Global thermohaline circulation

The possibility of future shut-down of convection in the North Atlantic Ocean, and a consequent reduction in the strength of the Gulf Stream, with associated cooling of western Europe has been much in the news in recent years. While the prospects for this occurring remain limited it is clear that the ocean circulation has changed significantly, and rapidly, at times in the past. I use models to explore the regimes under which change can occur, and the consequences of such change.

Highlights of this work include

• Discovery of the impact of opening of the sea connection between the Arctic and Atlantic through the Canadian Seaways. Without the freshening influence of Arctic waters on the Labrador Sea the latter becomes much saltier, and more subject to convection, allowing more northward heat transport. An open connection through the Canadian Archipelago therefore acts as a negative feedback on the North Atlantic overturning circulation.
• Discovery of a mechanism for producing intermediate water formation only during glacial periods in the northern Atlantic. Palaeoclimate models generally produce too much convection in the northern Atlantic, and so too much heat transport northwards. This is because of the extremely cold air temperatures associated with the massive ice sheets of the glacial Northern Hemisphere. However, palaeoceanographic data suggest a shallower, more limited production of water in the northern Atlantic. Modelling suggests that this is because of a greater input of freshwater to mid- and sub-polar latitudes of the northern Atlantic, from a combination of warmer air temperatures than previously suspected, and iceberg meltwater.
• Discovery that the convection is currently undergoing rapid change in the northern Atlantic. Using ocean models I have shown that convection is likely to have switched from the Labrador Sea to the Nordic Seas in the mid-1990s, and that the location of convection is important in deciding whether any salinity and temperature anomalies in the northern Atlantic penetrate into the wider ocean.

This work has been carried out with Martin Wadley, Eelco Rohling of Southampton University, and Steve Dye and Bob Dickson of the Centre for Fisheries and Aquaculture Science, and funded by NERC and the Leverhulme Trust.