Collaborative Research: The Influence of the Mendeleev Ridge and the Chukchi Borderland on the Large-scale Circulation of the Arctic Ocean

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The most important subsurface Arctic Ocean transport system, a cyclonic (here anticlockwise) boundary current, organized along the continental slopes and major trans-Arctic ridges, distributes waters, tracers and contaminants from the Atlantic (via Fram Strait and the Barents Sea) and the Pacific (via Bering Strait) around and into the deep Arctic basins. On its circum-Arctic pathway, parts of the topographically steered current are diverted away from the continental margin, generally along topographic ridges. The most complex obstacle the boundary current encounters is the Mendeleev Ridge/Chukchi Borderland complex, north of the Pacific entrance to the Arctic. This region is the cross- roads for Pacific-origin waters from the south and Atlantic waters carried from the west with the boundary current. The tortuous bathymetry offers many routes for a topographically steered current, and the spatial variability of the sparse data that exist clearly indicates the complexity of the region. These data also show significant interannual variability, in line with the major changes seen in the last decade throughout the Arctic, and they further suggest that the region diverts significant amounts of water into the deep basins, indicating this region's importance to shelf-basin exchange, deep basin ventilation, and circum- and trans-Arctic circulation (with feedback implications to the World Ocean circulation). Yet, the pathways and exchanges in this area are still unclear, both qualitatively and quantitatively, due to the lack of sufficiently concentrated observations. This research will conduct a high spatial resolution hydrographic and tracer survey, supported by short-term moored current and CTD measurements, in the region of the Chukchi Borderland and the southern end of the Mendeleev Ridge during August/September 2002.The objectives are to:- delineate the pathways of the boundary current carrying the Atlantic water past the Mendeleev Ridge and through the Chukchi Borderland;- ascertain the input from the boundary current and the shelves to the deep Arctic Ocean in the vicinity of the Mendeleev Ridge and the Chukchi Borderland;- understand and quantify the pathways and transformations of the Pacific waters through this region;- describe the horizontal and vertical structure of the boundary current, and estimate its transport; and- quantify recent temporal changes in this region by combining the spatially sparse data extending through most of the past decade with new detailed synoptic measurements.On a 35-day expedition on an ice-breaking research vessel, measurements will be made of temperature, salinity, dissolved oxygen, nutrients, CFCs, Ba, and 18-O on 12 sections that cross both the boundary flow and the Pacific inputs to the region before and after topographic junctions and hypothesized regions of flow diversion. This tracer suite will enable identification of the pathways of the boundary current and the Pacific-origin waters, and quantification of the different Atlantic and Pacific influences, as well as freshwater input from ice melt and different rivers. In addition, three moorings will be deployed, spanning the boundary current for the duration of the cruise. Current meters and moored conductivity and temperature sensors will quantify the vertical and horizontal extent of the boundary current, its structure and variability, and will yield an estimate of the transport and a description of eddies carried with or across the boundary current. To give a comprehensive picture of the system, the entire data set will be analyzed collectively and in tandem with hydrographic, tracer, and moored time series data from the last decade. Since the transit time of signals through this region is 2-4 years, the older data provide a temporal background for the new high spatial resolution data, whilst the newer data will supply an essential spatialframework for understanding the variability of the older surveys.The work will yield a substantially increased understanding of the role of this region in the Arctic circulation, including a determination of pathways, a quantification of exchanges, and an assessment of temporal change. Its timing in 2002 will fill a pending hiatus in hydrographic surveys in the Canadian Basin at a time when the most dramatic changes ever observed in the Arctic are propagating through this region. The project will provide necessary background and mechanistic information to the potential SEARCH and Arctic-Subarctic Ocean Flux programs, and essential far-field information to the SBI Phase II field program in the Chukchi and Beaufort seas. In addition, the results will be pivotal to validating and improving high resolution computer and conceptual models of the Arctic, and will offer insights to physical mechanistic problems, such as the driving mechanism of the boundary current and the interaction of an equivalent barotropic current with steep and sharp topography.