Coastal Upwelling Investigation in the Gulf of Thailand Using Ekman Transport and Sea Surface Temperature Upwelling Indices

Abstract

Two different upwelling indices: (1) Ekman transport upwelling index (UI_ET) and (2) sea surface temperature upwelling index (UI_SST) were evaluated determine likely locations of coastal upwelling occurring in the Gulf of Thailand (GoT). GoT covers the area between longitude 98.0°E to 106.0°E and latitude 5.0°N to 14.0°N. In addition, inter-annual and annual variability of UI_ET along the east and west coasts were also investigated. UI_ET was estimated using monthly averaged wind velocity during 2003--2018, while UI_SST was calculated based on the difference between coastal and oceanic monthly mean sea surface temperature at the same latitude. Based on spatial UI_ET, favorable upwelling conditions existed mainly along the east and west coasts during northeast and southwest monsoons, respectively. Furthermore, the favorable upwelling conditions were also found sparsely along the east coast and the west coast during the first and second inter-monsoon. The spatial UI_SST showed favorable upwelling condition along the west and east coasts around Ca Mau Cape during northeast and southwest monsoons. Meanwhile, during first and second inter-monsoons the favorable upwelling conditions rarely occurred. Disagreement of spatiotemporal coastal upwelling between UI_ET and UI_SST were likely due to the shallowness and bottom friction in the GoT. Considering inter-annual variability of meridionally averaged UI_ET along east and west coasts, it was found that favorable/unfavorable upwelling conditions were associated with Multivariate ENSO Index signal. Interestingly, annual cycle of meridionally averaged UI_ET along west and east coasts showed alternation of favorable and unfavorable upwelling conditions, respectively.