No abstract

No abstract

The ice-edge region of the southeast Bering Sea was studied in terms of the hydrographic regime, phytoplankton biomass, and primary productivity during the springs of 1975 through 1977. The results showed that a phytoplankton bloom occurs at the ice edge just as the spring ice-decay period begins, and that this accounts for a significant proportion of the annual carbon input over the shallow shelf. The bloom is intensified in time and space by the influence of the ice edge on the physical structure of the water column. Specifically, melting ice seems to increase the stability of the water column, near and under the ice, by lowering the salinity. Frontal structure in salinity and temperature are apparent at the ice edge and are attributed to the melting ice but also, at times, to wind-driven Ekman-type upwelling. These data are also related to recent short term (ca. months-year) climatic fluctuations that seem to control the seasonal position of the ice-edge zone relative to the shelf break. In "cold" years, the ice edge comes southward to the shelf break and overlies the more nutrientrich Alaska Stream/Bering Sea source water. In "warm" years, the ice-edge zone does not reach this nutrient-rich water. This may be important to the biology of the ice-edge ecosystem.The edge zones of seasonal sea ice are biologically productive, serving as focal points for the congregation of large numbers of marine birds and mammals and thus represent ecologically critical habitats in subpolar regions. The presence of a cold and solid boundary influences the biological regime in several ways, including effects on the physical and chemical regimes in the adjacent seawater. At least three structural features of the water column often associated with high oceanic primary production can occur in such an area simultaneously:water column stability, frontal structures and, under certain conditions, ice-edge upwelling. We show here that there is enhanced primary production due to water column stability and frontal structure at the edge of the seasonal sea ice in spring. Our data also suggest that ice-edge upwelling occurs. We show further that the intense

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 E1 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 E1 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 E1 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.As indicated by the sea ice cycle as well as the monthly mean sea surface temperature (SST) around the Pribilof Islands (Figure 1) that seasonally varies from less than iøC in April to almost 9øC in August, the Bering Sea is characterized by strong variability due primarily to its high latitude and associated seasonal variability in weather and insolation. However, in addition to the strong annual variabilit...

Abstract. 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 E1 Nifio 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 Nifia 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 E1 Nifios 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 E1 Nifio and La Nifia conditions was about even. Since the regime shift, E1 Nifio 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.