Effects of solar irradiance forcing on the ocean circulation and sea-ice in the North Atlantic in an isopycnic coordinate ocean general circulation model

The sensitivity of the ocean circulation to changes in solar irradiance is examined using an isopycnic coordinate, global ocean general circulation model (OGCM) coupled to a thermodynamic/dynamic sea-ice model. In the experiments, changes in the surface radiation forcing are calculated based on orbital data assuming that the atmospheric conditions are otherwise unchanged. Two sensitivity experiments are run with the ocean–sea-ice model: one with high and one with low solar irradiance representative of the last interglacial and glacial periods, respectively. The results show that the Atlantic merdional overturning circulation (AMOC) is increased (reduced) in response to lower (higher) summer solar irradiance. It is found that changes in the Arctic sea-ice volume and area are the main reason for the response. For the low solar irradiance case, less sea-ice is melted in summer leading to a saltier Arctic Ocean. This saltier water is then advected into the sinking regions in the winter, enhancing the intermediate and deep water formation. For the high solar irradiance case, a similar, but opposite, response occurs. The results thus confirm that the AMOC is very sensitive to external forcing. It is suggested that the scheme used for calculating changes in solar irradiance could prove useful when conducting glacial inception studies with fully coupled atmosphere–ocean models.