Active AMOC–NAO coupling in the IPSL-CM5A-MR climate model

Abstract

The atmospheric response to the AMOC variability is investigated in the IPSL-CM5A medium resolution climate model, using lagged maximum covariance analysis (MCA) of a control simulation. A robust atmospheric response is detected in winter, with a negative NAO-like response following by about 9-year an AMOC intensification in the North Atlantic, with a pattern broadly resembling the second mode of AMOC variability. The response is established through the SST footprint of the AMOC and the associated surface heat flux damping, with a dipole of SST anomalies made of cold SST in the Gulf Stream region and warm SST further northeast around the North Atlantic Current. The dipole SST anomaly pattern evolves synchronously with the AMOC changes at its dominant 20-year period, so that the lagged NAO-like response detected by MCA actually reflects the near-synchronous AMOC influence on the atmosphere, which is masked at short time lag by the stronger atmospheric forcing of the AMOC. The atmospheric response to an intensification of the AMOC is thus a positive NAO-like pattern, together with an anomalous low pressure over the Aleutians, opposite to that detected at 9-year lag by the MCA. Since the NAO also contributes to force the AMOC, there is a positive feedback between AMOC and NAO in the model, with the atmospheric feedback strength about 1/4 of that of the atmospheric forcing, which enhances the low frequency variability of AMOC. This is further confirmed by the lead-lag relation between the dominant mode of ocean and atmosphere, and by the robust 20-year period of the NAO.