Tropical Atlantic SST Forcing of Coupled NAO-SST Tripole Responses

Shiling Peng

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Recent observational studies reveal that a fall Pan-Atlantic sea-surface temperature (SST) anomaly, composed of a horseshoe-like dipole in the North Atlantic and a southern center in the equatorial Atlantic, tends to precede the winter North Atlantic Oscillation (NAO) and its related SST tripole. We seek to understand this relationship using first large ensembles of AGCM experiments and experiments with the AGCM coupled to a mixed-layer ocean (AGCM_ML). The models are forced either by the North Atlantic horseshoe (NAH) or by the tropical SST anomalies over the boreal winter months.

The AGCM results suggest that the NAH anomaly is ineffective in forcing the NAO, and hence it cannot account for the observed lagged relationship. In contrast, in the AGCM_ML, the tropical anomaly induces a strongly seasonal coupled response with a NAO-SST tripole developed in late winter. In early winter, the coupled response is characterized by a wave-train anomalous height accompanied by a NAH-like SST anomaly. These AGCM_ML results suggest that the observed lagged relationship between the fall NAH SST and winter NAO may result from the seasonal march of coupled responses to persistent tropical forcing.

To determine the possible effects of Ekman transport on the coupled response to the tropical forcing, the mixed-layer ocean model is subsequently extended to include the Ekman heat advection. Parallel experiments with the extended coupled model, AGCM_EML, reveal that the inclusion of Ekman transport causes a seasonal shift in the coupled response. The NAO-SST tripole response develops a couple of months earlier, compared to that in the AGCM_ML, due to a reinforcement between the anomalous Ekman transport and the surface heat flux. The fast development of the NAO response in the AGCM_EML suggests that tropical Atlantic SST anomalies should be able to influence the NAO, in nature, on the seasonal time scale, and that efficient interactions with the extratropical ocean play a significant role in determining the coupled response.

Collaborators: Walter A. Robinson*, Shuanglin Li, Martin P. Hoerling, and Michael A. Alexander

NOAA-CIRES Climate Diagnostics Center, University of Colorado, Boulder, Colorado *Department of Atmospheric Sciences, University of Illinois, Urbana, Illinois

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19 October 2005
2 PM/ DSRC 1D 403
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