Tuesday, 15 January 2002: 11:00 AM
Diagnostics of the Thermohaline Circulation in a Coupled Climate Model
Two 200-year integrations using a global coupled model consisting of the GISS AGCM and the HYCOM OGCM are carried out -- a control run assuming fixed atmospheric composition, and a perturbation run assuming gradual doubling of CO_2. The model does not use flux corrections at the air-sea interface, nor is the ocean 'spun up' prior to coupling. The control run stabilizes after several decades. When doubling CO_2 at the rate of 1% per year, the model responds with a 2 C increase in 200 years in the global mean surface air temperature and a virtually unchanged Atlantic meridional overturning circulation. The latter is maintained by a salinity increase which counteracts the effect of global warming on the surface buoyancy. This talk dwells on geographic details of the 3-D thermohaline circulation (THC) which, due to the predominantly isopycnic character of HYCOM, can be constructed in (x,y,density) space with relative ease. In both runs, we see strong similarities between the simulated and the observed THC. This is consistent with -- and partially explains -- the relatively small warming signal seen in the CO_2 doubling run. The largest deviations between the simulated and observed THC are found in the bottom-water source regions of the world ocean and appear to be related to deficiencies in the modeling of sea ice.