Wave Trains Excited by Cross-Equatorial Passage of the Monsoon Annual Cycle

Abstract

In this paper the authors illustrate wave trains that are excited during the equatorial passage of the annual cycle of monsoonal convection. Twice a year, during roughly the months of December?January and March?April, the annual cycle of monsoonal convection crosses the equator. A principle axis of annual cycle monsoon precipitation extends from the Java Sea to the eastern Himalayas. Monsoonal convection makes a north?south seesaw roughly along this axis each year. Near-equatorial convection provides a tropospheric heat source somewhat akin to that of El Niño over the equatorial Pacific Ocean. This equatorial passage of the monsoonal heat source excites a wave train, somewhat similar to the familiar Pacific?North American pattern. Monsoonal wave trains were extracted from a 9-yr dataset, and a composite geometry was constructed. This note also illustrates excitation of short-period wet and dry spells associated with excitation of this wave train. This is illustrated for several trough and ridge locations of the wave train by examining rainfall for a sequence of days some 10 days prior to and 10 days subsequent to passage of this wave train. There is a strong suggestion that equational passage of monsoon convection does influence short-term dry and wet spells along the wave train; that is, beneath upper troughs (ridges), wet (dry) weather prevails.