Influence of penetrating solar radiation on the heat budget of the equatorial Pacific Ocean

Abstract

SIGNIFICANT interannual variations in global climate, such as the El Niño/Southern Oscillation phenomenon (ENSO), result from interactions between ocean and atmosphere in the tropical Pacific. These interactions are mediated to a large degree by variations in the temperature of the sea surface, particularly in the warm, western portion of the basin^1–4. Changes in sea surface temperatures (SSTs) provide precursors for the prediction of climate variability on ENSO timescales. But current ocean–atmosphere models, which estimate SSTs using climatological surface heat fluxes derived from ship observations^5–7, consistently overestimate SSTs in the western Pacific by as much as 3 K. Here we use recent satellite observations of ocean transparency, coupled with climatological surface heat fluxes and ocean density profiles, to show that solar radiation in visible frequencies, usually assumed to be absorbed at the sea surface, in fact penetrates to a significant degree to below the upper mixed layer of the ocean which interacts actively with the atmosphere. The net effect is a reduction of the heat input into the upper layer; for a 20-m-thick mixed layer this is equivalent to an annual reduction in temperature of about 5–10 K. Our results provide a natural explanation for the discrepancy between the SSTs predicted by models and those observed.