Improving our climate models, one bias at a time

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Just submitted to the Journal of Climate, our paper on “The impact of SST biases in the tropical east Pacific and Agulhas current region on atmospheric stationary waves in the Southern Hemisphere”.

Climate models in the Coupled Model Intercomparison Project, Phase 5 (CMIP5) vary significantly in their ability to simulate the phase and amplitude of atmospheric stationary waves in the midlatitude Southern Hemisphere. These models also suffer from a double inter-tropical convergence zone (ITCZ), with excessive precipitation in the tropical eastern South Pacific, and many also suffer from a biased simulation of the dynamics of the Agulhas Current around the tip of South Africa. The intermodel spread in the magnitude of the strength and phasing of SH midlatitude stationary waves in the CMIP archive is shown to be significantly correlated with the double ITCZ bias and biases in the Agulhas Return Current. An idealized General Circulation Model (GCM) is used to demonstrate the causality of these links by prescribing an oceanic heat flux out of the tropical East Pacific and near the Agulhas Current. A warm bias in tropical east Pacific SSTs associated with an erroneous ‘‘double” ITCZ leads to a biased representation of midlatitude stationary waves in the austral hemisphere, capturing the response evident in CMIP models. Similarly, an overly diffuse sea surface temperature gradient associated with a weak Agulhas Return Current leads to an equatorward shift of the Southern Hemisphere jet by more than 3 degrees and weak stationary wave activity in the austral hemisphere. Hence, rectification of the double ITCZ bias and a better representation of the Agulhas Current should be expected to lead to an improved model representation of the austral hemisphere.