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.

The western South China Sea (WSCS) exhibits intense mesoscale eddy activity, especially in summer. However, the influence of the eddies on regional ecosystems is not well understood. By analyzing historical in situ observations, satellite‐derived near‐surface chlorophyll (CHL) data and eddy data, we investigated the composite response of CHL to eddies in the WSCS. The results show that eddies decrease CHL in the WSCS and that nearly half of this decrease is attributed to the activity of anticyclonic eddies during summer when they are much more energetic than cyclonic eddies. A comparison between local and nonlocal eddies suggests that the CHL response to the eddies is dominated by local dynamics rather than being trapped and transported from neighboring regions. In situ observations provide evidence that intense convergence and downwelling in anticyclonic eddies deepen the mixed layer depth and subsurface CHL maximum depth. The decreased nutrients in the euphotic layer inhibits the growth of phytoplankton, resulting in a weakened subsurface CHL maximum (by 54%) and decreased CHL in the entire euphotic layer (by 41% for the upper 100 m). Considering the greater number and area of anticyclonic eddies compared with cyclonic eddies, eddy activity might cause an upper ocean CHL decrease of ~7% in the WSCS in summer. This study demonstrated that the response of CHL to eddies in the WSCS differs in both spatial distribution and seasonal variation from those in other subregions of the SCS, emphasizing the importance of considering regional variability in eddies and large‐scale environments when evaluating the biogeochemical influences of eddies.

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.

Marine heatwaves have attracted great attention for their devastating impacts on marine organisms. However, the influence of marine heatwaves on phytoplankton community structure, a key aspect of pelagic ecosystems, remains elusive, especially in the biologically productive eastern boundary upwelling systems. Here, using a combination of multi-satellite observations and model outputs, we discover that phytoplankton in these upwelling systems exhibit not only a marked reduction in biomass (~50%), but also a significant shift toward smaller species in community structures during marine heatwaves, featuring a substantially increased/decreased dominance in picophytoplankton/microphytoplankton. This change in community structure is positively related to the intensity and duration of marine heatwaves, and is likely a result of community competition under reduced nutrient supply due to weakened coastal upwelling associated with poleward wind anomalies. These findings invite us to rethink the dominance of phytoplankton in the eastern boundary upwelling systems under extreme conditions, and may have important implications for local fisheries and carbon cycle under the ongoing warming.