpass filters were used here, and they should be identified by equation or code package in the\nmethods.\nA: Thanks. The shift speed and confidence intervals have been added in Figure 4 in the revised\nmanuscript. For the significance test, the effective sample size should be n-2 rather than n when\naccounting for autocorrelation. We have accordingly revised both the significance testing\nmethodology and the related content in the revised manuscript. We confirm that the same method\nand filtering scale of 150km was applied in Figure 4, and Methods is revised accordingly for\nclarification.\n7. I believe the paper is strengthened by new figures 3a and 3b. However, please remove the plotting\nof the “mean” as its own separate bar plot (in teal). The mean is not an additional source of data but\na statistical summary of the available datasets. Instead, plot something like a box and whisker\noverlying the bar plots which shows the mean and variability of the datasets. In the case of figure\n3a, the mean and variability would show the summary of the calculation in AVISO, CMEMS, OFES,\nand GLORYS datasets.\nA: Thanks for your suggestion. As suggested, we have replotted Figures 3a and 3b.\nMinor comment:\n8. Line 18: remove the backslash, replace with “and”\nA: Done. Thanks for your comment.\n9. Line 21: because “coastal” is ill-defined, remove and just say “…analysis indicated that a\nshoreward shift of baroclinic energy conversion and wind power under global warming …”\nA: Done. Thanks for your suggestion.\n10. Line 22: I do not feel the authors have established causality, so avoid the word “drive.”\nAppropriate wording could be “has occurred concurrent with the EKE onshore migration.”\nA: Done. Thanks.\n11. Line 65: again with “drive.” Suggest “… both surface wind power and baroclinic energy\nconversion have also shifted onshore.”\nA: Revised. Thanks for your comment.\n12. Line 85: unclear. Perhaps you mean to say “greater thermal amplification is observed near the\ncoasts compared to adjacent open oceans”\nA: Revised. Thanks for your comment.\n13. Line 87: replace “dynamic” with “momentum.” Suggest replacing “thermodynamic” with\n“temperature.”\nA: Thanks for your comment, we have replaced..\n14. Line 89: please add the phrase “…in the global average.” Suggestion: “Coherent shoreward\nshift … appears in the global average across all observational and reanalysis products…”\nA: Rephrased. Thanks for your suggestion.\n15. Line 91: report a statistic representing the variability. 37 km per decade +/- ?? and 42.7 km per\ndecade +/- ??\nA: Thanks. The shoreward shift speed with variability of EKE and ∇SST zonal centroid is 35.4 ±\n22.5 km per decade and 34.4 ±20.3 km per decade, respectively. Text is revised accordingly.\n16. Line 92: OFES is a model hindcast, which is not a reanalysis. Describe OFES as such.\nA: Done. Thanks for your comment.\n17. Line 95: State that the climate models indicate a shoreward shift in EKE but not SST. What is\nthe significance level of the climate model shift rate? Notice that the rate reported is less than a grid\npoint of change per decade for SST.\nA: Thanks. The significance level of all the CMIP6 result is added in the revised Figure 3c. In the\nhistorical experiments, however, the shoreward shift if EKE is only significant in one experiment.\nTherefore, we delete the CMIP6 results, and only discuss CMIP6 implications slightly at the end.\n18. Line 114: The text refers to Extended Data Fig. S1 but that figure has no information of the\nenhanced gradients of SST readily evident to the reader. Please elaborate or include another plot.\nA: Thanks for your comment. The revised Figure 2a can more clearly show the gradients of SST\nnear the shore.\n19. Line 118: Here, I believe the authors should add the following sentence at the end of the\nparagraph: “Overall, observations from satellite observations are consistent with the results of\nGLORYS reanalysis, as well as the model hindcast (OFES).”\nA: Thanks for your suggestion. Done.\n20. Line 124-127: report a rate and confidence interval for the shifting of WP and BC centroids\nA: Done. Thanks for your suggestion.\n21. Line 128-130: This should be rephrased to say that “one possibility for intensified wind stress\nnear coastal boundaries is amplified land-ocean thermal contrast …” Extended Data Fig. S2 does\nnot demonstrate the land-sea amplification in particular.\nA: Thanks for your comment. We have rephrased.\n22. Line 129-132: This has not been demonstrated. Please remove.\nA: Removed. Thanks.