The 1997–98 EL Niño in the Bering Sea as Compared with Previous ENSO Events and the “Regime Shift ” of the Late 1970’s

• Main Author: H. J. Niebauer
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: Figure 1. Winter; Bering Sea; Pacific; Soi; aleutian low; Sea ice; Alaska
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.360.1176; http://www.pices.int/publications/scientific_reports/Report10/niebauer.pdf

as well as extreme interannual variability. In the mean, winter sea ice advance exceeds 1000 km while in summer the Bering Sea is ice free. However, interannual ice variability is as great as 400 km. The interannual variability of the Bering Sea depends in large part on the winter variability of the Aleutian low. Results of analyses (e.g., Niebauer, 1988) of the Aleutian low, monthly mean ice cover from the Bering Sea, and the Southern Oscillation Index (SOI), show that before a regime shift in the late 1970s, below normal ice cover in the Bering Sea was typically associated with El Niño. That is, El Niño conditions caused the Aleutian low to move eastward of normal carrying warm Pacific air from the south over the Bering Sea. Sea and air temperatures were above normal under winds from the south or less strongly from the north. Conversely, above normal ice cover was associated with La Niña conditions which caused the Aleutian low to move westward of normal allowing higher pressure to move over the Bering Sea. Sea and air temperatures were below normal under stronger winds from the north. In the late 1970’s, 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 (Figure 1) as well as shifts in the position of the Aleutian low. The overall reduction in ice cover can be traced to an overall increase in El Niño conditions which has resulted in an increase in the influence of the Aleutian low over the eastern Bering Sea (Niebauer, 1998). However, during El Niños since the regime shift, the Aleutian low has been moving even farther to the east causing winds to blowfrom the east and north off Alaska instead of from the south. This has caused ice advance during El Niños (as during this last El Niño of 1997–98) whereas before the regime shift, the ice retreated during El Niño (Niebauer, 1998). This shift in the organization of ENSO (El Niño– Southern Oscillation) events has had a strong effect on the ...