Frictional moisture convergence in a composite life cycle of the Madden-Julian oscillation

A composite life cycle of the Madden-Julian oscillation (MJO) is constructed using an index based on the first two EOFs of the bandpass-filtered (20-80 days) 850-mb zonal wind averaged from 5 degrees N to 5 degrees S every 2.5 degrees around the equator. Precipitation, 1000-mb convergence, 850-mb wind, and 200-mb wind are composited for the period 1979-95. Water vapor integrated from the surface to 300 mb is composited for the period 1988-92. Frictional moisture convergence at the equator is shown to play an important role in the life cycle of the Madden-Julian oscillation (MJO). Regions of boundary layer convergence foster growth of positive water vapor anomalies to the east of convection. This convergence coincides with 850-mb easterly wind anomalies, as is consistent with Kelvin wave dynamics. Drying of the atmosphere occurs rapidly after the passage of convection with the onset of 850-mb westerly perturbations. Possible mechanisms for this drying include boundary layer divergence and subsidence or horizontal advection from the west or extratropics associated with Rossby wave circulations. Frictional convergence in front of convection helps to slowly moisten the atmosphere to a state that is again favorable for convection. This moistening may set the timescale for the reinitiation of convection in the Indian and west Pacific Oceans after strong drying and provides a mechanism for slow eastward propagation. A significant correlation exists between surface convergence and column water vapor anomalies in the west Pacific and Indian Oceans. Weaker correlations exist between 850-mb convergence and water vapor anomalies. Over the west Pacific, surface convergence leads positive water vapor anomalies, while 850-mb convergence lags positive water vapor anomalies. Northern Hemisphere summer (May-October) composites show that the phases of the MJO coincide with "active" and "break" periods of the Indian summer monsoon at intraseasonal timescales. The northward propagation of precipitation across India during the summer monsoon is associated with northward and westward movement of Rossby wave features trailing the main center of equatorial convection associated with the MJO.