The Structure and Predictability of the “High-Latitude Mode” in the CSIRO9 General Circulation Model

John W. Kidson National Institute of Water and Atmospheric Research Ltd., Wellington, New Zealand

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Ian G. Watterson CSIRO Division of Atmospheric Research and Cooperative Research Centre for Southern Hemisphere Meteorology, Melbourne, Australia

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Abstract

The CSIRO9 general circulation model shows a zonally symmetric mode of variability, which closely resembles the high-latitude mode (HLM) in middle and high latitudes of the Southern Hemisphere. The leading EOF of the zonal mean zonal wind between 30° and 68°S, whose amplitude has been taken as an index of the HLM, shows opposing variations centered near 40° and 60°S accounting for 43% of the daily variance. Analysis has concentrated on composites for periods when the index changed quickly between significant peaks of the opposite sign or persisted with a large amplitude for an extended period. The momentum flux variations are small at the northern and southern boundaries and the principal variations are centered near 49°S between the maxima in the zonal wind. The changes in angular momentum content are around 30% smaller in the southern band. Eddy heat fluxes are less coherent but help in maintaining the zonal wind anomalies against friction.

A simple model of the zonal wind index with stochastic forcing and linear damping reproduces its short period variations well but is less successful in simulating the observed continuity over 10- to 20-day lags.

Corresponding author address: Dr. John W. Kidson, NIWA, P.O. Box 14-901, Wellington, New Zealand.

Email: j.kidson@niwa.cri.nz

Abstract

The CSIRO9 general circulation model shows a zonally symmetric mode of variability, which closely resembles the high-latitude mode (HLM) in middle and high latitudes of the Southern Hemisphere. The leading EOF of the zonal mean zonal wind between 30° and 68°S, whose amplitude has been taken as an index of the HLM, shows opposing variations centered near 40° and 60°S accounting for 43% of the daily variance. Analysis has concentrated on composites for periods when the index changed quickly between significant peaks of the opposite sign or persisted with a large amplitude for an extended period. The momentum flux variations are small at the northern and southern boundaries and the principal variations are centered near 49°S between the maxima in the zonal wind. The changes in angular momentum content are around 30% smaller in the southern band. Eddy heat fluxes are less coherent but help in maintaining the zonal wind anomalies against friction.

A simple model of the zonal wind index with stochastic forcing and linear damping reproduces its short period variations well but is less successful in simulating the observed continuity over 10- to 20-day lags.

Corresponding author address: Dr. John W. Kidson, NIWA, P.O. Box 14-901, Wellington, New Zealand.

Email: j.kidson@niwa.cri.nz

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