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Over the past decade, several studies based on coupled ocean–atmosphere simulations have shown that the oceanic surface current feedback to the atmosphere (CFB) leads to a slow-down of the mean oceanic circulation and, overall, to the so-called eddy killing effect, i.e., a sink of kinetic energy from oceanic eddies to the atmosphere that damps the oceanic mesoscale activity by about 30%, with upscaling effects on large-scale currents. Despite significant improvements in the representation of western boundary currents and mesoscale eddies in numerical models, some discrepancies remain when comparing numerical simulations with satellite observations. These discrepancies include a stronger wind and wind stress response to surface currents and a larger air–sea kinetic energy flux from the ocean to the atmosphere in numerical simulations. However, altimetric gridded products are known to largely underestimate mesoscale activity, and the satellite observations operate at different spatial and temporal resolutions and do not simultaneously measure surface currents and wind stress, leading to large uncertainties in air–sea mechanical energy flux estimates. ODYSEA is a new satellite mission project that aims to simultaneously monitor total surface currents and wind stress with a spatial sampling interval of 5 km and 90% daily global coverage. This study evaluates the potential of ODYSEA to measure surface winds, currents, energy fluxes, and ocean–atmosphere coupling coefficients. To this end, we generated synthetic ODYSEA data from a high-resolution coupled ocean–wave–atmosphere simulation of the Gulf Stream using ODYSIM, the Doppler scatterometer simulator for ODYSEA. Our results indicate that ODYSEA would significantly improve the monitoring of eddy kinetic energy, the kinetic energy cascade, and air–sea kinetic energy flux in the Gulf Stream region. Despite the improvement over the current measurements, the estimates of the coupling coefficients between surface currents and wind stress may still have large uncertainties due to the noise inherent in ODYSEA, and also due to measurement capabilities related to wind stress. This study evidences that halving the measurement noise in surface currents would lead to a more accurate estimation of the surface eddy kinetic energy and wind stress coupling coefficients. Since measurement noise in surface currents strongly depends on the square root of the transmit power of the Doppler scatterometer antenna, noise levels can be reduced by increasing the antenna length. However, exploring other alternatives, such as the use of neural networks, could also be a promising approach. Additionally, the combination of wind stress estimation from ODYSEA with other satellite products and numerical simulations could improve the representation of wind stress in gridded products. Future efforts should focus on the assessment of the potential of ODYSEA in quantifying the production of eddy kinetic energy through horizontal energy fluxes and air–sea energy fluxes related to divergent and rotational motions. [ABSTRACT FROM AUTHOR]

The mesoscale interaction between sea surface temperature (SST) and wind is a crucial factor influencing oceanic and atmospheric conditions. To investigate the mesoscale coupling characteristics of the Yellow Sea and East China Sea, we applied a locally weighted regression filtering method to extract mesoscale signals from Quik-SCAT wind field data and AMSR-E SST data and found that the mesoscale coupling intensity is stronger in the Yellow Sea during the spring and winter seasons. We calculated the mesoscale coupling coefficient to be approximately 0.009 N·m−2/°C. Subsequently, the Tikhonov regularization method was used to establish a mesoscale empirical coupling model, and the feedback effect of mesoscale coupling on the ocean was studied. The results show that the mesoscale SST–wind field coupling can lead to the enhancement of upwelling in the offshore area of the East China Sea, a decrease in the upper ocean temperature, and an increase in the eddy kinetic energy in the Yellow Sea. Diagnostic analyses suggested that mesoscale coupling-induced variations in horizontal advection and surface heat flux contribute most to the variation in SST. Moreover, the increase in the wind energy input to the eddy is the main factor explaining the increase in the eddy kinetic energy. [ABSTRACT FROM AUTHOR]

Downstream of Cape Hatteras, the vigorously meandering Gulf Stream forms anticyclonic warm core rings (WCRs) that carry warm Gulf Stream and Sargasso Sea waters into the cooler, fresher Slope Sea, and forms cyclonic cold core rings (CCRs) that carry Slope Sea waters into the Sargasso Sea. The Northwest Atlantic shelf and open ocean off the U.S. East Coast have experienced dramatic changes in ocean circulation and water properties in recent years, with significant consequences for marine ecosystems and coastal communities. Some of these changes may be related to a reported regime shift in the number of WCRs formed annually, with a doubling of WCRs shed after 2000. Since the regime shift was detected using a regional eddy‐tracking product, primarily based on sea surface temperatures and relies on analyst skill, we examine three global eddy‐tracking products as an automated and potentially more objective way to detect changes in Gulf Stream rings. Currently, global products rely on altimeter‐measured sea surface height (SSH), with WCRs registering as sea surface highs and CCRs as lows. To identify eddies, these products use either SSH contours or a Lagrangian approach, with particles seeded in satellite‐based surface geostrophic velocity fields. This study confirms the three global products are not well suited for statistical analysis of Gulf Stream rings and suggests that automated WCR identification and tracking comes at the price of accurate identification and tracking. Furthermore, a shift to a higher energy state is detected in the Northwest Atlantic, which coincides with the regime shift in WCRs. Plain Language Summary: In the Northwest Atlantic Ocean, a specific type of eddy, known as a ring, is formed by the Gulf Stream, a strong ocean current that closely follows the U.S. East Coast between Florida and Cape Hatteras. Northeast of Cape Hatteras, the Gulf Stream separates from the continental shelf and starts to meander. Sometimes, these meanders grow into oxbow‐like structures that pinch off from the Gulf Stream and become rings. Warm core rings (WCRs) are formed north of the Gulf Stream, with warm centers that create sea surface hills. In contrast, cold core rings (CCRs) form south of the stream, with cold centers and sea surface valleys. Since 1980 an analyst, who studies the Gulf Stream, has used primarily sea surface temperature to identify WCRs. A regional Ring Census based on this work found a doubling in WCR formations after 2000. Other eddy data sets are based on satellite data, such as sea surface height (SSH), and are created by automated tracking algorithms. This study compares global eddy data sets with the Ring Census. Our findings suggest global data sets using SSH alone cannot replicate the results of the Ring Census. Additionally, we find the Northwest Atlantic region has become more energetic since 2000. Key Points: Global altimetry‐based products evaluated to study Gulf Stream rings differ from regional data products based on sea surface temperatureGlobal products do not detect an observed regime shift in the number of Warm Core Ring formations and do not reflect the known seasonalityA shift to a higher energy state in the Northwest Atlantic around 2000 is detected via satellite altimetry‐based geostrophic velocities [ABSTRACT FROM AUTHOR]

The Met Office Forecast Ocean Assimilation Model (FOAM) ocean–sea‐ice analysis and forecasting operational system has been using an ORCA tripolar grid with 1/4° horizontal grid spacing since December 2008. Surface boundary forcing is provided by numerical weather prediction fields from the operational global atmosphere Met Office Unified Model. We present results from a 2‐year simulation using a 1/12° global ocean–sea‐ice model configuration while keeping a 1/4° data assimilation (DA) set‐up. We also describe recent operational data assimilation enhancements that are included in our 1/4° control and 1/12° simulations: a new bias‐correction term for sea‐level anomaly assimilation and a revised pressure correction algorithm. The primary effect of the first is to decrease the mean and variability of sea‐level anomaly increments at high latitudes, whereas the second significantly reduces the vertical velocity standard deviation in the tropical Pacific. The level of improvement achieved with the higher resolution configuration is moderate but consistently satisfactory when measured using neighbourhood verification metrics that provide fairer quantitative comparisons between gridded model fields at different spatial resolutions than traditional root‐mean‐square metrics. A comparison of the eddy kinetic energy from each configuration and an observation‐based product highlights the regions where further system developments are most needed. [ABSTRACT FROM AUTHOR]

The study of Rossby wave propagation in strong jet stream waveguides is essential, as extreme weather events are associated with persistent atmospheric patterns at the surface which may be favored by quasi stationary Rossby waves in the upper troposphere through these pathways. But so far, all the studies are mostly for winter and summer seasons. Therefore, in the present study, we extended earlier works to the transition seasons. The waveguide patterns in both hemispheres during the spring and autumn transition seasons are explored using numerical simulations from a baroclinic model with six selected forcings in the 1979–2016 period. The results show that stronger subtropical jet streams are found in boreal and austral spring associated with stronger wave propagation. Particularly, stronger eddy kinetic energy and wave activity flux are found in boreal spring from the north of Middle East to eastern North Pacific, associated with stronger subtropical Asian jet, and in austral autumn in western Pacific region, associated with greater extension of polar jet. Interhemispheric propagation is verified in spring season in both hemispheres, through the equatorial eastern Pacific and Atlantic ducts, with a northwest-southeast orientation. [ABSTRACT FROM AUTHOR]

