Modelling the thermohaline circulation

A course resolution, single hemisphere, single basin ocean general circulation model (OGCM) showed multiple modes of the thermohaline circulation. During the integration the thermohaline circulation underwent an initial adjustment phase, 2 different haline dominated modes, 2 strengths of thermal flushes, deep decoupled oscillations (DDOs) and salt loop oscillations. For each mode of circulation the stability and longevity were related to the strength of the stabilising and destabilising feedbacks.The OGCM domain was divided into 4 boxes and parameterisations of the heat and salt fluxes between the boxes were derived. Fluxes of heat and salt due to advection were reasonably modelled by a 'Stommel type' parameterisation where the tracer flux was linearly dependent on the mean velocity and tracer gradient. In accordance with this parameterisation the strength of the meridional overturning cell was highly correlated with the meridional deep water density gradient. A coupled convection and surface boundary condition scheme was developed which provided a direct connection between the surface forcing and deep water.the parameterisations derived from the OGCM data were incorporated into a number of box models. Sensitivity studies of these box models showed that the stability and strength of the thermohaline circulation was strongly dependent on the parameterisation of the strength of the meridional overturning cell. A quantitative comparison of the box model and OGCM results suggested that the box model could be used predictively over a limited range of thermohaline circulation modes.