Downscaling Future Changes of the Global Warming Hot Spots in the Western Boundary Current Systems

Ocean warming most obvious in the upper 200-300 m due to extra heat being absorbed is an important manifestation of anthropogenic climate change. Global mean sea surface temperature (SST) warming is projected to be between 0.8 and 3.1 ^oC over 2081-2100 relative to 1986-2005, as reported in the Fifth Assessment Report of IPCC based on Coupled Model Intercomparison Project Phase 5 (CMIP5) climate models. However, SST changes are not expected to be geographically uniform, and there are several hot spots with much faster warming than the global mean, especially in the western boundary currents (WBCs) and their extension regions. However, with coarse resolution (~1^o in the ocean component), CMIP5 climate models cannot resolve these WBCs and their eddy field very well. Here we use a near-global eddy-resolving (0.1^o resolution) ocean general circulation model (OGCM) to downscale future climate changes over the 21^st century, by applying atmospheric anomaly fields derived from the ensemble mean of 17 CMIP5 models under the Representative Concentration Pathway 8.5. Localized strong upper ocean warming (~5 ^oC), not only in the surface layer but often extending down to several hundred of meters, can be found in all WBCs and their extensions (namely, Kuroshio, Gulf Stream, Agulhas Current, East Australian Current and Brazil Current). Through examining changes of ocean fields (velocity, temperature and salinity), mean and eddy kinetic energy and upper ocean heat budget, we study those warming hot spots in detail and find that they can be explained to the first order by the poleward expansion and/or intensification of subtropical ocean gyres. With embedded biogeochemical fields in the OGCM, we further investigate how the WBCs and associated eddy activity changes affect nutrient supply, biogeochemical response and primary productivity.