Eddy activity is particularly prominent in the Southern Ocean due to the instabilities of the Antarctic Circumpolar Current, which plays a critical role in energy transport of the global ocean. In this study, a systematic energetics analysis framework is employed to investigate notable anomalies of an intensified Eddy Kinetic Energy (EKE) event observed in the southwest region of the Kerguelen Plateau in the Indian sector of the Southern Ocean in 2017, utilizing a reanalysis product. The EKE anomalies, presenting across all depths, emerged in April, peaked during the austral winter, and persisted into the subsequent summer. Energetics analysis indicates that the pronounced EKE anomalies are primarily determined by baroclinic instability, with distinct governing mechanisms at the surface and in the internal ocean. The anomalous intrusion of warm Circumpolar Deep Water intensified the baroclinic energy conversion in the subsurface, contributing significantly to the EKE anomalies. Moreover, strong anomalous wind‐induced Ekman pumping served to amplify the lifting of isopycnals, which enhanced the baroclinic instability and subsequently intensified the EKE anomalies. This study sheds new light on underlying mechanisms governing local polar dynamics and provides insights into the intricate interaction between ocean dynamics and energy distribution in the Antarctic region. Plain Language Summary: The Indian sector of the Southern Ocean is known for its dynamic variability, often manifested as jets and eddies. Eddy Kinetic Energy (EKE) is widely used to measure the kinetic energy (KE) as the difference between the total KE and the KE of mean currents. This study found an anomalous event of significant increases in EKE in the southwest region of the Kerguelen Plateau in the Indian sector of the Southern Ocean in 2017. We applied a systematic energetics analysis framework to a reanalysis product to investigate the processes responsible for the observed anomalous event. The results suggest that the main cause was the anomalous intrusion of warm water masses in the upper and deeper ocean layers, which led to the increases in density gradients and then intensified the energy conversion from available potential energy to EKE. Moreover, changes in wind patterns have an impact on the variations of EKE in the upper ocean. This study enhances the understanding of the energy conversions and underlying mechanisms for EKE in polar regimes. Key Points: The subpolar region to the southwest of the Kerguelen Plateau was characterized by anomalous strong Eddy Kinetic Energy (EKE) in 2017The strong regional anomalies in EKE can be primarily attributed to baroclinic instability, with inverse barotropic energy conversionBaroclinic instability is mainly caused by anomalous intrusion of Circumpolar Deep Water, particularly at depths between 500 and 2,000 m [ABSTRACT FROM AUTHOR]

The monsoon is the primary force behind the upper‐layer circulation system and regulates eddy activity in the South China Sea (SCS). Using satellite observations and a state‐of‐the‐art oceanic reanalysis, this study examines transferring processes of eddy kinetic energy (EKE) during the SCS Summer Monsoon (SCSSM) onset (20th May on average). The SCSSM onset transforms surface winds from easterly to southwesterly, increasing EKE in the southwestern SCS by surface wind work. Despite the persistent energy input from wind work, EKE growth ceases during the first 20 days after SCSSM onset. An EKE budget analysis suggests that most energy input dissipates in the mixed layer due to the high wind speed, which explains the EKE stagnation. Additionally, eddy‐mean flow interactions transfer kinetic energy from eddies to mean flows during the SCSSM onset. The Vietnam coastal upwelling system provides available potential energy to EKE on the offshore side through buoyancy work. Our study reveals energy‐transferring processes from the summer monsoon to eddy activity and dissipation in the SCS, with implications for understanding multiscale dynamic evolutions during the monsoon transition. Plain Language Summary: The South China Sea (SCS) has received much attention for its unique ocean dynamics impacted by monsoons. While the SCS monsoon is known to modulate the seasonal variation in SCS circulation, the effects of the intraseasonal monsoon oscillations on the oceanic eddy activity remain unclear. Generally, ocean circulation and eddy activity get stronger when surface winds strengthen. However, this study reports a stagnation of eddy kinetic energy (EKE) after the SCS summer monsoon onset in the southern SCS despite the persistent energy input from the surface winds. EKE budget results show that high‐speed wind changes the current structure and greatly intensifies ocean dissipation. This study provides a new perspective on ocean energy‐transferring processes under the SCS monsoon transition and will benefit ocean circulation modeling and prediction in the future. Key Points: Eddy kinetic energy (EKE) growth stagnates for 20 days in eastern Vietnam after the South China Sea Summer Monsoon onsetDissipation due to the high wind speed explains the EKE stagnation as a dominant energy sinkThe formation of the upwelling system changes the energy transfer process and modulates the eddy‐mean flow interaction [ABSTRACT FROM AUTHOR]

Mesoscale eddies play an important role in driving the dynamics of the Arctic Ocean. Understanding their behavior is crucial for comprehending the ongoing changes in the region. In this study, by using a novel decade‐long simulation at 1 km resolution with the unstructured‐mesh Finite volumE Sea ice‐Ocean Model, we evaluate the spatial and temporal variability of eddy kinetic energy in the Eurasian Basin. We find that monthly, annual, and interannual variability of EKE near the surface is predominantly influenced by changes in sea ice cover, while the eddy activity at deeper depth, being shielded from the surface by ocean stratification, is more strongly influenced by local baroclinic energy conversion. Moreover, our research demonstrates that eddies in the Eurasian Basin can transport ocean heat from the Atlantic Water layer toward sea ice and cause local basal melting in the order of about 20 cm per month even in wintertime. Our study suggests that eddy activity in the Arctic Ocean will strengthen along with future sea ice decline, and that the impact of ocean heat of the Atlantic Water layer on sea ice retreat may become prominent. Plain Language Summary: Mesoscale eddies are swirling currents that are generally considered the "weather" of the ocean. They can be found everywhere in the ocean and are crucial for understanding many processes such as the transport of heat and salt, the structure of ocean stratification, but also the distribution of nutrients that influence the biological system from algae blooms to micro and macro fauna. Distinct from major eddies in lower latitudes, most eddies in the Arctic Ocean are only about 10 km in diameter. This makes it incredibly hard to simulate them in numerical models because a very high spatial resolution is needed to adequately represent them. In this study we use a simulation with 1 km horizontal resolution to analyze the role of eddies in the Eurasian Basin of the Arctic Ocean and to understand what drives their variability. We found that the dynamics differ greatly between the surface and deeper water layers, with the surface being influenced strongly by sea ice cover, while the deeper layer is shielded from this effect by the stratification. We have also shown that eddies can pump warm water from the Atlantic Water layer up to the mixed layer, thus melting sea ice even in winter. Key Points: Mesoscale eddies in the Eurasian Basin can bring ocean heat from the Atlantic Water layer to sea ice and cause basal melting even in winterVariability of surface eddy kinetic energy is mainly controlled by sea ice coverEddy activity at depth is mainly controlled by baroclinic instability and ocean stratification [ABSTRACT FROM AUTHOR]

The mesoscale interaction between sea surface temperature (SST) and wind is a crucial factor influencing oceanic and atmospheric conditions. To investigate the mesoscale coupling characteristics of the Yellow Sea and East China Sea, we applied a locally weighted regression filtering method to extract mesoscale signals from Quik-SCAT wind field data and AMSR-E SST data and found that the mesoscale coupling intensity is stronger in the Yellow Sea during the spring and winter seasons. We calculated the mesoscale coupling coefficient to be approximately 0.009 N·m−2/°C. Subsequently, the Tikhonov regularization method was used to establish a mesoscale empirical coupling model, and the feedback effect of mesoscale coupling on the ocean was studied. The results show that the mesoscale SST–wind field coupling can lead to the enhancement of upwelling in the offshore area of the East China Sea, a decrease in the upper ocean temperature, and an increase in the eddy kinetic energy in the Yellow Sea. Diagnostic analyses suggested that mesoscale coupling-induced variations in horizontal advection and surface heat flux contribute most to the variation in SST. Moreover, the increase in the wind energy input to the eddy is the main factor explaining the increase in the eddy kinetic energy.

The Agulhas Return Current (ARC), as a primary component of the Agulhas system, contributes to water exchange and mass transport between the southern portions of the Indian and Atlantic Ocean basins. In this study, satellite altimeter data and reanalysis datasets, and a new set of criteria for the piecewise definition of the jet axis are used to explore the long-term change of the ARC's axis position in recent three decades. It is found that the ARC axis exhibits a significant slanting trend with its western part (35–48°E) migrating northward and the eastern part (48–70°E) migrating southward. The meridional movement of the ARC path could be attributed to large-scale wind forcing. The anomalous surface wind stress curl, by Ekman pumping mechanism, leads to positive–negative–positive sea surface height anomalies in the western section and negative–positive–negative anomalies in the eastern section, thus the ARC axis tilts accordingly, in a northwest–southeast direction. Further analysis suggests that this ARC slanting trend is more dependent on the southward shift of the downstream axis and less on the topographic steering upstream. The downstream axis is more likely to interact with the ACC fronts and its migration could dominate the local EKE pattern by changing the background circulation and energy cascade direction. For the headstream west of 35°E, the ARC axis is more subject to topography, thus the EKE change is more dominated by eddy activity processes, including shedding, propagation and merging. This study provides some new insights into the long-term change of ARC and its interaction with the local EKE variability. [ABSTRACT FROM AUTHOR]

Abstract Several studies reported some aliasing errors of Ocean bottom pressure (OBP) data from Gravity Recovery and Climate Experiment (GRACE), although this data have been widely used to estimate the oceanic transports. In this study, the performances of monthly OBP data from six GRACE products with two different solutions are evaluated by comparisons with the observed records at the Drake Passage. Result shows that spherical harmonic products have a better ability to capture monthly OBP variability than mascon products at the Drake Passage. In all GRACE products, higher skills occur to the south of Polar Front than those in the northern Drake Passage, and the correlations with observations reach minimum in the Local Dynamics Array (LDA) region. Such spatial differences are mainly attributed to local mesoscale processes, accompanied with high-frequency bottom eddy kinetic energy (EKE). It indicates that the monthly OBP variations from GRACE products are not reliable in the eddy-rich regions.

