To better understand eddy‐induced heat and salt transport, a targeted joint hydrographic investigation focusing on an anticyclonic eddy was carried out in July 2017 in the northern South China Sea. In situ and satellite observations together with Hybrid Coordinate Ocean Model (HYCOM) output show the transport of subsurface saline Kuroshio water into the northern South China Sea by an anticyclonic eddy. Subsurface high‐salinity cores are consistent with the anticyclonic eddy centers. The transport of saline Kuroshio water occurs in two stages accompanied by eddy shedding. First, saline Kuroshio water is trapped within the anticyclonic eddy at its generation location. Then, although the salinity within the eddy gradually weakens as the eddy carries the saline water westward, the quantity of saline water (>34.70 psu) shows a sharp increase, dominated by eddy‐induced salinity advection. A diagnosis of the salinity budget further confirms that the contribution of eddy flow advection is greater than that of mean flow advection. The saline Kuroshio water is trapped and conveyed in this anticyclonic eddy, providing vital evidence and important implications for eddy‐induced salt transport and water exchange.

The properties and heat budget of marine heat waves (MHWs) on the northern South China Sea (SCS) continental shelf are investigated. MHWs with warming amplitudes above 1.5°C occur mainly along the coast, and their temperature anomaly decreases toward the open sea. MHWs with 1°–1.5°C warming and duration < 20 days dominate the northern SCS continental shelf. A heat budget analysis indicates that the main heat source is the sea surface net heat flux. Oceanic processes are dominated by the advection of mean temperature by the anomalous horizontal velocity (advha). The net contribution of advha always cools the upper layer of the ocean, resulting in the decay of MHWs. Active cross-slope water exchanges exist at the east and west sides of the northern SCS continental shelf edge, which makes the dominant contributions to the advha. In the MHW developing phase, the west (east) side makes a positive (negative) contribution to the advha. In the decay phase, both sides make a negative contribution to the advha, resulting in the rapid decay of MHWs. Although the contribution of advha to the heat budget varies along the northern SCS continental shelf edge, its net effect always cools the MHWs over the shelf. These results provide new insight into the characteristics and formation mechanism of MHWs on the northern SCS continental shelf; in particular, they clarify the respective contributions of air–sea flux and oceanic processes to MHWs. Significance Statement: Marine heat waves (MHWs) are unusual warming events in oceans that heavily affect marine ecosystems and arouse great concern from citizens. MHWs are active in the northern South China Sea (SCS) continental shelf. On the northern SCS continental shelf, the sea surface net heat flux is the main heat source of MHWs, and ocean current anomalies always cool the upper layer of the ocean. Active cross-slope water exchange at the east and west sides of the northern SCS continental shelf edge is the main oceanic way that cools the water on the shelf, eventually resulting in the decay of MHWs.

In situ data suggest that there is a subsurface speed maximum between 70 and 120 m in the Kuroshio, which is first revealed by the directly velocity observation. The subsurface speed maximum can also be advected into the largest marginal sea in the tropics, that is, the South China Sea (SCS). As it enters the SCS, its depth shoals to between 40 and 70 m and then finally disappears in the western northern SCS. Strong subsurface speed maximum intrusion results in strong baroclinic instability, which triggers vigorous baroclinic conversion, consequently the active mesoscale eddy variability in the northern SCS, and vice versa. Our study highlights this intuitive bridge connecting the Kuroshio intrusion and the mesoscale eddy activities in the northern SCS.

The vertical propagation of middepth (200–1,500 m) zonal velocity in the equatorial Indian Ocean (EIO) and its relationship with surface wind forcing are investigated. We focus on semiannual time scales, using in situ mooring observations and an oceanic reanalysis product that can reasonably well reproduce the structure and magnitude of the observed middepth zonal velocity in the EIO. The pronounced semiannual cycle in observed middepth zonal currents indicates a clear upward phase propagation with particularly robust signatures at depths of 500–1,000 m between 3°S to 3°N, 70 to 85°E. The main characteristics of the middepth currents are consistent with vertically propagating first‐meridional‐mode Rossby wave at semiannual period. Theory suggests that equatorial Kelvin waves, forced by zonal wind variations in the western half of the basin, transfer energy downward and eastward, reflecting into Rossby waves at the eastern boundary. These reflected Rossby waves transfer energy downward and westward, contributing greatly to observed phase propagation at middepths in the EIO.