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Simulations of Processes Associated with the Fast Warming Rate of the Southern Midlatitude Ocean

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  • 1 Wealth from Oceans National Research Flagship, CSIRO, Aspendale, Victoria, Australia
  • | 2 Wealth from Oceans National Research Flagship, CSIRO, Hobart, Tasmania, Australia
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Abstract

Significant warming has occurred across many of the world’s oceans throughout the latter part of the twentieth-century. The increase in the oceanic heat content displays a considerable spatial difference, with a maximum in the 35°–50°S midlatitude band. The relative importance of wind and surface heat flux changes in driving the warming pattern is the subject of much debate. Using wind, oceanic temperature, and heat flux outputs from twentieth-century multimodel experiments, conducted for the Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC), the authors were able to reproduce the fast, deep warming in the midlatitude band; however, this warming is unable to be accounted for by local heat flux changes. The associated vertical structure and zonal distribution are consistent with a Sverdrup-type response to poleward-strengthening winds, with a poleward shift of the Southern Hemisphere (SH) supergyre and the Antarctic Circumpolar Current. However, the shift is not adiabatic and involves a net oceanic heat content increase over the SH, which can only be forced by changes in the net surface heat flux. Counterintuitively, the heat required for the fast, deep warming is largely derived from the surface heat fluxes south of 50°S, where the surface flux into the ocean is far larger than that of the midlatitude band. The heat south of 50°S is advected northward by an enhanced northward Ekman transport induced by the poleward-strengthening winds and penetrates northward and downward along the outcropping isopycnals to a depth of over 1000 m. However, because none of the models resolve eddies and given that eddy fluxes could offset the increase in the northward Ekman transport, the heat source for the fast, deep warming in the midlatitude band could be rather different in the real world.

Corresponding author address: Wenju Cai, CSIRO Marine and Atmospheric Research, PMB 1, Aspendale, VIC 3195, Australia. Email: wenju.cai@csiro.au

Abstract

Significant warming has occurred across many of the world’s oceans throughout the latter part of the twentieth-century. The increase in the oceanic heat content displays a considerable spatial difference, with a maximum in the 35°–50°S midlatitude band. The relative importance of wind and surface heat flux changes in driving the warming pattern is the subject of much debate. Using wind, oceanic temperature, and heat flux outputs from twentieth-century multimodel experiments, conducted for the Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC), the authors were able to reproduce the fast, deep warming in the midlatitude band; however, this warming is unable to be accounted for by local heat flux changes. The associated vertical structure and zonal distribution are consistent with a Sverdrup-type response to poleward-strengthening winds, with a poleward shift of the Southern Hemisphere (SH) supergyre and the Antarctic Circumpolar Current. However, the shift is not adiabatic and involves a net oceanic heat content increase over the SH, which can only be forced by changes in the net surface heat flux. Counterintuitively, the heat required for the fast, deep warming is largely derived from the surface heat fluxes south of 50°S, where the surface flux into the ocean is far larger than that of the midlatitude band. The heat south of 50°S is advected northward by an enhanced northward Ekman transport induced by the poleward-strengthening winds and penetrates northward and downward along the outcropping isopycnals to a depth of over 1000 m. However, because none of the models resolve eddies and given that eddy fluxes could offset the increase in the northward Ekman transport, the heat source for the fast, deep warming in the midlatitude band could be rather different in the real world.

Corresponding author address: Wenju Cai, CSIRO Marine and Atmospheric Research, PMB 1, Aspendale, VIC 3195, Australia. Email: wenju.cai@csiro.au

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