Mesoscale eddies dominate the ocean kinetic energy reservoir. However, how and where this energy flows out from the mesoscale remains uncertain. Here, a simplified mesoscale energy budget is used where sources due to baroclinic instability are balanced by all the dissipative processes approximated as a linear damping rate. In this simple model, the eddy kinetic energy (EKE) dissipation is computed from a climatological mean field of density and satellite altimeter data, and is proportional to an eddy efficiency parameter α. Assuming an eddy efficiency of α = 0.1, we find a global EKE dissipation rate of 0.66 ± 0.19 TW. The results show an intense dissipation near western boundary currents and in the Antarctic Circumpolar Current, where both large levels of energy and baroclinic conversion occur. The resulting geographical distribution of the dissipation rate brings new insights for closing the ocean kinetic energy budget, as well as constraining future mesoscale parameterizations and associated mixing processes. Plain Language Summary: The ocean is home to abundant and large swirls from tens to hundreds of kilometers, called "mesoscale eddies." These eddies contain more momentum than most ocean currents and can thus impact the climate evolution. There are now good reasons to believe the effect of mesoscale eddies is directly related to their strength, and so to their kinetic energy. However, how the energy is removed from these eddies is still unclear mostly due to instrumental and theoretical limitations. In this work, a simplification of the eddy energetic behavior is used to indirectly estimate the dissipation from observations of temperature, salinity and surface currents. Our results confirm intensified dissipation near strong ocean currents and hence constitute a new attempt for the global reconstruction of the eddy kinetic energy dissipation in the world ocean. The work presented here is consistent and complementary to other studies and can help us to understand the ocean energy cycle. Key Points: Global mesoscale eddy kinetic energy dissipation rate estimated to 0.66 ± 0.19 TW from observation‐based and statistically analyzed data setsMore than 25% of the total dissipation occurs in the western boundary currents and 38% is found in the Antarctic Circumpolar CurrentEstimation of the eddy dissipation timescale from observations to inform future parameterization developments [ABSTRACT FROM AUTHOR]

Based on the eddy-resolving Four-dimensional variational Ocean Re-Analysis for the Western North Pacific over 30 years (FORA-WNP30) product, this study investigates the seasonal evolution of mesoscale eddy characteristics and underlying mechanisms in the Kuroshio Extension (KE) region. In the upstream region, eddies are stretched in the zonal direction and characterized by higher activity in summer and autumn. Energy analysis illustrates that baroclinic instability associated with the horizontal buoyancy flux is responsible for the seasonal variability. In the downstream region, in comparison, eddies tend to be stretched in the meridional direction. Eddy kinetic energy (EKE) level in this region is mainly regulated by the upstream through pressure work. During 1993–2013, the EKE level does not change in the upstream region but it depicts an increasing trend in the downstream region. This evolution reflects the decadal variability of the KE system associated with the Pacific Decadal Oscillation. [ABSTRACT FROM AUTHOR]

Several studies reported some aliasing errors of Ocean bottom pressure (OBP) data from Gravity Recovery and Climate Experiment (GRACE), although this data have been widely used to estimate the oceanic transports. In this study, the performances of monthly OBP data from six GRACE products with two different solutions are evaluated by comparisons with the observed records at the Drake Passage. Result shows that spherical harmonic products have a better ability to capture monthly OBP variability than mascon products at the Drake Passage. In all GRACE products, higher skills occur to the south of Polar Front than those in the northern Drake Passage, and the correlations with observations reach minimum in the Local Dynamics Array (LDA) region. Such spatial differences are mainly attributed to local mesoscale processes, accompanied with high-frequency bottom eddy kinetic energy (EKE). It indicates that the monthly OBP variations from GRACE products are not reliable in the eddy-rich regions. [ABSTRACT FROM AUTHOR]

The development of large-amplitude planetary waves (PWs) in March in the upper stratosphere during the easterly phase of the equatorial quasi-biennial oscillation (QBO) was investigated using ERA-interim reanalysis data for 1979–2019. During the 10-hPa easterly QBO, the amplitude at 3 hPa was significantly larger than that during the westerly QBO for cases of large-amplitude PWs. Case studies were conducted for individual events of the wave number 1 (wave-1) PW growth: an easterly case in 1994 and a westerly case in 1995. During the easterly QBO in March 1994, a developing perturbation at middle latitudes moved rapidly northeastward to replace the decaying high-latitude wave. In the early stage, conversion from the zonal mean to eddy kinetic energy in the subtropical region was crucial for wave development. This energy conversion was dependent on the sign of the meridional shear of the zonal wind in the middle latitudes. Negative shear was produced by the secondary meridional circulation associated with the equatorial QBO. After the perturbation started to develop in the middle latitudes, it moved northeastward over a few days due to potential vorticity flux, and the growth of the high-latitude waves was enhanced. A composite analysis also showed that the meridional shear of the zonal wind in the middle latitudes was negative during the easterly QBO in March. This study improves our understanding of the dynamic mechanism underlying the equatorial-polar relationship in the stratosphere in March. [ABSTRACT FROM AUTHOR]

Mesoscale eddies of the ocean (with a characteristic diameter of about 100 km and a life time‐span of about several weeks) are habitats of plankton organisms, many of which are bioluminescent. The spatial heterogeneity of bioluminescence of the upper mixed layer associated with the impact of mesoscale eddies is poorly studied. The 45‐year historical data set was retrieved, in order to select the bathy‐photometric surveys carried out in the form of station grids and transects across eddies. Data from 71 expeditions deployed in 1966–2022 to the Atlantic Ocean, Indian Ocean and Mediterranean Sea basin were analyzed, in order for the spatial heterogeneity of bioluminescent fields to be elucidated across eddy fields. The stimulated bioluminescence intensity was characterized by the bioluminescent potential, which represented the maximal amount of radiant energy emitted in a given volume of water by bioluminescent organisms. The normalized bioluminescent potential over oceanographic station grids exhibited correlation with the eddy kinetic energy and zooplankton biomass (r = 0.8, at P = 0.001 and r = 0.7, at P = 0.05, respectively), in a broad range of energy and bioluminescence units (0.02–0.2 m2 s−2; 0.4–92.0 × 10−8 W cm−2 L−1, respectively). Overall, estimates of bioluminescent potential variability on the mesoscale contribute to the assessment of the multiple‐scale variation of the bioluminescent field of the World Ocean. [ABSTRACT FROM AUTHOR]

• Enhanced Kuroshio looping path across the Luzon Strait is observed during 1993–2020. • Eddy Kinetic Energy shows long-term increasing in the Northeastern South China Sea. • Energy from baroclinic instability is dominant for the EKE long-term increasing. The seasonal to interannual variability of eddy kinetic energy (EKE) in the Northeastern South China Sea (NE-SCS) has been widely studied and it is recognized that they are strongly related to the state of the Kuroshio pathway in the Luzon Strait. While, due to the lack of long-term observations and high-resolution simulations, the decadal change of EKE in NE-SCS remains unexplored. In this study, we show the EKE trend in the past ∼ 30 years in the NE-SCS by using satellite observation and global HYbrid Coordinate Ocean Model reanalysis with the Navy Coupled Ocean Data Assimilation. It is found that due to the weakening of the Kuroshio in the Luzon Strait since 1990 s, the Kuroshio shows an enhanced looping path in the NE-SCS, inducing stronger EKE in this region. Further analysis confirms that the energy transfer by baroclinic instability is dominant for the increasing of EKE, when the Kuroshio intrudes into the NE-SCS and brings more potential energy inside the circulation. The Kuroshio state along the Luzon Strait is the key for modulating the EKE in the NE-SCS. Furthermore, the long-term weakening of Kuroshio current along the Luzon strait during 1993–2020 is determined by the decreasing of subtropical mode water, corresponding to the positive phase of the Atlantic Multidecadal Oscillation. This study provides insight into the interaction between marginal sea (i.e., the SCS) and the open ocean (i.e., the western Pacific Ocean), finally linking to the global climate change. [ABSTRACT FROM AUTHOR]

• The EKE exhibits vigorous interannual variations in western equatorial Pacific. • The EKE variations exhibit different characteristics in different flavors of El Niño. • Barotropic instability is the main mechanism for EKE interannual variations. • Variations of barotropic instability is closely related to the background currents. The interannual variations of eddy kinetic energy (EKE) in the western equatorial Pacific Ocean are investigated based on satellite observations and model outputs in this study. Results reveal that the EKE exhibits vigorous interannual variations, especially in the region of North Equatorial Countercurrent (NECC) and north of New Guinea, and the variations differ between the two types of El Niño events. The energy budget diagnosis indicates that the EKE variations are mainly attributed to the barotropic instability associated with the background currents. Specifically, the energetic NECC behaves northward shift and a stronger meander path, which favors the enhancement of EKE variations due to the enhanced barotropic instability. The interannual fluctuations of the strength of the New Guinea Coastal Current/Undercurrent (NGCC/NGCUC) and the eastward current along the equator contribute to the significant EKE interannual variations north of New Guinea. Further, the distinct features of EKE variations in two types of El Niño events are as follows: EKE typically weakens in the western equatorial Pacific during Eastern Pacific El Niño (EP-El Niño) events, whereas it intensifies north of New Guinea during Central Pacific El Niño (CP-El Niño) events. The opposite features north of New Guinea are attributed to the wind work and a stronger eastward current along the equator in CP-El Niño events. These results can contribute to a better understanding of the low-frequency eddy-mean flow interactions. [ABSTRACT FROM AUTHOR]

Synoptic eddies (with a period of two to eight days) are active in the South China Sea-Philippine Sea (SCS-PS) and control weather variations. In addition, the intensity and frequency of synoptic eddies may change along with variations in sea surface temperatures (SST). This paper presented the influence of SST in the northwest Indian Ocean on synoptic eddies in the lower troposphere over the SCS-PS in June. Our statistical analysis showed a significant negative correlation between the SST in the northwest Indian Ocean and the synoptic scale eddy kinetic energy (EKE) in the SCS-PS. By analyzing the EKE budget of synoptic eddies, we found that the variation in the synoptic scale EKE over the SCS-PS is mainly due to the change in the monthly zonal wind gradient, which affects the barotropic energy conversion between the monthly mean flow and the synoptic eddies. Additionally, the northwest Indian Ocean SST modulates the monthly flow over the SCS-PS by alternating the strength of the Walker circulation in the west Pacific and Indian Ocean. Finally, the influence of SST in the northwest Indian Ocean on EKE in the SCS-PS was reproduced using the simplified atmospheric general circulation model, SPEEDY. [ABSTRACT FROM AUTHOR]

The East Australian Current (EAC) is an important western boundary current of the South Pacific subtropical Circulation with high mesoscale eddy kinetic energy (EKE). Based on satellite altimeter observations and outputs from the eddy-resolving ocean general circulation model (OGCM) for the Earth Simulator (OFES), the seasonal variability of EKE and its associated dynamic mechanism in the EAC region are studied. High EKE is mainly concentrated in the shear-region between the poleward EAC southern extension and the equatorward EAC recirculation along Australia's east coast, which is confined within the upper ocean (0-300 m). EKE in this area exhibits obvious seasonal variation, strong in austral summer with maximum (465±89 cm² s-²) in February and weak in winter with minimum (334±48 cm² s-²) in August. Energetics analysis from OFES suggests that the seasonal variability of EKE is modulated by the mixed instabilities composed of barotropic and baroclinic instabilities confined within the upper ocean, and barotropic instability (baroclinic instability) is the main energy source of EKE in austral summer (winter). The barotropic process is mainly controlled by the zonal shear of meridional velocities of the EAC southern extension and the EAC recirculation. The poleward EAC southern extension and the equatorward EAC recirculation are synchronously strengthened (weakened) due to the local positive (negative) sea level anomalies (SLA) under geostrophic equilibrium, and the barotropic instability dominated by zonal shear is enhanced (slackened), which results in a high (low) level of EKE in the EAC region.

