Convectively coupled Kelvin wave propagation past Sumatra: A June case and corresponding composite analysis

Abstract

The propagation of two sequential convectively coupled (CC) Kelvin waves past Sumatra in June 2006 is examined in reanalysis data and satellite observations. Rainfall data are presented that are consistent with a considerable damping of the first wave. Such damping is not unexpected; however, reanalysis data suggest that in some instances, including the episode described here, CC Kelvin waves can condition the low-level flow in a manner that promotes passage of subsequent waves past the island. The mechanism involves a sustained northward shift in the low-level equatorial westerly winds and the latitude of zero absolute vorticity, generated as a CC Kelvin wave passes. For the June 2006 case, Kelvin wave filtered low-level winds are presented which are consistent with the view that such conditioning associated with the first wave caused the second wave to be shifted northward, assisting its eastward propagation. A vorticity analysis points to the development of widespread low-level divergence within a vortex to the west of Sumatra and associated vortex expansion as key factors in the observed conditioning. A composite analysis is presented for the region demonstrating recurring divergence events of this nature coupled with concomitant northward shifts in the dynamic equator. It finds that such events often occur in association with CC Kelvin waves, though the climatological extent to which the propagation of successor waves is affected is not addressed. Possible causes of the observed divergence events, including equatorial Rossby waves and contributions from transitions to stratiform convection, are investigated. © 2011 by the American Geophysical Union.