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Evolution and Dynamics of the Flow Through Herald Canyon in The ARTICLE IN PRESS Deep-Sea Research II ] (]]]]) ]]]–]]] Contents lists available at ScienceDirect Deep-Sea Research II journal homepage: www.elsevier.com/locate/dsr2 Evolution and dynamics of the flow through Herald Canyon in the western Chukchi Sea Robert S. Pickart a,Ã, Lawrence J. Pratt a, Daniel J. Torres a, Terry E. Whitledge c, Andrey Y. Proshutinsky a, Knut Aagaard b, Thomas A. Agnew d, G.W.K. Moore e, Holly J. Dail a a Woods Hole Oceanographic Institution, Woods Hole, MA 02540, USA b Applied Physics Laboratory, University of Washington, Seattle, WA 98105, USA c School of Fisheries and Oceans, University of Alaska, Fairbanks, AK 99775, USA d Meteorological Service of Canada, Downsview, OT, Canada M3H 5T4 e University of Toronto, Toronto, OT, Canada M5S 1A1 article info abstract The flow of summer and winter Pacific water masses through Herald Canyon is investigated using data Keywords: from a high-resolution hydrographic/velocity survey conducted in summer 2004. The survey was part of Arctic canyons the Russian-American Long Term Census of the Arctic (RUSALCA) program, and consisted of four cross- Canyon dynamics canyon transects occupied over a 2-day period. At the time of the survey dense winter water was Pacific winter water entering the western side of the canyon from the Chukchi Sea, flowing alongside a poleward jet of Hydraulics summer water on the canyon’s eastern flank. As the dense water progressed northward it switched sides Polynyas of the canyon and underwent a sudden increase in layer thickness. This coincided with vertical mixing near the interface of the winter and summer water, producing a new water mass mode exiting the canyon. All of these features are consistent with the notion of hydraulic activity occurring in the canyon. A three-layer hydraulic theory is applied to the flow, which suggests that it is supercritical and that hydraulic control is likely. A lock-exchange formulation accurately predicts the northward transport of the winter water. The origin of the winter water and the manner in which it drains into the canyon is investigated using satellite ice-concentration data, atmospheric re-analysis fields, historical in-situ data, and a simple circulation model. Finally, the fate of the Pacific water exiting the canyon, and its connection to the Chukchi shelfbreak current, is discussed. & 2009 Elsevier Ltd. All rights reserved. 1. Introduction observations to pin down the precise mean circulation of the Chukchi Sea, but the evidence to date suggests that the bulk of the In order for Pacific water to reach the interior basin of the 0.8 Sv (1 Sv ¼ 106 m3 sÀ1) of Pacific water flowing northward Arctic Ocean it must first cross the wide and shallow Chukchi Sea through Bering Strait is carried toward the shelf-edge in distinct north of Bering Strait. Numerical models suggest differing degrees branches. This includes the three branches noted above, plus a of topographic steering of the flow as it progresses northward on fourth branch through Long Strait (Woodgate et al., 2005). This the shelf. In the barotropic model of Winsor and Chapman (2004) latter pathway does not appear in the above models, possibly the inflow from Bering Strait spreads out across the Chukchi Sea, because of the closed western boundary in each of the with some areas of intensification due to specific bathymetric simulations. On the other hand, Woodgate et al. (2005) had only features. By contrast, the depth-averaged streamlines in Spall’s a single mooring with which to estimate the net transport (2007) model show three distinct branches of Pacific water north through Long Strait, so the volume transport of this branch is of the strait (shown schematically in Fig. 1): in the western uncertain. However, the Long Strait branch is a known pathway Chukchi Sea through Herald Canyon, in the eastern Chukchi Sea for nutrients into the East Siberian Sea (Codispoti and Richards, through Barrow Canyon, and on the central shelf through the 1968), and synoptic velocity measurements in summer have channel between Hanna and Herald shoals (known as the Central revealed northwestward flow of Pacific water through the strait Channel, Weingartner et al., 2005). Presently there are not enough (Weingartner et al., 1999). One well-known aspect regarding the circulation of the Chukchi Sea is that the two canyons at the shelf-edge significantly à Corresponding author. influence the northward flow of Pacific water, resulting in two E-mail address: [email protected] (R.S. Pickart). areas of enhanced outflow (Coachman et al., 1975). Consequently, 0967-0645/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2009.08.002 Please cite this article as: Pickart, R.S., et al., Evolution and dynamics of the flow through Herald Canyon in the western Chukchi