RAPID: Long-term Trends of the Atlantic Interhemispheric SST Gradient in the CMIP5 20th Century Simulations

University Of California-Berkeley, Berkeley CA

Investigators

Abstract

This is one of 16 Rapid Response (RAPID) projects funded as the result of a Dear Colleague Letter (NSF 11-006) encouraging diagnostic analyses of climate model simulations prepared for the Intergovernmental Panel on Climate Change Fifth Assessment Report (IPCC AR5). Research conducted in these projects is expected to lead to more detailed model intercomparisons, better understanding of robust model behaviors, and better understanding and quantification of uncertainty in future climate simulations. This project examines the secular trend in the Atlantic interhemispheric sea surface temperature (SST) gradient occuring in 20th century observations and climate model simulations. Observations show a trend in the gradient in which the Southern Hemisphere tropical SSTs warm faster than their Northern Hemisphere counterparts. Previous work using an earlier ensemble of climate model simulations suggested that the SST gradient trend is forced in part, perhaps predominantly, by the radiative effects of anthropogenic sulfate aerosols. The behavior of the SST gradient trend in simulations prepared for the AR5 is of particular interest because the new generation of models incorporates significant advances in the simulation of both direct and indirect effects of aerosols. In this project, research focuses on 1) quantifying the magnitude of the SST gradient trend, and assessing the spread in the trend across the different multimodel ensemble members; 2) attributing the cause of the trend; 3) assessing the role of aerosol indirect effects in determining the magnitude of the trend; 4) determining the relative roles of different aerosol types in forcing the trend, particularly sulfate aerosols and black carbon; and 5) understanding the flattening of the trend in the late 20th century. The primary method for identifying the aerosol radiative contribution to the trend consists of comparisons between model simulations forced with all 20th-century climate forcings (including aerosols) and simulations in which only the 20th century increases in greenhouse gas concentrations are used as forcing. The broader impact of the project lies in its support of the IPCC AR5, which is intended to provide information on climate change and its consequences to decision makers worldwide. The Atlantic SST gradient exerts a significant influence on the climate of the surrounding landmasses, and is implicated in droughts over the Sahel and northeastern Brazil. The extent to which the SST gradient trend is forced by aerosols, and if so which aerosols are most responsible, is thus of great societal relevance to these regions.