Eddies in the California Current System (CCS) are generated via baroclinic instabilities of the upwelling jet and contribute to mesoscale variability. However, climate‐induced changes in mesoscale activity of the CCS remain poorly explored. We present future eddy kinetic energy (EKE) from an ensemble of three high‐resolution downscaled ocean projections covering 1980–2100. EKE is projected to increase throughout the CCS toward the end of the century (2071–2100) compared to a 1980–2010 reference period. Using measures of upper ocean stratification, we find an increasing EKE trend is highly correlated with a more stratified ocean. These findings support previous studies suggesting that mesoscale activity is modulated mainly by baroclinic instabilities, rather than changes in the wind components, which did not show a systematic change across the three projections. Enhanced eddy activity could lead to changes in the distribution and aggregation of biogenic material, having important implications for the CCS. Plain Language Summary: The California Current System (CCS) is characterized by seasonal coastal upwelling and energetic currents that have horizontal scales of 10 to a hundred kilometers, referred to as the mesoscale. Together, upwelling and mesoscale activity contribute to a diverse and productive ecosystem. Swirling currents and time varying components of the mesoscale field that occur on time scales of days to months play an important role in distributing ecosystem components. Eddy kinetic energy (EKE) is often used to quantify mesoscale variability and some of the key questions regarding its evolution in a warming ocean still remain poorly addressed. In this study, we use high resolution climate projections of the CCS from 1980 to 2100 to analyze the evolution of EKE. By the end of the century, we find an increase in EKE that is closely related to enhanced stratification of the CCS associated with long‐term ocean warming, rather than with changes in the coastal winds and currents. Intensification of eddy activity has important implications for the ecosystem since it significantly influences cross‐shore transport of nutrients and other biogenic elements like oxygen, chlorophyll, and planktonic organisms. Key Points: Eddy kinetic energy (EKE) is projected to increase throughout the California Current System by the end of the 21st centuryFuture changes in EKE are associated with a warmer, more stratified ocean [ABSTRACT FROM AUTHOR]

By using satellite altimeter observations and the eddy-permitting Estimating the Circulation and Climate of the Ocean, Phase II (ECCO2), the seasonal variation of eddy kinetic energy (EKE) along the East Australian Current (EAC) is investigated. Both observations and ECCO2 outputs indicate active intra-seasonal EKE along the EAC path. The ECCO2 result reveals that the intra-seasonal EKE is mainly concentrated in the upper 500 m layer, and shows a prominent seasonal cycle, strong in austral summer and weak in austral winter. Eddy energy budget diagnosis reveals that the evolution of EKE is controlled by barotropic instability of the mean EAC. The seasonal variation of baroclinic instability is opposite to the barotropic instability variation, but of a much smaller magnitude. Further analysis indicates that the seasonal cycle of mesoscale signals in this region is related to the transport variability of the EAC. [ABSTRACT FROM AUTHOR]

The seasonality of eddy kinetic energy (EKE) is analyzed in the north Indian Ocean by adopting high-resolution ocean reanalysis data. Significant eddy energy can be mainly spotted in six regions, including the Somali Current (SC) region, the Gulf of Aden, the Laccadive Sea, the east of Sri Lanka, the East Indian Coastal Current (EICC) region, and the northwest of Sumatra. As the most energetic region, the EKE averaged above 200 m could exceed 0.15 m2·s-2 in the SC region, whereas the mean EKE above 200 m is less than 0.04 m2·s-2 in the other regions. The barotropic and baroclinic instabilities are vital to eddy energy, and the contribution of each term in the barotropic/baroclinic equations varies with season and region. In the SC region and EICC region, EKE is primarily generated by barotropic conversion due to the sharp velocity shear caused by the strong SC during the summer monsoon and the EICC from March to June. For the other regions, the leading source of EKE is the eddy potential energy (EPE), which is extracted from available potential energy of mean flow via baroclinic conversion, and then the EPE is converted into EKE through vertical density flux. Once generated, EKE will be redistributed by pressure work and advection via eddy energy flux, which varies in sync with the monthly variation of total EKE, transporting EKE to the adjacent region or deeper layer. From the vertical aspect, eddy energy conversions are more prominent above 200 m. The maximal EKE and barotropic conversion mostly occur at the surface, whereas the EPE and baroclinic conversion may have two peaks, which lie at the surface and in the thermocline. Using the satellite altimeter data and wind data, we further investigate the impact of geostrophic eddy wind work, which reveals a slightly dampening effect to EKE in the north Indian Ocean.

The evolution of mesoscale eddies is crucial for understanding the ocean energy cascade. In this study, using global reanalysis sea surface velocity data and a mesoscale eddy trajectory product tracked by satellite altimeters, we aimed to reveal the coherence of eddy kinetic energy (EKE) variation to low-frequency ageostrophic energy during the eddy life span. The variation in EKE throughout the eddy life span was highly coherent to that of the seven-day low-passed ageostrophic kinetic energy, with a correlation coefficient of −0.94. The low-frequency ageostrophic motions supplied 38% of the EKE variation in the growing stage of mesoscale eddies and absorbed 42% in the decaying stage. The evolution rate of the EKE during the eddy life span was consistent with the barotropic conversion rate of the low-frequency ageostrophic motions, further confirming the dominant role of low-frequency ageostrophic motions in eddy growth and decay. [ABSTRACT FROM AUTHOR]

The performance of eddy‐resolving global ocean–sea ice models in simulating mesoscale eddies is evaluated using six eddy‐resolving experiments forced by different atmospheric reanalysis products. Interestingly, eddy‐resolving ocean general circulation models (OGCMs) tend to simulate more (less) energetic eddy‐rich (eddy‐poor) regions with a smaller (larger) spatial extent than satellite observation, which finally shows that larger (smaller) mesoscale energy intensity (EI) is simulated in the eddy‐rich (eddy‐poor) regions. Quantitatively, there is an approximately 27%–60% overestimation of EI in the eddy‐rich regions, which are mainly located in the Kuroshio–Oyashio Extension, the Gulf Stream, and the Antarctic Circumpolar Currents regions, although the global mean EI is underestimated by 25%–45%. Apparently, the eddy kinetic energy in the eddy‐poor region is underestimated. Further analyses based on coherent mesoscale eddy properties show that the overestimation in the eddy‐rich regions is mainly attributed to mesoscale eddies' intensity and is more prominent when mesoscale eddies are in their growth stage. Plain Language Summary: Realistic simulation of mesoscale eddies is crucial in correctly reproducing observed large‐scale circulation. However, the state‐of‐the‐art eddy‐resolving ocean general circulation models (OGCMs) simulate a less energetic surface ocean on the global scale. Previous studies show that the biases in simulating global mesoscale eddies by eddy‐resolving OGCMs are not uniformly distributed. Therefore, we investigate the performances of eddy‐resolving OGCMs in simulating mesoscale eddies in two kinds of regions, eddy‐rich and eddy‐poor. Our results show that the eddy‐resolving OGCMs tend to simulate more (less) energetic eddy‐rich (eddy‐poor) regions with a smaller (larger) spatial extent with the metric eddy kinetic energy intensity. To further understand the simulated biases in the eddy‐rich and eddy‐poor regions, we analyze coherent mesoscale eddy properties after eddies are identified and tracked. We find that the overestimation in the eddy‐rich regions is mainly contributed by coherent mesoscale eddies' intensity rather than frequency and is more prominent when mesoscale eddies are in their growth stage. Our results give an in‐depth understanding of how well eddy‐resolving OGCMs can perform in simulating the observed mesoscale eddies on the regional scale. Key Points: Surface mesoscale energy intensity in the eddy‐rich regions is significantly overestimated by eddy‐resolving ocean general circulation modelsThe overestimation in the eddy‐rich regions is mainly attributed to coherent mesoscale eddies' intensityThe positive model biases in the eddy‐rich regions are more prominent when mesoscale eddies are in their growth stage [ABSTRACT FROM AUTHOR]

This research investigated the eddy–mean flow interactions in the Agulhas leakage region by utilizing the Ocean General Circulation Model for the Earth Simulator (OFES) output and an energetic analysis tool called the multiscale energetics and vorticity analysis tool (MS-EVA). MS-EVA relies on multiscale window transform (MWT) functional analysis and canonical transfer. It is found that the climatological characteristics of the nonlinear interactions between the eddy and mean flow exhibit mixed canonical transfers, including both barotropic and baroclinic canonical transfers. These canonical transfers are related to barotropic and baroclinic instabilities, respectively. These transfers are highly inhomogeneous in space, reaching their maxima around 18°–22° E and 36°–42° S, where the Agulhas Ring forms. Besides, the barotropic canonical transfers from the mean flow to the eddy tend to dominate the entire Agulhas leakage region, with a contribution ratio of 1.55 between the barotropic and baroclinic canonical transfers. These results suggest that barotropic instability plays a more important role in producing eddy activities in this region. [ABSTRACT FROM AUTHOR]

