Climate Change and Ocean Circulation
Andreas Schmittner, Oregon State University
Ocean currents transport large amounts of heat, carbon, nutrients and other substances around the globe. This transport affects climate as well as the functioning of the marine ecosystem and biogeochemical processes. Here I try to give an overview of our knowledge on past and possible future changes of the ocean circulation and its ramifications. Surface currents are mainly driven by winds whereas the deep circulation is caused by subtle differences in the density of sea water. This density driven overturning circulation spans the world oceans in a conveyor belt like fashion with sinking at high latitude areas in the North Atlantic and near Antarctica. It responds sensitive to perturbations of surface salinities for example through changes in precipitation or melting of glaciers. Theory and models suggest that the overturning circulation exhibits hysteresis behavior with two steady states and rapid transitions between them. Such transitions can lead to abrupt warming or cooling events at high latitude regions such as the North Atlantic. Evidence from paleoclimate proxy records and modeling indicates that such changes actually happened during the last ice age with strong impacts on the oceans ecosystem and cycling of nutrients and other biogeochemical substances such as oxygen. According to simulations with climate models similar changes might occur in the future due to man made global warming and glacier melt. However, the climate model projections show a large spread and uncertainty in the future evolution of the overturning circulation. Much more work will be needed to improve these forecasts as well as to assess impacts on higher trophic levels in the food chain such as fish.