Global Monitoring of Net Solar Irradiance at the Ocean Surface: Climatological Variability and the 1982–1983 El Niño

Abstract

A new method has been used to generate the first satellite-based long-term climatology of surface solar irradiance over the world oceans. These monthly mean data cover the period November 1978 through October 1985 on a global, 9° latitude-longitude spatial grid. The large-scale variability of surface solar irradiance is assessed over the world oceans for the entire (84-month) record. The results demonstrate the ability of the method to reveal large-scale seasonal and interannual phenomena. The reduction in surface solar irradiance due to clouds is evaluated globally both on monthly and long-term climatological scales. Monthly cloud forcing anomalies are found to display eastward propagation over the course of the 1982?1983 El Niño event. The mean January climatology is found to he consistent with the climatology obtained from a general circulation model run in perpetual January mode. This study marks the first large-scale observation-based examination of cloud solar forcing at the ocean surface. In addition, empirical orthogonal function (EOF) analysis is employed to investigate modes of seasonal and nonseasonal variability. Nonseasonal EOF modes of surface solar irradiance are related to nonseasonal EOF modes of outgoing longwave radiation (OLR). The dominant modes during the 1982?1983 El Niño are associated with eastward propagation in both the shortwave and longwave fields. These dominant nonseasonal EOF modes of surface solar irradiance are found to display features and amplitude variations that are nearly identical to those of the corresponding nonseasonal EOF modes of OLR. The association of these modes with El Niño is quantified using the correlation of the mode amplitudes with the Southern Oscillation index (SOI). In each case modes 1 and 2 are positively correlated with the SOI, and mode I has a strong correlation of 0.75 for the shortwave and 0.76 for the longwave field. Finally, a study of the regionally averaged behavior of surface solar irradiance and sea surface temperature (SST) in a section of the tropical Pacific (9°N?9°5, 117°? 144°W) during this same period indicates that fluctuations of surface solar irradiance in the tropical Pacific are sometimes a regional response to underlying changes in SST (and associated changes in cloudiness), rather than a driving mechanism responsible for variations in SST.