The upwelling of cold water from the depths of the Southern Ocean to its surface closes the global overturning circulation and facilitates uptake of anthropogenic heat and carbon. Upwelling is often conceptualised in a zonally averaged framework as the result of isopycnal flattening via baroclinic eddies. However, upwelling is zonally non-uniform and occurs in discrete hotspots near topographic features. The mechanisms that facilitate topographically confined eddy upwelling remain poorly understood and thus limit the accuracy of parameterisations in coarse-resolution climate models.Using a high-resolution global ocean sea-ice model, we calculate spatial distributions of upwelling transport and energy conversions associated with barotropic and baroclinic instability, derived from a thickness-weighted energetics framework. We find that five major topographic hotspots of upwelling, covering less than 30% of the circumpolar longitude range, account for up to 76% of the southward eddy upwelling transport. The conversion of energy into eddies via baroclinic instability is highly spatially correlated with upwelling transport, unlike the barotropic energy conversion, which is also an order of magnitude smaller than the baroclinic conversion. This result suggests that eddy parameterisations that quantify baroclinic energy conversions could be used to improve the simulation of upwelling hotspots in climate models. We also find that eddy kinetic energy maxima are found on average 110 km downstream of upwelling hotspots in accordance with sparse observations. Our findings demonstrate the importance of localised mechanisms to Southern Ocean dynamics.

Satellite altimetry observations, including the upcoming Surface Water and Ocean Topography mission, provide snapshots of the global sea surface high anomaly field. The common practice in analyzing these surface elevation data is to convert them into surface velocity based on the geostrophic approximation. With increasing horizontal resolution in satellite observations, sea surface elevation data will contain many dynamical signals other than the geostrophic velocity. A new physical quantity, the available surface potential energy, is conceptually introduced in this study defined as the density multiplied by half of the squared deviation from the local mean reference surface elevation. This gravitational potential energy is an intrinsic property of the sea surface height field and it is an important component of ocean circulation energetics, especially near the sea surface. In connection with other energetic terms, this new variable may help us better understand the dynamics of oceanic circulation, in particular the processes in connection with the free surface data collected through satellite altimetry. The preliminary application of this concept to the numerically generated monthly mean Global Ocean Data Assimilation System data and Archiving, Validation, and Interpretation of Satellite Oceanographic altimeter data shows that the available surface potential energy is potentially linked to other dynamic variables, such as the total kinetic energy, eddy kinetic energy and available potential energy. [ABSTRACT FROM AUTHOR]

Intra-annual sea level fluctuations and variability of mesoscale processes based on eddy kinetic energy (EKE) were studied in the northern (northward of 41 N) Japan/East Sea (JES) using data from satellite altimetry for 1993–2020. Decomposition to empirical orthogonal functions (EOF) was performed of the high-pass filtered, with the cut-off period of 250 days, sea level anomalies. The leading mode accounting for the major fraction of the variance yielded sea level fluctuations which were simultaneous in the entire sea and occurred in the range from 70 to 250 days without any preferable timescale. EKE in the northern sea was also expanded to EOF and yielded the leading mode capturing mesoscale variability within the Primorye (Liman) Current and the Tsushima Warm Current. The seasonal signal was found in the simultaneous intra-annual sea level fluctuations, which matches that of EKE, and, as found in the earlier studies, of the mean currents. The sea level rises, the mean currents intensify and EKE increases in summer and fall and the opposite changes occur in winter and spring, with the seasonal extremes in October/November and March/April, respectively. This is in line with the EKE generation by instability of the mean currents. The intra-annual sea level fluctuations and EKE manifest rich variability on quasi-biennial, interannual and decadal timescales. However, in contrast with the seasonal signal, the low-frequency variability does not match, implying different kinds of forcing, probably by local wind in the northern JES and by the transport variations in the Korea – Tsushima Strait (KTS) in the southern JES. Intra-annual simultaneous SLA reveal changing relationship with Pacific Decadal Oscillation (PDO): both were in-phase in 1993–1994 and from late 2007 to 2013 and out-of-phase from 1997 to 2002, while there was no specific relationship in other times. However, the relationship of these SLA with the interannual KTS transport variation seems inconclusive.

In this study, the variability of eddy kinetic energy (EKE) in the Kuroshio large meander (LM) region is investigated using both satellite sea surface height observation and high‐resolution ocean reanalysis data. The results show that the EKE has a remarkable interannual variability and it is negatively leading correlated with the change in the Kuroshio latitudinal position at 140°E. The mechanism analysis suggests that the baroclinic instability and advection processes are responsible for the EKE interannual variability and its effect on the Kuroshio latitudinal position at 140°E. Specifically, before the high EKE level occurs, a cyclonic eddy generates at the east of Kii Peninsula in the background field. The rapid development of this eddy and its eastward movement to the LM region induce the isopycnal inclinations there and the associated horizontal density gradient, which leads to the strong baroclinic instability and promotes the evolution of the eddy field and the formation of the high EKE level. The developed strong eddies, especially the cyclonic eddies, move downstream to 140°E, which pushes the Kuroshio off the shore and causes the southerly Kuroshio latitudinal position at 140°E. On the contrary, when the cyclonic eddies do not appear in the LM region, the isopycnals are relatively flat, which is not conducive to the generation of baroclinic instability. Consequently, the EKE level is low and only weak eddies are advected to 140°E, which does not substantially shift the Kuroshio southward and thus results in the northerly Kuroshio position. Plain Language Summary: Ocean mesoscale eddies have important effects on physical and biological processes. The formation area of the Kuroshio large meander (LM) is one of the most active areas of mesoscale eddies in the middle latitudes of the western North Pacific. It is, therefore, of significance to investigate the variability of mesoscale eddy activities represented by eddy kinetic energy (EKE) in the LM region. Our results show that the EKE level is high in some years, while it is low in other years and this kind of EKE variability has a leading negative correlation with the Kuroshio latitudinal position at 140°E. The mechanism analysis indicates that the existence of cyclonic eddies in the LM region in the background field causes differences in the isopycnal inclinations and the related baroclinic instabilities, which eventually lead to different EKE levels. Subsequently, when the EKE level is high, strong cyclonic eddies move downstream to 140°E, which pushes the Kuroshio off the shore and causes the southerly Kuroshio latitudinal position at 140°E. Contrarily, when the EKE level is low, only weak eddies are advected to 140°E, which does not substantially shift the Kuroshio southward and thus results in the northerly Kuroshio position there. Key Points: Eddy kinetic energy(EKE) in the Kuroshio large meander (LM) region has a remarkable interannual variabilityEKE interannual variability in the LM area has a leading correlation with that of the Kuroshio latitudinal position at 140°EBaroclinic instability and advection are responsible for the EKE variability and its relationship to the Kuroshio position at 140°E [ABSTRACT FROM AUTHOR]

Satellite altimeters provide quasi‐global measurements of sea surface height, and from those the vertically integrated geostrophic velocity can be directly estimated, but not its vertical structure. This study discusses whether the mesoscale (30–400 km) dynamics of three regions in the South Atlantic can be described by the surface quasi‐geostrophic (SQG) theory, both at the surface and in depth, using outputs from an ocean general circulation model. At these scales, the model surface eddy kinetic energy (EKE) spectra show slopes close to k−5/3 (k−3) in winter (summer), characterizing the SQG and quasi‐geostrophic (QG) turbulence regimes. We use surface density and temperature to (a) reconstruct the stream function under the SQG theory, (b) assess its capability of reproducing mesoscale motions, and (c) identify the main parameters that improve such reconstruction. For mixed layers shallower than 100 m, the changes in the mixed‐layer depth contributes nine times more to the surface SQG reconstruction than the EKE, indicating the strong connection between the quality of the reconstruction and the seasonality of the mixed layer. To further explore the reconstruction vertical extension, we add the barotropic and first baroclinic QG modes to the surface solution. The SQG solutions reproduce the model density and geostrophic velocities in winter, whereas in summer, the interior QG modes prevail. Together, these solutions can improve surface correlations (>0.98) and can depict spatial patterns of mesoscale structures in both the horizontal and vertical domains. Improved spatial resolution from upcoming altimeter missions poses a motivating scenario to extend our findings into future observational studies. Plain Language Summary: Altimeters provide sea surface height measurements from which geostrophic velocities can be calculated. However, the measurements are strict to the ocean surface and obtaining its vertical structure is an ongoing challenge. Using outputs from an ocean general circulation model, we focus on describing the dynamics of mesoscale motions (30–400 km) in three regions of the South Atlantic under the surface‐quasi‐geostrophic (SQG) theory. We reconstruct the stream function taking a snapshot of density (and temperature) and assess the capability of the SQG method to correctly reproduce surface and vertical fields. Our results indicate that density may drive mesoscale dynamics under specific environmental conditions, and the role played by the seasonality of mixed‐layer depth and eddy kinetic energy is discussed. To further explore the vertical reconstruction, we include the barotropic and first baroclinic quasi‐geostrophic (QG) modes to the surface solution, yielding fields highly correlated (>0.98) to the model outputs. The upcoming new high‐resolution altimeters poses a motivating scenario to apply the SQG method of reconstruction and extend our findings into future observational studies. Key Points: The surface solution dominates the total stream function in winter. It has a small yet significant contribution in summerThe surface quasi‐geostrophic (SQG) and isQG methods depend more on the seasonality of the mixed‐layer depth than on the content of eddy kinetic energyThe isQG reconstruction on the South Atlantic can reproduce mesoscale motions at depths above 500 m with a threshold of 0.5 correlation [ABSTRACT FROM AUTHOR]

