Net Shortwave Fluxes over the Ocean

Net surface shortwave fluxes (Qsw) computed from National Aeronautics and Space Administration/Langley satellite data are compared with Qsw from reanalyses of the European Centre for Medium-Range Weather Forecasts (ERA) and the National Centers for Environmental Prediction (NCEP). The mean and variability of Qsw is examined for the period 1983–91, with a focus on the tropical and summer hemisphere oceans during June, July, August (JJA) and December, January, February (DJF). Both reanalyses exhibit a positive bias, indicating too much sunlight is absorbed at the surface, in regions where low-level stratiform clouds are most common, but a negative bias in regions where cumuliform clouds are the dominant cloud type. The ERA has a greater intermonthly variability during JJA than the satellite data over most of the Pacific, especially north of 408N and in the central and eastern equatorial Pacific. The NCEP variability in JJA is also larger than the satellite estimates over the North Pacific and the eastern equatorial Pacific, but is smaller over most of the western tropical and subtropical Pacific. During DJF, the ERA has more realistic variability in shortwave fluxes over the tropical oceans than the NCEP reanalysis, which underestimates the variability in the tropical Pacific and the Indian Ocean by a factor of 2. Ocean models using atmospheric forcing from reanalyses will be impacted not only by regional and seasonal Qsw biases but also by differences in Qsw variability. It is estimated that the largest impacts on SST due to differences in variability are in the North Pacific, eastern tropical Pacific, and western Atlantic during JJA and in the Indian Ocean and the tropical Pacific and Atlantic during DJF.