A heavy winter monsoon rainfall episode influenced by easterly waves, a westerly trough, blocking and the ITCZ

A recent five-day cricket Test Match between England and India held at Bangalore, India, from 19 to 23 December 2001 ended in a disappointing draw. England was leading with a very respectable first innings score when heavy rains started on Day 3 of the match and led to the cancellation of the game. Just before that, one of the authors of this paper had received an e-mail from a Bangalore colleague inquiring about the prospects of rain for the following days. This led to our interest in the examination of the synoptic situation during that period. Here we present a short account of our findings, which turned out to be quite intriguing. In this note we focus on the scenario that preceded the heavy rainfall episode over southeast India during the winter monsoon period in December 2001. A persistent anticyclonic circulation in the upper troposphere over east Africa, centred near 58N, 508E, contributed to the bifurcation of the strong westerlies near 208N into two streams. An intense and persistent blocking situation was present near 608E. One branch of the split flow in the westerlies moved equatorward in the Arabian Sea, resulting in a trough in the Northern Hemisphere and a ridge in the Southern Hemisphere, the latter residing over the Southern Hemisphere intertropical convergence zone (ITCZ) near 58S. This ridge± trough system provided a moist stream of south-westerly flow over the south-east coast of India where the frequent westward passage of easterly waves near 850mbar provides the setting for heavy rain. Another important feature within the easterly trades are the frequent wind surges. These surges move from east to west within the easterly trades at a speed of roughly 78 longitude per day. The average strength of the trades is around 12ms and the surges often carry a wind speed as high as 13 to 14ms. Commonly, convection and rain are found ahead of the surge, due to the convergence of mass and moisture. At the time of the heavy rain over Chennai (13.138N, 80.258E) we could clearly see the simultaneous arrival of an easterly wave and a trade surge over the south-east coast of India. Winter monsoon rains over the coast of south-east India usually occur during October, November and December, with heavier rains occurring in November and December. The large-scale flow field over the region exhibits a very strong vertical wind shear. At the surface levels, the flow is generally from the north-east (also called the north-east monsoon). As the winds veer with height, at the 850 mbar level the flow is more easterly. In the upper troposphere the winds generally tend to be more westerly (Krishnamurti et al. 1997a,b). The north-easterly monsoonal flow is very shallow, occupying only the lowest 2 km of the atmosphere.