A theoretical study of the effect of subsurface oceanic bubbles on the enhanced aerosol optical depth band over the southern oceans as detected from MODIS and MISR

Abstract

Submerged oceanic bubbles, which have a much longer life span than whitecaps or bubble rafts, have been hypothesized to increase the water-leaving radiance and thus affect satellite-based estimates of water-leaving radiance to non-trivial levels. This study explores this effect further to determine whether such bubbles are of sufficient magnitude to impact satellite aerosol optical depth (AOD) retrievals through perturbation of the lower boundary conditions. There has been significant discussion in the community regarding the high positive biases in retrieved AODs in many remote ocean regions. In this study, for the first time, the effects of oceanic bubbles on satellite retrievals of AOD are studied by using a linked Second Simulation of a Satellite Signal in the Solar Spectrum (6S) atmospheric and HydroLight oceanic radiative transfer models. The results suggest an insignificant impact on AOD retrievals in regions with near-surface wind speeds of less than 12 m s -1. However, the impact of bubbles on aerosol retrievals could be on the order of 0.02-0.04 for higher wind conditions within the scope of our simulations (e.g., winds < 20 m -1. This bias is propagated to global scales using 1 year of Moderate Resolution Imaging Spectroradiometer (MODIS) and Advanced Microwave Scanning Radiometer EOS (AMSR-E) data to investigate the possible impacts of oceanic bubbles on an enhanced AOD belt observed over the high-latitude southern oceans (also called the enhanced southern oceans anomaly, or ESOA) by some passive satellite sensors. Ultimately, this study is supportive of the null hypothesis: submerged bubbles are not the major contributor to the ESOA feature. This said, as retrievals progress to higher and higher resolutions, such as from airborne platforms, the uniform bubble correction in clean marine conditions should probably be separately accounted for against individual bright whitecaps and bubble rafts.