THE CHUKCHI BORDERLAND: An Arctic Cross-roads Rebecca Woodgate (UW), Knut Aagaard (UW), Jim Swift (Scripps), Bill Smethie (LDEO), Kelly Falkner (OSU) Corresponding author: Rebecca Woodgate (woodgate@apl.washington.edu)

NSF-Arctic Natural Sciences (OPP-0117480) With thanks to USCGC Polar Star Back to High Latitude Dynamics

CHUKCHI BORDERLAND BASICS Near top of the world, some 600 miles north of the Bering Strait, 800 miles south of the North Pole, the entrance to the Arctic Ocean is marked by a complex area of tortuous topography known as the Chukchi Borderland.  In this complex region of ridges and deep-sea plateaus, waters from the Pacific and the Atlantic meet and interact.   The Atlantic waters are warmer, saltier and deeper.  They have made their way anticlockwise around the Arctic Ocean, hugging the continental slope, in a journey that has taken them many years since leaving the Atlantic.     The Pacific waters are colder and fresher and carry a rich nutrient load.     The interplay of these water masses and their fate in the Arctic Ocean, both of which depend on the ice motion, the sea floor topography and the winds, is still much of a mystery today.   In an NSF-sponsored project, we seek to trace and understand these water pathways.  On 5-week field trip to the Arctic, aboard the USCGC Polar Star, we used state-of-the-art instrumentation and techniques to do the best-ever oceanographic survey of this region, to understand the role this Arctic Cross-roads plays in Arctic climate and world climate.

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.

CBL2002 CRUISE A 35-day NSF-sponsored cruise aboard the USCGC Polar Star has studied in depth the physical oceanography of the Chukchi Borderland and Mendeleev Ridge regions. An extensive hydrographic survey (126 CTD casts) was conducted. In addition to CTD profiles of temperature, conductivity, oxygen, and light scatter and L-ADCP profiles of water velocity, bottle samples were taken for nutrients (2662 samples), dissolved oxygen (2999 samples), salinity (3066 samples) and tracers CFCs (F11, F12, F113, ca. 2500 samples), O18 isotopes (ca.1000 samples), Barium (ca.1000 samples), Helium (ca.108 samples), Iodine-129 (96 samples) and Cesium-137 (27 samples). Twenty-one denitrification (N:Ar ratio) samples were also taken. A total of 47 XBTs were used both to increase spatial coverage over the shelf and to increase spatial resolution in the slope regions. To better map the boundary current regime, 3 oceanographic moorings carrying current meters and temperature and salinity sensors were deployed across the boundary current for the ca. 1 month duration of the cruise. For details, see the cruise report and appendices For better map, click the image above.  (dots = CTD casts; x=XBT casts; Moorings were deployed on section 2)

THE T-S 'ZIGZAG" FINGERPRINT - TRACING ATLANTIC WATER PATHWAYS A major aim of the CBL project is to eludicate pathways of Atlantic water through the CBL region.  These waters are tradiationally identified as a temperature maximum at ~ 300-600m.  These are the Fram Strait Branch Waters (FSBW).  There is a second type of Atlantic origin waters below this - the Barents Sea Branch Waters (BSBW) - which are colder.  A curious known feature of FSBW is peculiar structures in temperature (T) and salinity (S) that exist in the FSBW core.  In a vertical profile, both T and S zig zag between colder/warmer and fresher/saltier watesr. In T-S space, the structures also look like zigzags. Theory suggests they are due to double diffusive interleaving of water masses. Our work suggests that these structures can be used to fingerprint the Atlantic waters.  The newer waters are seen to have smaller zigzags, the older waters bigger zigzags and the core of the boundary current has a partly smoothed "Point and bump" stucture.  From these structures (and supporting tracer data, both dissolved oxygen and CFCs), we trace Atlantic water pathways through the CBL region.  For more details, see our Atlantic water zigzag paper.

HOW PACIFIC WATER CAN GET INTO THE ARCTIC FROM THE CHUKCHI SHELF Another major aim of the CBL project is to understand how Pacific waters exit from the Chukchi Shelf into the Arctic Ocean.  Pacific waters may be identified by high silicate values.  If we map silicate in the CBL region (left), we see that Pacific waters are present over much of the area, at depths greater than the depth of the Chukchi Shelf, and at densities greater than the Pacific waters entering through the Bering Strait.  How can this be?     Our work suggests that this is due to mixing processes over the Chukchi Sea.  Denser Atlantic waters are sloshed up the Chukchi Slope into the Chukch Sea where they mix with the Pacific waters.  Thus, when these waters return to the Arctic, they carry the Pacific silicate signal, but are denser and warmer than the original Pacific waters.  This transport of Altantic waters south into the Chukchi Sea has been observed in Barrow Canyon, and likely relates to wind events or wave propogation.      This mechanism seems to afffect a large region of the CBL region. The process appears to be much more wide spread than the hypothesised mechanism of ventilation by hypersaline polynya waters, a process that would leave a very different signature in T-S space.  In that case, we would expect the coldest waters at S > 33 psu to have the highest silicate, and that is not what is observed in the CBL data.      For more details, see our Arctic Ventilation by Pacific Waters paper.

