Drastic Thickening of the Barrier Layer Off the Western Coast of Sumatra Due to the Madden Julian Oscillation Passage during the Pre-Years of the Maritime Continent Campaign

The drastic thickening of the barrier layer in the marginal sea off the western coast of Sumatra during the passage of the Madden Julian Oscillation (MJO) observed during December 2015 is investigated. Before the MJO arrival, the halocline above 20 m depth was very strong and the barrier layer thickness was 5-10 m from based on R/V Mirai observations. During the MJO forcing of 13-16 December, the isothermal layer was drastically deepened from 20 m to 100 m. Meanwhile, the mixed layer deepening was lagged behind the isothermal layer deepening by 1 day, and the barrier layer underwent dramatic thickening to 60 m within 24 hours. An evaluation of the vertical salinity gradient tendency showed that the dramatic thickening of the barrier layer was due to the vertical oceanic mixing by the atmospheric MJO forcing and the vertical stretching by the oceanic downwelling coastal Kelvin wave intruding from the open ocean. One of the important factors in the drastic barrier layer thickening was concluded to be the atmospheric external forcing and the oceanic internal wave being in-phase. The downwelling oceanic Kelvin wave continuously lowered the thermocline from the middle of November to the end of December, and the salinity stratification in the vicinity of the thermocline was continuously mitigated by the vertical stretching. Under such conditions, the MJO forcing caused vertical mixing of the freshwater with the strong salinity stratification and temperature stratification near the surface. The combination of the two distinct processes caused the drastic thickening of the barrier layer, and the barrier layer thickness reached a maximum of 85 m 5 days after the MJO arrival.