Dynamical downscaling of future changes in the Tasman Sea – a climate change hot spot

Upper ocean warming due to extra heat uptake is an important manifestation of anthropogenic climate change. Global mean sea surface temperature (SST) is projected to increase between 0.8 and 3.1 ^oC over 2081-2100 relative to 1986-2005, based on Coupled Model Intercomparison Project Phase 5 (CMIP5) climate models. However, SST changes are not geographically uniform, and there are several hot spots with much greater warming trend than the global mean, such as in the western boundary currents (WBCs) and their extension regions. However, with coarse resolution (~1^o in the ocean component), CMIP climate models cannot resolve these WBCs and their eddy field well. Here we use a near-global eddy-rich (0.1^o resolution) ocean general circulation model (OGCM) to dynamically downscale future climate changes over 2006-2100, by applying atmospheric anomaly fields derived from the ensemble mean of 17 CMIP5 models under the Representative Concentration Pathway 8.5. The Tasman Sea, especially the East Australian Current (EAC) and its extension region, is one of the hot spots for future climate change. The future downscaling experiment indicates strong upper ocean warming (4~5 ^oC) occurring not only in the surface layer but extending down to several hundred of meters, in the Tasman Sea from about 30^o to 45 ^oS. Through examining changes of upper ocean heat budget, we find that the warming pattern in the Tasman Sea can be explained mainly by the poleward expansion and strengthening of EAC extension, and enhanced meso-scale eddy activities. Both high-frequency (< 30 days) and low-frequency (> 30 days) heat advection components are important and counteract each other in some areas, causing quite uniform warming in the West Tasman Sea. With embedded biogeochemical fields in the OGCM, we further investigate how the projected boundary current changes and upper ocean warming affect nutrient supply, biogeochemical response, primary productivity and fisheries recruitment processes in the Tasman Sea.