This paper presents an innovative approach to enhance the assimilation of high-resolution simulated observations, specifically targeting Surface Water Ocean Topography (SWOT) Ka-band Radar Interferometer Sea Surface Height (SSH) products, within the Regional Ocean Modeling System (ROMS). Responding to the demand for improved assimilation techniques, we developed a multi-scale Four-Dimensional Variational Data Assimilation (4DVAR) system, building upon validated fine-scale correction capabilities from prior studies. The multi-scale strategy was extended to the ROMS-4DVAR system, providing a comprehensive solution for assimilating high-resolution observations. Leveraging the Observing System Simulation Experiment (OSSE) framework, we conducted a twin experiment comprising a nature run and a free run case. Subsequently, synthetic SWOT SSH measurements were decomposed, considering the model configuration resolution. These components, derived from dense SSH observations, were integrated into a two-step 4DVAR assimilation scheme. The first cycle targets large-scale features for model field correction, and the updated analysis serves as the background for the second assimilation step, addressing fine-scale observation components. Comparisons with the primitive ROMS-4DVAR using a single-scale scheme highlight the superiority of the multi-scale strategy in reducing gaps between the model and the SSH observations. The Root Mean Squared Error (RMSE) is halved, and the Mean Absolute Percentage Error (MAPE) decreases from 2.237% to 0.93%. The two-step assimilation procedure ensures comprehensive multi-scale updates in the SSH field simulation, enhancing fine-scale features in the analysis fields. The quantification of three-dimensional-model dynamic fields further validates the efficiency and superiority of the multi-scale 4DVAR approach, offering a robust methodology for assimilating high-resolution observations within the ROMS.

Under global warming, the South China Sea (SCS) is experiencing increasingly severe marine heatwaves (MHWs), with impacts on marine ecosystems such as coral reefs and marine pastures becoming more evident. The numerous anticyclonic eddies (AEs) distributed in the SCS are important drivers of MHW generation and development, yet their impacts on MHWs are still not fully understood. In this study, the vertical structures of various types of MHWs inside the AEs and in the background field were mapped and compared, and we found that AEs of varying amplitudes have distinct impacts on the vertical structures of MHWs. MHWs inside the AEs can be divided into two categories: subsurface-reversed MHWs and subsurface-intensified MHWs. The former is manifested as anomalous cooling in the subsurface, driven by the uplift of thermocline due to the inhibition of downward mixing. The latter is characterized by anomalous warming in the subsurface, resulting from strong vertical warm-water subsidence induced by large-amplitude AEs. This process may penetrate the thermocline and produce maximum warming anomalies in the layer beneath the region of greatest temperature gradient change. Our research reveals characteristics of various vertical structures of MHWs in the SCS, attributing their differences to the stable water layer’s different response to varying intensities of vertical heat conduction, and deepening people’s understanding of the impact of AEs in the SCS on the vertical structure of MHWs.

Using satellite-observed data and reanalysis data, we studied the spatiotemporal variation characteristics and dynamic mechanisms of thermal fronts in the Beibu Gulf (TFIBG). TFIBG occur in December, reach their strongest point in January in the following year, and then gradually weaken until they completely disappear in May. Their formation is related to the bathymetry of the Beibu Gulf. In winter, the seawater in shallow-water areas (deep-water areas) cools down more (less), and Ekman currents concurrently transport warm water from the central basin of the Beibu Gulf to the west coast, which results in the formation of a thermal front at the junction of cold and warm water. The interannual variation in TFIBG intensity is related to the northeast monsoon. The strengthened (weakened) Ekman current caused by the northeast monsoon transports more (less) warm water from the central basin of the Beibu Gulf to the west coast, forming a strong (weak) thermal front at the junction of cold and warm water on an interannual scale. The upward trend of TFIBG intensity may be related to the regional heterogeneity of climate warming. This research systematically studied TFIBG, which will help improve people’s understanding of the thermal front in the South China Sea (SCS).