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Circulation in the Eastern Bering Sea: Inferences from a 2-Km-Resolution Model

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Circulation in the Eastern Bering Sea: Inferences from a 2-Km-Resolution Model Deep-Sea Research II 134 (2016) 48–64 Contents lists available at ScienceDirect Deep-Sea Research II journal homepage: www.elsevier.com/locate/dsr2 Circulation in the Eastern Bering Sea: Inferences from a 2-km-resolution model S.M. Durski a,n, A. Kurapov a, J Zhang b, G.G Panteleev c,d a College of Earth,Ocean and Atmospheric Sciences, Oregon State University, Burt Hall, Corvallis, OR 97331, USA b Polar Science Center, Applied Physics Laboratory, University of Washington, Seattle, WA 98105, USA c International Arctic Research Center, University of Alaska, Fairbanks, AK 99775, USA d National Research Tomsk Polytechnic University, Tomsk, Russia article info abstract Available online 7 February 2015 A 2-km-resolution model of the eastern Bering Sea is developed to capture dynamical processes on the Keywords: scale of the Rossby radius of deformation on tidal to seasonal time scales. The model spans the region 1 1 1 Aleutian Island passes from 178 E to the Alaskan coast and from roughly 50 to 66 N, including the Aleutian Islands in the ROMS south and the Bering Strait in the north. The high resolution throughout ensures that the mesoscale Tidal circulation dynamics of significant subregions of the domain, such as the Aleutian Island passes, Bering Sea slope, Tidal mixing and the shelf canyons, are captured simultaneously without the concern for loss of interconnectivity Eddy generation between regions. Simulations are performed for the ice-free season (June–October) of 2009, with tidal Cold pool and atmospheric forcing. The model compares favorably with observations from AVHRR and Envisat satellites, Argo drifters, and Bering Sea shelf moorings. The mesoscale dynamics of the mixing and exchange flow through the eastern Aleutian Island passes, which exhibit strong diurnal and two-week variability, are well represented. The two-week oscillation in volume flux through the largest of these passes, Amukta Pass, is found to be out of phase with the transport through the neighboring passes (e.g., Seguam and Samalga passes). Mesoscale structure is also found to be ubiquitous along the mixing front of the cold pool. Structures at the scale of O(20 km) persist and play a role in determining the pattern of erosion of the water mass as the shelf warms and mixes. On the Bering Sea shelf, tidal motions are dominant, and variability on the horizontal scale of the first-mode internal tide develops (O(30 km)) from the shelf break to the onshore edge of the Bering shelf cold pool. & 2015 Elsevier Ltd. All rights reserved. 1. Introduction Maslowski, 2012). The influx of the North Pacificwatersandstrong tidal mixing in the vicinity of these passes set up currents that The continuous advancement in computational capabilities pro- traverse the Aleutian Island Arc and then extend northwestardward vides ocean modelers the opportunity to simulate the physics of the in an unstable and intermittent current along the length of the Bering oceans with ever-increasing resolution and/or scale. Problematic shelf slope. Slope-shelf exchange is influenced by the presence of boundary conditions can be moved farther away from the region deep canyons along the shelf break (Clement-Kinney et al., 2009). of interest and dynamics that previously were unresolved or only Strong tides on the southern shelf make the outer shelf islands (the coarsely resolved can now be captured. This increase in computa- Pribilof Islands) regions of rectified currents and intense mixing that tional capacity is particularly beneficial in applications where finer- influence water column structure in some cases over 100 km away. scale processes determine important dynamics of a system at a larger Furthermore internal tides may propagate across the Bering Sea basin scale and where the system exhibits interconnectivity such that from strong generation sites like the Aleutian Islands (Cherniawsky modeling subregions of the domain individually with open boundary et al., 2001; Cummins et al., 2001) and possibly the shelf break and conditions is tenuous. The eastern Bering Sea is one such region canyons to influence the structure and patterns of variability across (Fig. 1). Large scale currents of the North PacificandGulfofAlaska the Bering Sea shelf. The model effort discussed here focuses on coastal currents connect with the Bering Sea basin and shelf thro- resolving these processes over the entire eastern Bering Sea so that a ugh narrow island passes (Stabeno et al., 2005; Clement-Kinney and more accurate description of the dynamics of the system over the ice- free season can be obtained. A general understanding of the mean circulation in the east- n Corresponding author. ern Bering Sea has developed through observations and mode- E-mail address: [email protected] (S.M. Durski). ling (e.g., Stabeno et al., 