Cut‐Off Lows (COLs) in the Southern Hemisphere and the mechanisms involved in their development are investigated in detail using the eddy kinetic energy (EKE) budget applied to data from the ERA‐Interim reanalysis. This approach considers the most important processes that are typical for the evolution of midlatitude disturbances such as the baroclinic (BRC) and barotropic (BRT) conversions, and the ageostrophic flux convergence (AFC), known as downstream development. Composites of the volume‐integrated EKE and its components are evaluated based on the 200 most intense SH COLs (>98th percentile) observed in a 36‐year period. Results show that the AFC together with the BRC conversion are the most important contributors to the EKE growth for the COLs, characterizing the downstream baroclinic development. The AFC plays an important role in genesis and intensification phases of the COLs, while the BRC conversion is important for the system maintenance. The dissipation of the COLs occurs due to dispersive fluxes (ageostrophic flux divergence) together with other processes not directly computed in the EKE equation, such as friction and latent heat release which are problematic in reanalysis datasets. The BRT conversion contributes negatively to the COL development by transferring EKE to the zonal flow kinetic energy, though this is not enough to dampen the intensification. Regional differences were found in the energetics, indicating that COLs originating upstream of the continents are clearly dominated by ageostrophic fluxes, while the systems over the Australian region are mostly driven by baroclinic processes. [ABSTRACT FROM AUTHOR]

The Agulhas retroflection (AR) region possesses the highest eddy kinetic energy (EKE) level in the Indian Ocean. However, mechanisms regulating EKE of the AR remain uncertain. Here, by analyzing an eddy-resolving coupled model simulation with improved EKE representation, we show that the upper-ocean EKE of the AR is mainly generated through barotropic instability in its upstream and leakage zones and is by nonlocal transport in its downstream zone. The interaction between mesoscale eddies and local winds plays a key role in EKE dissipation. The lack of eddy-wind interaction results in flawed EKE budget in the leakage zone in ocean-alone models, leading to severe biases in EKE distribution with overestimation and over-strong penetration into the South Atlantic. Our results highlight the essence of mesoscale air-sea interaction in the dynamics of the AR, with implications for understanding the inter-basin transport of the Agulhas leakage.

Among Western Boundary Currents, the East Australian Current (EAC) has a more energetic eddy field relative to its mean flow, however, the relationship between upstream transport and downstream eddy kinetic energy (EKE) is still unclear. We investigate the modulation of downstream EKE in the EAC's typical separation region (Tasman EKE Box) (33.1°S–36.6°S) based on a long‐term (22‐year), high‐resolution (2.5–6 km) model simulation and satellite altimeter observations from 1994 to 2016. Our results show that the poleward EAC transport at 28°S leads the EKE in the Tasman EKE Box by 93–118 days. Barotropic instabilities are the primary source of EKE, and they control EKE variability in the EAC system. Anticyclonic eddies shed from the EAC dominate from 33°S–36°S during high‐EKE periods, but in low‐EKE periods anticyclonic eddies penetrate even further south by ∼2°. Plain Language Summary: The East Australian Current (EAC) mean flow is typically coherent from ∼27°S–32°S (upstream), but eddies form after it separates from the coast typically at ∼32°S, associated with high eddy variability downstream. However, we know little about what drives changes in the downstream eddies and the correlation with transport upstream. Here, we use both satellite observations and model simulations to investigate the interannual variability in the eddy field. We find that the transport upstream of separation is well correlated with the variability of sea surface height within the typical EAC separation region. An anomalously higher EAC transport at 28°S corresponds to an anomalously higher sea surface height within the typical EAC separation region. The reverse is true when the EAC transport is weaker, but the current separates from the coast further to the south. We also show that the energy converted from the mean flow to the eddy fields is mainly through mean kinetic energy to eddy kinetic energy. Key Points: Barotropic instabilities dominate eddy shedding and control variability of eddy kinetic energy (EKE) in the East Australian CurrentThere is a clear (inverse) relationship between poleward transport at 28°S and the latitude that anticyclonic eddies are shedTransport at 28°S and sea level anomalies at ∼27°S–29°S (upstream) are significantly correlated with EKE at ∼33.1°S–36.6°S (downstream) [ABSTRACT FROM AUTHOR]

Previous studies demonstrated that eddy processes play an important role in ice shelf basal melting and the water mass properties of ice shelf cavities. However, the eddy energy generation and dissipation mechanisms in ice shelf cavities have not been studied systematically. The dynamic processes of the ocean circulation in the Amery Ice Shelf cavity are studied quantitatively through a Lorenz energy cycle approach for the first time by using the outputs of a high-resolution coupled regional ocean-sea ice-ice shelf model. Over the entire sub-ice-shelf cavity, mean available potential energy (MAPE) is the largest energy reservoir (112 TJ), followed by the mean kinetic energy (MKE, 70 TJ) and eddy available potential energy (EAPE, 10 TJ). The eddy kinetic energy (EKE) is the smallest pool (5.5 TJ), which is roughly 8% of the MKE, indicating significantly suppressed eddy activities by the drag stresses at ice shelf base and bottom topography. The total generation rate of available potential energy is about 1.0 GW, almost all of which is generated by basal melting and seawater refreezing, i.e., the so-called “ice pump.” The energy generated by ice pump is mainly dissipated by the ocean-ice shelf and ocean-bottom drag stresses, amounting to 0.3 GW and 0.2 GW, respectively. The EKE is generated through two pathways: the barotropic pathway MAPE→MKE→EKE (0.03 GW) and the baroclinic pathway MAPE→EAPE→EKE (0.2 GW). In addition to directly supplying the EAPE through baroclinic pathway (0.2 GW), MAPE also provides 0.5 GW of power to MKE to facilitate the barotropic pathway.

Previous model‐based studies and observation‐based studies suggest that increasing wind energy input into the Southern Ocean will primarily cause increases in eddy kinetic energy (EKE) with no significant change in the mean circulation, a result that has been named the "eddy‐saturation" hypothesis. However, due to the sparsity of the available observations, quantifying and understanding regional EKE changes in the Southern Ocean is challenging. In this study, we examine regional trends in the Southern Ocean EKE using altimetry crossover measurements and an ocean reanalysis product to quantify if undersampling by altimetry will bias EKE trend estimates and to test if previously observed EKE trends are homogenous throughout the Southern Ocean or concentrated in a few regions. To verify that the EKE computed from altimetry crossovers accurately represents yearly averaged EKE over the Southern Ocean, we first conduct a sampling experiment with the HYCOM Global Ocean Forecasting System 3.1 ocean reanalysis. We find that the crossover sampling is sufficient to represent the yearly averaged EKE when averaged over sectors of at least 30° of longitude. We find no coherent increase in EKE over the entire Southern Ocean from altimetry crossover measurements, but instead find significant EKE increase over only one region, primarily south of New Zealand and downstream of the Campbell Plateau. We conclude that the EKE change in the Southern Ocean is not as homogenous as implied by previous studies and more work is needed to understand if this is consistent with the "eddy‐saturation" hypothesis or related more to local dynamics. Plain Language Summary: Eddy kinetic energy (EKE) represents the energy of time‐variable currents. Previous model‐based studies and observational‐based studies have suggested that increasing wind energy input into the Southern Ocean is mainly offset by increases in the EKE without significant change in the mean circulation. However, there have been no studies to quantify if this signal is coherent throughout the Southern Ocean or is regionally biased. Here, we use satellite observations and models to study surface geostrophic EKE in the Antarctic Circumpolar Current region where mean EKE is the highest. We first conduct a sampling experiment with the model to confirm that the more limited sampling of the satellite does not bias EKE when averaged over sectors of at least 30° longitude band. We then use the satellite data to show that EKE has only increased significantly over one region between 1993 and 2019. This suggests that estimates of EKE trends over the Southern Ocean are not certain as previously assumed. Key Points: Eddy kinetic energy (EKE) computed from altimetry crossovers can well represent EKE changes in the Southern OceanNo coherent increase in EKE over all areas of the Southern Ocean was identified since 1993EKE in the Southern Ocean has increased since 1993 in one region that is downstream of the Campbell Plateau [ABSTRACT FROM AUTHOR]

The strong, meandering, and eddy‐shedding Gulf Stream is a large oceanic reservoir of both mean and eddy kinetic energy in the northwestern Atlantic. Since 2015, underwater gliders equipped with Doppler current profilers have collected over 20,000 absolute velocity profiles in and near the Gulf Stream along the US East Coast. Those observations are used to make three‐dimensional estimates of mean and eddy kinetic energy, substantially expanding the geographic coverage of prior estimates of subsurface kinetic energy in the Gulf Stream. Glider observations are combined via weighted least squares fitting with anisotropic and inhomogeneous length scales to reflect both circulation and sampling density; this averaging technique can be applied to other quantities measured by the gliders. Mean and eddy kinetic energy decay approximately exponentially away from the surface. Vertical decay scales are longest within the high‐speed core of the Gulf Stream and somewhat shorter on the flanks of the Gulf Stream. Plain Language Summary: Energy is a key metric of the Earth's climate system, of which the ocean is a major part. Kinetic energy, the energy of moving water in the ocean, is partitioned into mean kinetic energy that is associated with the time‐averaged ocean circulation and eddy kinetic energy that is associated with time‐varying motions. Here, a large set of velocity measurements collected by autonomous underwater gliders is used to make three‐dimensional estimates of mean and eddy kinetic energy in and near the Gulf Stream, one of the strongest currents in the global ocean. These new estimates of oceanic kinetic energy serve as a benchmark for numerical simulations of the ocean and climate system to reproduce. Key Points: Underwater glider observations are used to produce three‐dimensional estimates of mean and eddy kinetic energy in and near the Gulf StreamMean and eddy kinetic energy generally decay exponentially with depth and have somewhat longer decay scales within the Gulf StreamThree‐dimensional mean and eddy kinetic energy fields are available for further analyses and will be updated with future observations [ABSTRACT FROM AUTHOR]

