Study of secondary heat sources over India during southwest monsoon 2002

Abstract

The southwest monsoon of 2002 has three unusual features viz., (i) delay in advance over westernmost parts of India by one month, (ii) absence of depressions/storms over Bay of Bengal and Arabian Sea and (iii) -51% departure of all India rainfall in July. These features of intra-seasonal variability have been studied in this paper. Apparent heat source (Q1) and apparent moisture sink (Q2) over Indian region have been estimated using daily NCEP-reanalysis data, for June - September 2002 to study the intra-seasonal variations of the secondary heat sources in relation to observed intra-seasonal variations of circulation and rainfall over India.
The intensities of vertically integrated heat source < Q1 > and moisture sink < Q2 > are found to coincide with the excess rainfall zone over India in June 2002, whereas the intensities of < Q1 > and < Q2 > over central parts of the country in July 2002 are not comparable. This indicates absence of convective rainfall and increased sensible heating over India in July. The x-t diagram of heating and drying rates at 500 hPa along 22.5° N shows large positive heating and drying rates of the order of 16° C per day over the western trough region around 26 June. Following this event, low positive or negative values are observed over central India in July.
The y-t diagram of vertical velocity at 500 hPa shows strong subsidence near the foot hills of Himalayas during the season. This has affected the formation of depressions or storms over Bay of Bengal and Arabian Sea. Adiabatic and diabatic heating as well as drying in the troposphere has led to the break-like situation over India in July 2002 and weaker monsoon circulation during August and September. The analysis has brought out the impact of northern hemispheric mid-latitude circulation on intra-seasonal variability of southwest monsoon 2002 more clearly.