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Atmospheric and hydrospheric sciences
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
Moteki Q, Katsumata M, Yoneyama K, Ando K, Hasegawa T
Barrier Layer, Madden Julian Oscillation, Air-sea interaction
Schematic profiles of the salinity (blue, top axis), and temperature (red, bottom axis) (a) before the 1st wind burst (WB) forcing on 12 December (6 m/s, 36 mm/day), (b) during the 1st WB forcing peak on 13 December (9 m/s, 5 mm/day), (c) during the 1st weakened WB forcing on 14 December (6 m/s, 10 mm/day), (d) during the 2nd WB forcing on 15 December (9 m/s, 74 mm/day), (e) during the 2nd weakened WB forcing on 16 December (6 m/s, 61 mm/day), (f) after the 2nd WB forcing on 17 December (3 m/s, -1 mm/day) in 2015. The solid and dashed lines indicate the MLD and ILD, respectively.
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 a depth of 20 m was very strong, and the barrier layer thickness was 5–10 m based on R/V Mirai observations. During the MJO forcing of 13–16 December, the isothermal layer drastically deepened from 20 to 100 m. Meanwhile, the mixed layer deepening lagged behind the isothermal layer deepening by 1 day, and the barrier layer underwent dramatic thickening to 60 m within 24 h. 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. In addition, an evaluation of the vertical temperature gradient tendency showed that the temperature inversion in the barrier layer formed by losing heat to the atmosphere due to the MJO forcing and downward advection of the temperature gradient due to the downwelling Kelvin wave resulting in the dramatic isothermal layer deepening. One of the important factors in the drastic barrier layer thickening was 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 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.