\n23. Line 132-135: Please remove this sentence comparing WP and BC. Instead discuss which\nshifting rate matches the rate of EKE and SST shifting better. From the plot, it appears that the BC\nhas not shifted quickly at all.\nA: Thanks. The orginal sentence is removed. The spatial trend distribution of the shoreward shift of\nWP is more consistent with that of EKE and |∇SST| (Fig.1-2&4), implying that the WP is more\nresponsible for the onshore shift of mesoscale activity. Text is revised accordingly.\n24. Line 137: ORAS5 needs to be introduced earlier in the paper – we have no idea what this product\nis, even the description of the acronym.\nA: Thanks for your comment. The ORAS5 for calculating broadening index is removed.\nLine 137-139: ORAS5 and GLORYS disagree greatly. This section should be further investigated\nor removed from the paper.\nA: Thanks. As suggested, the broadening index is removed from the paper.\n25. Line 139-140: The authors write: “The boundary current broadening also contributes to the\nonshore intensification of mesoscale activity.” This assertion needs more quantitative analysis,\nor the thought needs to be written as a discussion point.\nA: Thanks. We agree that this assertion needs more quatitative analysis. The current results are not\nrobust enough. So we remove the broadening index part and leave it for future investigation.\n26. Line 147: “However” is unnecessary and can be deleted.\nA: Deleted. Thanks.\n27. Line 159: include a statistic describing the variability in the global average shift speeds\nA: Done. Thanks for your suggestion.\n28. Line 161: please adjust the language to say: “The shift speeds of EKE and |∇SST | zonal centroids\nare spatially correlated, a plausible explanation being that the shoreward shift of nearshore |∇SST |\ncan increase the nearshore baroclinic instability, providing favorable conditions for the generation\nand maintenance of oceanic mesoscale activity.”\nA: Done. Thanks for your suggestion.\n29. Line 165: remove “contributed dominantly” and replace with “have also shifted onshore” or\nsimilar phrasing\nA: Done. Thanks for your comment.\n30. Line 166: remove that “variability of baroclinic energy conversion is related to stratification\nchanges under warming.” There is no evidence for increased vertical stratification (d rho/dz), found\nin the denominator of your BC term, causing the spatial changes in BC. Remove that the wind\npower intensification is caused by coastal processes. No evidence in the paper is presented to\nsupport this claim.\nA: Removed. Thanks for your suggestion.\n31. Line 168: The CMIP6 climate models do not really project a shoreward shift of |∇SST | - only\nEKE seems to be “consistent” with historical observations. Please be more precise in your language.\nA: Thanks. The CMIP6 model results are not all significant. Text is revised to be more precise.\n32. Line 175-185: I would suggest reorganizing the last paragraph to be more coherent. Here are my\nedits: keep the first two sentences but move them to become the third and fourth sentences. The\nparagraph then becomes: “The global warming-induced changes in oceanic mesoscale activity may\nalter coastal material exchange 42 (Fig. 3d). For example, prolonged advection routes may reduce\noffshore nutrient transport by eastern boundary eddies, exacerbating subtropical ocean\n“desertification.” The observed shoreward shift of |∇SST| zonal centroids may extend to subsurface\nlayers, influencing deeper mesoscale dynamics. Eddy-driven subduction pumps nutrients vertically\nvia ageostrophic secondary circulation, enhancing nearshore heat and carbon fluxes and biological\nproductivity. Thus, reconfigured coast-open ocean exchanges may disrupt nutrient distribution …”\nA: Thanks for your comment. Text is revised as suggested.\nReferences:\nHelene T Hewitt, Malcolm Roberts, Pierre Mathiot, Arne Biastoch, Ed Blackley, Eric\nP Chassignet, Baylor Fox-Kemper, Pat Hyder, David P Marshall, Ekaterina\nPopova, Anne-Marie Treguier, Laure Zanna, Andrew Yool, Yongqiang Yu,\nRebecca Beadling, Mike Bell, Till Kuhlbrodt, Thomas Arsouze, Alessio Bellucci,\nFred Castruccio, Bolan Gan, Dian Pustrasahan, Christopher D Roberts, Luke Van\nRoekel, and Qiuying Zhiang. Resolving and parameterising the ocean mesoscale\nin earth system models. Current Climate Change Reports, September 2020. URL:\nhttps://doi.org/10.1007/s 06 1-020-0016 -w\nChelton, D. B., DeSzoeke, R. A., Schlax, M. G., El Naggar, K., & Siwertz, N. (199.).\nGeographical variability of the first baroclinic Rossby radius of deformation.\nJournal of Physical Oceanography, 2.(3), 33- 60.\nCastillo-Trujillo, A. C., Kwon, Y. O., Fratantoni, P., Chen, K., Seo, H., Alexander, M.