1999; Reed and Stabeno, 1999; Ladd and http://dx.doi.org/10.1016/j.dsr2.2015.02.002 0967-0645/& 2015 Elsevier Ltd. All rights reserved. S.M. Durski et al. / Deep-Sea Research II 134 (2016) 48–64 49 ° 65 N Bering Strait Norton Sound St Lawrence Isl. St. Matthew Isl. ° 60 N C55 S55 Pribilof Isls. ° Unimak Pass 55 N Amukta Pass Moorings Offset Mooring Amchitka Pass Model trans. ° ° ° 180 W 170 W 160 W Fig. 1. A map of the eastern Bering Sea model domain and bathymetry. Also marked are two BEST–BSIERP mooring sites, an offset mooring site (for model data comparison) and a model transect used in the analysis. Stabeno, 2009; Clement-Kinney et al., 2009; Zhang et al., 2010; Bering Sea shelf, tides are energetic leading to formation of a mixing Danielson et al., 2011; Panteleev et al., 2012). The basin circulation front in the summer at approximately the 50 m isobath (Kachel is generally a cyclonic gyre. In the south, along the north slope of et al., 2002). the Aleutian Island ridge, is the eastward flowing Aleutian North Dynamics in some parts of the Bering Sea are significantly inf- Slope Current (ANSC, Fig. 2). This flow turns sharply northwest- luenced by processes occurring on tidal time scales and on spatial ward becoming the Bering Slope Current (BSC) upon reaching the scales on the order of a few kilometers. For example, the eastern Bering Sea shelf in the southeast corner of the basin. The area passes that connect the North PacificoceanwiththeBeringSea along the slope is noted for enhanced eddy variability on the scale basin are typically only tens of kilometers wide but experience very of 20–100 km, detected in altimetry, satellite radiometer imagery, strong tidal forcing. The energetic mixing and exchange flows they and drifter motions (Sinclair and Stabeno, 2002). Exchange with introduce into the system determine the characteristics of the ANSC the North Pacific occurs through numerous Aleutian Island passes (Kowalik, 1999; Reed and Stabeno, 1999; Cummins et al., 2001; Ladd with source waters from the North Pacific gyral circulation (the et al., 2005; Foreman et al., 2006; Nakamura et al., 2010). Mixing can Alaskan Stream), and the fresher Gulf of Alaska shelf current (the be so vigorous that it may potentially mix growing phytoplankton Alaska Coastal Current). out of the euphotic zone, thereby reducing production, which can Mean flows on the Bering Sea shelf are northward and generally explain low chlorophyll areas around the islands seen in satellite strongest near the Bering Strait. Inflow onto the shelf occurs color imagery (Ladd et al., 2005). through Unimak Pass west of Unimak Island and the tip of the The scale of dominant baroclinic eddies in the Bering Sea, the Aleutian Peninsula, in the south, and by exchange with basin waters Rossby radius of deformation, is estimated to be as small as 20 km along the 1200 km shelf-slope boundary. Each winter a seasonal ice (Chelton et al., 1998; Ladd et al., 2005). However modeling studies sheet forms extending southward over a large portion of the Bering of the Bering Sea circulation published to date have generally lacked Sea shelf. During years in which spring winds from the north pre- the resolution to represent oceanic phenomena at this scale. Recent dominate, the ice cover can reach south of the Pribilof Islands and studies have nonetheless provided new and important information persist there into May (Stabeno et al., 2012a). A bottom cold pool on mean transports, and seasonal and interannual variability in the is formed when the ice retreats and stratification is established ocean circulation and ice extent and concentration. Among recent primarily through surface heat fluxes on the southern shelf. Both contributions, Danielson et al. (2011) provided model-data compar- salinity and temperature maintain strong stratification on the nor- isons in the Bering Sea using a 35-year simulation obtained with an thern shelf (Stabeno et al., 2012b). Particularly on the southern 10-km resolution ROMS-based coupled ice-circulation model of the 50 S.M. Durski et al. / Deep-Sea Research II 134 (2016) 48–64 Fig. 2. Seasonal average surface velocity magnitude and vectors from June through October 2009. Velocities less than 0.05 m sÀ1 are de-emphasized using gray-scale. Black contour lines indicate bathymetry at À50, À100, À200, À500 and À1000 m. The major currents are displayed schematically with colored arrows, ANSC –Aleutian North Slope Current, BSC – Bering Slope Current, AS – Alaskan Stream, ACC – Alaska Coastal Current. North-East Pacific, and focused on monthly to decadal variability in using the same model. An earlier study by Hermann et al. (2002) the area. Danielson et al. (2012) used the same model along with an involved development of a model focusing on eddy variability in the idealized barotropic representation of the Bering Sea shelf to study southeastern Bering Sea using a model with maximum resolution of the differences in onshelf transport produced by northwesterly 4km. Mizobata et al.
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