Role of Ocean-Atmosphere Coupling in Regional Climatic Impacts of Anthropogenic Sulfate Aerosols

Abstract

Using a suite of coupled and uncoupled climate model experiments, we explore the impacts of anthropogenic sulfate aerosols on tropical Pacific climate and its variability. The role of sea surface temperature (SST), ocean dynamics, and ocean-atmosphere interaction in climate response to aerosols is examined by appropriately choosing the ocean component namely, full ocean general circulation model (OGCM), slab ocean model (SOM) or prescribed climatological SST. The ensemble of shorter responses versus a long-term response highlights processes and coupled feedbacks that are active on seasonal-interannual timescales versus on multi-decadal timescales. We find that an abrupt increase in tropospheric sulfate aerosols gives rise to El Niño like warming of the eastern tropical Pacific on seasonal-interannual timescales. Dynamical interaction between the ocean and the atmosphere causes this equatorial warming, whereas the thermodynamic interaction is responsible for the off-equatorial warming. These two interactions are related to the presence of Bjerknes feedback over the equator versus the off-equatorial WES (Wind-Evaporation-SST) feedback in the tropical climate system. In long-term, ocean dynamics will remove the initial tropical warming leaving a weaker and negative SST in response to increased aerosols in the fully-coupled case. Absence of ocean dynamics in the partially-coupled case, on the other hand, will lead to amplification of the off-equatorial warming via positive WES feedback. These coupled feedbacks, therefore, control the intertropical convergence zone (ITCZ) shift in response to aerosols over the eastern tropical Pacific. In this study, we also identify a cloud microphysics based mechanism for the high cloud increase over the tropical Indian Ocean. This regional increase in high clouds results in local net positive radiative forcing in comparison to negative forcing elsewhere on the globe.