Abstract

The climate effect of the meridional overturning circulation (MOC) is discussed in terms of two separate examples, the connection with decadal heat anomalies in the North Atlantic and the longer-term connection with atmospheric CO2 . Firstly, historical temperate date over the last 60 years suggest that there are thermal anomalies extending to depths of 1 to 2 kilometres over the subtropical and subpolar gyres, which often have the opposing sign in each gyre. A dynamical assimilation of the historical data suggests that these decadal thermal anomalies are primarily formed by the convergence in ocean heat transport: the subtropical thermal anomalies are mainly controlled by the wind-induced Ekman convergence in the heat transport, while the subpolar thermal anomalies are instead controlled by the convergence in the MOC -Ekman heat transport. Secondly, coupled carbon and general circulation model experiments suggest that there is a long-term effect on atmospheric CO2 whenever overturning anomalies connect to changes in the Southern Ocean: increasing residual circulation leads to increasing long-term atmospheric CO2 . This relationship is a consequence of partly compensating processes, increased overturning enhances the subduction of carbon, but at the same time enhancing the upwelling of regenerated carbon.