Fast-changing upwelling off the west coast of Hainan Island

In this study, we investigated the short-term dynamics of upwelling off the west coast of Hainan Island (UWH) by combining Moderate Resolution Imaging Spectroradiometer (MODIS) infrared sea surface temperature (SST) images and Regional Ocean Modeling System (ROMS) simulations. MODIS observations showed that the UWH could exhibit rapid temporal variations, where the SST could increase or decrease by more than 1 degrees C within 3 h. Further investigations based on ROMS simulations suggested that flood tides could rapidly intensify the UWH, whereas ebb tides could rapidly weaken the UWH. Strong horizontal advection by intense tidal currents and high vertical velocity due to divergence/convergence triggered by progressive tidal waves were identified as the intrinsic physical mechanisms responsible for the rapid SST variations in the UWH. These findings were verified based on a MODIS-observed fast-cooling UWH event and a MODIS-observed fast-warming UWH event. Analyses of the timing of occurrence, tidal phase, and spatial patterns of the significant variations in the SST for these events all agreed with the hypothesis stated above. Model-data fusion analysis indicated that when a neap tide or transition period for tidal currents occurs, the influence of the tidal flow on the UWH is weak because of the associated weak horizontal advection and divergence/convergence.