UNDERSTANDING SOUTHERN HEMISPHERE CIRCULATION AND RAINFALL TRENDS: THE ROLE OF SSTS

Wednesday, December 11, 2019 - 08:45
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The Southern Hemisphere large-scale circulation and rainfall distribution has exhibited significant trends over the past century. Australia in particular has experienced significant drying in the southwest and increases in the northwest. One difficulty in disentangling the role of anthropogenic forcing from natural variability in these changes is that historical coupled climate model simulations exhibit quite different trends in sea surface temperature (SST) patterns from those that have been observed. An alternative method of untangling the roles of natural variability and anthropogenic climate change in recent rainfall trends is to use atmospheric GCMs forced with observed SSTs. It is expected that constraining the models by observed SSTs should enhance their ability to reproduce the regional patterns of change over the historical period. However in some regions, the lack of coupling may lead to an incorrect atmospheric response. Recent studies have utilised a method that attempts to combine these two approaches, by prescribing SSTs in certain regions and allowing a freely evolving coupled model elsewhere. These so-called ‘pacemaker’ experiments allow coupling to occur in regions where the atmosphere is largely driving the SST variability, but crucially retain some information of the observed SST evolution (e.g. from the tropical Pacific) to drive the atmosphere. Here we examine a hierarchy of modelling frameworks to understand the role of SSTs in long-term changes in the SH atmospheric circulation and associated rainfall changes over Australia. We find that SST-forced models are unable to capture the observed rainfall trends from 1960-present over Australia and in fact get the opposite response. In austral winter in particular, the SH atmospheric circulation response in the models resembles a wave train rather than the more zonal response observed. The pacemaker experiments are better able to capture these trends, suggesting that both tropical Pacific SSTs are contributing to the rainfall trends and that coupling is needed to reproduce them.

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