RESPONSE OF TROPICAL CLOUDS TO THE INTERANNUAL VARIATION OF SEA-SURFACE TEMPERATURE

Connections between the large-scale interannual variations of clouds, deep convection, atmospheric winds, vertical thermodynamic structure, and SSTs over global tropical oceans are examined over the period July 1983-December 1990. The SST warming associated with El Nino had a sig nificant impact on the global tropical cloud field, although the warmi ng itself was confined to the equatorial central and eastern Pacific. Extensive variations of the total cloud field occurred in the northeas tern Indian, western and central Pacific, and western Atlantic Oceans. The changes of high and middle clouds dominated the total cloud varia tion in these regions. Total cloud variation was relatively weak in th e eastern Pacific and the Atlantic because of the cancellation between the changes of high and low clouds. The variation of low clouds domin ated the total cloud change in those areas. The destabilization of the lapse rate between 900 and 750 mb was more important for enhancing co nvective instability than was the change of local SSTs in the equatori al central Pacific during the 1987 El Nino. This destabilization is as sociated with anomalous rising motion in that region. As a result, con vection and high and middle clouds increased in the equatorial central Pacific. In the subtropical Pacific, both the change of lapse rate be tween 900 and 750 mb associated with anomalous subsidence and the decr ease of boundary-layer buoyancy due to a decrease of temperature and m oisture played an important role in enhancing convective stability. Co nsequently, convection, as well as high and middle clouds, decreased i n these areas. The change of low clouds in the equatoral and southeast ern Atlantic was correlated to both local SSTs and the SST changes in the equatorial eastern Pacific. In this area, the increase of low clou ds was consistent with the sharper inversion during the 1987 El Nino. The strengthening of the inversion was not caused by a local SST chang e, although the local SST change appeared to be correlated to the chan ge of low clouds. The coherence between clouds and SST tendency shows that SST tendency leads cloud variation in the equatorial Pacific. Thu s, the change of clouds does nor dominate the sign of SST tendency eve n though the cloud change was maximum during the 1987 El Nino. In some areas of the Indian, subtropical Pacific, and North Atlantic Oceans, cloud change leads SST tendency. Cloud change might affect SST tendenc y in these regions.