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Synoptic Flow and Density Observations Near an Arctic Shelf Break

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Synoptic Flow and Density Observations Near an Arctic Shelf Break 1402 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 27 Synoptic Flow and Density Observations near an Arctic Shelf Break ANDREAS MUÈ NCHOW Institute of Marine and Coastal Sciences, Rutgers University, New Brunswick, New Jersey EDDY C. CARMACK Institute of Ocean Sciences, Sidney, British Columbia, Canada (Manuscript received 8 July 1996, in ®nal form 14 January 1997) ABSTRACT Analyses of data from three shipborne surveys describe the quasi-synoptic density and velocity ®elds near Barrow Canyon, Alaska. The canyon parallels the northwestern coast of Alaska and contains three different water masses. These are 1) warm and fresh Alaskan coastal waters that originate from the Bering Strait; 2) cold and moderately salty waters that originate from the Chukchi shelf; and 3) warm and salty waters that originate from the Atlantic layer of the Arctic Ocean. A halocline separates the Chukchi shelf and Atlantic layer waters. The halocline slopes upward into the canyon where it is then twisted to slope across the wide canyon. An intensi®cation of the Beaufort gyre near the shelf break just seaward of Barrow Canyon raises the halocline more than 100 m toward the surface. Locally upwelling favorable winds raise the Arctic halocline, which thus is ventilated within Barrow Canyon adjacent to the coast. In the absence of winds the halocline slopes across- canyon in the thermal wind sense due to a northward ¯owing coastal current. Velocity measurements from a towed acoustic Doppler current pro®ler reveal a northward ¯owing jet that transports about 0.3 Sv (Sv [ 106 kg m23) of Bering Sea summer water into the Arctic Ocean at speeds that exceed 0.7 m s21. Total northward transports through the canyon exceed 1.0 Sv. The warm waters of this coastal current supply more than 100 W m22 of heat to the atmosphere. The jet separates both from the bottom and from the coast. Hence, a laterally and vertically sheared jet forms, which breaks into three branches at about 71.88N latitude. 1. Introduction In the summer and early fall warm and fresh Alaskan coastal waters from the eastern Bering Sea generally A steric height difference of about 0.5 m (relative to arrive at Point Barrow and pass through Barrow Canyon a reference level of 1000 m) between the Paci®c and into the Arctic Ocean (Paquette and Bourke 1974; Ahl- Arctic Oceans drives a mean northward ¯ow across the naÈs and Garrison 1984; Aagaard and Roach 1990). Aa- Bering and Chukchi shelves (Stigebrandt 1984). The gaard (1984) and Hufford (1973) use the pronounced observed annual mean transport through the 50-m-deep temperature signal of these waters to infer an alongshore 6 3 21 Bering Strait is about 0.8 Sv (Sv [ 10 m s ) (Coach- current over the slope of the Beaufort Sea. Current ¯uc- man and Aagaard 1988), which agrees well with the tuations correlate with both the atmospheric pressure transports predicted by a barotropic model (Overland difference along the western coast of Alaska and the and Roach 1987). In order to reach the Arctic Ocean, local alongshore winds (Mountain et al. 1976; Coach- however, the ¯ow through Bering Strait must cross the man and Aagaard 1988). This generally strong corre- wide and shallow Chukchi shelf. An important part of lation breaks down, however, when buoyant waters ar- this ¯ow is the northward setting coastal current that rive from the south in the summer and fall. We argue ¯ows along the west coast of Alaska from Bering Strait below that during this season buoyancy-forced motions to Barrow Canyon (Paquette and Bourke 1974). Here contribute to the dynamics of the eastern Chukchi Sea we discuss data from shipborne hydrographic (CTD) and Barrow Canyon. and acoustic Doppler current pro®ler (ADCP) surveys The presence of Bering Strait waters in the Arctic to describe the structure and variability of this ¯ow as interior was noted by Coachman and Barnes (1961), it encounters the shelf break over Barrow Canyon. who searched early hydrographic data from the Arctic basins and found a persistent temperature maximum at about 70-m depth throughout much of the eastern Can- Corresponding author address: Dr. Andreas MuÈnchow, Institute ada Basin. Tracing similar water masses along the con- of Marine and Coastal Sciences, Rutgers University, P.O. Box 231, tinental slope of the Beaufort Sea, Hufford (1973, 1974), New Brunswick, NJ 08903. Mountain et al. (1976), and Aagaard (1984) postulate q1997 American Meteorological Society JULY 1997 MUÈ NCHOW AND CARMACK 1403 that the subsurface temperature maximum over the slope unclear where, when, and how the submesocale eddies of the Beaufort Sea is due to warm waters from Bering of the Arctic Ocean form. D'Asaro (1988b) proposed Strait entering through Barrow Canyon. Property dis- strong currents and lateral current shears in Barrow Can- tributions (Hufford 1974) and direct velocity