Atmospheric response to the North Atlantic Ocean variability on seasonal to decadal scales

A control simulation with the IPSL-CM5 climate model is used to assess the influence of ocean-atmosphere coupling in the North Atlantic region at seasonal to decadal time scales. At the seasonal scale, the air-sea interaction patterns are similar in the model and the observations. In both cases, a summer SST anomaly with a horseshoe shape significantly leads an atmospheric signal in winter that resembles the North Atlantic Oscillation (NAO). The air-sea interactions in the model thus seem realistic, although the amplitude of the atmospheric signal is twice too small. In both cases, the North Atlantic horseshoe SST anomaly pattern is in part generated by the spring and summer internal atmospheric variability. In the model, the ocean dynamics also contributes at decadal scales, as the North Atlantic SST anomaly that follows an intensification of the Atlantic meridional overturning circulation (AMOC) by about 9 years resembles the horseshoe pattern, and is also similar to the model Atlantic multidecadal oscillation. As in the model the AMOC has a significant impact on the winter NAO, most strongly so when it leads by 9 years, the seasonal and decadal interactions are consistent. In the observations, there is also a strong correlation between the Atlantic multidecadal oscillation and the SST horseshoe pattern that influences the NAO, suggesting that the natural variability of the AMOC significantly influences the climate of the North Atlantic sector at the decadal scale.