A process-based analysis of ocean heat uptake in an AOGCM

About 90% of the anthropogenic increase in heat stored in the climate system 6 is found the oceans. Therefore it is relevant to understand the details of ocean heat up7 take. Here we present a detailed, process-based analysis of ocean heat uptake (OHU) pro8 cesses in HiGEM1.2, an atmosphere-ocean general circulation model (AOGCM) with an 9 eddy-permitting ocean component of 1/3 resolution. Similarly to various other models, 10 HiGEM1.2 shows that the global heat budget is dominated by a downward advection of heat 11 compensated by upward isopycnal diffusion. Only in the upper tropical ocean do we find 12 the classical balance between downward diapycnal diffusion and upward advection of heat. 13 The upward isopycnal diffusion of heat is located mostly in the Southern Ocean, which thus 14 dominates the global heat budget. We compare the responses to a 4xCO2 forcing and an en15 hancement of the windstress forcing in the Southern Ocean. This highlights the importance 16 of regional processes for the global ocean heat uptake. These are mainly surface fluxes and 17 convection in the high latitudes, and advection in the Southern Ocean mid-latitudes. Changes 18 in diffusion are less important. In line with the CMIP5 models, HiGEM1.2 shows a band 19 of strong OHU in the mid-latitude Southern Ocean in the 4xCO2 run, which is mostly ad20 vective. By contrast, in the high-latitude Southern Ocean regions it is the suppression of 21 convection that leads to OHU. In the enhanced windstress run, convection is strengthened 22 at high Southern latitudes, leading to heat loss, while the magnitude of the OHU in the 23 Southern mid-latitudes is very similar to the 4xCO2 results. Remarkably, there is only very 24 small global OHU in the enhanced windstress run. The wind stress forcing just leads to a 25 redistribution of heat. We relate the ocean changes at high southern latitudes to the effect of 26 climate change on the Antarctic Circumpolar Current (ACC). It weakens in the 4xCO2 run 27 and strengthens in the wind stress run. The weakening is due to a narrowing of the ACC, 28 caused by an expansion of the Weddell Gyre, and a flattening of the isopycnals, which are 29 explained by a combination of the wind stress forcing and increased precipitation. 30 T. Kuhlbrodt, J. M. Gregory and L. C. Shaffrey NCAS, Department of Meteorology University of Reading Earley Gate, PO Box 243 Reading RG6 6BB, UK E-mail: [email protected] J. M. Gregory Met Office Hadley Centre Exeter, UK !"#$%&'() !"#$%&'('%)*%+*,-!*.+%/.-01#("2)3%&"4'5 6789:)';% !"#$%&'('%)*%+"',%!"-$'.%/'0'('-#'1