In 3 consecutive years from 2016 to 2018, extreme ocean warming events, or marine heatwaves (MHWs), occurred during boreal summers in the East China Sea (ECS) and South Yellow Sea (SYS), which is unprecedented in the past four decades based on the satellite record. In this study, we used a high‐resolution hydrodynamic model based on Finite Volume Community Ocean Model (FVCOM) to simulate the evolution of these warming events. An upper ocean temperature budget (0–20 m) analysis based on the model results shows that the shortwave radiation and the ocean advection anomalies jointly contributed to the anomalous warming in the three successive summers (June–August) in the SYS and the north part of the ECS. In addition, the reduction of surface wind speeds during the 2016 and 2017 summers further weakened the vertical mixing, thereby enhancing the anomalous warming in the north part of the ECS adjacent to the SYS. During the three summers, the increases of shortwave radiation were closely related to the East Asian Summer Monsoon (EASM) variability, which reduced the cloud cover in the ECS and SYS, whereas the advection anomalies were mostly associated with regional wind anomalies. In summer 2018, upper ocean heat was transported into the central trough of the SYS, accumulated in an anticyclonic eddy generated by the anomalous wind stress curls. Therefore, despite the primary driver of the MHWs is the EASM variation, regional processes are critical to driving the spatial pattern of the MHW intensity in the ECS and SYS.

In this study, the numerical wave model Simulating Waves Nearshore (SWAN), which resolves nearshore wave processes, and a hydrodynamic model, the Finite-Volume Community Ocean Model (FVCOM), were coupled to simulate waves and currents during Typhoon Fung-wong (2014) and Typhoon Chan-hom (2015) around the Zhoushan Islands. Both of these models employ the same unique unstructured grid. In particular, the influence of sea-surface currents, e.g., typhoon-induced and tidal currents, as well as the sea-water level, on wave simulation was studied. The composite wind field, which is derived from the parametric Holland model and European Centre for Medium-Range Weather Forecasts (ECMWF) winds (H-E winds), was taken as the forcing field. TPXO.5 tide data, sea-surface temperatures from the HYbrid Coordinate Ocean Model (HYCOM), HYCOM sea-surface salinity, and HYCOM sea-surface current were treated as open-boundary conditions. The comparison of sea-surface-current speed between the FVCOM simulation and the National Centers for Environmental Prediction (NCEP) Climate Forecast System Version 2 (CFSv2) data revealed a root-mean-square error (RMSE) of about 0.1206 m/s, with a correlation (Cor) more than 0.8, while the RMSE of the simulated sea-water level when compared with the HYCOM data was around 0.13 m, with a Cor of about 0.86. The validation indicated that the simulated results in this study were reliable. A sensitive experiment revealed that the sea-water level affected the typhoon-induced wave simulation. Validation against the measurements from the moored buoys showed an RMSE of <0.9 m for the sea-water level, which specifically reflected less overestimation during the high-sea state. Moreover, the significant-wave-height (SWH) difference (SWH without the sea-water level minus SWH with the sea-water level) was as great as −0.5 m around the Zhoushan Islands during the low-sea state. Furthermore, we studied the typhoon-induced waves when Typhoon Fung-wong passed the Zhoushan Islands, revealing that the reduction of SWH could be up to 1 m in the Yangtze Estuary and tidal flats when the maximum waves occurred.

Moored current measurements were made at one mooring site in the northern Gulf of Tonkin for about 1 year during 1988–1989. Analyses were performed to examine characteristics and variability of tidal and subtidal flows. Rotary spectra showed two peaks at diurnal and semidiurnal periods, with higher diurnal energy. Complex demodulations of diurnal and semidiurnal tidal currents indicated that the tidal current magnitudes varied significantly with seasons: more energetic in the stratified summer than in the vertically well‐mixed winter. The observed subtidal currents were highly correlated with the surface wind in winter but not in summer; challenging the conceptual summertime anticyclonic circulation pattern derived using wind‐driven homogenous circulation theory. The computed currents from a global ocean model were in good agreement with the observed currents. Similar to the current observations, the model‐computed flow patterns were consistent with the conceptual wind‐driven circulation pattern in winter but opposite in summer. Process‐oriented experiments suggest that the summertime cyclonic circulation in the northern Gulf of Tonkin forms as a result of the combination of stratified wind‐driven circulation and tidal‐rectified inflow from Qiongzhou Strait. The interaction between the southwest monsoon and buoyancy‐driven flow from Hong River can significantly intensify the cyclonic circulation near the surface, but its contribution to the vertically averaged flow of the cyclonic circulation is limited.

The Subantarctic Mode Water (SAMW) is a thick low‐stratified subsurface water mass located north of the Antarctic Circumpolar Current, and it is important for heat uptake. How the SAMW has changed and what drives that change remain to be quantified. By synthesizing observations, eddy‐resolving ocean model hindcasts and climate model simulations, we show that the SAMW has become warmer, fresher, lighter, and weaker in recent decades. As revealed by partially coupled model experiments, the SAMW core has remained at a constant depth since 2004 due to a balance between the enhanced wind stress curl, which deepens the SAMW core, and the buoyancy gain, which shallows the core. The lightening and weakening of the SAMW mainly result from the increased surface buoyancy gain due to climate warming, while the enhanced westerlies play a secondary role in slowing this trend. As greenhouse warming continues, the increased stratification will keep weakening the SAMW.

As discrete prolonged extreme warm water events, marine heatwaves (MHWs) have become more frequent, stronger and longer‐lasting during the past several decades. The relative contributions of external forcing and internal variability to these changes and their underlying drivers remain unclear. Here, analyses of 90 simulations in CESM2 reveal that external forcing dominates the increasing frequency by causing the mean warming of sea surface temperature (SST), accounting for 82% of the observed trends. Both the mean warming and increased variance of SST contribute to the longer‐lasting MHWs during 1982–2021, with external forcing contributing 38% of the increase in the SST variance for global average. Internal variability, especially the Inter‐decadal Pacific Oscillation (IPO), is closely associated with regional MHW changes. The observed negative IPO trend during 1982–2021 is related to increasing, strengthening and longer‐lasting MHW over Kuroshio Extension, but decreasing and shorter‐lasting MHW over the Northeast Pacific Coast.