The impact of surface flux anomalies on the mid-high latitude Atlantic Ocean Circulation in HADCM3. RAPID Project – The Role of Air-Sea Forcing in Causing Rapid Changes in the North Atlantic Thermohaline Circulation Report No. 1

forcing in the Hadley Centre coupled ocean-atmosphere model (HadCM3) are reported; the study forms part of a Natural Environment Research Council (NERC) Rapid programme project. An analysis of 100 years of the HadCM3 control run indicates that deep convection occurs in the Greenland Sea, the Irminger Basin and the Labrador Sea. However, a composite analysis of mixed layer depth only reveals a clear connection between deep convection and air-sea flux anomalies in the Greenland Sea, and we have focused on this region in our subsequent analysis. Evaluation of the different components of the density flux in the Greenland Sea shows that the net heat flux is a more important influence on surface density than both net evaporation and ice melt. A composite analysis of the ocean circulation was carried out for years with anomalously strong and weak heat loss over the Greenland Sea. Years of strong heat loss are associated with increased Greenland Sea convection and a rapid increase in the southward flow through the Denmark Strait by about 30%. Evidence of more widespread changes in the circulation at mid-high latitudes was also found but we have not yet established whether they are directly linked to the anomalous Greenland Sea forcing.