Some Aspects of Western Hemisphere Circulation and the Madden–Julian Oscillation

Abstract

lthough the greatest variance in convection associated with the Madden?Julian oscillation (MJO) occurs over the Indo-Pacific warm pool, the MJO is associated with substantial circulation patterns in the tropics and the extratropics of the Western Hemisphere. Reanalysis data suggest that upper-tropospheric easterly wind anomalies on the equator between 40° and 140°W precede 86% of active convective phases of MJO events greater than one standard deviation in amplitude over the Indian Ocean basin during the Northern Hemisphere winter. Composites of those MJO events that are preceded by westerly wind anomalies and those events preceded by easterly wind anomalies are compared. Results show that those events that are preceded by westerly wind anomalies fail to thrive and do not yield the amplitude in convection or the canonical atmospheric circulation response that is associated with those preceded by easterly wind. The composite of events preceded by easterly winds reveals that these winds amplify coincident with arrival of an anticyclone into the tropics from a wave train that extends across the middle latitudes of the Pacific Ocean and North America. The resultant easterlies then radiate eastward across Africa to the Indian Ocean basin at the phase speed of convectively coupled Kelvin waves, where they are joined by other anticyclones propagating into the tropics, apparently facilitating westward outflow from the amplifying Indian Ocean basin convection.