Aspects of the 40–50 Day Oscillation during the Northern Summer as Inferred from Outgoing Longwave Radiation

Abstract

Intraseasonal variability Of tropical convection over the Indian Ocean/Pacific region during northern summer is studied using outgoing longwave radiation (OLR). OLR anomalies are found to propagate eastward along the equator from the Indian 0cean to the western Pacific and northward towards the Indian subcontinent and southern China. It is found that the dominant mode of tropical convection consists of a dipole with centers located over the Indian 0cean and the western Pacific/South China Set. This dipole undergoes complex structural changes over a broad period range centered around 40–50 days. During a typical oscillation, an anomaly first develops over the equatorial Indian Ocean. This anomaly then extends eastward to the equatorial western and central Pacific to form an elongated convection zone, while its center is displaced progressively northward from the Indian Ocean into the Indian subcontinent by an anomaly of the opposite sign. The elongated convection zone over the western Pacific then propagates northwestward over the South China Sea into southern China. The simultaneous northward propagation of the dipole centers ova India and South China suggests that the two components of the summer monsoon are subject to the same large-scale control. The 40–50 day oscillation is also found to be phase-locked to the monsoon onset over India, the Mei-yu regime over South China, and in general the seasonal variation of convection over the equatorial Indian-west, Pacific Ocean. However, the eastward propagation along the equator over the Indian Ocean appears to be present all year round. This suggests that the eastward propagation may be a basic property of equatorially trapped wave modes and that the meridional propagation arises as a result of the interaction between these equatorial modes and the monsoon circulations.