Sea. Deep-Sea Research II (2009), doi:10.1016/j.dsr2.2009.08.002 ARTICLE IN PRESS 2 R.S. Pickart et al. / Deep-Sea Research II ] (]]]]) ]]]–]]] 74 74 72 72 430 70 70 68 68 -185 -180 -175 -170 -165 -160 -155 -150 Fig. 1. Study area of the Chukchi Sea, including names of geographical features. Three branches of the Bering Strait inflow are represented schematically, the western-most of which feeds Herald Canyon. Circles denote stations occupied as part of the RUSALCA program in August 2004. The four transects in Herald Canyon are the focus of the paper. Also shown is the section across the Chukchi slope at 1661W (squares), occupied in September 2004 as part of the Western Arctic Shelf-Basin Interactions (SBI) program. these regions are of particular importance for understanding the (Coachman et al., 1975; Kirillova et al., 2001; Weingartner et al., impacts that the shelf-derived waters have on the interior Arctic 2005). basin, including the ventilation of the cold halocline and the What is the source of water on the western side of Herald offshore flux of nutrients, carbon, and zooplankton. As summar- Canyon? In the near-surface layer Weingartner et al. (2005) ized in Pickart et al. (2005), Barrow Canyon, on the northeast observed a cold and fresh water mass during summer, believed to Chukchi shelf, has been studied extensively, and there is a result from ice-melt, on three different cruises from 1993 to 1995. significant body of literature describing various aspects of the The deep layer seems to consist of winter-transformed water canyon flow and the water masses within it. This includes the role throughout much of the year (Kirillova et al., 2001; Weingartner of winds and upwelling (e.g. Aagaard and Roach, 1990), the impact et al., 2005; Woodgate et al., 2005). Weingartner et al. (2005) state of polynyas (e.g. Weingartner et al., 1998), and the vorticity that this deep water mass is simply the resident Chukchi Sea dynamics as they relate to eddy formation (D’Asaro, 1988; winter water (which in summer is a remnant from the previous Munchow and Carmack, 1997; Pickart et al., 2005). By contrast, winter), while Kirillova et al. (2001) refer to it as a ‘‘Wrangel type’’ comparatively little is known about Herald Canyon, on the of water mass. This terminology implies that the formation region northwest Chukchi shelf. For example, while numerous mooring is near Wrangel Island, where in wintertime it is well known that records have been obtained in Barrow Canyon over the years (e.g. persistent polynyas form in the lee of the island (Cavalieri and Garrison and Becker, 1976; Aagaard and Roach, 1990; Weingartner Martin, 1994; Winsor and Bjork,¨ 2000), producing brine-enriched et al, 1998, 2005), only a single long-term mooring record exists bottom water (e.g. Weingartner et al., 1998). There is evidence of for Herald Canyon (Woodgate et al., 2005). such polynya-driven water formation in the Long Strait mooring Nonetheless, previous shipboard programs and historical data record of Woodgate et al. (2005). analyses have shed light on some aspects of the flow through As is the case with the water masses, there also seem to be Herald Canyon. Significant differences seem to exist between the distinct flow regimes on either side of Herald Canyon. A year-long two sides of the canyon, both in terms of the water masses and the current meter record from 1990 to 1991 obtained from the currents. As mentioned above, this canyon is one of the main canyon’s eastern flank showed predominantly northward flow conduits for the flow emanating from Bering Strait (Coachman (with reduced amplitude in winter), consistent with the idea of a et al., 1975). For the most part, the Bering Sea-origin water in this direct route from Bering Strait to this side of the canyon branch is believed to flow along the eastern flank of the canyon. In (Woodgate et al., 2005). By contrast, the corresponding current winter this consists of cold, salty winter-transformed Pacific water meter record from the western flank showed weaker and more originating from the Bering Sea (Muench et al., 1988), modified variable flow (although this record ended after only 3.5 months). further by cooling and brine-rejection within the Chukchi Sea These differences are mirrored in the summertime shipboard (Weingartner et al., 1998; Woodgate et al., 2005). In summer, the velocity data presented by Weingartner et al. (1999), which flow in this branch is mainly summertime Bering Sea Water showed persistent strong northward flow on the eastern flank, (Coachman et al., 1975),1 which is separated by a frontal boundary and sluggish, variable flow on the western flank. If the flow on the from the water masses in the western portion of Herald Canyon western flank is not part of the Bering Strait branch that passes through the canyon, then what forces it? Kirillova et al.
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