Variation of the surface coastal current in the marginal sea off the west coast of Sumatra during the Pre-YMC and YMC periods was investigated by the R/V “Mirai” cruises (Pre-YMC [Years of the Maritime Continent]: MR15-04 and YMC: MR17-08). One of the major differences between the cruises of MR15-04 and MR17-08 was the direction of the surface coastal current. According to the LADCP observation, the southeastward current along the coast was continuously dominant under the forcing of a weak easterly winds in the MR15-04, whereas in the MR17-08, the current direction was rotated counterclockwise around 10 days under the strong westerly wind forcing after the passage of the MJO, suggesting the passage of meso-vortices. The difference was confirmed by the oceanic reanalysis of the ECCO2, and it was confirmed that the meso-vortex was slowly moved along the coast (about 300 km / 10 days). It was suggested that the presence or absence of the meso-vortex formation caused a large difference in not only the horizontal advection but also the up or down welling in the mixed layer. The meso-vortex was also accompanied by a clear maximum of the sea surface height, and a large vertical motion in the mixed layer is estimated in ECCO2. The coastal meso-vortex was gradually formed and developed under the continuous westerly wind forcing more than 10 days after the MJO passage.

Years of the Maritime Continent or YMC was designed to improve our knowledge of the weather-climate systems over the MC and their numerical simulation and prediction skill. Since its first field campaign in November 2017, many intensive observations have been carried out under the coordination with the MC countries. Although currently some observations have been postponed due to COVID-19 pandemic, field campaigns are expected to continue beyond 2021. Up to now, some key information could be obtained, which suggest future approaches. In particular, diurnal cycle of rain near the coast is one of major targets, as they are dominant component of precipitation in this region. Some studies suggest the important role of temperature contrast between land and ocean, and inaccurate initial sea surface temperature conditions might cause a delay of offshore propagation of rainfall region. This fact provides a clue to improve simulation of precipitation behavior over the MC. In addition, during the campaign, several new observation tools have been introduced. For example, autonomous surface vehicle (ASV) had been deployed and measured surface meteorology as well as sea surface condition. Besides, GNSS-derived water vapor measurement was successfully carried out. Those results suggest that such ASVs equipped with new tools can be used to monitor accurate sea surface condition without significant cost and time. However, usually those instruments are not allowed to operate freely in the MC. Thus, based on collaboration with the MC countries, researchers in the MC are highly expected to take this role as regional representative. In this presentation, we will also show other results which provide tips for future direction.