Temporal variations of oceanic surface layer and atmospheric profiles in the southern equatorial Indian Ocean during CINDY/DYNAMO

During two-month period in October and November 2011, we deployed our research vessel “Mirai” at (8S, 80E) to participate CINDY/DYNAMO field campaign. During the period, the CINDY/DYNAMO observation array successfully captured initiation of the convectively active phase in Madden-Julian Oscillation (MJO). On R/V Mirai, various instruments were operated to obtain detailed temporal variations of both atmospheric and oceanic status at the station. While Mirai was continuing observation at the station, two active phases of MJO appeared in the end of October and in the end of November.

Before the convectively active period in October, a gradual deepening of the thermocline with increasing surface-layer temperature was observed for about two-weeks. The period is also characterized by the very dry atmosphere with clear sky and no significant precipitating clouds. Under such atmospheric condition, diurnal warming of the surface seawater was clearly observed. The condition continued at (8S, 80E) until Mirai temporally left the station on Oct.24, while the active convection of MJO was appearing around and north of the equator. The prevailing southeasterly wind at (8S, 80E) suggests that the heat and moisture from the ocean surface were transported toward the convectively active region in north of the station.

In contrast to the October event, the event in November accompanied no significant increase of SST or heat contents in the oceanic surface layer. Especially in the latter half of November, cold and less-saline water stayed between surface layer and thermocline. After the event, the temperature in the surface layer dropped and not increased again until the convectively active phase in the end of November. In spite of such oceanic condition, the atmospheric profiles gradually evolved toward the convectively active phase, by deepening the moist layer.

These observed temporal variations indicate that, at 8S, no event in the period accompanied both gradual moistening of the atmosphere and gradual increase of SST, as in traditional conceptual model of the MJO. The meridional difference of both atmospheric and oceanic conditions between R/V Mirai at 8S and R/V Revelle at the equator suggests the needs to investigate meridional structure to better understand the mechanism of the observed MJO.