Rotational atmospheric circulation during North Atlantic-European winter: the influence of ENSO
The dominant variability modes of the North Atlantic-European rotational flow are examined by applying a principal component analysis (PCA/EOF) to the 200 hPa streamfunction mid-winter anomalies (Jan–Feb monthly means). The results reveal that, when this norm is used, the leading mode (EOF1) does not correspond to the traditional North Atlantic Oscillation (NAO, which appears in our analysis as the second leading mode, EOF2) but is the local manifestation of the leading hemispheric streamfunction EOF. The regression of this regional mode onto the global SST field exhibits a clear El Niño signature, with no signal over the Atlantic, while the associated upper height anomalies resemble the Tropical/Northern Hemisphere (TNH) pattern. East of North America, this TNH-like wavetrain produces a meridional dipole-like pattern at lower levels. Although in some ways this pattern resembles the NAO (EOF2), the dynamics of these two modes are very different in that only EOF2 is associated with a latitudinal shift of the North Atlantic stormtrack. Thus, the choice of the streamfunction norm in the EOF analysis allows the separation of two different phenomena that can produce similar dipolar surface pressure anomalies over the North Atlantic but that have different impact on European climate. These two modes also differ on their contribution to variability at lower levels: while NAO-EOF2 is mostly confined to the North Atlantic, TNH-EOF1 has a more annular, global character. At upper levels NAO-EOF2 also produces a global pattern but with no annular structure, reminiscent of the “circumglobal” teleconnection.
KeywordsRotational circulation Atmospheric teleconnection ENSO NAO
We are grateful to Dr Adam Scaife (Met Office-Hadley Centre, UK), Dr Tercio Ambrizzi (Universidade de Sao Paulo, Brazil) and Dr David Karoly (University of Melbourne, Australia) for useful discussions, and to Dr Paul Berrisford (University of Reading, UK) for kindly providing the ERA40 datasets. We thank two anonymous reviewers for their invaluable help improving a previous version of this paper. The anonymous referees reviewing the present manuscript were also very helpful. This research was supported by the national CGL2005-06600-C03-02, CGL2006-04471 and CGL2009-06944 projects of the Spanish Ministry of Education and Science. PZG is supported by the Ramón y Cajal program and AdlC is supported by a FPI-UCM grant.
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