Sea surface height responding to tropical cyclones based on COAWST and SAR imagery

Cyclonic structure is visible on synthetic aperture radar (SAR) during a tropical cyclone (TC). Based on coupled ocean-atmosphere-wave-sediment transport (COAWST) model hindcasts, fused wind data from the Haiyang-2 (HY-2) constellation, the Advanced Scatterometer (ASCAT), the Soil Moisture Active-Passive (SMAP) radiometer, and the Advanced Microwave Scanning Radiometer-2 (AMSR2) are assimilated into the COAWST model using a three-dimensional variational (3D-Var) data assimilation scheme. Validation against along-track measurements gives root mean square errors (RMSEs) of 3.76 m/s for wind speed against the Stepped-Frequency Microwave Radiometer (SFMR), 0.12 m for sea surface height (SSH) against the HY-2 altimeter, and 0.93°C for sea surface temperature (SST) against National Data Buoy Center (NDBC) buoys, respectively. Model simulations reveal a spatially asymmetric SSH response to TCs, with a pronounced rise on the right of the storm track, weaker signals on the left and rear, and a radially outward decrease. Notably, a temporary negative SSH anomaly can occur near the storm center when the TC undergoes a recent directional change, reflecting the dominant influence of wind-driven upwelling along the prior trajectory. Furthermore, the peak sea surface height anomaly (SSHA) typically lags the maximum wind speed by 12 to 48 h. In addition, this study evaluated the SAR backscatter coefficient (σ0) response to key oceanic parameters during TCs. σ0 correlated positively with sea surface wind speed and SSH, with correlation coefficients (COR) of 0.79 and 0.73. Partial correlation yields COR of 0.51 for wind speed and 0.50 for SSH, confirming independent relationships, indicating sea surface roughness is primarily wind-driven yet positively linked to SSHA (wind-induced convergence/divergence). Furthermore, the study demonstrates a notable response of σ0 to ocean-atmosphere stratification intensity, particularly in relation to the Brunt-Väisälä frequency (N). A sharp increase in σ0 is observed under low N conditions, which may be associated with enhanced vertical mixing.