Monsoon Disturbances, Intraseasonal Oscillations, Teleconnection Patterns, Blocking, and Storm Tracks of the Global Atmosphere during January 1979: Linear Theory

Jorgen S. Frederiksen CSIRO, Division of Atmospheric Research, Mordialloc, Victoria, Australia

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Carsten S. Frederiksen Bureau of Meteorology Research Centre, Melbourne, Victoria, Australia

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Abstract

The results of a study are presented that indicate that a wide variety of atmospheric disturbances, including those associated with storm tracks and blocking in both hemispheres, quasi-stationary global teleconnection patterns, and localized monsoon disturbances, as well as intraseasonal oscillations, may be generated through the instability of the three-dimensional global basic state for January 1979 including a wave-CISK cumulus heating parameterization. The analysis has been conducted with a two-level primitive equation eigenvalue model, and the growing disturbances for various specifications of the strengths of the cumulus heating have been analyzed.

Within the parameter range studied, inclusion of explicit moisture in the basic state has little effect on the structures of the storm track and onset-of-blocking modes in both hemispheres, but it increases growth rates as expected. It is also responsible for generating a significant amplitude of the tropical shear streamfunction of low-frequency and quasi-stationary teleconnection pattern modes, particularly in the Australian/South Pacific region where a coupling to the Australian monsoon appears to occur. Remarkable localized quasi-stationary monsoon disturbances of fairly small scale are found, which tend to produce either break or active periods of the Australian monsoon depending on their phase. The CISK heating focuses these modes in the Australian region and increases their growth rates.

A group of intraseasonal oscillation modes with quite complex structures and periods between about 20 and 60 days is also found. They have eastward-propagating velocity potentials, peaking in the equatorial regions with a zonal wavenumber 1, and possibly 2, envelope wave within which are embedded smaller-scale structures. They have equivalent barotropic streamfunctions in extratropical regions with a baroclinic structure, typical of the first internal mode, in the tropical regions. At certain phases they can produce break or active periods of the Australian monsoon. Both a three-dimensional large-scale basic-state flow and a cumulus heating parameterization appear to be necessary for generating, through instability, intraseasonal oscillation modes with realistic structures.

Abstract

The results of a study are presented that indicate that a wide variety of atmospheric disturbances, including those associated with storm tracks and blocking in both hemispheres, quasi-stationary global teleconnection patterns, and localized monsoon disturbances, as well as intraseasonal oscillations, may be generated through the instability of the three-dimensional global basic state for January 1979 including a wave-CISK cumulus heating parameterization. The analysis has been conducted with a two-level primitive equation eigenvalue model, and the growing disturbances for various specifications of the strengths of the cumulus heating have been analyzed.

Within the parameter range studied, inclusion of explicit moisture in the basic state has little effect on the structures of the storm track and onset-of-blocking modes in both hemispheres, but it increases growth rates as expected. It is also responsible for generating a significant amplitude of the tropical shear streamfunction of low-frequency and quasi-stationary teleconnection pattern modes, particularly in the Australian/South Pacific region where a coupling to the Australian monsoon appears to occur. Remarkable localized quasi-stationary monsoon disturbances of fairly small scale are found, which tend to produce either break or active periods of the Australian monsoon depending on their phase. The CISK heating focuses these modes in the Australian region and increases their growth rates.

A group of intraseasonal oscillation modes with quite complex structures and periods between about 20 and 60 days is also found. They have eastward-propagating velocity potentials, peaking in the equatorial regions with a zonal wavenumber 1, and possibly 2, envelope wave within which are embedded smaller-scale structures. They have equivalent barotropic streamfunctions in extratropical regions with a baroclinic structure, typical of the first internal mode, in the tropical regions. At certain phases they can produce break or active periods of the Australian monsoon. Both a three-dimensional large-scale basic-state flow and a cumulus heating parameterization appear to be necessary for generating, through instability, intraseasonal oscillation modes with realistic structures.

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