Funds are provided to study the impact of physical variability on the processes and structure of the Bering shelf ecosystem, with special emphasis on how freshwater redistributed by the shelf circulation or introduced from sea ice melt modifies stratification and nutrient distributions. The principal investigators inquire how changes in sea ice affect advection and mixing; how variable fluxes of low-salinity, nutrient-deficient coastal waters may affect production; how cross-shelf fluxes are established and altered; how these fluxes might respond to climate change; how the seasonal stratification cycle is controlled; and how the buoyant coastal flow evolves.
The 2008-2010 field effort will focus on the central Bering Sea shelf and will employ both hydrography, including extensive d18O sampling, and a nine-mooring array that spans most of the central shelf. The moorings variously will carry ADCPs, T/C recorders, fluorometers, and temperature chains. The measurements will be augmented by hydrographic and d18O sampling and drifter studies under other support. Analysis will also incorporate wind forcing, coastal discharges, surface buoyancy fluxes, and ice thickness and drift into the synthesized data set and its interpretation.
These data, in conjunction with another mooring program funded elsewhere, will provide much of the background circulation and stratification information necessary to place the biological data from BEST in context.
The eastern Bering Sea shelf is one of the most productive marine ecosystems in the global ocean. This shelf, through the Bering Strait at its northern end, connects the sub-arctic North Pacific Ocean with the Arctic Ocean. This connection is critical in the global hydrologic cycle, in maintaining Arctic Ocean sea ice, and in sustaining the productive Chukchi Sea marine ecosystems. For these reasons we undertook an NSF-supported study to the physical oceanography of the Bering Sea shelf. Our study investigated seasonal and interannual variations in the circulation and distribution of temperature and salinity and the processes that govern these variations. The study addressed these issues using a combination of observations and numerical ocean circulation models. The observations included moored oceanographic instruments that sampled temperature, salinity, and ocean velocity every hour for the period between July 2008 and July 2010 at 8 locations on the Bering Sea shelf. These data were supplemented by shipborne measurements of temperature and salinity made by many scientists involved in this multi-investigator study of the Bering Sea. Those measurements were made between the months of March and September and thus cover most of the year, but most importantly, during the spring and summer when biological production is greatest. In addition we used similar shipbased and moored data sets collected over many years and archived in national data bases to assess interannual variations and for comparisons with the numerical ocean models. The models simulate the time-varying circulation, temperature and salinity fields of the ocean. Our comparisons indicate that the models and observations agree quite well with respect to seasonal and inter-annual variations in temperature, currents, and sea-ice distribution, but with less agreement in the salinity distributions. As a consequence of these results, several improvements are being made to improve the salinity predictability of the models. Both observations and numerical integrations show that much of the shelf flow reverses between northwesterly and southeasterly winds. While southeasterly winds are less frequent, they are associated with large on-shelf transport across most of the shelfbreak, and in October–April they are also accompanied by a reversal of the normally eastward flow near Cape Navarin. In contrast, northwesterly winds promote off-shelf transport across most of the shelfbreak, along with increased eastward transport near Cape Navarin. Variations in the cross-shelf transport are important because this transport carries nutrient-rich water from the deep basin onto the Bering Sea shelf and ultimately through Bering Strait. Interannual changes in the cross-shelf transport will affect the amount of nutrients carried onto the shelf and thus the total amount of biological produced in a given year. Our work has also identified the primary temporal-spatial patterns of variability in the temperature and salinity fields over the Bering Sea and northern Gulf of Alaska for near-surface (0-20m) and subsurface (40-100m) depth layers. Correlations between these patterns and various climate indices show that the Pacific Decadal Oscillation (PDO), the North Pacific Gyre Oscillation (NPGO) and the Bering Sea annually integrated ice area anomalies are important indices of thermohaline variability. Moreover, the spatial structure of these modes provide insights on their potential ecosystem impacts. We have identified a number of ecologically and economically important marine species whose temporal variability is significantly correlated with the identified spatial patterns. This project has provided a number of undergraduate and graduate students with experience at sea during the research cruises. It has also supported one graduate student who successfully completed his PhD using data from this project. We also participated in a 3-day professional development workshop for teachers from around the US and from the Bering Sea region. As a consequence of this workshop we have also participated in "Bering Sea Days" in the viallge of St. Paul on St. Paul Island in the Bering Sea. This event provides learning activities for students (and their teachers) on oceanography, with an emphasis on processes occurring in the Bering Sea.
