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Examining Mechanisms of Variability within the Pacific Storm Track: Upstream Seeding and Jet-Core Strength

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  • 1 Department of Atmospheric Sciences, University of Washington, Seattle, Washington
  • | 2 Department of Earth and Space Sciences, University of Washington, Seattle, Washington
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Abstract

This paper examines how variations in two mechanisms, upstream seeding and jet-core strength, relate to storminess within the cold season (October–April) Pacific storm track. It is found that about 17% of observed storminess covaries with the strength of the upstream wave source, and the relationship is robust throughout the cold season and for both the Pacific and Atlantic basins. Further analyses of the intraseasonal variability in the strength and structure of the wintertime [December–February (DJF)] Pacific jet stream draw upon both Eulerian-variance and feature-tracking statistics to diagnose why winter months with a strong-core jet stream have weaker storminess than those with a weak-core jet stream. Contrary to expectations, it is shown that the basic spatial patterns actually conform to a simple linear picture: regions with a weaker jet have weaker storminess. The overall decrease in storminess is most strongly linked to the weaker amplitude of individual storms in strong-core months. Previously proposed mechanisms are evaluated in the context of these new results. Last, this analysis provides further evidence that the midwinter suppression in storminess over the North Pacific Ocean is primarily due to a notable lack of storminess upstream of the Pacific storm track in the heart of winter.

Corresponding author address: Sandra Penny, University of Washington, 408 ATG Building, Box 351640, Seattle, WA 98195. E-mail: smpenny@atmos.washington.edu

Abstract

This paper examines how variations in two mechanisms, upstream seeding and jet-core strength, relate to storminess within the cold season (October–April) Pacific storm track. It is found that about 17% of observed storminess covaries with the strength of the upstream wave source, and the relationship is robust throughout the cold season and for both the Pacific and Atlantic basins. Further analyses of the intraseasonal variability in the strength and structure of the wintertime [December–February (DJF)] Pacific jet stream draw upon both Eulerian-variance and feature-tracking statistics to diagnose why winter months with a strong-core jet stream have weaker storminess than those with a weak-core jet stream. Contrary to expectations, it is shown that the basic spatial patterns actually conform to a simple linear picture: regions with a weaker jet have weaker storminess. The overall decrease in storminess is most strongly linked to the weaker amplitude of individual storms in strong-core months. Previously proposed mechanisms are evaluated in the context of these new results. Last, this analysis provides further evidence that the midwinter suppression in storminess over the North Pacific Ocean is primarily due to a notable lack of storminess upstream of the Pacific storm track in the heart of winter.

Corresponding author address: Sandra Penny, University of Washington, 408 ATG Building, Box 351640, Seattle, WA 98195. E-mail: smpenny@atmos.washington.edu
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