Mesoscale eddies are important for regulating oceanic energy. Variation in eddy shapes leads to uncertainty in calculations of heat or energy content. In this study, we investigated the deformation of a warm eddy in the northern South China Sea from April to June 2018 and elucidated the mechanism governing the deformation. Satellite altimetry images showed that the warm eddy originated from Luzon Strait (LS eddy), migrated westward, and then moved along 500‐m isobaths until it approached the east of Hainan Island. Thereafter, the LS eddy deformed, moved southward, merged with other eddies, and finally dissipated. Using ship and virtual‐mooring Chinese underwater glider observations, we examined the three‐dimensional structure of the LS eddy. The warm eddy had a low‐density core that reached a depth of 250 m. The LS eddy gave rise to a front along its eastern edge, and two associate submesoscale eddies with horizontal radii of approximately 10 km were found at the front. The warm eddy was circular before deformation but morphed into an egg shape after deformation. This shape change allowed the eddy to entrain additional water mass (approximately 1014 kg). The deformation event was able to be forecasted by the vorticity and deformation index, as the eddy deformed by leaking into the zone with a high vorticity and deformation index. We used modeling and energy transformation calculations to analyze the mechanism of the warm eddy deformation. Our results revealed that baroclinic instability played a primary role in the deformation event.

Some mesoscale eddies intrude over the continental slope in the northern South China Sea, supporting cross‐shelf matter transport. We investigated the characteristics, the intruding tracks and formation mechanisms, of slope intrusion mesoscale eddies using satellite altimeter data and model outputs. In total, 36 and 22 slope intrusion anticyclonic and cyclonic eddies (SAEs/SCEs) are found, respectively. Slope intrusion eddies have longer lifetimes (~58 days), smaller size (~110 km), and greater eddy kinetic energy and vorticity compared to ordinary eddies but are more unstable and more easily deformed during their life cycles. The statistical results show that more slope intrusion eddies are generated during winter than other seasons. It is found that slope intrusion eddies mainly propagate westward/northwestward, and southwestward along the continental slope and shelf. Eddy intrusions occur mainly near the Dongsha Islands, east of Hainan, and north of the Xisha Islands. SAEs continue to propagate onshore after arrival at the continental slope, while SCEs dissipate more quickly. Using mooring data, we found that eddy‐ambient flow interaction could cause the differences between SAEs and SCEs around the Dongsha Islands. Energy conversion was analyzed in these three regions using numerical products. During intrusion, eddies lose eddy kinetic energy and ambient flows gain energy.

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Material transport caused by mesoscale eddies has been revealed much in the open ocean; however, it is still unclear how much eddy‐induced mass transport in the slope region of the northern South China Sea (SCS). Using the LASG/IAP Climate System Ocean Model (LICOM) from 2007 to 2017, we identified 47 anticyclonic eddies and 97 cyclonic eddies that intruded onto the continental slope, termed slope intrusion eddies. The slope intrusion eddies are more horizontally asymmetric and energetic than those without entering the slope. These eddies induced cross‐slope heat and salt transport of O (1012) W and O (104) kg s−1 owing to their horizontal asymmetry in both Xisha Islands and Dongsha Islands, where are the intrusion zones of mesoscale eddies. Based on the potential vorticity budget, we found that the horizontal asymmetry of velocity was caused by the asymmetry of potential vorticity, which was mainly generated by eddy‐current nonlinear effect in the Dongsha Islands and topographic beta effect in the Xisha Islands, respectively. This study may promote our understanding on the mesoscale dynamics and oceanic energy redistribution in the continental shelf zone of marginal sea.