\nA., & Saba, V. S. (2023). An evaluation of eight global ocean reanalyses for the\nNortheast US continental shelf. Progress in Oceanography, 219, 103126."}, {"round": 3, "reviewer_comments": {"reviewer_1": "I am satisfied with the revisions and have no further comments to add.", "reviewer_2": "See attached document.\n[Editorial Note: See end of file]"}, "author_response": "Point-to-point responses to the reviewers’ comments\nOn\n“Shoreward Shift of Oceanic Mesoscale Activity over the Recent Three Decades”\nWe wish to thank the Editor and two anonymous reviewers for their valuable time, careful\nevaluation, and constructive comments. We have taken all comments seriously and revised the\nmanuscript accordingly. Following are our point-to-point responses, where our responses are\nin blue, and the revised text in the manuscript is indicated in blue as well.\nReviewer #1 (Remarks to the Author):\nI am satisfied with the revisions and have no further comments to add.\nA: We thank the reviewer for all the comments and for confirming that our revisions\nhave addressed all the concerns.\nReviewer #2 (Remarks to the Author):\n1. Here, I do not feel confident that the authors have correctly adjusted their methods\nand that this is reflected in the figures. I and the other reviewer asked for the\nsignificance of the trends over time to be reassessed using statistics that consider serial\nautocorrelation. The method above would appear to change the standard deviation used\nin the t-statistic for the significance test. Perhaps the authors can confirm in the text of\nthe manuscript that this is what they in fact have implemented.\nA: Thanks for the comments. We apologize for not correctly presenting the methods\nand results. The confidence interval of WNA should be 29.2 in previous Figure 1. In\nthe previous version, we first conducted the Ljung-Box Q-test (LBQ) to assess the\npresence of significant autocorrelation in each time series. For sequences exhibiting\nstatistically significant autocorrelation (p-value < 0.05), we implemented the Newey-\nWest heteroskedasticity and autocorrelation consistent (HAC) covariance matrix\nestimator to correct standard errors and adjust confidence intervals. Since the majority\nof the data series did not exhibit significant autocorrelation, their confidence intervals\nremained largely unchanged.\nIn this revision, we don’t make any prior judgment on the significance of\nautocorrelation, but instead processed all the data series using the Newey-West method.\nAfter the correction, the confidence intervals are all updated, since the Newey-West\nmethod accounts for both autocorrelation and heteroscedasticity. Importantly, these\ncorrections do not affect our study’s overall conclusion. The text and the figure are all\nupdated accordingly.\nThe use of n-2 is not appropriate to alleviate serial autocorrelation, though it probably\nhelps due to the relatively small sample size of 30.\nA: We apologize for the confusion. To clarify, we address autocorrelation not by\nreducing the effective degrees of freedom, but by using the Newey-West HAC method\nmentioned above. The change in the effective degrees of freedom from n to n-2 was an\nindependent correction of a previous typo in the older version of the manuscript.\nThe figures 1 and 2 have both changed since the last revision. Figure 1 plots the change\nin eddy kinetic energy. From the response to reviewer 1, I assume that the changes are\ndue to a low-pass filter of the results from the previous draft. What are the new black\ndots on the figure? This should be written in the figure caption or anywhere in the\nmanuscript (and why are they included? Are the trends for each region using only the\n“significant” grid cells now?).\nA: Thanks for your comment. The black dots in Figure 1 indicate regions where the\nEKE trends are significant. However, this local significance does not directly influence\nthe zonal centroid shift, as the centroid is calculated as an integral over the entire\ndomain using data from all grid points. The figure caption is revised to clarify.\nFigure 2 is also modified, but for a reason not explained. Figure 2 captures changes in\nthe SST gradient. The rates of zonal centroid movement are now slightly different, the\nsignificance of the rates of movement are now slightly changed for different regions,\nand the SST changes plotted in figure A also show a different pattern. My guess is that\nthe authors also low-pass filtered the SST gradient one more time, but I have no clue as\nto why. This needs to be explained. Black dots have also been added on Figure 2a,\nwhich need to be explained.