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This paper analyzes the differences between ERA-Interim and ERA5 surface winds fields relative to Advanced Scatterometer (ASCAT) ocean vector wind observations, after adjustment for the effects of atmospheric stability and density, using stress-equivalent winds (U10S) and air–sea relative motion using ocean current velocities. In terms of instantaneous root mean square (rms) wind speed agreement, ERA5 winds show a 20 % improvement relative to ERA-Interim and a performance similar to that of currently operational ECMWF forecasts. ERA5 also performs better than ERA-Interim in terms of mean and transient wind errors, wind divergence and wind stress curl biases. Yet, both ERA products show systematic errors in the partition of the wind kinetic energy into zonal and meridional, mean and transient components. ERA winds are characterized by excessive mean zonal winds (westerlies) with too-weak mean poleward flows in the midlatitudes and too-weak mean meridional winds (trades) in the tropics. ERA stress curl is too cyclonic in midlatitudes and high latitudes, with implications for Ekman upwelling estimates, and lacks detail in the representation of sea surface temperature (SST) gradient effects (along the equatorial cold tongues and Western Boundary Current (WBC) jets) and mesoscale convective airflows (along the Intertropical Convergence Zone and the warm flanks for the WBC jets). It is conjectured that large-scale mean wind biases in ERA are related to their lack of high-frequency (transient wind) variability, which should be promoting residual meridional circulations in the Ferrel and Hadley cells.

Changes in the position and strength of the Southern Hemisphere surface westerlies have significant implications for ocean circulation and the global carbon cycle. Here we compare the climatologies, as well as the trends, in the position and strength of the surface westerly wind‐stress jet in reanalyses with the Coupled Model Intercomparison Project (CMIP) phase 3 and phase 5 models over the historical period from 1979–2010. We show that both the CMIP3 and CMIP5 models exhibit an equatorward biased climatological jet position. The reanalyses and climate models both show significant trends in annual mean jet strength, though the climate models underestimate the strengthening. Neither reanalyses nor models show a robust trend in annual mean jet position over the historical period, though significant trends do occur in the Austral summer position. We also compare the response of the CMIP3 and CMIP5 model wind‐stresses to a range of anthropogenic forcing scenarios for the 21st century. Key Points A strengthening of the SH westerly jet has occurred since 1979 There is not a robust shift in annual‐mean jet position Climate models have an equatorward biased jet, and underestimate strengthening

The monthly mean sea level along the U.S. Mid‐Atlantic Coast varies seasonally, reaching a minimum in January and a maximum in September during the 1960–2020 period. However, this seasonal cycle has changed significantly on multi‐decadal timescales. In the last two decades, the annual minimum has shifted from January to February. The amplitude of seasonal changes increased by 65% from 14.16 cm in 1980–1999 to 23.16 cm in 2000–2020. Even more concerning, the maximum sea level in September rose by 82%, from 6.81 to 12.38 cm, potentially exacerbating coastal flooding over the past 20 years. A two‐layer ocean model effectively replicates both the phase and magnitude of the observed changes and attributes these shifts to changes in wind stress near the coast, with relatively minor influence from deep ocean forcing. Both alongshore and cross‐shore wind stress changes are found to contribute to changes in the sea level's seasonal cycle. Plain Language Summary Sea level height varies seasonally in response to external forces and internal processes. Along the U.S. Mid‐Atlantic Coast, it varies about 19 cm between the seasonal low in January/February and the high in September, according to observations from the past six decades. However, this seasonal cycle has changed markedly on decadal timescales. Between 2000 and 2020, the amplitude of seasonal sea level variation increased by more than 65% compared to the previous two decades. Our analysis suggests that changes in wind stress, especially in coastal areas, are the primary driver of these changes. Key Points The seasonal cycle of sea level along the Mid‐Atlantic Coast has undergone significant changes in the last 6 decades The main driver for changes in the seasonal cycle is wind stress along the coastal areas Influences from deep open ocean make only minor contributions to changes in the seasonal cycle

Spatial and temporal variations of nutrient-rich upwelled water across the major eastern boundary upwelling systems are primarily controlled by the surface wind with different, and sometimes contrasting, impacts on coastal upwelling systems driven by alongshore wind and offshore upwelling systems driven by the local wind-stress-curl. Here, concurrently measured wind-fields, satellite-derived Chlorophyll-a concentration along with a state-of-the-art ocean model simulation spanning 2008-2018 are used to investigate the connection between coastal and offshore physical drivers of the Benguela Upwelling System (BUS). Our results indicate that the spatial structure of long-term mean upwelling derived from Ekman theory and the numerical model are fairly consistent across the entire BUS and closely followed by the Chlorophyll-a pattern. The variability of the upwelling from the Ekman theory is proportionally diminished with offshore distance, whereas different and sometimes opposite structures are revealed in the model-derived upwelling. Our result suggests the presence of sub-mesoscale activity (i.e., filaments and eddies) across the entire BUS with a large modulating effect on the wind-stress-curl-driven upwelling off Lüderitz and Walvis Bay. In Kunene and Cape Frio upwelling cells, located in the northern sector of the BUS, the coastal upwelling and open-ocean upwelling frequently alternate each other, whereas they are modulated by the annual cycle and mostly in phase off Walvis Bay. Such a phase relationship appears to be strongly seasonally dependent off Lüderitz and across the southern BUS. Thus, our findings suggest this relationship is far more complex than currently thought and seems to be sensitive to climate changes with short- and far-reaching consequences for this vulnerable marine ecosystem.

