Downstream Amplification and Formation of Monsoon Disturbances

Abstract

In this paper we present many examples (based on 43 years of data) of a phenomenon of downstream amplification over the monsoonal belt. The specific finding here is the following sequence of events: 1) During northern summer pressure drops in the vicinity of the North Vietnam coast (near 20°N) as a typhoon or a tropical storm arrives; 2) during the ensuing week pressure rises over Indochina and Burma by some 5?7 mb; and 3) during the following week a monsoon disturbance forms near the northern part of the Bay of Bengal. On an x-t (or Hovmöller) diagram this sequence of low-high-low formation is similar to the downstream amplification phenomenon of the middle latitudes. The following are some interesting differences: over the middle latitudes the eastward propagating phase velocity is of the order of 10° longitude day?1, while the eastward propagating group velocity (the speed of propagation of the amplification) is around 30° longitude day?1. The tropical counterparts are westward propagating, and the phase and group velocity are, respectively, around 6° and 2° longitude day?1. In meteorological literature one frequently notes reference to in situ formation of monsoon depressions over the northern part of the Bay of Bengal. Our study illustrates the superposition of stationary long waves with progressive short waves, the latter arriving from the western Pacific. This result is contrary to this notion of in situ formation. In this paper we examine some aspects of this slowly westward propagating group velocity phenomenon.