Diagnostics of the oceanic thermohaline circulation in a coupled climate model

Two century-scale integrations of a global coupled model consisting of the GISS atmospheric model and the HYCOM ocean model are carried out: a control run assuming fixed atmospheric composition, and a perturbation run assuming gradual doubling of CO2. The model does not use flux corrections at the air–sea interface, nor is the ocean “spun up” prior to coupling. When increasing CO2 at the rate of 1% per year to twice its original level and keeping it constant thereafter, 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 (MOC). Due to the predominantly isopycnic character of HYCOM, geographic details of the 3-D thermohaline circulation in potential density space can be extracted from the model output with relative ease. The analysis confirms that even regional details of the MOC in this experiment are rather insensitive to the climate change brought on by CO2 doubling. Furthermore, the analysis reveals strong similarities between the simulated and the observed MOC.