Interannual variability of cloud forcing and meridional energy transport for the Northern Hemisphere winter from 1984 to 1990

Broadband radiative flux data from the Earth Radiation Budget Experiment (E RBE) 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 Hemisph ere winter from 1984 to 1990. This period captures the El Nino season of 19 86/87 and the La Nina season of 1988/89, with an average state biased towar d La Nina-type conditions. Clouds are found to cool the Pacific basin by ap proximately 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 durin g the El Nino and an increase during the La Nina. A key aspect of this study is the concurrent analysis of cloud type and amo unt 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 subsequ ent analysis of ERBE net radiation measurements reveals that the geographic redistribution of cloudiness observed in the ISCCP data results in an incr ease of the meridional energy gradient during the El Nino and a decrease du ring the La Nina. ECMWF reanalyses data are used to document and discuss the interannual vari ability of the meridional transport of atmospheric energy and the atmospher ic circulation. During the 1986/87 El Nino, it is found that the transport of atmospheric energy from the Tropics and subtropics to higher latitudes i ncreases by approximately 6% from the study average. Conversely, the transp ort decreases by about 2% during the 1988/89 La Nina. An investigation of t he variability of the structure and the strength of the meridional energy t ransport by the mean meridional circulation, stationary eddies, and transie nt eddies is then used to diagnose changes to the atmospheric circulation.