Surface ocean temperature and velocity anomalies at meso‐ and sub‐meso‐scales induce wind stress anomalies. These wind‐front interactions, referred to as thermal (TFB) and current (CFB) feedbacks, respectively, have been studied in isolation at mesoscale, yet they have rarely been considered in tandem. Here, we assess the combined influence of TFB and CFB and their relative impact on surface wind stress derivatives. Analyses are based on output from two regions of the Southern Ocean in a coupled simulation with local ocean resolution of 2 km. Considering both TFB and CFB shows regimes of interference, which remain mostly linear down to the simulation resolution. The jointly‐generated wind stress curl anomalies approach 10−5 N m−3, ∼20 times stronger than at mesoscale. The synergy of both feedbacks improves the ability to reconstruct wind stress curl magnitude and structure from both surface vorticity and SST gradients by 12%–37% on average, compared with using either feedback alone. Plain Language Summary Surface ocean temperature and velocity anomalies at 0.1–100 km scales imprint their signatures on the surface wind stress, which in turn supplies the ocean with momentum. This process is called wind‐front interaction and typically referred to as thermal and current feedbacks when generated by temperature gradients and velocity gradients, respectively. Previously, studies using satellite observations and regional numerical models have studied either feedback in isolation; consideration of both feedbacks in tandem remains immature. Here, we present an approach that assesses both feedbacks' combined and relative impact on the surface wind stress, using output from an air‐sea coupled simulation. This approach allows us to identify constructive and destructive patterns of how the two feedbacks interact, which remain mostly linear down to the simulation resolution. The jointly‐generated wind stress derivative anomalies are 20 times stronger than observed previously at larger scales. Considering both feedbacks, reconstructions of wind stress derivatives are viable and have 10%–40% less error on average compared with using either feedback by itself. Contributions from either feedback in modifying wind stress fields vary temporally and can be related to physical properties such as surface wind speed and air‐sea temperature difference across the studied area. Key Points Surface temperature gradients and vorticity anomalies at scales down to O(10) km impact wind stress curl and divergence jointly Wind‐front interactions at sub‐mesoscales are at least an order of magnitude stronger than at mesoscales Reconstruction of the wind stress curl using both thermal and current feedback is 10%–40% more accurate than relying on a single feedback

Relative roles of wind stress and buoyancy forcing in shaping Pacific circulation and sea level remain unclear. Using large‐ensemble simulations from Community Earth System Model version 2, we disentangle the contributions of wind and buoyancy fluxes during 1960–2014. Wind stress accounts for 81% of barotropic circulation changes and explains 54% of regional sea‐level trend, while buoyancy forcing contributes 19% of barotropic circulation changes but 46% of regional sea‐level trend. Circulation changes diagnosed from the barotropic stream function match estimations from the Sverdrup stream function, underscoring the reliability of wind‐driven frameworks. Wind stress drives ocean heat redistribution through meridional transport and subduction, inducing sea‐level rise along the poleward flanks of subtropical gyres. Buoyancy forcing partially offsets wind‐driven changes in the North Pacific, while exerting a weaker but synergistic influence in the South Pacific. These findings highlight the dominant yet regionally modulated role of wind stress in shaping Pacific circulation and sea level.

Central Pacific (CP) El Niño (i.e., CP El Niño) events have occurred more frequently during recent decades. Wind stress patterns are argued to have significant effects on the generation and evolution of CP El Niño. However, the winds differ in different CP El Niño events, making it hard in previous studies to avoid overgeneralizing the timing and location of the winds that indeed matter. In this study, the theoretically favorable wind perturbations (FWPs) that may warm the Niño-4 region, in terms of their directions, horizontal structures, and bounds, in each month before the peak month (December) of CP El Niños are determined, using an adjoint sensitivity method. The mechanisms of the FWPs are interpreted. Primarily, zonal temperature advection via the equatorial wave–associated velocity anomalies is responsible. In particular, easterly FWPs over the central equatorial Pacific with off-equatorial westerly FWPs (constituting a wind structure with a strong north–south gradient) during the first half year can play a positive role in warming the Niño-4 region and so can the westerly FWPs over the western tropical Pacific, while westerly FWPs in the western-central tropical Pacific in the second half year show higher efficiency. Meanwhile, the particular wind structure of the first half year (i.e., the easterly anomaly over the central equatorial Pacific with strong wind stress curl off the equator) has also been verified to be able to produce a CP-type warming in an intermediate coupled model (ICM); similar wind stress anomalies had been observed in some CP El Niño events. Thus, the FWPs provide helpful guidance in analyzing the generation and evolving processes of the wind-driven CP El Niño.

