Mechanisms of the formation of intraseasonal oscillations and Australian monsoon disturbances: The roles of convection, barotropic and baroclinic instability

A study is made of the roles of barotropic instability, baroclinic instability and convection in the formation and structures of intraseasonal oscillations and low frequency anomalies. The analysis has been conducted with barotropic and two-level primitive equation models using as basic states the threedimensional global flow for January 1979, including a Kuo-type cumulus convection parameterization and observed moisture mixing ratio in the baroclinic model. The resulting moist static stability parameter, used in this study, is positive everywhere so that pure wave-CISK instability is not possible. Anomaly correlations have been calculated to provide quantitative measures of the similarity of barotropic modes and dry and moist low frequency baroclinic modes. The differences and similarities in the tropical and extra-tropical structures of the modes have been examined for the different cases. Baroclinic processes appear to be essential for generating small scale localized stationary Australian monsoon disturbances; both the horizontal and vertical shears and moisture appear to be important in determining the structures, growth rates and periods of these modes. Barotropic instability of the zonally varying 300 hPa basic state streamfunction produces a number of modes with qualitatively similar extra-tropical structure to baroclinic model 20-30 day oscillations, 40-60 day intraseasonal oscillations and quasi-stationary teleconnection patterns. These tend to be of slightly larger scale than both corresponding dry and moist baroclinic modes. Dry baroclinic intraseasonal oscillation and quasi-stationary teleconnection pattern modes have relatively poor vertical structures in the tropical regions. A baroclinic three-dimensional basic state and a cumulus convection parameterization are required to capture the crucial first internal mode structure in the tropical regions of intraseasonal oscillations.