From the Southern Ocean to the Tropics: How Can Climate Model Errors Amplify?

Coupled atmosphere-ocean general circulation models (CGCMs) show important systematic errors.  In the tropics, precipitation tends to be excessive over the oceans south of the equator, and marine low clouds are generally underestimated above too warm sea surface temperatures (SSTs).  In the extratropics, SSTs are also too warm over the Southern Ocean.  We argue for remote links between these tropical and extratropical errors and present a possible amplifying mechanism involving the sensitivity of marine clouds to underlying SST.  Our arguments are supported by the very different response obtained by two CGCMs that greatly differ in their simulation of the response to an identical artificial decrease in solar radiation flux incident at the top of the atmosphere.  In both models  (UCLA CGCM and NorESM), reduced insolation over the Southern Ocean results in large-scale circulation changes consistent with less heating of the southern hemisphere and in reduced local SSTs. In the model with the stronger SST/marine cloud feedbacks, these colder SSTs over the Southern Ocean are advected to the subtropical regions, which combined with the circulation changes result in increased low level clouds in the major stratocumulus regions and an associated reduction of the SST in these regions. In the model with the weaker feedbacks (NorESM) the effects on the subtropics are very small.  A CGCM that underestimates SST/marine cloud feedbacks, therefore, may not benefit from improvements in the Southern Ocean simulation.