Oceanic biophysical response to large-scale wind forcing observed southwest of Sumatra during December 2017

The water southwest of Sumatra is characterized by the seasonal and interannual occurrence of coastal upwelling, which drives biological variability and active fisheries. Although satellite observations show active biophysical variability at the surface, no observations of surface–subsurface physical and biogeochemical variations were available. Based on a field experiment conducted southwest of Sumatra (4.2°S, 101.5°E) by the research vessel Mirai from December 5, 2017 to January 1, 2018, this study reports observational results for ocean temperature, salinity, nutrients, and biological variations. During the observation period, westerly winds dominated the eastern equatorial Indian Ocean, with northwesterly winds southwest of Sumatra. This wind forcing was unfavorable for local coastal upwelling. Time series of in situ observations indicated gradual shoaling of the thermocline from mid-to late December 2017. This thermocline displacement was attributed to the propagation of equatorial and coastal Kelvin waves in response to the appearance of easterly winds (weakening of westerly winds) in the central (eastern) equatorial Indian Ocean. Concurrent with thermocline shoaling, we observed active mixing at the top of the thermocline, upward transport of nutrients to the euphotic layer, and subsurface phytoplankton growth. These biophysical responses demonstrate that even if local wind forcing is unfavorable for coastal upwelling, remote wind forcing can affect the nutrient supply that supports biological activity off Sumatra through Kelvin wave propagation. These results will provide fundamental data for validating biophysical models of the eastern Indian Ocean.