Circulations of the West Pacific Jet Stream

George P. Cressman Australian Numerical Meteorology Research Centre, Melbourne. Victoria, Australia

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Abstract

The jet stream of the west Pacific is a very steady feature of the winter circulation, with almost continuousintensification over East China and Japan and weakening in the central Pacific. In this study the National Meteorological Center spectral forecast model with normal mode initialization is used as a tool to provide diagnostic elements in a region of nonstandard data sources. Cross sections transverse to the jet stream in its intensification zone show a deep single-celled direct solenoidal circulation around a single frontal zone to be producing large increases of kinetic energy. Farther east, in the central Pacific, a strong indirect solenoidal circulation produces a strong decrease of kinetic energy.

The irrotational wind components at jet stream levels are suggestive of an inertial cycle as part of the jet stream dynamics. A simple model of a modified inertial cycle is integrated in which the geostrophicwind is allowed to vary as a function of the ageostrophic flow transverse to the jet axis. This produces dimensions of the cycle in agreement with observations. It leads to a conclusion that the indirect circulationcell and its climatological aspects are an inertial consequence of the constant baroclinic activity near Japan.

Abstract

The jet stream of the west Pacific is a very steady feature of the winter circulation, with almost continuousintensification over East China and Japan and weakening in the central Pacific. In this study the National Meteorological Center spectral forecast model with normal mode initialization is used as a tool to provide diagnostic elements in a region of nonstandard data sources. Cross sections transverse to the jet stream in its intensification zone show a deep single-celled direct solenoidal circulation around a single frontal zone to be producing large increases of kinetic energy. Farther east, in the central Pacific, a strong indirect solenoidal circulation produces a strong decrease of kinetic energy.

The irrotational wind components at jet stream levels are suggestive of an inertial cycle as part of the jet stream dynamics. A simple model of a modified inertial cycle is integrated in which the geostrophicwind is allowed to vary as a function of the ageostrophic flow transverse to the jet axis. This produces dimensions of the cycle in agreement with observations. It leads to a conclusion that the indirect circulationcell and its climatological aspects are an inertial consequence of the constant baroclinic activity near Japan.

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