STABILITY AND VARIABILITY OF THE OCEAN THERMOHALINE CIRCULATION AND CLIMATE IN SIMPLE COUPLED MODELS

The behavior of the thermohaline circulation and climate is explored in two new simple, ocean-atmosphere models. The first model, a one-hemisphere, steady-state box model, highlights the role of thin, ocean surface layers in the communication between the atmosphere and the subsurface ocean. Poleward heat and moisture transports in the atmosphere depend linearly on the north-south gradient of sea surface temperature. For small vertical exchange in the ocean, the model exhibits only one solution: a relatively cold, mid-high latitude climate associated with a weak, salinity-driven circulation ("off" mode). For large vertical and horizontal exchange in the ocean, the model also exhibits only one solution: a relatively warm, mid-high latitude climate associated a strong, thermally-driven circulation ("on" mode). These results suggest that the wind-driven circulation in consort with diapycnal mixing suppresses the "off" mode in the modern ocean-atmosphere system.

The second model, a two-hemisphere, time-dependent model with continuous vertical stratification, highlights the competition of different water mass formation and mixing processes in determining water properties and flow structure within the ocean. Building on the results of the first model, ocean processes are expanded to include small-scale vertical diffusion, wind-driven circulation, open ocean convection, brine rejection/shelf water convection around Antarctica as well as the equatorward Ekman transport at the southern tip of South America. The model atmosphere transports heat and moisture poleward with parameterizations based on mid-latitude, baroclinic instability and the temperature dependence of water vapor content. The ice albedo feedback is included through sea ice/snow cover parameterizations. For certain parameter ranges, this model exhibits millennium-scale fluctuations of the thermohaline circulation which bear resemblance to "deep decoupling" oscillations found in more complex ocean models. These fluctuations tend to be suppressed and the "on" mode is favored for large vertical diffusion, strong Ekman transport and strong salt rejection. The structure and evolution of these fluctuations compares favorably with millennium-scale variability found in climate proxy records.