Southern Midlatitude Zonal Wind Vacillation and Its Interaction with the Ocean in GCM Simulations

I. G. Watterson CSIRO Atmospheric Research and CRC for Southern Hemisphere Meteorology, Aspendale, Australia

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Abstract

Much of the variance of monthly and zonal mean zonal wind anomalies in the Southern Hemisphere troposphere can be attributed to the vacillation of a barotropic dipole-like “mode.” In four multidecadal simulations of the CSIRO general circulation model, the vacillation is also associated with variations in atmospheric temperature, surface pressure, cloud cover, rainfall, surface, and top-of-atmosphere energy fluxes, surface stresses, and ocean currents. Surface heat fluxes reach 6 W m−2 in composites of months of high value of the vacillation index. In simulations of the model that include either a mixed-layer or general circulation ocean model, these fluxes produce zonal mean SST anomalies, reaching 0.14°C at 45°S, which persist for several months. Ocean dynamics modifies this response, particularly farther south. The atmosphere then responds, although weakly, to these ocean anomalies, as is demonstrated by comparing the interacting-ocean runs with runs where SSTs follow either a climatological annual cycle or observed variations, and also by an additional experiment in which the ocean anomalies are specified. This response clearly alters the evolution of lower-tropospheric temperatures in the months after the index peak. It also leads to a small southward shift in the decaying vacillation wind pattern, as well as a weakly anomalous subtropical jet. The SST anomalies are broadly consistent with a simple model of the air–sea interaction, in which the atmospheric vacillation index is stochastically generated. By adding a second, atmospheric response mode, with specified latitudinal structure, the simple model is also able to reproduce the evolving temperature and wind patterns. It is noted that the GCM vacillation and its interaction with the ocean are broadly consistent with the National Centers for Environmental Prediction observational dataset, within the limitations of the record.

Corresponding author address: Dr. I. G. Watterson, Division of Atmospheric Research, CSIRO, 107-121 Station Street, Aspendale, Vic 3195, Australia.

Email: igw@dar.csiro.au

Abstract

Much of the variance of monthly and zonal mean zonal wind anomalies in the Southern Hemisphere troposphere can be attributed to the vacillation of a barotropic dipole-like “mode.” In four multidecadal simulations of the CSIRO general circulation model, the vacillation is also associated with variations in atmospheric temperature, surface pressure, cloud cover, rainfall, surface, and top-of-atmosphere energy fluxes, surface stresses, and ocean currents. Surface heat fluxes reach 6 W m−2 in composites of months of high value of the vacillation index. In simulations of the model that include either a mixed-layer or general circulation ocean model, these fluxes produce zonal mean SST anomalies, reaching 0.14°C at 45°S, which persist for several months. Ocean dynamics modifies this response, particularly farther south. The atmosphere then responds, although weakly, to these ocean anomalies, as is demonstrated by comparing the interacting-ocean runs with runs where SSTs follow either a climatological annual cycle or observed variations, and also by an additional experiment in which the ocean anomalies are specified. This response clearly alters the evolution of lower-tropospheric temperatures in the months after the index peak. It also leads to a small southward shift in the decaying vacillation wind pattern, as well as a weakly anomalous subtropical jet. The SST anomalies are broadly consistent with a simple model of the air–sea interaction, in which the atmospheric vacillation index is stochastically generated. By adding a second, atmospheric response mode, with specified latitudinal structure, the simple model is also able to reproduce the evolving temperature and wind patterns. It is noted that the GCM vacillation and its interaction with the ocean are broadly consistent with the National Centers for Environmental Prediction observational dataset, within the limitations of the record.

Corresponding author address: Dr. I. G. Watterson, Division of Atmospheric Research, CSIRO, 107-121 Station Street, Aspendale, Vic 3195, Australia.

Email: igw@dar.csiro.au

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