[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

*堀井 孝憲¹、植木 巌¹、茂木 耕作¹、Richards Kelvin²、松本 和彦¹、安藤 健太郎¹ (1.国立研究開発法人海洋研究開発機構、2.International Pacific Research Center/University of Hawaii)

キーワード：スマトラ島、沿岸湧昇、クロロフィル-a、栄養塩

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.