measure- yon as key elements for the generation of submesoscale ments (Aagaard 1989) indicate a subsurface eastward vortices. Commenting on the similarity of these eddies ¯ow against the local winds over the slope of the Beau- with those found near the Mediterranean out¯ow, he fort Sea. This current bears strong similarities to the hypothesized lateral friction to shear a strong boundary subsurface slope currents found off California, Ireland, current that subsequently separates from the coast. Stern and northwest Africa (Huthnance 1992). The dynamics and Whitehead (1990) simulated the separation of a bar- and spatial distribution of slope currents in the Arctic otropic jet from a sharp boundary in a rotating system Ocean are largely unknown even though Aagaard (1989) both analytically and with a rotating tank. They ®nd that postulates that they constitute the major circulation fea- separation occurs only for certain upstream lateral ve- ture of this ocean. locity pro®les in conjunction with a critical corner angle. Submarine canyons bordering the Arctic Ocean fa- Klinger (1994) generalized the laboratory experiments cilitate the exchange of mass, heat, and momentum be- by allowing a sloping bottom as well as density strat- tween the wide continental shelves and the deep basins. i®cation to enter the problem. His results, however, are Their width often exceeds the internal deformation ra- less clear as the parameter range has increased vastly dius, and geostrophically balanced baroclinic ¯ows in and no unifying interpretation was given. the opposite direction are theoretically possible on op- This study provides a ®rst synoptic description of the posing sides of the canyon (Klinck 1989). Little is spatially variable ¯ow and density ®eld of Barrow Can- known, however, on the spatial distribution of currents yon. Our study focuses on the northern terminus of the in wide Arctic canyons. For example, recirculation and northward shelf ¯ow that extends from Bering Strait in enhanced upwelling may occur in any or all of the wide the south to Barrow Canyon in the north. Section 2 Arctic canyons, for example, the Santa Anna (at 758E), details our study area and instrumentation consisting of Kolyma (at 1608E), Herald (at 1708W), Barrow (at a towed ADCP and standard hydrographic pro®ling sen- 1558W), and Mackenzie (at 1408W) Canyons. Hanzlick sors (CTD). In section 3 we describe the water masses and Aagaard (1980) interpret hydrographic data from and their spatial distribution in and near the canyon. the Santa Anna Canyon off Siberia in terms of a recir- Across-shelf exchange processes involve both the along- culating ¯ow within this canyon. Only Mountain et al. and across-canyon density structures. Section 4 dis- (1976) and Aagaard and Roach (1990) report direct ob- cusses the synoptic circulation over and near the canyon servations from one and two current meter moorings, and shows how Bering Sea waters exit the Chukchi shelf respectively, deployed in Barrow Canyon off Alaska. through Barrow Canyon. Section 5 synthesizes hydro- Their data describe a rectilinear, seasonally averaged graphic and ¯ow ®eld observations, presents quantita- ¯ow of about 0.2 m s21 and peak currents that exceed tive estimates of property ¯uxes through the canyon, 0.8 m s21. These are exceptionally large currents when and speculates on the dynamics of the ¯ow. compared to the generally quiescent interior Arctic Ocean where measured ¯ows rarely exceed 0.05 m s21 2. Study area and data sources (Aagaard 1989); however, no spatial ¯ow ®eld obser- vations are available from any Arctic canyon. This study Barrow Canyon connects the Chukchi and Beaufort of Barrow Canyon provides the ®rst synoptic obser- shelves with the deep Canada Basin of the Arctic Ocean. vations of both the spatial velocity and density ®elds of The canyon constitutes a deep, wide, and long inter- a wide Arctic canyon. section across the Chukchi shelf that runs almost parallel Canyon through¯ows may also constitute a source of to the coastline of northwestern Alaska and intersects mesoscale variability in the ocean interior. Aagaard and the east±west extending continental slope of the Arctic Carmack (1994) suggest that such motions are a key Ocean near Point Barrow, Alaska (Fig. 1). Here the mechanism of shelf±basin exchange in the Arctic Ocean. coastline changes its orientation by 908 and the canyon Newton et al. (1974), Manley and Hunkins (1985), and separates the Chukchi shelf to the west of Point Barrow D'Asaro (1988a) all describe isolated submesoscale ed- from the Beaufort shelf to the east. The two adjacent dies as an ubiquitous feature of the interior circulation shelves differ substantially. The Chukchi shelf is shal- of the Canada Basin. Hart and Killworth (1976) em- low (,50 m), wide (.200 km), and gently sloping. In ployed linear stability analysis to show that these eddies contrast, the Beaufort shelf is deeper (,100 m), nar- could not have formed within the interior ocean. Their rower (,40 km), and more steeply sloping.
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