El Niño and Atmospheric Water Vapor: Observations from Nimbus 7 SMMR

Abstract

Atmospheric water vapor over the global oceans is remotely sensed from the Nimbus 7 Scanning Multichannel Microwave Radiometer (SMMR) measurements for about five years—January 1979 to September 1983. Based on the data for three years, 1979–81, preceding the recent El Niño event, we have derived monthly and seasonal mean maps of the water vapor over the oceans. These seasonal mean maps show five prominent features in the water vapor distribution throughout the year with some seasonal changes. Four of these features are associated with the subtropical anticyclones in the Pacific and Atlantic Oceans in the Northern and Southern Hemispheres. The fifth feature is apparently associated with the Walker circulation in the tropical Pacific Ocean. This feature is a cell of maximum values in the water vapor stretched along the equator from about 60°E to the dateline. Interannual variability of the water vapor over the oceans occurs mostly on the peripheries of these atmospheric circulation features. Anomalies, with respect to the three year monthly mean, in the water vapor distribution over the oceans during the 1982–83 El Niño event lead us to infer significant changes in the circulation of the lower layers of the atmosphere at that time. The intense phase of the El Niño is accompanied by well-organized subsidence to the west, north, and south of the convectively active zone that is over the near-equatorial regions of the central Pacific. As the convective zone intensifies and moves eastward the associated subsidence intensifies and moves with it. These observations imply, in a simplistic sense, that the “Walker circulation” that is normally present in the equatorial Pacific region appears to be reversed at the time of the intense phase of El Niño. It appears that the decay phase of El Niño is marked by decoupling of the subsidence zone from the convective zone.