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On the Source of Midlatitude Low-Frequency Variability. Part I: A Statistical Approach to Persistence

Robert VautardLaboratoire de Météorologie Dynamique, Ecole Normale Supérieure. Paris, France

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Bernard LegrasLaboratoire de Météorologie Dynamique, Ecole Normale Supérieure. Paris, France

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Michel DéquéC.N.R.M., Météorologie Nationale, Toulouse, France

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Abstract

The forcing of low-frequency variability by synoptic transient traveling perturbations is investigated within a quasi-geostrophic channel forced by a localized baroclinic jet. Spontaneously generated baroclinic perturbations grow and decay along a storm track; at the end of the track a maximum of low-frequency variability is obtained, in agreement with atmospheric observations. The structure of low-frequency variability is studied with a combination of statistical methods, using a multivariate red noise model as a random reference. We show that the anomalies are preferentially linked with local stationary structures or long-wave vacillations according to their location and their sign.

A systematic study of persistence is conducted with a criterion based on rms of the streamfunction variations. The interesting quantity is the probability of persistence which shows a very inhomogeneous distribution in phase space and several separated maxima. The composites based on these maxima exhibit the characters of zonal and blocking regimes. The transient feedback has a positive role in extending the jet downstream but the primary effect is the maintenance of the blocking circulation.

Abstract

The forcing of low-frequency variability by synoptic transient traveling perturbations is investigated within a quasi-geostrophic channel forced by a localized baroclinic jet. Spontaneously generated baroclinic perturbations grow and decay along a storm track; at the end of the track a maximum of low-frequency variability is obtained, in agreement with atmospheric observations. The structure of low-frequency variability is studied with a combination of statistical methods, using a multivariate red noise model as a random reference. We show that the anomalies are preferentially linked with local stationary structures or long-wave vacillations according to their location and their sign.

A systematic study of persistence is conducted with a criterion based on rms of the streamfunction variations. The interesting quantity is the probability of persistence which shows a very inhomogeneous distribution in phase space and several separated maxima. The composites based on these maxima exhibit the characters of zonal and blocking regimes. The transient feedback has a positive role in extending the jet downstream but the primary effect is the maintenance of the blocking circulation.

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