The lower limb of the Atlantic meridional overturning circulation (AMOC) is the equatorward flow of dense waters formed through the cooling and freshening of the poleward‐flowing upper limb. In the subpolar North Atlantic (SPNA), upper limb variability is primarily set by the North Atlantic Current, whereas lower limb variability is less well understood. Using observations from a SPNA mooring array, we show that variability of the AMOC's lower limb is connected to poleward flow in the interior Irminger Sea. We identify this poleward flow as the northward branch of the Irminger Gyre (IG), accounting for 55% of the AMOC's lower limb variability. Over 2014–2018, wind stress curl fluctuations over the Labrador and Irminger Seas drive this IG and AMOC variability. On longer (>annual) timescales, however, an increasing trend in the thickness of intermediate water, from 2014 to 2020, within the Irminger Sea coincides with a decreasing trend in IG strength. Plain Language Summary In the subpolar North Atlantic, warm salty waters get transported northwards by the upper branch of the meridional overturning circulation. As they travel northwards, they transform: cooling, densifying, and sinking. The cooler deeper waters then get transported back southwards toward the equator in the lower branch of the overturning circulation. The transformation and transport of these waters plays a critical role in our climate system. However, the lower branch of the overturning circulation and the mechanisms controlling how it changes are still not well understood. Observations from a fixed array of moorings between Greenland and Scotland are used here to identify the interior (away from land boundaries) Irminger Sea as a region important for the overturning's lower branch. Specifically, we find that a closed system of currents in the western Irminger Sea, known as the Irminger Gyre, plays an important role in the overturning's variability. Gyre strength is then linked to the recirculation of newly transformed waters that get exported as part of the overturning's lower branch. Finally, we investigate the impact of the atmosphere on Irminger Sea circulation and find that fluctuations of the winds are important drivers of change in this gyre and the overturning. Key Points The interior Irminger Sea, where the poleward limb of the Irminger Gyre (IG) dominates, is a hotspot for the overturning's lower limb variability A trend in IG transport is linked to deep intermediate water masses found in the Irminger Sea Wind stress curl over the Labrador and Irminger Seas drives IG and Atlantic meridional overturning circulation variability

Ocean mesoscale thermal feedback (TFB) is the influence of mesoscale sea surface temperature (SST) anomalies on the overlying atmosphere and its feedback to the ocean. Over the past few decades, TFB has been shown to affect the atmosphere by inducing low-level wind and surface stress anomalies and modulating ocean–atmosphere heat fluxes ubiquitously over the global oceans. These anomalies can alter the climate variability. However, it is not clear yet to what extent heat and momentum flux anomalies modulate the mesoscale ocean activity. Here, using coupled ocean–atmosphere mesoscale simulations over a realistic subtropical channel centered on the equator in which the TFB can be turned off by spatially smoothing the SST as seen by the atmosphere, we show that TFB can damp the mesoscale activity, with a more pronounced effect near the surface. This damping appears to be sensitive to the cutoff filter used: on average, the surface mesoscale activity is attenuated by 9% when smoothing the SST using an ∼1000-km cutoff but by only 2% when using an ∼350-km cutoff. We demonstrate that the mesoscale activity damping is primarily caused by a sink of available eddy potential energy that is controlled by the induced-anomalous heat fluxes, the surface stress anomalies having a negligible role. When TFB is neglected, the absence of sink of potential energy is partly compensated by a more negative eddy wind work. We illustrate that TFB filtering in a coupled model must be done carefully because it can also impact the large-scale meridional SST gradients and subsequently the mean large-scale wind stress curl and ocean dynamics.

The build-up of decadal timescale upper ocean heat content in the off-equatorial western tropical Pacific can provide necessary conditions for interannual El Niño/Southern Oscillation (ENSO) events to contribute to decadal timescale transitions of tropical Pacific SSTs to the opposite phase of the Interdecadal Pacific Oscillation (IPO). This can be viewed as a corollary to subseasonal westerly wind burst events contributing to El Niño interannual timescale transitions. A long pre-industrial control run with CESM1 is analyzed to show that there is a greater chance of ENSO activity to contribute to an IPO transition when off-equatorial western Pacific Ocean heat content reaches either a maximum (for El Niño to contribute to a transition to positive IPO) or minimum (for La Niña to contribute to a transition to negative IPO) as seen in observations. If above a necessary ocean heat content threshold, the convergence associated with westerly anomaly near-equatorial surface winds associated with El Niño activity can draw that heat content equatorward to sustain anomalously warm western and central Pacific SSTs. These are associated with positive precipitation and convective heating anomalies, a Gill-type response and wind stress curl anomalies that continue to feed warm water into the near-equatorial western Pacific. These conditions then sustain the decadal-timescale transition to positive IPO (with the opposite sign for transition to negative IPO). Associated central equatorial Pacific convective heating anomalies produce SLP and wind stress anomalies in the North and South Pacific that can excite westward-propagating off-equatorial oceanic Rossby waves to contribute to the western Pacific thermocline depth and consequent heat content anomalies.