Large Scale Atmospheric Circulations and Polar Low Genesis over the Gulf of Alaska

Polar lows in the North Pacific Ocean form during the advection of cold air over the warmer ocean. Determining the strength and duration of these favorable atmospheric conditions could be used as a method of forecasting polar low frequency. Therefore a study was performed to identify relationships between the frequency of number of polar lows over the Gulf of Alaska to indices of atmospheric and oceanographic changes over an eight winter season period (1992-2000). The indices include the Pacific Decadal Oscillation Index, Aleutian Low Pressure Index, Atmospheric Forcing Index, Multivariate El Nino/Southern Oscillation Index, Pacific-North American Index and the Arctic Oscillation Index. Polar low frequency showed small fluctuations early in the period under investigation. The number of polar lows was less than 30 up to the 1996-1997 winter season. An increase in polar lows took place until the 1999-2000 season when there were only 9 polar lows observed. The largest (52) occurred during the 1997-1998 winter season. Certain atmospheric/oceanographic patterns provide an optimum environment for polar lows to generate. The Atmospheric Forcing Index (AFI) is strongly linked to polar low frequency (R² adjusted value=.73; correlation value=0.88). Positive values of AFI represent an intense Aleutian Low, above average frequency of westerly winds, cooling of sea surface temperatures in the central North Pacific and warm SSTs within North American coastal waters and parts of the Gulf of Alaska. Increased polar low frequency was also linked to the Pacific-North American Index (R² adjusted value=.54; correlation value=0.78)where the long wave pattern promotes instability over the region that aids in polar low genesis. On the other hand, a positive mode of the Arctic Oscillation Index coincided with decreased polar low frequency (R² adjusted value=.15; correlation value=-.52). This suggests that the negative mode of the AO is associated with increase polar low activity because cold arctic air is able to move equatorward over warmer oceanic waters creating regions of low level instability. In either case, the results from this study suggest that polar lows form over the North Pacific Ocean during preferred modes of large-scale, atmospheric circulation patterns which will provide more information on how to forecast them.