The Changing Relationship between Interannual Variations of the North Atlantic Oscillation and Northern Tropical Atlantic SST

The relationship between interannual variations of boreal winter North Atlantic Oscillation ( NAO) and northern tropical Atlantic ( NTA) sea surface temperature ( SST) experienced obvious interdecadal changes during 1870-2012. Similar interdecadal changes are observed in the amplitude of NTA SST anomalies. The mean NTA SST change may be a plausible reason for several changes in the NAO-NTA SST connection. Under a higher mean NTA SST, NTA SST anomalies induce larger wind anomalies over the North Atlantic that produce a tripole SST anomaly pattern and amplify NTA SST anomalies. Comparison of the evolution of anomalies between 1970-86 and 1996-2012 unravels changing roles of El Nino-Southern Oscillation ( ENSO) and extratropical atmospheric disturbances in the formation of NTA SST anomalies. During 1970- 86, ENSO events play a key role in initiating NTA SST anomalies in the preceding spring through atmospheric circulation changes. With the decay of ENSO, SST anomalies in the midlatitude North Atlantic weaken in the following summer, whereas NTA SST anomalies are maintained up to winter. This leads to a weak NAONTA SST connection in winter. During 1996- 2012, the preceding spring atmospheric circulation disturbances over the midlatitude North Atlantic play a dominant role in the genesis of a North Atlantic horseshoe ( NAH)like SST anomaly pattern in the following summer and fall. This NAH- like SST anomaly pattern contributes to the development of the NAO in late fall and early winter. The atmospheric circulation anomaly, in turn, is conducive to the maintenance of NTA SST anomalies to winter via changing surface latent heat flux and shortwave radiation. This leads to a close NAO- NTA SST connection in winter.