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Flow of Pacific water through the Western Chukchi: Dynamics and biological implications

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July 1, 2004 through June 30, 2005

Robert S. Pickart
Woods Hole Oceanographic Institution
Woods Hole, MA 02543-1541

Program Manager: John Calder NOAA

Related NOAA Strategic Plan Goal:
Goal 2. Understand climate variability and change to enhance society’s ability to plan and respond.

In summer 2004 the Russian icebreaker Khromov carried out the inaugural cruise of the Russia-US Long-term Census of the Arctic (RUSALCA) program. RUSALCA resulted from the 2003 Memorandum of understanding for World Ocean and Polar Regions Studies between NOAA and the Russian Academy of Sciences. The three-leg expedition was a great success, during which US and Russian scientists carried out an array of multidisciplinary projects aimed at furthering our understanding of climate change in the Arctic. Leg 2 focused on the region from Bering Strait northward into the Chukchi Sea, and our part of the hydrographic component of this leg consisted of a detailed survey of the flow through Herald Canyon (Figure 1). This is the first time that the canyon, which lies in Russian territorial waters, has been sampled at high cross-stream resolution, enabling us to resolve fully the currents and water masses. The survey was done using a conductivity/temperature/depth (CTD) package with the addition of dissolved oxygen, turbidity, fluorescence, a lowered acoustic Doppler current profiler, and a video plankton recorder for measuring zooplankton and particle (marine snow) content in the water.

Pacific water enters the Arctic through Bering Strait and is largely steered through the Chukchi Sea by the topography of the shelf. In particular, the two canyons on either side of the Chukchi Sea−Barrow canyon in the east and Herald canyon in the west−tend to concentrate the flow before it reaches the shelfbreak. Presently we know very little about the flow through Herald canyon, although this branch contains a significant amount of the transport of the Pacific water, and carries most of the nutrient load into the Arctic. The topography of the canyon is such that hydraulic control may be active, which in turn could influence the downstream entrainment of the flow as well as its ability to form eddies. These are the kind of things we need to know in order to determine the ultimate fate of the Pacific water when it reaches the open Arctic, and the associated implications for climate change.

The final processing, calibration, and de-spiking of the RUSALCA CTD data set were completed, and a data report was written and distributed to the PIs in the RUSALCA group. The processed data, station profile plots, and vertical section plots of the data were made available to the group on a website. The lowered ADCP velocity data are still being processed, and will soon be complete.

The analysis of the CTD data are now underway. Figure 2 shows the vertical sections of potential temperature, turbidity, and fluorescence from the crossing at the head of Herald canyon. On the western side of the canyon one sees cold, dense winter water (magenta color) above the bottom, adjacent to warm summertime Bering water on the eastern side of the canyon. Presumably both of these water masses are flowing northward. The turbidity and fluorescence distributions show a strong plume emanating from the bottom beneath the summer water (near station 54) while the fluorescence shows a distinct plume surrounding the winter water. These features may arise in part due to the dynamics of the flow through the canyon−for instance bottom boundary layer detachment or convergence and downwelling of the jet. We will pursue these ideas in the coming months, and describe the overall adjustment of the Pacific water, including the ramifications for the chemistry and biology of this region.

Last updated: August 19, 2008

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