Characteristics of the 1997/1998 El Nino cloud distributions from SAGE II observations

The present study examines the characteristics of cloud distributions with emphasis on cloud longwave radiative forcing (CLRF) during the peak of the 1997/1998 El Nino in relation to climatological conditions, based on measurements from the Stratospheric Aerosol and Gas Experiment (SAGE) II. The observed distinct cloud occurrence and CLRF during this unusual 1997/1998 El Nino constitutes a unique data set for validating and improving cloud-radiation-climate interactions in general circulation and climate models. Using the solar occultation technique, the SAGE II satellite instrument is capable of providing measurements with a 1-km vertical resolution facilitating the analysis with sufficient vertical as well as near global scale (70degreesS-70degreesN) details. The present study indicates (1) above normal high-altitude opaque cloud occurrence over the eastern tropical Pacific and an opposite situation over the Pacific warm pool, leading to a distribution of the cumulative opaque cloud anomalies above 3 km generally consistent with the pattern of observed tropical sea surface temperature and precipitation anomalies; (2) a similar behavior in the subvisual cloud distributions near the tropical tropopause; (3) a zonally averaged cloud distribution that is characterized by reduced opaque clouds at low latitudes, except in the southern tropics below 10 km, and by enhanced opaque clouds at high latitudes, along with increased subvisual clouds in the southern tropics and decreased subvisual clouds in the northern subtropics in the upper troposphere; and (4) a geographic distribution of model-calculated CLRF anomalies that resembles closely that inferred from the Earth Radiation Budget Experiment and the Clouds and the Earth's Radiant Energy System. A discussion on the influence of the El Nino on large-scale mean tropospheric circulations is also provided.