Peter Winsor
Figure 1. Map of the northern Bering Sea. Shown are positions of two moored current meters (black squares); F3 south of St. Lawrence Island (SLI) and A3 located just north of Bering Strait. Also shown is the drift track (from 1 September 2002 to 28 February 2003) of a holey-sock drifter deployed north of Nunivak Island (NI). Numbers 1 to 3 show three identified stages of particular interest discussed in the text. Gray dots show NCEP grid points used for meteorological forcing. Bathymetry is from ETOPO5.

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Project - Numerical Modeling - Circulation features in northern Bering Sea
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Peter Winsor, S. Danielson, T. Weingartner, K. Aagaard, and L. King

The motivation for this study comes from a small set of drifters deployed in the northern Bering Sea during autumn of 2002. The drift track of one of these drifters (Fig. 1) suggests that the drifter encountered three stages (numbered 1 to 3 in Fig. 1), each dominated by different flow dynamics. In short, stage 1 shows how winds can drive nearshore, relatively fresh coastal waters far (~150 km) offshore within a short time period (~6 days). Stage 2 suggests that coastal water advected offshore may produce a baroclinic northwesterly flow towards St. Lawrence Island (SLI). The third stage shows evidence that water south of SLI is advected eastward, flows around the Northeast Cape, and is then rapidly drawn northward by the flow through Bering Strait.

We used the Regional Ocean Model System (ROMS); a free-surface, hydrostatic, primitive equation ocean model that uses stretched, terrain-following coordinates in the vertical and orthogonal curvilinear coordinates in the horizontal, in a idealized configuration to understand some first-order dynamics of the circulation in the northern Bering Sea.

Figure 2. Steady-state barotropic current vectors, forced by a steady northward flow of 0.8 Sv through Bering Strait. Current vectors are plotted every second grid point. The location of current meter mooring F3 is shown by the black square (see Fig. 1).

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To investigate stage 3, the origin of the current south of SLI, we performed some simple numerical model calculations, using the Regional Ocean Model System (ROMS) in a two-dimensional barotropic mode, forced only by a steady northward flow of 0.8 Sv through Bering Strait. Fig 2 shows steady-state barotropic current vectors after 20 days of steady flow through Bering Strait. The area shown is a small potion of the much larger model domain (see description in Section 2). The vectors indicate an intense western boundary current, with large current speeds in Anadyr Strait (between SLI and the Russian coast). This is consistent with the results of Kinder et al. (1986). Somewhat surprising is the bifurcation of the western flow, with a weak branch flowing along the southern coast of SLI, sweeping around the northeastern corner (Northeast Cape), and then eventually rejoining the northward flow north of SLI. North of Anadyr Strait the western boundary current spreads out laterally over much of the shelf to form a broader, weaker flow. The width of the model current along the south coast of SLI is ~50 km at the location of the F3 mooring, with typical nearshore current speeds of ~5 cm s-1. The steady-state transport south of SLI is ~0.08-0.12 Sv, roughly 10-15% of the total (imposed) transport through Bering Strait.

This is ongoing work. Please contact me for further information.

Related publications:
Winsor, P., D. C. Chapman, S. Danielson, T. Weingartner, K. Aagaard, and L. King, 2005: Circulation features in northern Bering Sea inferred from drifters, moored current meters, and numerical modeling, in prep.

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