\nA: We apologize for any confusion caused by our previous revisions. To maintain\nconsistency with the temporal resolution of the EKE data, this revision utilizes daily\nGLORYS SST data, replacing the monthly data used in the previous version. We do not\napply a low-pass filter to the SST gradient one more time. The change in data resolution\nis the reason for the slightly altered spatial patterns and the modified rates of zonal\ncentroid movement. The figure caption is revised to clarify the meaning of the black\ndots.\nMore concerning is that it makes it difficult to assess any modification to the result from\naddressing autocorrelation versus making the trend line with a new data input.\nA: We sincerely apologize for making the review assessing difficult. We hope the above\nclarification can well address the concerns.\n2. The authors need to add units to their reported rate. Their explanation is inadequate\nfor addressing the previous comment. The reason is that the rate reported for shifting is\n0.36 +/- 0.32 for the global shifting of surface wind power, 0.07 +/- 0.06 for the global\nshifting of baroclinic instability, and -0.21 +/- 0.33 for the global shifting of barotropic\ninstability. If these are rates reported in km per decade, as the figures 1 and 2 report for\neddy kinetic energy and SST gradient, then they are insignificant. This would change\nan entire section of the manuscript. The authors further should report the shift rate in\nthe manuscript rather than just in the figure. Also, I assume that the single asterisk in\nthe figure denotes significance at the 90% confidence level, but it should be reported in\nthe figure caption.\nA: Thanks for your suggestions. The shift speeds of WP, BC and BT is indeed in km\nper decade. It is important to note that the zonal centroid shifts of these energy terms\n(Eq. 1; e.g. Wang et al., 2024) represent the primary processes contribute to the\ntemporal variation of EKE, not the EKE field itself.\n𝜕𝐸𝐾𝐸\n= 𝑊𝑃+𝐵𝐶 +𝐵𝑇+𝜀 (1)\n𝜕𝑡\nwhere 𝜀 is the dissipation term. The trends of these energy terms exhibit intense and\nheterogeneous changes (Fig. 4a,c&e). We herein use their zonal centroids as an\nintegrated metric to evaluate whether the net energy forcing is shifting onshore. In other\n!\"#\"\nwords, the shift speeds of these terms correspond to the shifts of rather than EKE.\n!$ \nTherefore, the qualitative direction of these centroid movements is of greater\ninterpretive value than their precise magnitudes. The shift rates have been added to the\nrevised manuscript for completeness, and the text has been rephrased to clarify this\nrationale.\nWang, S., Jing, Z., Wu, L. et al. A more quiescent deep ocean under global\nwarming. Nat. Clim. Chang. 14, 961–967 (2024). https://doi.org/10.1038/s41558-\n024-02075-2\n3. In general, trends that are insignificant at either the 90% or 95% confidence level\nshould not be discussed as results in the text of the manuscript, except to say that a\nrelationship was not found\nA: Thanks for your comment. Text is revised accordingly to clarify.\n4. There is inconsistent presence of error bars on the new figure 3. The figure caption\nstates “The errorbars indicate significance at the 90% level,” but this is very confusing.\nAre some trends significant and others not? An error bar is a separate piece of\ninformation compared to the significance of the trend. This needs further clarification.\nA: We apologize for the unclear writing. We only added the error bars to the trends\nwith statistical significance 90% and higher. Text is revised accordingly to clarify.\n5. This was not done by the authors. The authors still need to respond to this comment\nby adding a sentence in the text about the shifting rates of wind power (WP) and\nbaroclinic instability (BC).\nA: Thanks for your suggestion. We have reported the shifting rates in the revised\nmanuscript.\nReviewer #2 (Remarks on code availability):\nWe are not able to install and run the code to reproduce the results, but the code allows\nus to do the major calculation utilized in the paper. I have asked for additional text in\nthe methods so that the paper results can be reproduced. In my opinion, I do not need\nfurther code provided.\nA: We thank the reviewer for the careful review of our code repository and for this\nconstructive suggestion. We agree that enhancing the methodological description is\ncrucial for reproducibility. As requested, we have revised the Methods section to\nprovide a more detailed description of the key calculations. We hope these textual\nrevisions now provide a comprehensive pathway to reproduce our results."