A seasonal bridge for winter to winter variability of the North Atlantic Oscillation : Role of Atlantic summer sea surface temperatures.
Part I : Observations
C. Cassou, L. Terray, C. Deser, M. Drevillon, J.W. Hurrell
National Center for Atmospheric Research
P. O. Box 3000
Boulder, CO 80307
email: cassou@ucar.edu
voice: (303) 497 1710
Introducing temporal lags in the relationship between the ocean and the North Atlantic-Europe winter atmosphere reveals that the phase of the winter North Atlantic Oscillation (NAO) can be inferred from the strength during the preceding summer of two North Atlantic Sea Surface Temperature (SST) anomalous patterns. Both are characterized by anomalous interhemispheric SST gradient in the tropical basin coupled to a meridional shift of the Inter Tropical Convergence Zone, but differ by their extratropical signature. The first one is known as the horseshoe mode with an anomalous core off Newfoundland surrounded by opposite sign anomalies. The second one is a remainder of the previous winter tripole to which it is spatially and temporally highly correlated. It is suggested here that the extratropical signature of the summer horseshoe mode can be interpreted as primarily a response to anomalous local atmospheric circulation whose origin is found in the alteration of the tropical Atlantic atmosphere.
The persistence of the two anomalous SST patterns from summer until early winter is hypothesized to feedback to the North Atlantic-Europe atmosphere at the end of autumn and ultimately to favor the transition to either positive or negative NAO winter regimes. The delayed oceanic forcing upon the winter atmosphere is controlled by the climatological atmospheric background state, in particular the strength and position of the sub tropical jet and the intensity of the storm tracks. Their alterations are found to be related to the anomalous extratropical meridional ST gradient associated with persistent summer modes which change the local baroclinicity and modify the late fall planetary and synoptic atmospheric wave activity. Persistent changes in the overturning tropical atmospheric cells in the Atlantic associated with the summer oceanic modes are expected as well to have a remote impact onto the midlatitude atmosphere. Asymmetries between the two phases of the NASO and their lagged oceanic relationship are also documented.
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Cassidy P. Rush: cassrush@ucar.edu