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Effects of Ekman Transport on the NAO Response to a Tropical Atlantic SST Anomaly
Shiling Peng, NOAA/CIRES/CDC, Boulder, CO; and W. A. Robinson, S. Li, and M. A. Alexander
Our earlier study showed that a tropical Atlantic sea-surface temperature (SST) anomaly induces a significant coupled NAO-SST tripole response in late winter (February-April) in a coupled model, in which an atmospheric general circulation model is coupled to a slab mixed layer ocean (AGCM_ML). This simulated NAO response developed one or two months later in the model than in observations. To determine the possible effects of Ekman heat transport on the development of the coupled response to the tropical forcing, an extended coupled model (AGCM_EML), including Ekman transport in the slab mixed-layer ocean, is now used. Large ensembles of AGCM_EML experiments are performed, parallel to the previous AGCM_ML experiments, with the model forced by the same tropical Atlantic SST anomaly over the boreal winter months (September-April).
The inclusion of Ekman heat transport is found to result in an earlier development of the coupled NAO-SST tripole response in the AGCM_EML, compared to that in the AGCM_ML. The mutual reinforcement between the anomalous Ekman transport and the surface heat flux causes the tropical forcing to induce an extratropical SST response in early winter (November-January) in the AGCM_EML that is twice as strong as that in the AGCM_ML. The feedback of this stronger extratropical SST response on the atmosphere in turn drives the development of the NAO response in early winter. In late winter, the sign of the anomalous surface heat flux is reversed in the Gulf Stream region such that it opposes the anomalous Ekman transport. The resulting equilibrium NAO response in the AGCM_EML is similar to that in the AGCM_ML, but it is reached one-to-two months sooner in the AGCM_EML. The presence of Ekman transport hence causes a seasonal shift in the evolution of the coupled response. The faster 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.
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Session 1, Coupled Ocean–Atmosphere Basin–Scale And Decadal Variability
Monday, 30 January 2006, 9:00 AM-11:45 AM, A309
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