Similar characteristics between the early winter of 2002–2003 and December warmings in Alaska: Role of polar/mid-latitude interactions

The first half of the 2002-2003 winter (SON) over Alaska and adjacent maritime regions was characterized by persistent and unusual warmth, accompanied by copious precipitation over the southern Alaskan coastal regions and extremely dry conditions over interior and northern sections of the state. These conditions were associated with a persistent planetary wave pattern featuring a broad trough over the northwestern Pacific Ocean, a strong ridge extending from the California coast into the northern Yukon Territory, and a persistent mean southerly flow extending from the mid-latitudes into the Alaskan Arctic through virtually the entire troposphere. This meridional flow pattern occasionally extended from the upper tropics to the Beaufort and Chukchi Seas and has been associated with an unusually high occurrence of liquid and freezing precipitation.

Analysis of the mean atmospheric and oceanic conditions during this period shows a remarkable similarity to another phenomenon that has not been well documented in the literature, namely, a long-term mean tendency for warming conditions to develop over Alaska and parts of the Yukon Territory during mid December. Promiment among the conditions associated with this pattern, besides the aforementioned planetary wave structure, are an unusally broad East Asian mid latitude (winter monsoonal) anticyclone, a stationary lower tropospheric cold pool over eastern Siberia, a somewhat weak, but Pan-Arctic, mid tropospheric polar vortex, and a tripolar themal anomaly structure over the North Pacific.

Although such structure in the high-latitudes has been tentatively linked to ENSO warm phase conditions in the past, the mean December warming signatures appear to occur irrespective of the existence of the ENSO warm phase or its strength, with a weak warm anomaly in the central Equatorial Pacific the only common signature.

In this paper we will present an overview of the conditions described above and an atmospheric and oceanic analysis of the contributing factors. Through this analysis we will speculate as to contributing physical mechanisms, the role played by polar/mid latitude and atmosphere/ocean interaction processes, and the role of synoptic scale systems as regards the persistence of the planetary wave patterns.