The influence of the 1998 El Nino upon cloud-radiative forcing over the Pacific warm pool

Clouds cool the climate system by reflecting shortwave radiation and warm i t by increasing the atmospheric greenhouse. Previous studies have shown tha t in tropical regions of deep convection there is a near cancellation betwe en cloud-induced shortwave cooling and longwave warming. The present study investigates the possible influence of the 1998 El Nino upon this near canc ellation for the tropical western Pacific's warm pool; this was accomplishe d by employing satellite radiometric measurements (Earth Radiation Budget E xperiment, and Clouds and the Earth's Radiant Energy System). With the excl usion of the 1998 El Nino, this study also finds near cancellation between the shortwave and longwave cloud forcings and demonstrates that it refers t o the average of different cloud types rather than being indicative of a si ngle cloud type. The shortwave cooling slightly dominates the longwave warm ing, and there is considerable interannual variability in this modest domin ance that appears attributable to interannual variability of tropopause tem perature. For the strong 1998 El Nino, however, there is a substantially gr eater tendency toward net radiative cooling, and the physical mechanism for this appears to be a change in cloud vertical structure. For normal years, as well as for the weaker 1987 El Nino, high clouds dominate the radiation budget over the warm pool. In 1998, however, the measurements indicate the radiation budget is partially governed by middle-level clouds, thus explai ning the net cooling over the warm pool during the 1998 El Nino as well as emphasizing differences between this event and the weaker 1987 El Nino.