Optimal Relationship of Large-Scale Flow Patterns and the Barotropic Feedback Due to High-Frequency Eddies

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  • 1 Meteorological Institute, University München, Federal Republic of Germany
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Abstract

A canonical correlation analysis is used to study the optimal relationship between low-frequency variations of the 500 mb height and the barotropic feedback effect in terms of cyclone-scale eddy-induced height tendencies. The analysis is performed separately for the Atlantic and Pacific sector using eight winters of observed data.

Three significant modes consisting of pairs of optimally correlated height patterns and associated height tendency patterns are obtained for the Atlantic sector but only one mode for the Pacific sector. These modes are found to be related to the EA, WA and PNA teleconnection patterns and to Atlantic blocking highs. Generally, the mutual orientation of the height and the height tendency patterns is such that a center in the height pattern is associated with an eddy tendency center of the same polarity, i.e., both variables are positively correlated. Maximal canonical correlations are obtained if the height field lags the height tendency field by one to two days. The efficiency of the eddy-induced height tendency to force the canonical height modes is evaluated as the ratio of the associated barotropic kinetic energy conversion to that due to the conversion from the time-mean flow. This ratio is about 0.4, and the characteristic time scale of the eddy tendency forcing is of the order of 14 days while that due to the time-mean flow conversion is about 6 days.

It is suggested that the obtained canonical correlation structure is primarily due to a steering effect of the teleconnections on the baroclinic eddies, and to a close coupling between the maximal cyclone-scale eddy activity and the barotropic eddy feedback effect.

Abstract

A canonical correlation analysis is used to study the optimal relationship between low-frequency variations of the 500 mb height and the barotropic feedback effect in terms of cyclone-scale eddy-induced height tendencies. The analysis is performed separately for the Atlantic and Pacific sector using eight winters of observed data.

Three significant modes consisting of pairs of optimally correlated height patterns and associated height tendency patterns are obtained for the Atlantic sector but only one mode for the Pacific sector. These modes are found to be related to the EA, WA and PNA teleconnection patterns and to Atlantic blocking highs. Generally, the mutual orientation of the height and the height tendency patterns is such that a center in the height pattern is associated with an eddy tendency center of the same polarity, i.e., both variables are positively correlated. Maximal canonical correlations are obtained if the height field lags the height tendency field by one to two days. The efficiency of the eddy-induced height tendency to force the canonical height modes is evaluated as the ratio of the associated barotropic kinetic energy conversion to that due to the conversion from the time-mean flow. This ratio is about 0.4, and the characteristic time scale of the eddy tendency forcing is of the order of 14 days while that due to the time-mean flow conversion is about 6 days.

It is suggested that the obtained canonical correlation structure is primarily due to a steering effect of the teleconnections on the baroclinic eddies, and to a close coupling between the maximal cyclone-scale eddy activity and the barotropic eddy feedback effect.

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