Circulation on the central Bering Sea shelf, July 2008 to July 2010

Abstract

circulation over the central Bering Sea shelf using measurements at eight instrumented moorings, hindcast winds and numerical model results. At sub-tidal time scales, the vertically integrated equations of motion show that the cross-shelf balance is primarily geostrophic. The along-shelf balance is also mainly geostrophic, but local accelerations, wind stress and bottom friction account for 10-40% of the momentum balance, depending on season and water depth. The shelf exhibits highly variable flow with small water column average vector mean speeds (<5 cm s À1 ). Mean/peak speeds in summer (3-6 cm s À1 /10-30 cm s À1 ) are smaller than in winter and fall (6-12 cm s À1 /30-70 cm s À1 ). Low frequency flows (<1/4 cpd) are horizontally coherent over distances exceeding 200 km. Vertical coherence varies seasonally, degrading with the onset of summer stratification. Because effects of heating and freezing are enhanced in shallow waters, warm summers increase the cross-shelf density gradient and thus enhance northward transport; cold winters with increased ice production and brine rejection increase the (now reversed) cross-shelf density gradient and enhance southward transport. Although the baroclinic velocity is large enough to influence seasonal transports, wind-forced Ekman dynamics are primarily responsible for flow variations. The system changes from strong northward flow (with coastal convergence) to strong southward flow (with coastal divergence) for northerly and easterly winds, respectively. Under northerly and northwesterly winds, nutrient-rich waters flow toward the central shelf from the north and northwest, replacing dilute coastal waters that are carried south and west.

Currents and transport on the Eastern Bering Sea shelf: An integration of over 20 years of data

“…17 in Danielson et al, 2012a), with northward winds generally forcing northward flow, and with southward winds resulting in weaker northward or even southward flow. This was also true for the inner mooring (at a depth of 40 m) on the S line.…”

“…Mean transport in the summer of 1997 and 1998 was $ 0.05 Sv toward the northwest, with stronger transport ( $ 0.1 Sv) in the fall of 1997. Danielson et al (2012a) examined currents from eight yearlong moorings deployed on the 55 -m, 40 -m, and 25 -m isobaths in July 2008 and again in July 2009. Six of these moorings were deployed north of the IF moorings on the central (C) line and northern (N) line (Fig.…”

“…In addition to the magnitude of the wind stress, the relative orientation of the wind with respect to the orientation of the shelf break and coastline is critical for determining the response of the shelf currents to wind forcing (Danielson et al, 2012a(Danielson et al, , 2012b. Together, these determine how the synoptic wind-forced flows modify the mean flow field.…”

“…Wind stress drives the dominant portion of the synoptic-scale (35-100 h) variability in the coastal domain and north of 62°N (Danielson et al, 2012a.…”

Advection in polar and sub-polar environments: Impacts on high latitude marine ecosystems

“…Variability in ice cover on the Bering Sea shelf is associated with wind-driven variability in the advection of warmer waters onto the shelf during winter (Stabeno et al, 2001), with enhanced onshelf flow when winds are predominantly from the southeast (Danielson et al, 2012a(Danielson et al, , 2012b. Model projections suggest more ice-free years in the southeast Bering Sea (Wang and Overland, 2015), while the winter cover of sea ice in the northern Bering Sea north of 60°N is not expected to decline significantly in the foreseeable future (Stabeno et al, 2012).…”