How much of the NAO monthly variability is from ocean–atmospheric coupling: results from an interactive ensemble climate model

The chaotic atmospheric circulations and the ocean–atmosphere coupling may both cause variations in the North Atlantic Oscillation (NAO). This study uses an interactive ensemble (IE) coupled model to study the contribution of the atmospheric noise and coupling to the monthly variability of the NAO. In the IE model, seven atmospheric general circulation model (AGCM) realizations with different initial states are coupled with a single realization of the land, ocean and ice component models. The chaotic noise from the atmosphere at the air-sea interface is therefore reduced. The time variances of monthly NAO index in the ensemble AGCM mean of the IE model is found to be about 20.1 % of that in the SC model. Therefore, more than 79.9 % of the simulated monthly variability of NAO is caused by atmospheric noise. The coupling between sea surface temperature (SST) and NAO is only found in regions south of about 40°N in the North Atlantic Ocean. The IE strategy highlighted the interaction between the NAO and the SST in the region (28°–38°N, 20°W–50°W) to the southeast of the Gulf Stream extension. While the ocean–atmosphere coupling explains <1/5th of the NAO variability in the IE model, it shows slightly larger persistence than the SC model, consistent with the hypothesis of a slower mode of variability from ocean–atmosphere coupling that has larger predictability than the variability driven by the atmosphere.

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