by Niall C. Slowey and Thomas J. Crowley
. . .Continued from Part 1
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
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.
Updated December 20, 1995