}, {"round": 4, "reviewer_comments": {"reviewer_3": "Comments on the revised manuscript by Zhou et al. (2025)\nI have carefully read the revised manuscript and the authors’ response to reviewer’s comments. Overall, I think the authors have well addressed the issues proposed by reviewer #2. But, I think the authors also consider the following comments to further improve their paper.\n\nDetailed comments\n1.The trends in Figs. 1a and 2a show that only a small portion of the points in the chosen 10 regions are significant at 90%. This situation is particularly bad for the gradient of SST gradient (i.e. Fig. 2a). I suggest the authors clarify the percentage of the significant points in each region and add more discussions on the statistical limitations in the manuscript.\n2.Line 36: The mentioning of global warming in this sentence is very abrupt. You’d better first mention global warming before this sentence.\n3.Lines 73-74: I do not understand why the authors choose to use the model outputs (i.e. GLORYS) rather than the satellite altimeter observations to calculate the EKE trend. For Figure 2a, you can also use satellite SST to calculate the trend.\n4.Line 120: Could you add a percentage to quantify the “most regions” here ?\n5.Lines 123-152: The mechanism interpretations here are not enough and many arguments are speculative. What are the mesoscale activities mentioned here refer to? Are they related to mesoscale eddies? If so, is it possible that the zonal migration of oceanic mesoscale activity is associated with the changing zonal propagation of mesoscale eddies? In addition to the energetics analysis shown here, I think more discussions are needed.\n6.Lines 175-177: The authors attribute the shoreward shift of mesoscale activity to increasing nearshore baroclinic instability associated with shoreward shift of nearshore SST gradient. But, it is also possible that the enhanced stirring of mesoscale processes increase the nearshore SST gradient. So, it is dissicult to tell which is the reason and which is the result. If the reason is the increasing nearshore SST gradient, could the authors further explain its mechanism?"}, "author_response": "Point-to-point responses to the reviewers’ comments\nOn\n“Shoreward Shift of Oceanic Mesoscale Activity over the Recent Three Decades”\nWe wish to thank the Editor and two anonymous reviewers for their valuable time,\ncareful evaluation, and constructive comments. We have taken all comments seriously\nand have modified the manuscript accordingly. Followings are our point-to-point\nresponses, where the comments are in black and our responses are in blue.\nReviewer #1 (Remarks to the Author):\nI am satisfied with the revisions and have no further comments to add.\nA: We thank the reviewer for all the comments and for confirming that our revisions\nhave addressed all the concerns.\nReviewer #2 (Remarks to the Author):\n1. Here, I do not feel confident that the authors have correctly adjusted their methods\nand that this is reflected in the figures. I and the other reviewer asked for the\nsignificance of the trends over time to be reassessed using statistics that consider serial\nautocorrelation. The method above would appear to change the standard deviation used\nin the t-statistic for the significance test. Perhaps the authors can confirm in the text of\nthe manuscript that this is what they in fact have implemented.\nA: Thanks for the comments. We apologize for not correctly presenting the methods\nand results. The confidence interval of WNA should be 29.2 in previous Figure 1. In\nthe previous version, we first conducted the Ljung-Box Q-test (LBQ) to assess the\npresence of significant autocorrelation in each time series. For sequences exhibiting\nstatistically significant autocorrelation (p-value < 0.05), we implemented the Newey-\nWest heteroskedasticity and autocorrelation consistent (HAC) covariance matrix\nestimator to correct standard errors and adjust confidence intervals. Since the majority\nof the data series did not exhibit significant autocorrelation, their confidence intervals\nremained largely unchanged.\nIn this revision, we don’t make any prior judgment on the significance of\nautocorrelation, but instead processed all the data series using the Newey-West method.\nAfter the correction, the confidence intervals are all updated, since the Newey-West\nmethod accounts for both autocorrelation and heteroscedasticity. Importantly, these\ncorrections do not affect our study’s overall conclusion. The text and the figure are all\nupdated accordingly.