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

Abstract

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.

Coastal Upwelling Events, Salinity Stratification, and Barrier Layer Observed Along the Southwestern Coast of Sumatra

“…As the large‐scale ocean condition in the eastern Indian Ocean associated with IOD (Du & Zhang, 2015; Durand et al., 2013; Grunseich et al., 2011; Kido & Tozuka, 2017), the upper‐layer salinity southwest of Sumatra tended to be fresh (saline) during negative (positive) IOD. Consistent with past CTD observations (Moteki et al., 2018), upper‐ocean salinity stratification was common throughout the observation period likely due to continuous precipitation off Sumatra, forming an almost constant shallow mixed layer above the isothermal layer. Except for July–November 2015 and August–September 2017, the isothermal layer was deeper than the mixed layer, and a thick barrier layer was observed (Table 2).…”

“…As the large-scale ocean condition in the eastern Indian Ocean associated with IOD (Du & Zhang, 2015;Durand et al, 2013;Grunseich et al, 2011;Kido & Tozuka, 2017), the upper-layer salinity southwest of Sumatra tended to be fresh (saline) during negative (positive) IOD. Consistent with past CTD observations (Moteki et al, 2018) Notes. See Section 3.1 for how to estimate the distance to Sumatra.…”

“…Moteki et al. (2018) used CTD observation obtained by Research Vessel Mirai stationed approximately 50 km off the southwestern coast of Sumatra (4°S, 102°E) and presented the ocean vertical structure during the period from late‐November to mid‐December 2015. Although they reported salinity stratification, barrier layer, and their dramatic temporal variation, their results were based on short time series of less than one month and not during a season of coastal upwelling.…”

“…Southwest of Sumatra, because of the lower latitudes (approximately 4°S) compared to those of Java (approximately 8°S–9°S), we expect a larger spatial scale and stronger impact from equatorial Kelvin waves on the upwelling system. The constant density stratification and complicated vertical structure due to the large freshwater input (Moteki et al., 2018) should be elucidated using a longer time series in order to understand intraseasonal variations of the coastal upwelling system together with the background condition of seasonal and year‐to‐year variations. In particular, we focus on the possible process by which the previously reported upper ocean stratification (e.g., Du et al., 2005) affects the mixed layer heat balance off Sumatra through changing ocean vertical process on intraseasonal timescale.…”

“…Susanto et al (2016) presented ocean vertical profiles of temperature, salinity, and currents from conductivity-temperature-depth (CTD) casts in the Sunda Strait (around 105°E-106°E, 6.6°S-5.8°S), but they focused on water properties in terms of the Indonesian Throughflow (ITF). Moteki et al (2018) used CTD observation obtained by Research Vessel Mirai stationed approximately 50 km off the southwestern coast of Sumatra (4°S, 102°E) and presented the ocean vertical structure during the period from late-November to mid-December 2015. Although they reported salinity stratification, barrier layer, and their dramatic temporal variation, their results were based on short time series of less than one month and not during a season of coastal upwelling.…”