Oceanic Feedback in Regional Climate Response to Sulfate Aerosol Forcing

There is considerable uncertainty in quantifying climate response to aerosol radiative forcing owing to lack of complete understanding and/or inability to represent aerosols and their effect on the climate system. Differences in aerosol burden, representation and its interaction with the climate is often stated as a major reason for disagreement among various climate model projections of future climate change as well as of historical changes. In particular, the role of oceanic feedback on the regional climatic response due to changes in atmospheric aerosol forcing remains poorly understood.

In this study, we conduct fully coupled global model experiments using the NCAR CESM (version-1.1.2, with minimal representation of aerosols using MAM3, with aerosol direct, indirect and semi-direct effect) to study the climate response to sulfate aerosols. A 40-year long control run is carried out using pre-industrial forcing and emissions. To assess the sensitivity to modern aerosol forcing, a series of year-long perturbed integrations are carried out, using present day sulfate emissions. The perturbed simulations are restarted every January from the control trajectory. This year-long simulation approach allows only the upper ocean to respond to the forcing, thus improving the signal to noise ratio over the Tropical Pacific region.

In addition to finding widespread cooling of the Northern Hemisphere and associated changes in tropical circulation, a typical response to sulfate aerosols, a small warming signal in Tropical Pacific is also observed. This warming is shown to be strongly linked to dynamical interaction between the tropical atmosphere and ocean, which is known to give rise to ENSO, exhibiting consistent responses in wind stress and upper ocean circulation.

There is also a significant cooling and hydrological response over South-East Asia and Western Pacific which can be directly attributed to sulfate aerosols effect on radiation and clouds. We examine whether the above two responses, (1) cooling and reduction of precipitation over region with high sulfate burden and (2) warming over fairly clean environment of the Tropical Pacific, are inter-connected.