\nThe use of n-2 is not appropriate to alleviate serial autocorrelation, though it probably\nhelps due to the relatively small sample size of 30.\nA: We apologize for the confusion. To clarify, we address autocorrelation not by\nreducing the effective degrees of freedom, but by using the Newey-West HAC method\nmentioned above. The change in the effective degrees of freedom from n to n-2 was an\nindependent correction of a previous typo in the older version of the manuscript.\nThe figures 1 and 2 have both changed since the last revision. Figure 1 plots the change\nin eddy kinetic energy. From the response to reviewer 1, I assume that the changes are\ndue to a low-pass filter of the results from the previous draft. What are the new black\ndots on the figure? This should be written in the figure caption or anywhere in the\nmanuscript (and why are they included? Are the trends for each region using only the\n“significant” grid cells now?).\nA: Thanks for your comment. The black dots in Figure 1 indicate regions where the\nEKE trends are significant. However, this local significance does not directly influence\nthe zonal centroid shift, as the centroid is calculated as an integral over the entire\ndomain using data from all grid points. The figure caption is revised to clarify.\nFigure 2 is also modified, but for a reason not explained. Figure 2 captures changes in\nthe SST gradient. The rates of zonal centroid movement are now slightly different, the\nsignificance of the rates of movement are now slightly changed for different regions,\nand the SST changes plotted in figure A also show a different pattern. My guess is that\nthe authors also low-pass filtered the SST gradient one more time, but I have no clue as\nto why. This needs to be explained. Black dots have also been added on Figure 2a,\nwhich need to be explained.\nA: We apologize for any confusion caused by our previous revisions. To maintain\nconsistency with the temporal resolution of the EKE data, this revision utilizes daily\nGLORYS SST data, replacing the monthly data used in the previous version. We do not\napply a low-pass filter to the SST gradient one more time. The change in data resolution\nis the reason for the slightly altered spatial patterns and the modified rates of zonal\ncentroid movement. The figure caption is revised to clarify the meaning of the black\ndots.\nMore concerning is that it makes it difficult to assess any modification to the result from\naddressing autocorrelation versus making the trend line with a new data input.\nA: We sincerely apologize for making the review assessing difficult. We hope the above\nclarification can well address the concerns.\n2. The authors need to add units to their reported rate. Their explanation is inadequate\nfor addressing the previous comment. The reason is that the rate reported for shifting is\n0.36 +/- 0.32 for the global shifting of surface wind power, 0.07 +/- 0.06 for the global\nshifting of baroclinic instability, and -0.21 +/- 0.33 for the global shifting of barotropic\ninstability. If these are rates reported in km per decade, as the figures 1 and 2 report for\neddy kinetic energy and SST gradient, then they are insignificant. This would change\nan entire section of the manuscript. The authors further should report the shift rate in\nthe manuscript rather than just in the figure. Also, I assume that the single asterisk in\nthe figure denotes significance at the 90% confidence level, but it should be reported in\nthe figure caption.\nA: Thanks for your suggestions. The shift speeds of WP, BC and BT is indeed in km\nper decade. It is important to note that the zonal centroid shifts of these energy terms\n(Eq. 1; e.g. Wang et al., 2024) represent the primary processes contribute to the\ntemporal variation of EKE, not the EKE field itself.\n𝜕𝐸𝐾𝐸\n= 𝑊𝑃+𝐵𝐶 +𝐵𝑇+𝜀 (1)\n𝜕𝑡\nwhere 𝜀 is the dissipation term. The trends of these energy terms exhibit intense and\nheterogeneous changes (Fig. 4a,c&e). We herein use their zonal centroids as an\nintegrated metric to evaluate whether the net energy forcing is shifting onshore. In other\n!\"#\"\nwords, the shift speeds of these terms correspond to the shifts of rather than EKE.\n!$ \nTherefore, the qualitative direction of these centroid movements is of greater\ninterpretive value than their precise magnitudes. The shift rates have been added to the\nrevised manuscript for completeness, and the text has been rephrased to clarify this\nrationale.\nWang, S., Jing, Z., Wu, L. et al. A more quiescent deep ocean under global\nwarming. Nat. Clim. Chang. 