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 clim ate of the North Pacific, causing, among many other effects, a 5% redu ction 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 mo nthly mean northern hemisphere sea level pressure for winters (Decembe r-March) for 1947-1996 are presented and compared with monthly mean ic e cover from the Bering Sea for 1952-1996, the Southern Oscillation In dex, and the western Pacific oscillation. Before the regime shift, bel ow 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. Co nversely, above normal ice cover was associated with La Ni (n) over ti lde a conditions, which caused the Aleutian low to move westward of no rmal, 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 nor th off Alaska and resulting in above normal ice in the Bering Sea. Bef ore the regime shift the occurrence of El Ni (n) over tilde o and La N i (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 eviden ce 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-ye ar oscillation in a North Pacific atmospheric-ocean coupled system.