Intraseasonal fluctuations in low-level meridional winds over the Indian Ocean and monsoonal convection over South Asia

^*Contribution no. 87-03, Department of Meteorology,University of Hawaii,USA

Abstract

Some aspects of 30-60 day meridional wind fluctuations over the mid-latitude Indian Ocean were investigated using lag correlation maps for the 5 summers of 1979-1983. At times, the equatorial low-frequency (30-60 day) modes exhibit a systematic eastward propagation. However, they are occasionallyinterrupted by irregular movements or even westward propagation. This study concentrates only on those periods with a systematic eastward propagation and a well-defined tropical-extratropical atmospheric teleconnection pattern.

Low-level meridional wind fluctuations in the low-frequency domain are prominent over the western Indian Ocean off the east coast of Africa. The anomalous southerly flows, which are initiated around northern Madagascar move northward and, after crossing the equator, contribute to the intensification of the monsoonal westerlies over South Asia, thus, enhancing convection over the monsoon region. In comparison, 30-60 day southerlies (anomaly) over the eastern Indian Ocean off the west coast of Australia tend to enhance equatorial convection over the western Pacific between about 150°E and the date line.

Perturbations with periods shorter than 30 days are considered transient disturbances. Spectral analysis of transient eddy kinetic energy in the extratropical regions reveals a significant peak in the low-frequency range. This is associated with an amplitude modulation of transient disturbances. Thus, it is plausible that transient disturbances, as a group, provide energy for the enhancement of 30-60 day planetary-scale perturbations over the subtropics.