14, 961–967 (2024). https://doi.org/10.1038/s41558-\n024-02075-2\n3. In general, trends that are insignificant at either the 90% or 95% confidence level\nshould not be discussed as results in the text of the manuscript, except to say that a\nrelationship was not found\nA: Thanks for your comment. Text is revised accordingly to clarify.\n4. There is inconsistent presence of error bars on the new figure 3. The figure caption\nstates “The errorbars indicate significance at the 90% level,” but this is very confusing.\nAre some trends significant and others not? An error bar is a separate piece of\ninformation compared to the significance of the trend. This needs further clarification.\nA: We apologize for the unclear writing. We only added the error bars to the trends\nwith statistical significance 90% and higher. Text is revised accordingly to clarify.\n5. This was not done by the authors. The authors still need to respond to this comment\nby adding a sentence in the text about the shifting rates of wind power (WP) and\nbaroclinic instability (BC).\nA: Thanks for your suggestion. We have reported the shifting rates in the revised\nmanuscript.\nReviewer #2 (Remarks on code availability):\nWe are not able to install and run the code to reproduce the results, but the code allows\nus to do the major calculation utilized in the paper. I have asked for additional text in\nthe methods so that the paper results can be reproduced. In my opinion, I do not need\nfurther code provided.\nA: We thank the reviewer for the careful review of our code repository and for this\nconstructive suggestion. We agree that enhancing the methodological description is\ncrucial for reproducibility. As requested, we have revised the Methods section to\nprovide a more detailed description of the key calculations. We hope these textual\nrevisions now provide a comprehensive pathway to reproduce our results.\nPoint-to-point responses to the reviewers’ comments\nOn\n“Shoreward Shift of Oceanic Mesoscale Activity over the Recent Three Decades”\nReviewer #3 (Remarks to the Author):\nComments on the revised manuscript by Zhou et al. (2025)\nI have carefully read the revised manuscript and the authors’ response to reviewer’s\ncomments. Overall, I think the authors have well addressed the issues proposed by\nreviewer #2. But, I think the authors also consider the following comments to further\nimprove their paper.\nWe are very grateful to the reviewer for the positive comments for our previous\nresponse to the other reviewer and for the new constructive comments. Our point-by-\npoint responses are provided in blue below.\nDetailed comments\n1.The trends in Figs. 1a and 2a show that only a small portion of the points in the chosen\n10 regions are significant at 90%. This situation is particularly bad for the gradient of\nSST gradient (i.e. Fig. 2a). I suggest the authors clarify the percentage of the significant\npoints in each region and add more discussions on the statistical limitations in the\nmanuscript.\nA: Thanks for the valuable comment. The significant local trends in EKE and |∇SST|\n(approximately 40% and 30% of grid points in Figs. 1a and 2a, respectively) are distinct\nfrom the significance of zonal centroid shifts. The former are local metrics, while the\nlatter evaluates large-scale spatial redistribution. Consequently, the local trends cannot\nbe used to directly determine the significance of changes in the zonal centroids. Text is\nrevised to clarify.\n2.Line 36: The mentioning of global warming in this sentence is very abrupt. You’d\nbetter first mention global warming before this sentence.\nA: Thanks for your suggestion. Text is revised accordingly.\n3.Lines 73-74: I do not understand why the authors choose to use the model outputs (i.e.\nGLORYS) rather than the satellite altimeter observations to calculate the EKE trend.\nFor Figure 2a, you can also use satellite SST to calculate the trend.\nA: Thanks for asking. The GLORYS dataset was chosen for its relatively high\nresolution and widespread use in mesoscale ocean analysis. Our methodology also\nincorporates satellite data (AVISO, CMEMS, OISST), with a multi-dataset summary\nprovided in Figure 3a. Furthermore, as requested in previous revisions, Figures 1 and 2\ndisplay only GLORYS results to ensure consistency and clarity.\n4.Line 120: Could you add a percentage to quantify the “most regions” here?\nA: Thanks. The percentage has been added in the revised manuscript.\n5.Lines 123-152: The mechanism interpretations here are not enough and many\narguments are speculative. What are the mesoscale activities mentioned here refer to?