Processes Governing the Recovery of a Perturbed Thermohaline Circulation in HadCM3

In an experiment with the latest version of the Hadley Centre climate model the model response has been analyzed after the thermohaline circulation (THC) in the Atlantic Ocean has been suppressed. The suppression is induced by a strong initial perturbation to the salinity distribution in the upper layer of the northern North Atlantic. The model is then allowed to adjust freely. Salinity gradually increases and deep water formation in the Greenland and Norwegian Seas restarts, later also in the Labrador Sea. The meridional overturning recovers after about 120 yr. In the first few decades when the overturning is very weak surface air temperature is dominated by cooling of much of the Northern Hemisphere and weak warming of the Southern Hemisphere, leading to maximum global cooling of 0.9°C. The disruption to the atmosphere's radiation balance results in a downward flux anomaly at the top of the atmosphere, maximally 0.55 W m–2 in the first decade then decreasing with the THC recovery. The processes responsible for the recovery of the THC is examined in detail. In future model development this will help to reduce uncertainty in modeling THC stability. The recovery is driven by coupled ocean–atmosphere response. Northward salt transport by the subtropical gyre is crucial to the recovery of salinity in the North Atlantic. A southward shift of the ITCZ creates positive salinity anomalies in the tropical North Atlantic. This supports the northward salt transport by the subtropical gyre that helps to restart deep water formation and the THC.