Response of tropical clouds to the interannual variation of sea surface temperature

Abstract

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 Niño had a significant impact on the global tropical cloud field, although the warming itself was confined to the equatorial central and eastern Pacific. Extensive variations of the total cloud field occurred in the northeastern Indian, western and central Pacific, and western Atlantic Oceans. The changes of high and middle clouds dominated the total cloud variation in these regions. Total cloud variation was relatively weak in the eastern Pacific and the Atlantic because of the cancellation between the changes of high and low clouds. The variation of low clouds dominated the total cloud change in those areas. The destabilization of the lapse rate between 900 and 750 mb was more important for enhancing convective instability than was the change of local SSTs in the equatorial central Pacific during the 1987 El Niño. This destabilization is associated with anomalous rising motion in that region. As a result, convection and high and middle clouds increased in the equatorial central Pacific. In the subtropical Pacific, both the change of lapse rate between 900 and 750 mb associated with anomalous subsidence and the decrease of boundary-layer buoyancy due to a decrease of temperature and moisture played an important role in enhancing convective stability. Consequently, convection, as well as high and middle clouds, decreased in these areas. The change of low clouds in the equatoral and southeastern Atlantic was correlated to both local SSTs and the SST changes in the equatorial eastern Pacific. In this area, the increase of low clouds was consistent with the sharper inversion during the 1987 El Niño. The strengthening of the inversion was not caused by a local SST change, although the local SST change appeared to be correlated to the change of low clouds. The coherence between clouds and SST tendency shows that SST tendency leads cloud variation in the equatorial Pacific. Thus, the change of clouds does not dominate the sign of SST tendency even though the cloud change was maximum during the 1987 El Niño. In some areas of the Indian, subtropical Pacific, and North Atlantic Oceans, cloud change leads SST tendency. Cloud change might affect SST tendency in these regions.