Model study of the North Atlantic region atmospheric response to autumn tropical Atlantic sea-surface-temperature anomalies

Lead-lag Maximum Covariance Analysis (MCA) between National Centers for Environmental Prediction reanalysis sea surface temperature (SST) and 500 hPa geopotential-height fields shows that autumn tropical Atlantic SST anomalies are significantly linked with the following-winter North Atlantic Oscillation (NAO). The ability of the Météo-France atmospheric general circulation model ARPEGE to reproduce this relationship is tested, by forcing it with autumn tropical SST anomalies derived from lead-lag MCA analysis results. The autumn SST forcing induces a strong wave-like simultaneous response in October and November. The occurrence of the autumn weather regimes is also affected. in agreement with the significant spatial correlation of the midlatitude part of the wave response with the NAO pattern. By coupling the model with a slab ocean in midlatitudes, we show that the thermal coupling between the ocean and the atmosphere allows a better representation of the midlatitude part of the response. A negative autumn tropical SST anomaly triggers an interaction between the midlatitude SST, the low-frequency circulation and the storm-track activity, which reinforces and maintains a positive phase of the NAO until winter.