Sub-seasonal scale fluctuations of the ITCZ over the Indo-Pacific region during the summer monsoon season. Part I. Features over the Indian region

Abstract

Monex-79 and ISMEX-73 data have been analysed to study the sub-seasonal scale fluctuation of near equatorial oceanic intertropical convergence zone ( ITCZ) over the North Indian ocean during the summer monsoon of 1979 and 1973. The oceanic ITCZ is characterised by a narrow shear zone between the equatorial westerlies and the tropical easterlies, associated with organised convective clouds. Synoptic analysis presented in this paper shows the steady northward propagation of the oceanic ITCZ from its near equatorial position (5-10°N) to the continental position (20-25°N) during the onset and mid-season revivals of monsoon after breaks. The northward propagation is initiated by the strengthening of the equatorial westerlies which result in the intensification of the shear zone and the embedded disturbances. The establishment of the northward propagating mode near normal monsoon trough position over the continent characterises the active phase of monsoon. As the monsoon cycles from active to weak/break phase, the monsoon trough (continental ITCZ) dissipates near the foothills of the Himalayas and the oceanic ITCZ gets emphasised once again near the equatorial region. The major phase changes in the ITCZ occur at an interval of about 30-50 days which dominantly control the intra-seasonal fluctuation of the Indian summer monsoon. The paper also discusses the characteristic features of the oceanic ITCZ during different phases of the monsoon.