The extra-tropical atmospheric response to El Niño events-a multivariate significance analysis

Up to now, a well-defined and consistent image of the extra-tropical atmosphere's response to intense El Niño warmings of the sea surface was missing—in contrast to the tropical area near the forcing source. We attribute this to the currently usual inadequate analysis methods which lead to a misinterpretation of multidimensional signals in the presence of large random fluctuations. In this paper, we demonstrate how the multi-variate significance test procedures, reviewed herein, serve to detect the El Niño signal in the GCM atmosphere of the ECMWF as well as in the observed circulation in 1982/83. The results of the rigorous statistical tests are illustrated by stability and univariate considerations and are “synoptically” compared with respective response patterns generated by a few other GCMs.

It is concluded that a stable and highly significant extra-tropical response in terms of 500 mbar height to an El Niño warming exists. It is found that the January 1983 observed and the GCM generated patterns are highly coherent. This signal covers the 270° sector of longitude from the North-East Pacific to Eurasia. The model response to a cold SST anomaly is marginally significant, less stable, and different in structure, so that a strong non-linearity outside the Pacific region is evident. Furthermore, we hypothesize that an intense El Niño event induces an intensification of the north-westerly Atlantic daily variability.