The Graveyard of storms in the North Pacific

A paper in press in the Journal of Climate by Michel d. S. Mesquita et.al. describes a complete climatology of extra-tropical storm tracks in the North Pacific. This includes the description of “lysis density”, a variable which shows where storms die, shedding more light on the Gulf of Alaska.

Extra-tropical storms

The North Pacific and Bering Sea regions represent loci of cyclogenesis (where storms are born) and storm track activity. In this paper climatological properties of extra-tropical storms in the North Pacific/Bering Sea are presented based upon aggregate statistics of individual storm tracks calculated by means of the feature-tracking algorithm by Hodges for the period from 1948(49) to 2008.

Results show that the inter-seasonal variability is not as large during the spring and autumn seasons. Most of the storm variables – where storms start, their intensity, and where they travel to – exhibited a maxima pattern that was oriented along a zonal axis. From season to season this axis underwent a north-south shift and, in some cases, a rotation to the northeast. This was determined to be a result of zonal heating variations and mid-tropospheric moisture patterns. Summer storms tended to be longest in duration. Temporal trends tended to be weak over the study area. SST did not emerge as a major cyclogenesis control in the Gulf of Alaska.
 

Storms in the Gulf of Alaska

Barotropic processes (when pressure is a function of density only) have an influence in shaping the downstream end of storm tracks and, together with the blocking influence of the coastal orography of northwest North America, result in high lysis concentrations, effectively making the Gulf of Alaska the “graveyard” of Pacific storms.

The results suggest that most of the storm systems that end up in the Gulf of Alaska do not form locally. A targeted assessment confirms that for all seasons with the exception of summer, for which a source region in the Aleutian Islands is indicated. There are indications of tropical cyclones undergoing extra-tropical transitions and reaching the Gulf of Alaska, especially during summer and autumn (see figure below).
 

Plots of all autumn (September, October and November) storms   possessing end-points in the Gulf of Alaska from 1948(9) to 2008. The   figure also shows tropical systems undergoing extratropical transition   during that season, an important finding in the paper. The color   scheme represents the T42 intensities with background removed – units   of 10^-5 s^-1. Figure created by Dr. Kevin Hodges (University of   Reading, UK) based on his tracking algorithm called TRACK (illustration by Dr. Kevin Hodges).

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