\nAre they related to mesoscale eddies? If so, is it possible that the zonal migration of\noceanic mesoscale activity is associated with the changing zonal propagation of\nmesoscale eddies? In addition to the energetics analysis shown here, I think more\ndiscussions are needed.\nA: Thanks for raising the important concern. Our study defines ‘mesoscale activity’\nmore comprehensively, using a high-pass filter to capture all mesoscale signals, which\nincludes but is not limited to mesoscale eddies. A complex interplay of eddies, fronts,\njets, and filaments are all included. Even though, the behavior of mesoscale eddies is\nan important component of the overall signals.\nWe agree that the zonal migration of oceanic mesoscale activity could be associated\nwith the changing zonal propagation of mesoscale eddies. To investigate, we analyzed\nthe zonal centroid of number of mesoscale eddies from the global altimeter eddy\nproduct META3.2 DT (Mesoscale Eddy Trajectory Atlas 3.2 Delayed Time,\nhttps://doi.org/10.24400/527896/a01-2022.005). It is found that the mesoscale eddies\nare more shoreward in most areas (Fig. R1). However, the accuracy of satellite-\naltimetry derived eddy detection near coastlines is limited, making this analysis results\n(Fig. R1) questionable.\nTherefore, although the behavior of mesoscale eddies is an important component\nof the overall signals, the extent to which their propagation changes account for the full\nzonal migration we observe remains an open question worthy of further study. We have\nadded the discussion and clarify the mesoscale activity in the revised manuscript.\nFigure R1. a), Distribution of global eddy number trends from 1993 to 2021 from\nMETA3.2. b-k), Time-series of zonal centroid anomalies (compared to the mean\nlocation over the time period) of eddy number (grey) in west boundary regions (b-f),\neast boundary regions (g-k), and the global ocean (l), superimposed with the linear\ntrends. Grey shadings are the 95% confidence intervals for the linear fit based on the\nStudent’s t-test. The figure labeled with * and ** indicates significance at the 90% and\n95% confidence level, respectively. ‘V=’ denotes the shift speed in km per decade.\n6.Lines 175-177: The authors attribute the shoreward shift of mesoscale activity to\nincreasing nearshore baroclinic instability associated with shoreward shift of nearshore\nSST gradient. But, it is also possible that the enhanced stirring of mesoscale processes\nincrease the nearshore SST gradient. So, it is dissicult to tell which is the reason and\nwhich is the result. If the reason is the increasing nearshore SST gradient, could the\nauthors further explain its mechanism?\nA: Thanks for your insightful suggestion. We agree that it is possible that the enhanced\nstirring of mesoscale processes increase the nearshore SST gradient. Furthermore, the\nincreased nearshore |∇SST| may be linked to the intensification of alongshore winds.\nAccording to the established hypothesis of Bukun (Bukun, 1990), intensified\nalongshore winds enhance upwelling in eastern boundary current systems, which\ninhibits nearshore SST rise and amplifies the cross-shelf gradient. In western boundary\nregions, which lack major upwelling systems, intensified alongshore winds may\nenhance mixing in the upper ocean, thereby suppressing the SST increasing rate and\nalso strengthening the SST gradient.\nAdditionally, under global warming, enhanced ocean stratification can lead to\nonshore intensification of western boundary currents (Yang et al, 2025), and thereby\ncontribute to the |∇SST| shoreward shift.\nAll the discussion is added in the text accordingly, along with the possibility that\nmesoscale stirring may also increase the nearshore SST gradient.\nReferences:\nBakun, A. (1990). Global climate change and intensification of coastal ocean upwelling.\nScience, 247(4939), 198-201.\nYang, H. et al. Onshore intensification of subtropical western boundary currents in a\nwarming climate. Nature Climate Change 1-7 (2025)."}, {"round": 5, "reviewer_comments": {"reviewer_3": "The authors have well addressed my comments. I recommend acceptance in the present form."}, "author_response": "Point-to-point responses to the reviewers’ comments\nOn\n“Shoreward Shift of Oceanic Mesoscale Activity over the Last Three Decades”\nReviewer #3 (Remarks to the Author):\nThe authors have well addressed my comments. I recommend acceptance in the present\nform.\nA: We thank the reviewer for all the comments and for confirming that our revisions\nhave addressed all the concerns."}]

Decision letter