Abstract

This study investigates the variations in the last decades of underwater sunlight attenuation coefficient (Kd) and its forcing (W/m2¬) on the Earth’s surface heat budget. It first compares the trends of essential climate variables Kd and sea surface temperature (SST) to gauge their connection. To estimate an equivalent global forcing, it also finds the variation of radiant heating rate (RHR) profiles under the ocean surface. The trends over a different period (14 to 30 years) for Kd, SST and diurnal SST (DSST) were extracted using the statistical Seasonal Mann-Kendal test, from remote sensing datasets available respectively on ESA OC-CCI, NOAA and REMSS public FTP servers. The results were geographically compared on 360°x180° gridded maps. They were used to estimate global linear regressions of their correlation coefficients. A positive correlation of 0.38 between Kd and SST was found. This result highlights the relevance to investigate the weight of the fate of irradiance decay on the surface heat budget. Using results of Kd trends over the 18-year period, changed incoming solar radiation (I0) decay in the upper 50 meters, and associated changed RHR profiles, it was possible to map the critical depth (Zcrit) at which the (1998) and (2017) RHR curves crossed path. A slightly better negative correlation of -0.42 was found between SST and Zcrit. The latter was used to calculate and map the theorical forcing on the air-sea interface attributed to ocean color changes. Averaged over the Earth’s surface, the forcing is 0.33 W/m2. This order of magnitude compares to the 3.1 W/m2 attributed to greenhouse gases (GHG) forcing. It suggests that the optically responsive aquatic components accumulated into the oceans may significantly drive the surface energy transfer by retaining the solar irradiance closer to the surface. It is recommended to further the study by including Kd in an OAGCM to account for the feedback of the four main fluxes (shortwave, longwave, latent, and sensible heat) composing the budget. The weighting of the forcing attributed to Kd in the budget will improve the significance of the result. Further investigation toward the management of Shortwave (SW), the black carbon (BC) and the ocean color are also recommended. Solutions against anthropogenic warming are proposed, such as the mixing enhancement of stratified waters.