Located between the sub-Antarctic Front and the Antarctic Polar Front, the Prince Edward Islands (PEIs) provide an essential breeding ground for top predators and are an ideal location to investigate disturbances linked to climate change. This study provides the first seasonal characterisation of surface hydrography at the PEIs, using satellite and reanalysis products from 1993 to 2016. Sea surface temperature (SST) showed consistently higher values to the north, while wind, currents and eddy kinetic energy all showed higher values to the south of the islands. The highest SST (8 °C) occurred in summer and the lowest (2 °C) in winter, with a one-to two-month delay between the maximum incoming solar radiation (in December–January) and the highest SST (in February). The highest wind speed occurred in July (10.8 m s−1) and the minimum in February (7 m s−1). Geostrophic currents were four-times larger than Ekman currents, showing lower speeds between April and June (0.25–0.30 m s−1) and a peak in August (0.45 m s−1). There were no significant differences for SST and Ekman currents between the regions upstream and downstream of the PEIs. In contrast, surface total and geostrophic current velocities were weaker downstream because the islands act as a barrier to the flow. A zone of lower wind speed, coinciding with enhanced positive wind stress curl (WSC), favouring downwelling, occurred directly upstream throughout the year. Although WSC over the PEIs was negative (upwelling-favourable), no corresponding cooling was evident in SST. This seasonal characterisation provides a baseline against which interannual/decadal variability and future changes in these parameters can be assessed. [ABSTRACT FROM AUTHOR]

FAV in pump sump has been a matter of focus for years for the major effects on the efficiency of the pump, wasting a lot of energy. In order to explore the mechanism of the dynamic characteristics of FAV and provide the help to eliminate FAV,the flow field under the bell of an axial flow pump was measured by V3V. The velocity gradient in the vortex area, vortex intensity, and eddy kinetic energy during the evolution of FAV were analyzed. The experimental results show that a large velocity gradient is a key factor to the formation of a vortex. The process of FAV formation and development is the process of vortex shedding. The evolution process of FAV can be divided into five stages: inception, development, continuance, collapse, and disappearance. The velocity gradient, vortex intensity, and eddy kinetic energy of FAV increase with time, reaching maximum values, maintain these values for 0.4s, and then decrease rapidly. The increase rate was less than decrease rate, which is different from the calculation results of the vorticity transfer equation. The vortex intensity meets the vortex tube intensity conservation theorem in the continuance stage. The research results can provide reference value for the design of pump station and vortex elimination, and then realize the energy saving. • The 3D velocity fields of the floor-attached vortices were measured by using V3V. • The floor-attached vortices evolution has development, continuance, collapse stages. • The key factor of formation of floor-attached vortices was large velocity gradient. • Intensity of vortices meets vortex tube intensity conservation in continuance stage. • The intensity increase rate of floor-attached vortices is less than decrease rate. [ABSTRACT FROM AUTHOR]

Eddy kinetic energy (EKE) in the Celebes Sea (CS) is investigated using a global, eddy-resolving, ocean general circulation model (OGCM) output. The OGCM simulation shows that a strong EKE is confined in the upper 300-m layer. The period of EKE variability varies with depth. In the 0–100-m layer, EKE has a distinct annual cycle that is strong in winter (December–February) and weak in summer (June–August). However, in the 100–300-m layer, semi-annual variation is dominant, which shows stronger EKE in spring and fall and weaker EKE in summer and winter. An eddy energy budget analysis reveals that the barotropic eddy energy conversion rate has a vertical structure similar to that of EKE. Compared to the barotropic eddy energy conversion rate, the baroclinic eddy energy conversion rate is much smaller and does not match the EKE vertical pattern. The budget analysis indicates that the variation in EKE in the CS is governed by barotropic instability of the background circulation. Further analysis reveals that the variation in the regional background circulation with depth is due to a combination of the forcing of the local monsoon and the Mindanao Current (MC) in the western tropical Pacific. [ABSTRACT FROM AUTHOR]

Mixing by mesoscale eddies profoundly impacts climate and ecosystems by redistributing and storing dissolved tracers such as heat and carbon. Eddy mixing is parameterized in most numerical models of the ocean and climate. To reduce known sensitivity to such parameterizations, observational estimates of mixing are needed. However, logistical and technological limitations obstruct our ability to measure global time‐varying mixing rates. Here, we extend mixing length theory with mean‐flow suppression theory, and first surface modes, to estimate mixing from readily available observational‐based climatological data, of salinity, temperature, pressure, and eddy kinetic energy at the sea surface. The resulting full‐depth global maps of eddy mixing can reproduce the few available direct estimates and confirm the importance of mean‐flow suppression of mixing. The results also emphasize the significant effect of eddy surface intensification and its relation to the vertical density stratification. These new insights in mixing dynamics will improve future mesoscale eddy mixing parameterizations. Plain Language Summary: Large whirls of hundreds of kilometers can mix water with different temperatures, salinity, and other properties. These whirls are called mesoscale eddies and are very difficult to include in numerical simulation of ocean and climate. Therefore, we include them using a simplified representation: a parameterization. These parameterizations need as input, the strength with which these eddies mix. Ideally, we would thus measure these mixing strengths globally and over the full depth of the ocean. However, this is impossible due to technological, logistical, and financial limitations. To avoid these limitations, we instead indirectly estimate mixing from variables that we can measure globally over the full depth of the ocean. We here present a new way to indirectly estimate mixing from widely available observations of temperature, salinity, pressure, and surface eddy kinetic energy. This results in three‐dimensional maps of eddy mixing strengths. We find that eddies mix much stronger near the surface than in the deep ocean and that this is partly caused by the vertical stratification of ocean density. These new insights and maps can be used to improve mixing parameterizations and thus significantly improve all kinds of calculations that are important for the Earth's climate and ecosystems. Key Points: We present a new parameterization for mesoscale eddy mixing that combines mixing‐length and flow‐suppression theory with vertical modesGlobal, full‐depth mesoscale eddy mixing estimates are obtained from observations of temperature, salinity, pressure, and eddy kinetic energyMesoscale eddy mixing is surface intensified and strongly influenced by the vertical stratification of ocean density [ABSTRACT FROM AUTHOR]

At the ocean surface, satellite observations have shown evidence of a large spectrum of waves at low latitudes. However, very little is known about the existence and properties of the deep variability. Most of the subsurface observations rely on localized measurements, which do not allow for a global estimation of this variability. In this study, we use velocity estimates, provided by Argo float drifts at 1,000 m, to analyze the spatial and temporal distribution of the deep eddy kinetic energy (EKE) and its spectral signature with an unprecedented time and space coverage. In the tropical Pacific, high EKE is found along the equator, at the western boundary and poleward of 7°N. EKE meridional distribution is also found to vary at the scale of the meridionally alternating mean zonal jets: it is higher inside eastward currents. We develop an original statistical scale analysis to determine the temporal and spatial scale dependence of this deep EKE footprint. We show the presence of periodic features whose characteristics are compatible with theoretical equatorial waves dispersion relations. Annual and semiannual Rossby waves are observed at the equator, as well as ∼30‐day Yanai waves, consistent with surface tropical instability waves. The location and intensification of these waves match the downward energy propagation predicted by ray tracing linear theory. Short‐scale variability (with ∼70‐day periods and 500‐km wavelength) has also been detected poleward of 7°N. The generation mechanisms of this variability are discussed, as well as its potential importance for the mean circulation. Plain Language Summary: Energy in the deep ocean is important as it is a potential driver of the deep circulation, which has important climate feedbacks. Because of its singular dynamics, the equatorial ocean is a preferential region of transfer of energy from the surface to the interior of the ocean. Very little is known, however, about the energy content in the deep equatorial oceans. In this study, we use the large number of floats, called Argo floats, drifting at 1,000‐m depth in the ocean to describe the deep kinetic energy in equatorial regions. We show that various energetic waves are present at 1,000 m in the tropical Pacific, and we discuss their potential generation mechanisms as well as their implications for the circulation. These new observations may help to validate some theories or numerical simulations of the deep equatorial and tropical circulation. Key Points: Eddy kinetic energy at 1,000 m in the tropical Pacific is investigated using Argo float driftsDeep intra‐annual variability is evidenced with 30‐day period and 1,000‐km wavelength at the equator and 70‐day period, 500 km off the equatorEddy kinetic energy exhibits small‐scale features, suggesting interactions with the mean jet‐like circulation at 1,000 m [ABSTRACT FROM AUTHOR]

Simulating Arctic Ocean mesoscale eddies in ocean circulation models presents a great challenge because of their small size. This study employs an unstructured‐mesh ocean‐sea ice model to conduct a decadal‐scale global simulation with a 1‐km Arctic. It provides a basinwide overview of Arctic eddy energetics. Increasing model resolution from 4 to 1 km increases Arctic eddy kinetic energy (EKE) and total kinetic energy (TKE) by about 40% and 15%, respectively. EKE is the highest along main currents over topography slopes, where strong conversion from available potential energy to EKE takes place. It is high in halocline with a maximum typically centered in the depth range of 70–110 m, and in the Atlantic Water layer of the Eurasian Basin as well. The seasonal variability of EKE along the continental slopes of southern Canada and eastern Eurasian basins is similar, stronger in fall and weaker in spring. Plain Language Summary: Ocean mesoscale eddies play crucial roles in the ocean, climate, and ecosystem. While this is presumably also true for Arctic eddies, their dynamics and impacts are far less understood than for lower latitudes: It is a great challenge to resolve the very small Arctic eddies in numerical simulations. Just now, owing to the development of new‐generation models, it becomes possible to resolve Arctic eddies in realistic global ocean configurations. The study presents the results from a first‐ever decadal‐scale global simulation with the Arctic Ocean at 1‐km resolution. An overview of the eddy kinetic energy and its generation is provided for the Arctic deep basin, with a focus on both spatial and seasonal variability. The current results fill some knowledge gaps in Arctic eddy energetics, and also help to identify questions that we will be able to answer with such frontier simulations in the future. Key Points: A decadal‐scale global simulation with the whole Arctic Ocean at 1‐km resolution is carried out using FESOM2A basinwide overview of Arctic eddy energetics is provided with a focus on spatial patterns and seasonal changes of eddy kinetic energyContinental slopes in southern Canada and eastern Eurasian basins are the most energetic regions featuring similar seasonality [ABSTRACT FROM AUTHOR]