THE FLOATING CLASSROOM    Throughout the cruise, our "teacher at sea", Gail Grimes from Lake Stevens High School, Washington, brought the Arctic into the Classroom, by a daily webdiary, explaining science and life on a top research ice-breaker.  This website was watched by her classes in Washington, and other classes throughout the US.     See her website and diary for details and photos.

A STREET GUIDE TO THE CHUKCHI BORDERLAND PROJECT

Large-Scale Arctic Circulation The Role of the Arctic Cross-roads Why do we care? How Atlantic waters affect the sea ice Pathways of nutrient-rich Pacific waters Arctic Climate Change? Chukchi Borderland Cruise 2002 Chukchi Borderland Moorings 2002 Chukchi Borderland CTD Work 2002 The Floating Classroom Summary This talk is downloadable as a PDF file

Aagaard, K., L.A. Barrie, E. C. Carmack, C. Garrity, E.P. Jones, D. Lubin, R.W. Macdonald, J.H. Swift, W.B. Tucker, P.A. Wheeler, and R.H. Whritner, 1996, U.S., Canadian researchers explore Arctic Ocean, EOS, 77, 209-213.

Aagaard, K., J.H. Swift and E.C. Carmack, 1985, Thermohaline circulation in the Arctic Mediterranean Seas, Journal of Geophysical Research, 90, 4833-4846.

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Carmack, E.C., K. Aagaard, J.H. Swift, R.W. Macdonald, F.A. McLaughlin, E.P. Jones, R.G. Perkin,J.N. Smith, K. Ellis, and L. Kilius, 1997, Changes in temperature and contaminant distributions within the Arctic Ocean, Deep-Sea Res. Part II, 44, 1487-1502.

Frank, M., W.M. Smethie, Jr., and R. Bayer, 1998, Investigation of subsurface water flow along the continental margin of the Eurasian Basin using the transient tracers tritium, He-3 and CFCs. Journal of Geophysical Research,103:30773-30792.

Guay, C.K., and K.K .Falkner, 1997, Barium as a tracer of Arctic halocline and river waters, Deep-Sea Res. Part II, 44, 1543-1569.

Guay, C.K., K.K. Falkner, R.D.Muench, M. Mensch, M. Frank and R.Bayer, 2001, Wind-driven transport pathways for Eurasian Arctic river discharge, Journal of Geophysical Research, 106 (C6), 11469-80.  

Jones, E.P., L.G. Anderson, and J.H. Swift, 1998, Distribution of Atlantic and Pacific waters in the upper Arctic Ocean: Implications for circulation, Geophys. Res. Lett., 25, 765-768.

Macdonald, R.W., E.C. Carmack, F.A. McLaughlin, K.K. Falkner, and J.H. Swift, 1999, Connections among ice, runoff and atmospheric forcing in the Beaufort Gyre, Geophys. Res. Lett., 26, 2223-2226.

Maslowski, W, B. Newton, P. Schlosser, A. Semtner, and D. Martinson, 2000, Modeling recent climate variability in the Arctic Ocean, Geophys. Res. Lett., 27, 3743-3746.

Smethie, W.M., Jr., D.W. Chipman, J. H. Swift, and K. P. Koltermann, 1988, Chlorofluoromethanes in the Arctic Mediterranean Seas:  Evidence for formation of bottom water in the Eurasian Basin and deep water exchange through Fram Strait.  Deep Sea Research, 35: 374-369.

Smethie, W.M., Jr., P. Schlosser, T.S. Hopkins, and G. Boenisch. 2000, Renewal and circulation of intermediate waters in the Canadian Basin observed on the SCICEX-96 cruise. Journal of Geophysical Research, 105:1105-1121.

Swift, J.H., E.P. Jones, K. Aagaard, E.C. Carmack, M. Hingston, R.W. Macdonald, F.A. McLaughlin, and R.G. Perkin, 1997, Waters of the Makarov and Canada basins, Deep-Sea Res. Part II, 44, 1503-1529.

Weingartner, T.J., D.J. Cavalieri, K. Aagaard, and Y. Sasaki, 1998, Circulation, dense water formation, and outflow on the northeast Chukchi shelf, J.  Geophys. Res., 103, 7647-7661.

Woodgate, R.A., K. Aagaard, R.D. Muench, J. Gunn, G. Björk, B. Rudels., A.T. Roach, and U. Schauer, 2001, The Arctic Ocean boundary current along the Eurasian slope and the adjacent Lomonosov Ridge: Water mass properties, transports and transformations from moored instruments, Deep-Sea Research I, 48, 1757-1792.

We gratefully acknowledge financial support for this work from  Arctic Natural Sciences, National Science Foundation (NSF).

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