As a major sink of anthropogenic heat and carbon, the Southern Ocean experienced pronounced warming with increasing extreme temperature events over the past decades. Mesoscale eddies that strongly influence the uptake, redistribution, and storage of heat in the ocean are expected to play important roles in these changes, yet observational evidence remains limited. Here, we employ a comprehensive analysis of over 500,000 historical hydrographic profile measurements combined with satellite-based eddy observations to show enhanced thermal eddy imprints in the Southern Ocean. Our observations reveal that anticyclonic (cyclonic) eddies are responsible for nearly half of the subsurface high (low)-temperature extremes detected, although only 10% of the profiles are located in eddy interiors. Over the past decade (2006 to 2019), both mean and extreme temperature anomalies within eddies in the Antarctic Circumpolar Current increased significantly, promoting the rise in subsurface ocean temperature variability. This enhanced role of eddies is likely a result of enhanced eddy pumping due to the increase in eddy intensity and ocean stratification caused by ocean warming. Our analysis underscores the crucial role of eddies in amplifying ocean temperature variability and extremes, with their effects expected to be even more pronounced as global warming persists.

Large‐scale circulations are quite typical processes in the northern South China Sea (SCS) deep basin, yet their impacts on the variation of internal solitary waves (ISWs) remains poorly understood. We, here, focus on impacts of the upstream SCS western boundary current (SCSwbc) and three typical circulation patterns associated with different Kuroshio intruding paths on ISW propagation in this region. We show that the ISW modulated by the upstream SCSwbc gets a speedup, which is comparable to that induced by Coriolis effect, while the ISW amplitude shows an obvious reduction. Statistically, there is approximately one‐third time of 23 years (1993–2015) when circulation‐induced changes of wave speed exceed Coriolis‐induced changes. Specifically, the looping circulation pattern has the highest impact on significant speedup and amplitude reduction among the three patterns, while the leaping pattern has the lowest impact. These differences in wave characteristics caused by different circulation patterns result from the wave scattering, focusing and Doppler effects.

The vertical structure of eddies in the Leeuwin Current system affects the eddy volume, heat, and salt transport, even the ecosystem. However, the understanding of eddy vertical structure and eddy‐induced transport here are still very limited. In this study, satellite observed sea surface heights were combined with decade‐long in situ measurements of Argo floats to study the vertical structure of mesoscale eddies in the LC system and their volume, heat, and salt transport. A novel eddy reconstruction method, which considers the influences of both the eddy and background flow, is devised to study the three‐dimensional structure of eddies. Result shows that, in LC system, anticyclonic eddies (AEs) are usually surface‐intensified, with the geostrophic velocity decreasing sharply below the mixed layer, while cyclonic eddies (CEs) are subsurface‐intensified, with a maximum speed at 240 m. The density anomaly core of the average AE (CE) is at a depth of 130 m (650 m) with a density anomaly of −0.51 (0.24) kg/m³. The volume‐integrated eddy kinetic energy and available potential energy of the average CE are much larger than those of the average AE. The average lifespan of CEs is significantly longer than that of AEs, which can be explained by the deeper vertical scale of CEs. The offshore volume transport by eddy drift across the coastal (107°E) section is 9.05 Sv (12.5 Sv). The heat and salt onshore transport by eddy drift across the coastal (107°E) section are, respectively, 10.6 Tw and 143.1 ton/s (17.1 Tw and 241.0 ton/s).

Internal solitary waves (ISWs) are a ubiquitous phenomenon in the dynamic ocean system, which play a crucial role in driving transport through turbulent mixing. Over the past few decades, numerical modelling has become a vital approach to investigate the generation mechanism and spatial distribution of ISWs. The northern South China Sea (NSCS) has been treated as a physical oceanographic focus of ISWs in massive numerical studies since the last century. However, there has been no systematic evaluation of a reliable three-dimensional (3D) model about accurately reproducing ISW characteristics in the NSCS. In this study, we implement a 3D ISW forecasting model in the NSCS and quantitatively evaluate the requirements of factors (i.e. model resolution, tidal forcing, and stratification selection) in accurately depicting ISW properties by comparison with observational data at a mooring station in the vicinity of the Dongsha Atoll. Firstly, the 500 m resolution model can basically reproduce the principal ISW characteristics, while the 250 m resolution model would be a better solution to identify wave properties, specifically increasing 40 % accuracy of predicting characteristic half-widths. Nonetheless, a 250 m resolution model spends nearly 5-fold the computational resources of a 500 m resolution model in the same model domain. Compared with the former two, the model with a lower resolution of 1000 m severely underestimates the nonlinearity of ISWs, resulting in an incorrect ISW field in the NSCS. Secondly, the model with 8 (or 13) primary tidal constituents can accurately reproduce the real ISW field in the NSCS, while the one with four main harmonics (M2, S2, K1 and O1) would underestimate averaged wave-induced velocity for about 38 % and averaged mode-1 wave amplitude for about 15 %. Thirdly, the model with the initial condition of field-extracted stratification gives a better performance in predicting some wave properties than the model with climatological stratification, namely 13 % improvement of arrival time and 46 % improvement of characteristic half-width. Finally, background currents, spatially varying stratification and external (wind) forcing are discussed to reproduce a more realistic ISW field in the future numerical simulations.