Variability in Bering Sea ice cover as affected by a regime shift in the North Pacific in the period 1947-1996

In the late 1970s, a "regime shift" or "step" occurred in the climate of the North Pacific, causing, among many other effects, a 5% reduction in the ice cover in the eastern Bering Sea as well as shifts in the position of the Aleutian low. Analyses of the Aleutian low from monthly mean northern hemisphere sea level pressure for winters (December-March) for 1947-1996 are presented and compared with monthly mean ice cover from the Bering Sea for 1952-1996, the Southern Oscillation Index, and the western Pacific oscillation. Before the regime shift, below normal ice cover in the Bering Sea was typically associated with El Ni (n) over tilde o conditions, which caused the Aleutian low to move eastward of normal, carrying warm Pacific air over the Bering Sea. Conversely, above normal ice cover was associated with La Ni (n) over tilde a conditions, which caused the Aleutian low to move westward of normal, allowing higher pressure to move over the Bering Sea. Since the regime shift, during El Ni (n) over tilde os the Aleutian low has been moving even farther east, causing winds to blow from the east and north off Alaska and resulting in above normal ice in the Bering Sea. Before the regime shift the occurrence of El Ni (n) over tilde o and La Ni (n) over tilde a conditions was about even. Since the regime shift, El Ni (n) over tilde o conditions are about 3 times more prevalent. In recent work [e.g., Mantua et al., 1997; Minobe, 1997] there is evidence that this regime shift is the latest in a series of climate shifts going back to at least the late 1800s and may be due to a 50- to 70-year oscillation in a North Pacific atmospheric-ocean coupled system.