Mechanisms determining the atmospheric response to the Atlantic overturning circulation

In climate models, an intensification of the Atlantic Meridional Overturning Circulation (AMOC) precedes by a few years a warming in the North Atlantic subpolar basin. Analysis of model simulations and observations suggested that such a warming of the Atlantic sea surface temperature (SST) may cause an atmospheric response in winter resembling a negative phase of the North Atlantic Oscillation (NAO). To further establish the causality links between the ocean and the atmosphere, and illustrate the mechanisms determining this atmospheric response, ensembles of atmosphere-only simulations are designed using the IPSL-CM5A-LR coupled model outputs and the corresponding atmospheric model component LMDZ5A.

The atmospheric model simulations use prescribed SST and sea-ice anomalies that follow an intensification of the AMOC in the coupled model. Despite limitations due to poor sea-ice representation in the atmospheric model, the SST and sea-ice anomalies are found to cause an atmospheric response similar, but weaker to that found in the coupled model. The main influence is due to warm subpolar Atlantic SST anomalies north of 30°N and the associated upward heat flux which are responsible for a decrease of the lower-tropospheric baroclinicity downstream of the region of maximum eddy growth. These changes are causing a weaker transient eddy feedback and evolve into a negative NAO signal. The sea-ice anomalies further amplify the atmospheric circulation anomalies, as they act to reinforce the upward heat flux in the eastern subpolar North Atlantic region. The atmospheric response seems to be strongly modulated by the coupling with the stratosphere, as it is associated with a stratospheric warming leading the tropospheric signal.

Finally, the SST and sea-ice influence is found to be non-linear in identical simulations using three times larger SST and Sea-ice anomalies.