Journal of Climate, submitted 2/13.
Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY.
Korean Institute of Ocean Science and Technology, Ansan, Korea.
Adam H. Sobel
Deapartment of Applied Physics and Applied Mathematics, and Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY.
Basin-wide convective anomalies over the Indian Ocean (IO) associated with the Madden-Julian oscillation (MJO) sometimes propagate eastward and reach the west Pacific (WP), but sometimes do not. Long-term observations of outgoing longwave radiation and recent reanalysis products are used to investigate the difference between the propagating and non-propagating MJO events. IO convection onset events associated with the MJO are grouped into three categories based on the strengths of the simultaneous dry anomalies over the eastern Maritime Continent and the WP. The IO convection anomaly lives longer and propagates further eastward when the dry anomaly is stronger. When the dry anomaly is weaker, the convection anomaly ceases before reaching the WP in most cases.
Analysis of the column integrated moist static energy (MSE) budget shows that horizontal advection moistens the atmosphere to the east of the positive MSE anomaly associated with the active convection, and is of sufficient magnitude to explain the eastward propagation of the positive MSE anomaly associated with the IO convection. Interpretation is complicated, however, by lack of closure in the MSE budget. A residual term, of smaller but comparable magnitude to the horizontal advection, also moistens the column to the east of the positive MSE anomaly. Nonetheless, we decompose the horizontal advection term into contributions from different scales. We find that a dominant contribution is from free-tropospheric meridional advection by the instraseasonal time-scale wind anomalies. The positive meridional advection in between the convective and dry anomalies is induced by the anomalous poleward flow, which we interpret as part of the Rossby wave response to the dry anomaly, and the climatological MSE pattern, which peaks at the equator. Our results suggest that the dry anomaly plays an active role in the propagation of the IO convection through horizontal moisture advection by the wind response to it.