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On the Dynamics of Strait Flows: an Ocean Model Study of the Aleutian Passages and the Bering Strait

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On the Dynamics of Strait Flows: an Ocean Model Study of the Aleutian Passages and the Bering Strait Ocean Dynamics (2013) 63:243–263 DOI 10.1007/s10236-012-0589-6 On the dynamics of strait flows: an ocean model study of the Aleutian passages and the Bering Strait Tal Ezer & Lie-Yauw Oey Received: 26 June 2012 /Accepted: 19 December 2012 /Published online: 8 January 2013 # Springer-Verlag Berlin Heidelberg 2013 Abstract A high-resolution numerical ocean circulation seems to originate from two different sources: a remote model of the Bering Sea (BS) is used to study the natural origin from variability in the Alaskan Stream that enters variability of the BS straits. Three distinct categories of the BS through the Aleutian passages and a local origin strait dynamics have been identified: (1) Shallow passages from the interaction of currents with the Bowers Ridge in such as the Bering Strait and the Unimak Passage have the Aleutian Basin. Comparisons between the flow in the northward, near barotropic flow with periodic pulses of Aleutian passages and flow in other straits, such as the larger transports; (2) wide passages such as Near Straits, Yucatan Channel and the Faroe Bank Channel, suggest Amukta Pass, and Buldir Pass have complex flow patterns some universal topographically induced dynamics in strait driven by the passage of mesoscale eddies across the strait; flows. and (3) deep passages such as Amchitka Pass and Kamchatka Strait have persistent deep return flows opposite Keywords Bering Sea . Aleutian passages . Numerical in direction to major surface currents; the deep flows persist ocean modeling . Flow–topography interaction . Strait independent of the local wind. Empirical orthogonal func- dynamics tion analyses reveal the spatial structure and the temporal variability of strait flows and demonstrate how mesoscale variations in the Aleutian passages influence the Bering 1 Introduction Strait flow toward the Arctic Ocean. The study suggests a general relation between the barotropic and baroclinic The Bering Sea (BS) is located between North America in Rossby radii of deformations in each strait, and the level the east and Asia in the west and between the North Pacific of flow variability through the strait, independent of geo- OceaninthesouthandtheArcticOceaninthenorth graphical location. The mesoscale variability in the BS (Fig. 1a). The BS is divided between the shallow Bering Shelf in the east and the deep Aleutian Basin in the south- west. The anti-clockwise coastal currents around the Aleutian Basin (Fig. 1b) include the Aleutian North Slope Responsible Editor: Yasumasa Miyazawa Current (ANSC; Stabeno et al. 2009), the Bering Slope This article is part of the Topical Collection on the 4th International Current (BSC; Johnson et al. 2004), and the Kamchatka Workshop on Modelling the Ocean in Yokohama, Japan 21-24 May Current (KC; Panteleev et al. 2006). South of the BS along 2012 the Aleutian Arc, an energetic western boundary current, the T. Ezer (*) Alaskan Stream (Favorite 1967; Reed 1968, 1984; Thomson Center for Coastal Physical Oceanography, 1972; Reed and Stabeno 1999), is found (Figs. 1 and 2). Old Dominion University, 4111 Monarch Way, There are evidences that the AS influences the flow through Norfolk, VA 23508, USA e-mail: [email protected] the Aleutian passages and the BS circulation (Favorite 1974; Royer and Emery 1984; Reed 1990; Reed and Stabeno L.-Y. Oey 1993; Stabeno and Reed 1992; Stabeno et al. 1999, 2005; Program in Atmospheric and Oceanic Sciences, Maslowski et al. 2008; Ladd and Stabeno 2009; Ezer and Princeton University, Sayre Hall, P.O. Box CN710, Princeton, NJ 08544-0710, USA Oey 2010), but the exact mechanism of its influence is not e-mail: [email protected] fully understood. North of the BS is the Chukchi Sea that 244 Ocean Dynamics (2013) 63:243–263 Fig. 1 a Map of the area of (a) Map of Bering Sea and Model Domain interest and locations of important passages; the model domain (dash line) extends south to about 40°N. b Annual mean model sea surface height and schematic of the major currents. Various sections, passages, and straits are marked by “A” to “H” in (b) BowersRidge (b) Model Sea Surface Height and Sections N) o Latitude ( Longitude (oE) leads to the Arctic Ocean. The Bering Strait provides the transferred through the Aleutian Passages; their study shows only connection between the Pacific and the Arctic Oceans, the need for better understanding of dynamics of strait flow, so it plays a crucial role in the global ocean circulation and which motivated the present study. climate (De Boer and Nof 2004). Long-term observations of Global climatic changes and the Pacific Decadal the Bering Strait flow indicate variability at a wide range of