By analyzing 22 years (1993–2015) of daily eddy data, statistics of surface eddy properties were refreshed in the South China Sea. More than 7,000 of historical Argo profiles were collocated into eddy‐centered coordinates to reveal the composite mean three‐dimensional structure of eddies. The results indicate that eddies of both polarities have long conical shape, with a maximum (minimum) density anomaly of 0.55 kg/m³ (−0.51 kg/m³) at 60 m (90 m) in the composite cyclonic (anticyclonic) eddy. Temperature and salinity anomalies also peak at eddy cores, with values of −1.5 °C and 0.15 psu in the cyclonic eddy and 1.4 °C and −0.16 psu in the anticyclonic eddy. The temperature and density anomalies extend vertically to 400–500 m, while the salinity anomalies are apparent only in the upper 150 m. The temperature anomalies contribute about 90% of the density anomalies. Mixed layer depths in cyclonic eddies are on average 15 m shallower than those in anticyclonic eddies. The rotation of the composite cyclonic (anticyclonic) eddy generates meridional heat transport of 1.4 × 10¹² W (−3.1 × 10¹² W) and salt transport of −4.0 × 10⁴ kg/s (5.6 × 10⁴ kg/s). More than 90% of the heat and salt transports are concentrated in the upper 300 and 100 m, respectively. Compared to the meridional transports, the westward propagation of eddies results in zonal heat and salt transports on the same orders of magnitudes. The westward propagation of eddies also generates a basin‐scale westward water transport of 1.4 Sv, equivalent to about 30% of the annual‐mean Luzon Strait transport.

Extreme ocean temperature events are becoming increasingly common due to global warming, causing catastrophic ecological and socioeconomic impacts^1,2,3,4,5. Despite extensive research on surface marine heatwaves (MHWs) and marine cold spells (MCSs) based on satellite observations^6,7, our knowledge of these extreme events and their drivers in the subsurface ocean—home to the majority of marine organisms—is very limited^8,9. Here we present global observational evidence for the important role of mesoscale eddies in the occurrence and intensification of subsurface MHWs and MCSs. We found that 80% of measured MHWs and MCSs below a depth of 100 m do not concur with surface events. In contrast to the weak link between surface MHWs (MCSs) and ocean eddies, nearly one-third of subsurface MHWs (MCSs) in the global ocean, and more than half of such events in subtropical gyres and mid-latitude main current systems, occur within anticyclonic (cyclonic) eddies. These eddy-associated temperature extremes have intensified at rates greater than background level in past decades, suggesting a growing impact of ocean eddies on subsurface MHWs and MCSs with ongoing global warming.

Oceanic barrier layer impedes the vertical exchanges of heat and biogeochemical substances between the mixed layer and deep ocean, playing essential roles in modulating air‐sea heat flux and oceanic biogeochemical cycles. Recent studies revealed that mesoscale eddies can modify the vertical motions of the mixed layer. But their influence on the barrier layer has not been determined. By analyzing more than a decade of in situ, satellite, and reanalysis data, this study investigated the influence of eddies on barrier layer in the Bay of Bengal. The results show that the barrier layer in anticyclonic eddies is significantly thickened in winter (by 50% on average). This thickening consists of the deepening of isothermal layer by eddy downwelling and the shoaling of mixed layer by near‐surface salinity restratification. The thickening of the barrier layer enlarges with latitude, in consistent with the reinforcement in salinity stratification and surface cooling. The thickened barrier layer impedes the upward supply of heat into the mixed layer from the below, promoting the winter surface cooling (by 0.7°C on average) and subsurface temperature inversion (by 0.6°C on average). The feature is contrary to the intuitive expectation that anticyclonic eddies possess high sea surface temperature in eddy interiors. As a consequence, both cyclonic and anticyclonic eddies result in negative sea surface temperature anomalies, indicative of a net decrease in the local sea surface temperature. These effects of eddies may be a common feature in salinity‐stratified oceans in winter and have potential implications for the local biogeochemistry and air‐sea coupled models.

With the combined analysis of synthetic aperture radar image and satellite altimeter data collected in the northeastern South China Sea (SCS), this study found one type of distorted phenomenon of internal solitary wave (ISW) with the long front caused by the oceanic mesoscale eddy. Motivated by these satellite observations, the authors carried out numerical experiments using the fully nonhydrostatic and nonlinear MITgcm to investigate the perturbation of ISWs by an isolated cyclonic or anticyclonic eddy. The results show that the ISW front is distorted by these oceanic eddies due to the retardation and acceleration effects at their two sides. The ISW energy along the front is focused onto (scattered from) the wave fragment where a concave (convex) pattern is formed, and the previously accumulated energy in the focusing region is gradually released after the ISW propagates away from the eddies. The ISW amplitude is modulated greatly by the eddies due to the energy redistribution along the front. Sensitivity results indicate that the magnitude of the modulated ISW amplitude in the focusing region can reach twice the incident ISW amplitude, while in the scattering region it can be reduced by more than a half. These results therefore suggest that models with eddies included, especially the energetic eddies, could further improve the amplitude predictions in the northeastern SCS. Additionally, the internal gravity wave formed behind the energy-focusing region by the anticyclonic eddies can steepen and break with the consequent formation of a secondary trailing ISW packet. Finally, this study shows that the model results of the distorted front and trailing packet are in qualitative agreement with that of the satellite observations in the northeastern SCS.