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Re Curving Hurricanes Away from the US Coastlines and what it means for winter
By Craig McPeck
The 2009 Hurricane Season, is too be remembered a very quiet season which had very few powerful hurricanes. However, the current season is not over, it is easy to assume that with the amount of shear and jet stream tracks, that the current Hurricane season will not progress to be as strong as forecasted as past years.
All seven of the named Tropical Cyclones; Danny, Claudette, Tropical Depression One, Erika, Bill , Ana, with the exception of the current active tropical cyclone, Fred; have nearly curved away from any of the United States coastlines out to sea. At least 6 out of the 7 named tropical cyclones for the 2009 Hurricane Season.
Now you are probably wondering, whats the big deal about this, its great that they are missing the United States, and yes it is great. Recurving Tropical Cyclones have a significant importance on the winter in the United States. It has been known, to past occurences of 12 or more re curving Tropical Cyclones that wetter then normal conditions were present across portions of the Northeastern, Ohio Valley and portions of the Midwestern United States during the winter months. With the Southeast and Southwestern United States staying pretty dry. With five or less re curving tropical cyclones, the above is usually the opposite. Take a look at the chart below regarding the above subject;
The above image is quoted in a paragraph;
"As shown in Figure 9, especially for the
winters following inactive recurving seasons, there is
a very notable change in the mean meridional
circulations and the jet streams that lie between each.
Following inactive recurving TC seasons, the winter
subtropical jetstream accelerates, the polar jet stream
decelerates, both of which are consistent with an
active baroclinic eddy waveguide. This result also
further supports existing research that the subtropical
jet stream is significantly modulated by midlatitude
eddies (Feldstein and Lee 1998). The anomalous
strength of the two jet streams implies a modulation of
the three cells of the mean circulation. Lastly, there
is some evidence of a modulation of the winter
Walker Circulation following anomalous TC
recurving activity. The obvious lack of symmetry
between the two panels in Figure 9 suggests that
response of the winter climate"
Feldstein, S. and S. Lee, 1998: Is the atmospheric zonal
index driven by an eddy feedback? Journal of Atm. Science, 55, 3077-3086.
In conclusion, Active recurving Tropical Cyclone seasons sometimes and do lead to more above average moisture over portions of the Ohio Valley and Northeastern United States during the winter time. While we are on a good start to achieving the 12 named tropical cyclones, we have yet too see if this comes to fruition until the 2009 Hurricane season finally ceases to exist.
Sources:
http://ams.confex.com/ams/88Annual/techprogram/paper_134979.htm