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Eddy–Zonal Flow Interactions Associated with the Southern Hemisphere Annular Mode: Results from NCEP–DOE Reanalysis and a Quasi-Linear Model

Harun A. RashidSchool of Earth Sciences, University of Melbourne, Melbourne, Victoria, Australia

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Ian SimmondsSchool of Earth Sciences, University of Melbourne, Melbourne, Victoria, Australia

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Abstract

Using a recent 22-yr (1979–2000) record of the NCEP–Department of Energy (DOE) reanalysis 2 dataset (hereafter, NCEP2 reanalysis) and a two-level spherical quasi-geostrophic, quasi-linear model, the eddy–zonal- mean flow interactions associated with the Southern Hemisphere annular mode (SAM) variability have been investigated. The main emphasis is to highlight the differing natures of the eddy–zonal-mean flow interactions (i) due to the high-frequency (2–8 days) eddies and medium-frequency (8–30 days) eddies, and (ii) during the extreme and transitional phases of the SAM. The results show that, in addition to a previously documented positive feedback due to the high-frequency eddies, the medium-frequency eddies provide an oscillatory feedback to the SAM variability, leading to an enhancement of the latter's spectral power in an intermediate frequency band. An examination of the latitude–lag composites of the transient eddy forcing (TEF) due to high-frequency eddies during the extreme and transitional phases of the SAM reveals that the composite negative phase anomalies have a longer lifetime and greater magnitudes than the positive phase anomalies. Evidence is also presented suggesting that the coherent variations of the zonal flow anomalies in middle and high latitudes are both forced by the high-frequency eddies at high latitudes.

A two-level quasi-geostrophic, quasi-linear model is shown to reproduce many of the features of the SAM revealed in the NCEP2 reanalyses, including the oscillatory feedback mentioned above. The differing features of the eddy–zonal-mean flow interaction during the positive and negative phases and between high and middle latitudes found in the NCEP2 reanalysis were also simulated well by the quasi-linear model. It is suggested that an interplay between the anomalous zonal wind shear and the anomalous downgradient momentum flux may be responsible for the oscillatory feedback found in both the NCEP2 reanalysis and the quasi-linear model.

Corresponding author address: Dr. M. Harun Ar Rashid, School of Earth Sciences, University of Melbourne, VIC 3010, Australia. Email: harun@earthsci.unimelb.edu.au

Abstract

Using a recent 22-yr (1979–2000) record of the NCEP–Department of Energy (DOE) reanalysis 2 dataset (hereafter, NCEP2 reanalysis) and a two-level spherical quasi-geostrophic, quasi-linear model, the eddy–zonal- mean flow interactions associated with the Southern Hemisphere annular mode (SAM) variability have been investigated. The main emphasis is to highlight the differing natures of the eddy–zonal-mean flow interactions (i) due to the high-frequency (2–8 days) eddies and medium-frequency (8–30 days) eddies, and (ii) during the extreme and transitional phases of the SAM. The results show that, in addition to a previously documented positive feedback due to the high-frequency eddies, the medium-frequency eddies provide an oscillatory feedback to the SAM variability, leading to an enhancement of the latter's spectral power in an intermediate frequency band. An examination of the latitude–lag composites of the transient eddy forcing (TEF) due to high-frequency eddies during the extreme and transitional phases of the SAM reveals that the composite negative phase anomalies have a longer lifetime and greater magnitudes than the positive phase anomalies. Evidence is also presented suggesting that the coherent variations of the zonal flow anomalies in middle and high latitudes are both forced by the high-frequency eddies at high latitudes.

A two-level quasi-geostrophic, quasi-linear model is shown to reproduce many of the features of the SAM revealed in the NCEP2 reanalyses, including the oscillatory feedback mentioned above. The differing features of the eddy–zonal-mean flow interaction during the positive and negative phases and between high and middle latitudes found in the NCEP2 reanalysis were also simulated well by the quasi-linear model. It is suggested that an interplay between the anomalous zonal wind shear and the anomalous downgradient momentum flux may be responsible for the oscillatory feedback found in both the NCEP2 reanalysis and the quasi-linear model.

Corresponding author address: Dr. M. Harun Ar Rashid, School of Earth Sciences, University of Melbourne, VIC 3010, Australia. Email: harun@earthsci.unimelb.edu.au

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