The switching between zonal and blocked mid-latitude atmospheric circulation: a dynamical system perspective

Atmospheric mid-latitude circulation is dominated by a zonal, westerly flow. Such a flow is 8 generally symmetric, but it can be occasionally broken up by blocking anticyclones. The subsequent 9 asymmetric flow can persist for several days. In this paper, we apply new mathematical tools based 10 on the computation of an extremal index in order to reexamine the dynamical mechanisms responsible 11 for the transitions between zonal and blocked flows. We discard the claim that mid-latitude circulation 12 features two distinct stable equilibria or chaotic regimes, in favor of a simpler mechanism that is well 13 understood in dynamical systems theory: we identify the blocked flow as an unstable fixed point (or saddle 14 point) of a single basin chaotic attractor, dominated by the westerlies regime. We also analyze the North 15 Atlantic Oscillation and the Arctic Oscillation atmospheric indices, whose behavior is often associated 16 with the transition between the two circulation regimes, and investigate analogies and differences with 17 the bidimensional blocking indices. We find that the Arctic Oscillation index, which can be thought as 18 a proxy for a hemispheric average of the Tibaldi-Molteni blocking index, keeps track of the presence of 19 Davide Faranda LSCE, CEA Saclay l’Orme des Merisiers, CNRS UMR 8212 CEA-CNRS-UVSQ, 91191 Gif-sur-Yvette, France Tel.: +33-169081142 Fax: +33-169087716 E-mail: [email protected] Giacomo Masato Department of Meteorology/NCAS, University of Reading, Reading, UK Nicholas R. Moloney London Mathematical Laboratory, 14 Buckingham Street, London WC2N 6DF, UK Yuzuru Sato RIES/Department of Mathematics, Hokkaido University, Kita-ku, Sapporo, Hokkaido 060-0812, Japan London Mathematical Laboratory, 14 Buckingham Street, London WC2N 6DF, UK Francois Daviaud Laboratoire SPHYNX, SPEC, CEA Saclay, CNRS UMR 3680, 91191 Gif-sur-Yvette, France Bérengère Dubrulle Laboratoire SPHYNX, SPEC, CEA Saclay, CNRS UMR 3680, 91191 Gif-sur-Yvette, France Pascal Yiou LSCE, CEA Saclay l’Orme des Merisiers, CNRS UMR 8212 CEA-CNRS-UVSQ, 91191 Gif-sur-Yvette, France 2 Davide Faranda et al. unstable fixed points. On the other hand, the North Atlantic Oscillation, representative only of local 20 properties of the North Atlantic blocking dynamics, does not show any trace of the presence of unstable 21 fixed points of the dynamics. 22