Vertical heat transports in the ocean and their effect an time-dependent climate change

In response to increasing atmospheric concentrations of greenhouse gases, t he rate of time-dependent climate change is determined jointly by the stren gth of climate feedbacks and the efficiency of processes which remove heat from the surface into the deep ocean. This work examines the vertical heat transport processes in the ocean of the HADCM2 atmosphere-ocean general cir culation model (AOGCM) in experiments with CO2 held constant (control) and increasing at 1% per year (anomaly). The control experiment shows that glob al average heat exchanges between the upper and lower ocean are dominated b y the Southern Ocean, where heat is pumped downwards by the wind-driven cir culation and diffuses upwards along sloping isopycnals. This is the reverse of the low-latitude balance used in upwelling-diffusion ocean models, the global average upward diffusive transport being against the temperature gra dient. In the anomaly experiment, weakened convection at high latitudes lea ds to reduced diffusive and convective heat loss from the deep ocean, and h ence to net heat uptake, since the advective heat input is less affected. R eduction of deep water production at high latitudes results in reduced upwe lling of cold water at low latitudes, giving a further contribution to net heat uptake. On the global average, high-latitude processes thus have a con trolling influence. The important role of diffusion highlights the need to ensure that the schemes employed in AOGCMs give an accurate representation of the relevant sub-grid-scale processes.