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Modes of Interannual Variability of the Southern Hemisphere Circulation Simulated by the CSIRO Climate Model

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  • 1 CSIRO Atmospheric Research, Aspendale, Victoria, Australia
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Abstract

This study examines the capability of the Commonwealth Scientific and Industrial Research Organisation (CSIRO) climate model in simulating the observed modes of interannual variability of the Southern Hemisphere circulation. Modes of variability in the 500-hPa geopotential height (Z500) field of the following three experiments are examined: 1) a coupled experiment, in which the atmosphere and the ocean are fully coupled, producing El Niño–Southern Oscillation (ENSO) cycles and allowing full air–sea interactions; 2) a mixed layer experiment, in which the atmosphere is coupled to an ocean mixed layer heat equation allowing limited air–sea interactions; and 3) a climatology experiment, in which the atmosphere is forced by an observed SST climatology with a fixed annual cycle, allowing no air–sea interactions. It is found that the observed modes are reasonably simulated in all three experiments, although the amplitude of the model modes is generally smaller than that of the observed. These modes include the high-latitude mode (i.e., the Antarctic Oscillation), the Pacific–South American (PSA) mode, and the wavenumber-3 mode. It is also found that the response of the mid- to high-latitude atmosphere circulation to the model ENSO forcing projects mainly onto the PSA mode. Many features of the PSA mode are similar to those associated with the Pacific–North American mode in the Northern Hemisphere. In response to these Z500 modes, the ocean produces coherent modes of variability, but the oceanic feedback effect appears to be weak. The amplitude of anomalies associated with each mode of the Z500 field in the three experiments shows little difference, suggesting that these Z500 modes can be generated by atmospheric internal dynamics alone, and that the ocean dynamics, air–sea interactions, and ENSO forcing are not essential.

Corresponding author address: Dr. Wenju Cai, CSIRO Atmospheric Research, 107-121 Station Street, Aspendale, Victoria 3195, Australia. Email: wjc@dar.csiro.au

Abstract

This study examines the capability of the Commonwealth Scientific and Industrial Research Organisation (CSIRO) climate model in simulating the observed modes of interannual variability of the Southern Hemisphere circulation. Modes of variability in the 500-hPa geopotential height (Z500) field of the following three experiments are examined: 1) a coupled experiment, in which the atmosphere and the ocean are fully coupled, producing El Niño–Southern Oscillation (ENSO) cycles and allowing full air–sea interactions; 2) a mixed layer experiment, in which the atmosphere is coupled to an ocean mixed layer heat equation allowing limited air–sea interactions; and 3) a climatology experiment, in which the atmosphere is forced by an observed SST climatology with a fixed annual cycle, allowing no air–sea interactions. It is found that the observed modes are reasonably simulated in all three experiments, although the amplitude of the model modes is generally smaller than that of the observed. These modes include the high-latitude mode (i.e., the Antarctic Oscillation), the Pacific–South American (PSA) mode, and the wavenumber-3 mode. It is also found that the response of the mid- to high-latitude atmosphere circulation to the model ENSO forcing projects mainly onto the PSA mode. Many features of the PSA mode are similar to those associated with the Pacific–North American mode in the Northern Hemisphere. In response to these Z500 modes, the ocean produces coherent modes of variability, but the oceanic feedback effect appears to be weak. The amplitude of anomalies associated with each mode of the Z500 field in the three experiments shows little difference, suggesting that these Z500 modes can be generated by atmospheric internal dynamics alone, and that the ocean dynamics, air–sea interactions, and ENSO forcing are not essential.

Corresponding author address: Dr. Wenju Cai, CSIRO Atmospheric Research, 107-121 Station Street, Aspendale, Victoria 3195, Australia. Email: wjc@dar.csiro.au

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