Effects of El Nino‐Southern Oscillation and North Pacific weather patterns on interannual variability in the subarctic Bering Sea

Abstract

Extraordinary interannual variability in ice cover, air and sea surface temperatures (SST), and surface winds in the eastern Bering Sea have been observed over recent years. To investigate the causes of this interannual variability, long‐term (20–30 years) time series of air, ocean, and ice parameters from the Bering Sea were cross‐correlated with the southern oscillation index (SOI), an index of El Nino‐Southern Oscillation (ENSO) events in the tropical southern hemisphere, as well as with an index of Pacific/North American (PNA) events in the north Pacific. Five to thirty percent of the interannual variability (linear regression with various smoothing) in the Bering Sea data sets, with the exception of surface winds, is explained by the SOI when the Bering Sea data lags the SOI. For comparison, 29–52% of the variability in the SST off South America can be explained by the SOI. The signs of the correlations all suggest that warming in the Bering Sea follows negative anomalies in the SOI, that is, ENSO events. Positive anomalies in the SOI, which tend to precede El Nino events, were found to precede cooling in the Bering Sea. Significant correlation persists for 18–20 months. Higher‐order polynomial regressions between the SOI and Bering Sea data can explain up to 40% of the Bering Sea variability. The mechanism for the connection between ENSO events and Bering Sea interannual variability appears to be of atmospheric nature and is associated with the winter position and intensity of the Aleutian low. The Aleutian low is intensified and eastward of normal in association with El Nino, i.e., warm events but is weaker and westward of normal during cool events. It is the regional winds associated with the variable position of the Aleutian low that cause the regional warming and cooling events. This seesaw in the Aleutian low is also used to explain the out‐of‐phase ice conditions between the Bering Sea and the Sea of Okhotsk. PNA events are also associated with an intensified Aleutian low as well as 700 mbar ridging over northwestern North America causing southerly flow over Alaska. However, while correlation between ENSO events and PNA was significant, the correlation between the Bering Sea and PNA was marginal except for surface winds. Here surface winds from the north were significantly correlated with the PNA. This apparent inconsistency, as well as the lack of correlation of surface winds with the SOI, is explained by the lack of preferred site of the Aleutian low during ENSO events. This lack of preferred site helps explain why the major El Nino event of 1982–1983 had little apparent effect on the Bering Sea.