Three dimensional structure and evolution of moisture and precipitation in the MJO

Abstract:

The large-scale circulation features that are responsible for the structure and evolution of the moisture and precipitation fields in the MJO are examined, using the analysis protocol of Adames and Wallace (2014a,b). The analysis reveals the dynamical processes that lead to the buildup of moisture to the east of the region of maximum ascent and drying to the west of it. To the east, a deep plume of ascent confined to the equatorial belt advects moisture upward from the surface toward the middle and upper troposphere while diffluence above the boundary layer acts to widen the anomalously moist region in the lower troposphere. To the west of the region of strongest ascent, equatorially-symmetric, shallow meridional circulations induce drying through anomalous descent beneath the 600 hPa layer while confluence above the boundary layer advects drier air from higher latitudes. The resulting drying is co-located with anomalous westerlies associated with forced equatorial Rossby waves. The major features that give rise to this systematic moistening and drying during the MJO cycle support the “bottom-up” evolution in vertical velocity and the characteristic “swallowtail” shape in vertical motion and precipitation.