Momentum input from westerly winds blowing over the Southern Ocean can be modulated by mesoscale surface currents and result in changes in large‐scale ocean circulation. Here, using an eddy‐resolving 1/20 degree ocean model configured near Drake Passage, we evaluate the impact of current‐wind interaction on vertical processes. We find a reduction in momentum input from the wind, reduced eddy kinetic energy, and a modification of Ekman pumping rates. Wind stress curl resulting from current‐wind interaction leads to net upward motion, while the nonlinear Ekman pumping term associated with horizontal gradients of relative vorticity induces net downward motion. The spatially averaged mixed layer depth estimated using a density criteria is shoaled slightly by current‐wind interaction. Current‐wind interaction, on the other hand, enhances the stratification in the thermocline below the mixed layer. Such changes have the potential to alter biogeochemical processes including nutrient supply, biological productivity, and air‐sea carbon dioxide exchange. Plain Language Summary: Momentum transfer between winds blowing over the Southern Ocean depends on the relative speed of the winds and surface currents. Mesoscale eddies with a scale of 100 km or less are very vigorous and thus can modulate momentum transfer. Here, we use an ocean model with sufficiently high horizontal resolution that it can resolve the mesoscale and hence capture the modulation. We find a reduction in the momentum transfer from the wind to the ocean and a reduction in eddy kinetic energy, together with a modification of wind‐driven vertical motion. Structural changes in the wind stress field modify patterns of upwelling and downwelling in a manner that can be understood from nonlinear Ekman theory. Moreover, current‐wind interaction results in an increase in the stratification below the mixed layer and hence a reduced communication between the surface and the interior ocean. There is thus a potential impact on biogeochemical processes and the climate of the Southern Ocean. Key Points: High‐resolution Southern Ocean simulations suggest that current‐wind‐stress interaction on the mesoscale reduces eddy kinetic energy by 25%Current‐wind interaction induces a net upward linear but downward nonlinear modulation of Ekman pumping ratesCurrent‐wind interaction enhances stratification near the bottom of the mixed layer by up to 10% [ABSTRACT FROM AUTHOR]

The Slope Sea is the dynamic ocean region located between the United States and Canadian northeast continental shelves and the northeastward flowing Gulf Stream (GS) located farther offshore. Here we define it as located between the 200-m isobath and the monthly GS sea surface temperature (SST) front from −75° to −55° E. Monthly mean near-surface eddy kinetic energy (EKE) was computed for the Slope Sea using surface geostrophic current anomalies derived from gridded 1993–2016 Copernicus Marine Environment Monitoring Service (CMEMS) sea height anomalies. Long-term, monthly mean Slope Sea EKE anomalies show a robust seasonal cycle with a winter (February) minimum and summer (June) maximum. This agrees with both seasonally-varying density stratification and dissipation and also the seasonal variation in the formation of GS WCRs within the Slope Sea. The RMS of the Slope Sea EKE seasonal cycle generally increased after 2002 by a factor of up to ∼2 relative to prior years. The seasonal cycle of Slope Sea EKE displayed higher EKE in the vicinity of the New England Seamount Chain (NESC) that extends towards the shelf break front from approximately −67° E to −63° E. Interannual variability of annual mean near-surface EKE from individual digitized GS warm core ring (WCR) observations from a Bedford Institute of Oceanography (BIO) WCR database is highly correlated with Slope Sea EKE. However, interannual variability of annual mean near-surface EKE computed from a census of all newly formed WCRs displayed only a weak correlation. Many of the WCRs from both the BIO and WCR census displayed anomalously low EKE values and were observed within the northern Slope Sea away from the GS. Some were located inshore of the position of the climatological mean shelf break front. WCRs with higher EKE were located throughout the Slope Sea, with higher numbers in the vicinity of the NESC. The many observations of the less energetic features located close to or inshore of the mean shelf break front suggest they are important to cross-shelf fluxes of heat, salt, nutrients, shelf biota. They therefore likely impact the shelf ecosystem, similar to the more energetic and typical WCRs impacting the outer shelf as discussed by earlier workers. • Altimeter-derived currents describe eddy kinetic energy changes in the slope sea. • Slope sea eddy kinetic energy displays strong seasonal and inter-annual variation. • Slope sea eddies display large variation in strength and formation location. • Eddy kinetic energy changes do not support the warm-core ring regime shift of 2000. [ABSTRACT FROM AUTHOR]

The Kuroshio Extension (KE) plays an important role in climate and environmental change in the North Pacific. In this paper, more than 20 years of merged absolute dynamic topography and merged sea level anomaly products from satellite altimetry are used to analyze the stability of the KE system. By analyzing the annually averaged sea surface topography, the variations of inter-annual path and annually averaged eddy kinetic energy at the KE region, the KE's two dynamic states are given as: the relatively stable state during 1993–1995, 2002–2005, and 2010–2012, and the unstable dynamic state among 1996–2001 and 2006–2009. During the stable state, the KE spindle had a shorter path length and smaller time-varying amplitude, as well as a trend to move northward. While during the unstable state, the KE spindle had a longer path length and an integral southward transport trend, and was observed to oscillate significantly over time. The analysis on the KE's upstream and downstream region gives the same variations, indicating that they are significantly affected by the El Nino events. The power spectrum of the mean latitudinal position variation of the KE's upstream and downstream shows significant quasi-decadal oscillation characteristics and strong annual signals. Furthermore, the correlation of the strength variation between the southern RG and the KE's upstream is calculated to be 0.50 after low-pass filtering, and that of the mean latitudinal position variation between the southern RG and the KE's upstream/downstream are 0.75/0.69 after low-pass filtering, respectively. The strong correlations demonstrated that the southern RG and the KE are closely linked.

The modulation of the seasonal variability of the surface eddy kinetic energy (EKE) in the Kuroshio extension (KE) region induced by the stable and unstable KE states is analyzed using satellite‐based observations and high‐resolution ocean reanalysis data. The two KE states are found to strongly modulate the seasonal variability of the local surface EKE: the latter reaches its maximum in May–July if the KE is in its stable state, while it peaks in August–October if the KE is in its unstable state. Further investigation indicates that such behavior is mainly caused by the different baroclinic energy transfers induced by the meridional density gradients associated with the two KE states. Plain Language Summary: Ocean mesoscale eddies have an important effect on physical and biological processes; it is, therefore, important to understand the main features of the mesoscale eddy activity. The Kuroshio extension (KE) region is the most active region of mesoscale eddies in the North Pacific Ocean. On the decadal time scale, the KE yields two main states: a stable and an unstable state. Whether these two states have an effect on the mesoscale eddy activity in the KE region is worth investigating. Our study shows that the KE bimodality affects considerably the seasonality of the mesoscale eddy activity, as represented by the surface eddy kinetic energy (EKE) field: when the KE takes the stable state, the EKE level attains its highest value in summer, while this occurs in autumn if the KE is in its unstable state. The different baroclinic energy transfers induced by the meridional density gradients associated with the two KE states are responsible for the differences in the EKE seasonal cycle. Key Points: The KE bimodality plays an important modulation role in the seasonal variability of surface EKE in the KE regionSurface EKE reaches its maximum in summer when the KE takes the stable state, while it peaks in autumn when the KE is in its unstable stateDifferent baroclinic instabilities related to meridional density gradients are responsible for the differences in the EKE seasonal cycle [ABSTRACT FROM AUTHOR]

Sudden stratospheric warming (SSW) events have been suggested to be followed by a surface impact, though this response varies between events. Using reanalysis data, we identify two types of tropospheric responses to SSWs: Two thirds of the SSW events are dominated by a zonally symmetric tropospheric response with an equatorward shift of the jet in the Atlantic, consistent with the canonical SSW response in the form of a negative signature of the North Atlantic Oscillation. For the remaining third of SSW events, a zonally asymmetric response is found, associated with a poleward shift of the jet in the Atlantic. The Pacific is found to contribute to the sign of the North Atlantic response, as synoptic wave propagation from the Eastern Pacific links the Pacific and Atlantic storm tracks for both equatorward and poleward jet responses. Plain Language Summary: The stratosphere, the layer of the atmosphere starting at about 10 km above the Earth's surface, can have a major impact on surface weather in winter, in particular during stratospheric extreme events, known as sudden stratospheric warmings. The tropospheric response is strongest in the North Atlantic and Europe; however, not all events exhibit the same surface response, which remains a major open research question. Here, we analyze the changes in surface weather after 26 sudden stratospheric warming events. We identify two types of responses: For two thirds of the events, the effect of the stratosphere leads to a southward shift of the westerly jet in the Atlantic and an eastward extension in the Pacific. For the remaining one third of events, sudden stratospheric warmings are associated with a northward shift of the jet in both basins. We find that the anomalous weather patterns in the Pacific may contribute to the sign of the Atlantic response by propagation of synoptic storms from the East Pacific toward the Atlantic. The results of this study can potentially improve the understanding of the coupling between the stratosphere and surface weather and help to extend weather prediction timescales. Key Points: On average over all events, SSW events are followed by a reduction in North Atlantic storm track activityOne third of the SSWs does not exhibit the canonical downward response but shows an increase in storm track activity in the North AtlanticThe Pacific is found to contribute to the sign of the North Atlantic response in both cases [ABSTRACT FROM AUTHOR]