[A-OS11] Physical, biogeochemical, and ecological processes and variability in the Indian Ocean

[AOS11-P03] Shoaling of the thermocline, nutrients supply, and biological variations southwest of Sumatra observed during December 2017

*Takanori Horii¹, Iwao Ueki¹, Qoosaku MOTEKI¹, Kelvin J. Richards², Kazuhiko Matsumoto¹, Kentaro Ando¹ (1.Japan Agency for Marine-Earth Science and Technology, 2.International Pacific Research Center/University of Hawaii)

Keywords:Sumatra, CoastalCoastal upwelling, Chlorophyll-a, nutrients

A field experiment southwest of Sumatra centered at 4.2°S, 101.5°E was conducted by R/V Mirai from 5 December 2017 to 1 January 2018 as a component of the international project "Years of the Maritime Continent (YMC)." Using data obtained by the cruise, this study reports observational results in ocean temperature, salinity, and biological variations. During the intensive observation period, westerly and northwesterly winds dominated in the equatorial eastern Indian Ocean and southwest of Sumatra. The wind forcing was unfavorable for local coastal upwelling. However, the in-situ temperature timeseries showed gradual shoaling of the thermocline (20°C-25°C isotherm) from mid to the end of December 2017. Concurrently chlorophyll-a concentration increased above the thickened thermocline. The shoaling of the thermocline was probably caused by remote wind forcing through the equatorial Kelvin wave. In December 2017, there were easterly winds in the western and central equatorial Indian Ocean that could generate upwelling Kelvin waves. Shipboard ADCP observation shows northwestward flows above the thermocline consistent with the first baroclinic upwelling Kelvin wave. The increase in chlorophyll-a and changes in the vertical distribution of nutrients related to the thermocline variation is discussed.