The Southern Ocean response to global warming in the CSIRO coupled ocean-atmosphere model

The Southern Ocean response to global warming is examined for a transient greenhouse gas integration using the Commonwealth Scientific and Industrial Research Organisation (CSIRO) coupled ocean-atmosphere model. The ocean component includes the Gent and McWilliams scheme for adiabatic eddy-induced transport. The atmospheric equivalent CO2 concentration (CO2e) follows the IPCC/IS92a radiative forcing scenario from 1880 to 2082, and is then maintained at a constant value of three times the 1880 level for seven centuries. The simulated changes which occur in the Southern Ocean under global warming are very profound, and they begin to separate clearly from the background climate noise in the model's 1990s. A major reduction in the depth and extent of convective mixing occurs by the time of CO2e doubling (year 2033), with near-cessation of convection by the time of CO2e tripling. Similarly major reduction occurs in the downwelling adjacent to Antarctica associated with Antarctic Bottom Water formation, which also nearly ceases by the time of CO2e tripling. These changes are associated with a marked reduction in surface density and salinity. By the time of CO2e tripling, both the pycnocline and halocline south from 60 degrees S intensify over the control by about a factor of four. The changes in surface salinity and density continue to intensify for several centuries during the subsequent period of elevated stable CO2e, and convection and Antarctic downwelling do not recover at all for the duration of the transient run. The water of the entire global ocean below about 1.5 km depth remains stagnant for the duration of the period of elevated stable CO2e, retaining a density which is too great to allow renewal from any source. Possible caveats on the realism of these results are discussed, and potential consequences of the above changes are noted. (C) 1999 Elsevier Science Ltd. All rights reserved.