The Aleutian Low and Winter Climatic Conditions in the Bering Sea. Part I: Classification

S. N. Rodionov Joint Institute for the Study of the Atmosphere and Oceans, University of Washington, Seattle, Washington

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J. E. Overland NOAA/Pacific Marine Environmental Laboratory, Seattle, Washington

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N. A. Bond Joint Institute for the Study of the Atmosphere and Oceans, University of Washington, Seattle, Washington

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Abstract

The Aleutian low is examined as a primary determinant of surface air temperature (SAT) variability in the Bering Sea during the winter [December–January–February–March (DJFM)] months. The Classification and Regression Tree (CART) method is used to classify five types of atmospheric circulation for anomalously warm months (W1–W5) and cold months (C1–C5). For the Bering Sea, changes in the position of the Aleutian low are shown to be more important than changes in its central pressure. The first two types, W1 and C1, account for 51% of the “warm” and 37% of the “cold” months. The W1-type pattern is characterized by the anomalously deep Aleutian low shifted west and north of its mean position. In this situation, an increased cyclonic activity occurs in the western Bering Sea. The C1-type pattern represents a split Aleutian low with one center in the northwestern Pacific and the other in the Gulf of Alaska. The relative frequency of the W1 to C1 types of atmospheric circulation varies on decadal time scales, which helps to explain the predominance of fluctuations on these time scales in the weather of the Bering Sea. Previous work has noted the prominence of multidecadal variability in the North Pacific. The present study finds multidecadal variations in frequencies of the W3 and C3 patterns, both of which are characterized by increased cyclonic activity south of 51°N. In general, the CART method is found to be a suitable means for characterizing the wintertime atmospheric circulation of the North Pacific in terms of its impact on the Bering Sea. The results show that similar pressure anomaly patterns for the North Pacific as a whole can actually result in different conditions for the Bering Sea, and that similar weather conditions in the Bering Sea can arise from decidedly different large-scale pressure patterns.

Corresponding author address: S. N. Rodionov, Joint Institute for the Study of the Atmosphere and Oceans, Box 354235, University of Washington, Seattle, WA 98195-4235. Email: sergei.rodionov@noaa.gov

Abstract

The Aleutian low is examined as a primary determinant of surface air temperature (SAT) variability in the Bering Sea during the winter [December–January–February–March (DJFM)] months. The Classification and Regression Tree (CART) method is used to classify five types of atmospheric circulation for anomalously warm months (W1–W5) and cold months (C1–C5). For the Bering Sea, changes in the position of the Aleutian low are shown to be more important than changes in its central pressure. The first two types, W1 and C1, account for 51% of the “warm” and 37% of the “cold” months. The W1-type pattern is characterized by the anomalously deep Aleutian low shifted west and north of its mean position. In this situation, an increased cyclonic activity occurs in the western Bering Sea. The C1-type pattern represents a split Aleutian low with one center in the northwestern Pacific and the other in the Gulf of Alaska. The relative frequency of the W1 to C1 types of atmospheric circulation varies on decadal time scales, which helps to explain the predominance of fluctuations on these time scales in the weather of the Bering Sea. Previous work has noted the prominence of multidecadal variability in the North Pacific. The present study finds multidecadal variations in frequencies of the W3 and C3 patterns, both of which are characterized by increased cyclonic activity south of 51°N. In general, the CART method is found to be a suitable means for characterizing the wintertime atmospheric circulation of the North Pacific in terms of its impact on the Bering Sea. The results show that similar pressure anomaly patterns for the North Pacific as a whole can actually result in different conditions for the Bering Sea, and that similar weather conditions in the Bering Sea can arise from decidedly different large-scale pressure patterns.

Corresponding author address: S. N. Rodionov, Joint Institute for the Study of the Atmosphere and Oceans, Box 354235, University of Washington, Seattle, WA 98195-4235. Email: sergei.rodionov@noaa.gov

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