Interannual Variability of Cloud Forcing and Meridional Energy Transport for the Northern Hemisphere Winter from 1984 to 1990

Abstract

Broadband radiative flux data from the Earth Radiation Budget Experiment (ERBE) are used to document the interannual variability of net cloud forcing over the lifetime of the ERBE scanner instruments. The study focuses on the high variability observed in the Pacific basin during the Northern Hemisphere winter from 1984 to 1990. This period captures the El Niño season of 1986/87 and the La Niña season of 1988/89, with an average state biased toward La Niña?type conditions. Clouds are found to cool the Pacific basin by approximately 23 W m?2 over the study period. Interannual variations of net cloud forcing vary by less than 5%, with a decrease of cloud cooling during the El Niño and an increase during the La Niña. A key aspect of this study is the concurrent analysis of cloud type and amount information from the International Satellite Cloud Climatology Project (ISCCP). It demonstrates that the variability of middle-thick and high-thin clouds is integral to the observed changes in net cloud forcing. A subsequent analysis of ERBE net radiation measurements reveals that the geographic redistribution of cloudiness observed in the ISCCP data results in an increase of the meridional energy gradient during the El Niño and a decrease during the La Niña. ECMWF reanalyses data are used to document and discuss the interannual variability of the meridional transport of atmospheric energy and the atmospheric circulation. During the 1986/87 El Niño, it is found that the transport of atmospheric energy from the Tropics and subtropics to higher latitudes increases by approximately 6% from the study average. Conversely, the transport decreases by about 2% during the 1988/89 La Niña. An investigation of the variability of the structure and the strength of the meridional energy transport by the mean meridional circulation, stationary eddies, and transient eddies is then used to diagnose changes to the atmospheric circulation.