Advection of Dry Air by Transient Eddies within an MJO

Temporal variations in tropical convection occur on various scales ranging from hours to seasons and beyond. The Madden–Julian oscillation (MJO; Madden and Julian 1971, 1972) is one of the most prominent phenomena regulating the tropical convection. In order to improve our understanding of an MJO, a field observational campaign called CINDY/DYNAMO was conducted over the central equatorial Indian Ocean in October-December 2011. During the campaign, large-scale-organized convection associated with an MJO broke out, and formation and development of transient eddies were prominent in the mid-level (around 550 hPa or melting level) during the MJO active phase over the Indian Ocean. Eddies have a zonal scale of 2000-3000 km and time scale of 8-10 days. Following the shift of the convective active area associated with an MJO, the eddy packet moves to the east, although each eddy slowly propagates to the west. Associated with the development of the eddy, dry air is transported from the higher latitude into the equator in the mid-level. Vertical transport of water vapor moistens the mid troposphere during almost the entire period of CINDY/DYNAMO campaign. However, the drying associated with the eddy exceeds the moistening by the vertical motion, when the eddy is reinforced within an MJO convection envelope, and suppresses the convection around the equator. As a result, convectively-active regions move to the east or higher latitudes.