Thermohaline circulation at three key sections in the North Atlantic over 1985-2002

Efforts are presently underway to monitor the Thermohaline Circulation (THC) in the North Atlantic. A measuring strategy has been designed to monitor both the Meridional Overturning Circulation (MOC) in the subtropics and dense outflows at higher latitudes. To provide a historical context for these new observations, we diagnose an eddy-permitting ocean model simulation of the period 1985 - 2002. We present time series of the THC, MOC and heat transport, at key hydrographic sections in the subtropics, the northeast Atlantic and the Labrador Sea. The simulated THC compares well with observations. We find considerable variability in the THC on each section, most strikingly in the Labrador Sea during the early 1990' s, consistent with observed changes. Overturning in the northeast Atlantic declines by similar to 20% over the 1990' s, coincident with an increase in the subtropics. We speculate that MOC weakening may soon be detected in the subtropics, if the decline continues in mid-latitudes.