Oscillations (PDO) may influence the ecosystem in the BS scales (Aagaard et al. 1985; Coachman and Aagaard 1988; (Hunt et al. 2002; Jin et al. 2009). However, it is not clear if the Roach et al. 1995; Woodgate et al. 2005; 2006). The mod- BS–Pacific connection is mostly through air–sea interactions, eling study of Ezer and Oey (2010; hereafter EO10) sug- in which seasonal and interannual variations in the atmosphere gests that some of the variability in the Bering Strait are over the North Pacific impact the BS (Pickart et al. 2009), or related to the variability generated in the Pacific Ocean and potentially there is also oceanic connections such that, for Ocean Dynamics (2013) 63:243–263 245 Fig. 2 a Surface temperature (degrees Celsius, in color) and (a) velocity trajectories obtained from the annual model fields. b The mean flow at a north–south section across 190°E (see a for location), showing the westward flowing Alaskan Stream (AS, blue core) and the eastward flowing Aleutian North Slope Current (ANSC, red core) on both sides of the Aleutian Ridge (b) AS ANSC Aleutian Ridge AleutianA Trench example, PDO-related variations in circulation in the Gulf of For comparison, other basins are often connected to each other Alaska (Royer 1975) impact the Alaskan Current and associated through only a few straits that are more easily monitored; strait flows into the BS, as suggested by EO10. The exchange of examples are the Bering Strait, Denmark Strait, Faroe Bank momentum and heat between the Pacific Ocean and the BS is Channel, Gibraltar Strait, Yucatan Channel, Florida Strait, etc. complex, as it involves flows crossing the Aleutian Ridge into The large variability of the flow in individual passages the BS through many passages and straits of different geometry. makes it difficult to estimate the mean flow from sparse 246 Ocean Dynamics (2013) 63:243–263 observations, and observations of flows in deep straits often intrinsic dynamics of strait flows. Comparing the various BS cover only the upper layers. As a result, strait transports straits, with their wide range of topographies and geographical reported in the literature sometimes differ by an order of locations, could serve as a case study to see if there are any magnitude from each other, and in some cases do not agree common mechanisms that govern the dynamics of strait on the direction of the mean flow (see a summary of 13 flows. For example, the shallow (∼70 m) Unimak Pass (see observational studies of strait flows in Table 1 of EO10). Fig.1a for locations and names of all passages analyzed in this Therefore, ocean circulation models are important tools in study) in the east has northward flow directly into the Bering complementing measurements and in shedding light on the Shelf, to feed the Alaskan Coastal Current (Fig. 1b), but on the BS circulation and strait flows (Overland et al. 1994; Liu and other (western) end of the Aleutian Arc is the deep (∼4,500 m) Leendertse 1982; Wang et al. 2009; Hu and Wang 2010; Kamchatka Strait (Panteleev et al. 2006), with a southward EO10). We will use here the same ocean circulation numerical surface flow of the KC toward the Pacific. Although these two model as in EO10, but the focus will be on understanding the straits are connected through complex current systems Table 1 Characteristics of straits and passages in the BS unforced model, ordered by decreasing aspect ratio (“s” in third column) and labeled by the three categories discussed in the text ¼ LðÞkm −1 T T Strait/passage Longitude, latitude s HðÞkm V (ms ) R (km) L/R Transport SD/mean (in Sv) ΔH (m) RC (km) L/RC g′ (ms−2)Fr B “Shallow” straits Bering Strait 169.9°W, 65.7°N ~85/0.05=~1,700 V~0.2 RT~170 L/RT~0.5 0.09/0.35=~0.26 ΔH~20 RC~1 L/RC~85 g′~0.0007 Fr~1.6 B=0.07 Unimak Pass 165.2°W, 54.3°N ~50/0.07=~700 V~0.5 RT~260 L/RT~0.2 0.21/0.47=~0.45 ΔH~25 RC~3.5 L/RC~14 g′~0.007 Fr~1.2 B=0.06 “Wide” straits Amukta Pass 172.0°W, 52.5°N ~250/0.75=~330 V~0.1–0.2 RT~720 L/RT~0.35 0.69/1.2=~0.58 ΔH~50 RC~5.1 L/RC~49 g′~0.007 Fr~0.1 B=0.46 Buldir Pass 184.2°W, 52.4°N ~350/1.5=~230 V~0.1–0.5 RT~1,050 L/RT~0.33 1.2/4.1=~0.29 ΔH~100 RC~7.3 L/RC~48 g′~0.007 Fr~0.4 B=0.51 Near Strait 190.0°W, 54.0°N ~500/3.5=~150 V~0.1–0.5 RT~1,330 L/RT~0.38 3.2/2.64=~1.21 ΔH~100 RC~7.3 L/RC~68 g′~0.007 Fr~0.4 B=0.53 “Deep” straits Amchitka Pass 180.0°W, 51.8°N ~120/1.8=~65 V~0.1–0.2 RT~1,050 L/RT~0.1 0.78/3.2=~0.24 ΔH~100 RC~7.3 L/RC~16 g′~0.007 Fr~0.2 B=0.48 Kamchatka Strait 195.0°W, 56.0°N ~200/4.5=~45 V~0.5–1 RT~1,740 L/RT~0.1 5.5/1.1=~5 ΔH~200 RC~9.8 L/RC~20 g′~0.007 Fr~0.6–1 B=0.32 Other straits Faroe Bank Channel 6.0°W, 60.0°N ~20/0.8=~25 V~0.5–1 RT~700 L/RT~0.03 1.5/3=~0.5 ΔH~200 RC~20 L/RC~1 g′~0.007 Fr~0.6–1 Yucatan Channel 85.8°W, 21.5°N ~150/2=~75 V~1–1.5 RT~2,600 L/RT~0.06 5/30=~0.17 ΔH~200 RC~40 L/RC~4 g′~0.03 Fr~0.3 Data from models of the Faroe Bank Channel (Ezer 2006) and the Yucatan Channel (Ezer et al.
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