Quarterdeck 3.3
Quarterdeck 3.3
by Niall C. Slowey and Thomas
J. Crowley
. . .Continued from Part 1
[35K] When the PNA index has a positive
(+) value, the jet stream and thus the flow of air over North America has
a meridional or north-south orientation, bringing cold conditions from Canada,
called "northers," to the gulf region. When the PNA index has
a negative (-) value, the jet stream has a zonal or east-west orientation.
(Redrawn from Leathers and Palecki, 1992)
The atmospheric circulation of the extratropical Northern Hemisphere
displays several preferred patterns of interannual and interdecadal variability.
The most prominent is called the Pacific/North American (PNA) teleconnection
because it is characterized by coincident changes in the heights of mid-tropospheric
air pressure levels which extend from the central Pacific to eastern North
America.
The pattern has two extreme phases. A positive PNA index value corresponds
to an expanded ridge of high atmospheric pressure over western North America
and deepened troughs of low atmospheric pressure over the Aleutian Islands
and the southeast United States, resulting in a north-south (merid-ional)
oriented flow of air over North America. A negative PNA index value corresponds
to east-west (zonal) oriented flow.
The winter climate of the Gulf of Mexico and southeastern United States
is extremely sensitive to shifts between the two phases of the PNA pattern.
Indeed, changes in the region's climate are used to characterize the PNA
pattern itself.
The prevail-ing winter climatic regime is intim-ately linked to the minimum
temperatures associated with fronts and the frequency of frontal passage.
The presence of humid subtropical air from the Gulf of Mexico generally
results in relatively warm winters. On the other hand, the passage of fronts
to the region from the open interior of North America can bring very cold
and dry Arctic air from Canada, causing extremely low minimum air temperatures,
a phenomenon locally known as a "norther" or "blue norther."
As a result, the gulf coast of the southeastern United States experiences
the coldest winters of any locality in the world at the same latitude and
elevation, and its average winter temperature has varied by 10°C or
more over the last several decades.
Oceanography, Texas A&M
University
rshatto@ocean.tamu.edu
URL=http://oceanography.tamu.edu/Quarterdeck/QD3.3/Slowey/slowey-b.html
Updated December 20, 1995