Seth L. Danielson

The Siberian Coastal Current: A wind‐ and buoyancy‐forced Arctic coastal current

We describe circulation and mixing in the Siberian Coastal Current (SCC) using fall shipboard measurements collected between 1992 and 1995 in the western Chukchi Sea. The SCC, forced by winds, Siberian river outflows, and ice melt, flows eastward from the East Siberian Sea. It is bounded offshore by a broad (∼60 km) front separating cold, dilute Siberian Coastal Water from warmer, saltier Bering Sea Water. The alongshore flow is incoherent, because the current contains energetic eddies and squirts probably generated by frontal (baroclinic) instabilities. These enhance horizontal mixing and weaken the cross-shore density gradient along the SCC path. Eventually, the SCC converges with the northward flow from Bering Strait, whereupon it deflects offshore and mixes with that inflow. Deflection occurs where the alongshore pressure gradient vanishes. That location varies on synoptic and seasonal timescales, because this gradient depends on the winds, buoyancy fluxes, and the sea level difference between the Pacific and Arctic Oceans. Deflection usually occurs on the Chukchi shelf, but the SCC occasionally flows southward through Bering Strait. Such events are short lived (1–10 days) and occur mainly in fall and winter under northerly winds. SCC transport is likely small (∼0.1 Sv), but its dilute waters could substantially freshen the Bering Strait inflow and affect the disposition of Pacific waters in the Arctic Ocean. Arctic river outflows should preferentially form surface-advected fronts rather than bottom-advected fronts because vertical-mixing energy is low on arctic shelves. Surface-advected fronts are more susceptible to upwelling winds (and for the SCC, the pressure gradient between the Pacific and Arctic Oceans) than bottom-advected fronts. The SCC never developed in fall 1995 because of anomalously steady upwelling winds. The western Chukchi shelf could have formed upper halocline source water in the winter of 1995–1996.

Lateral mixing across ice meltwater fronts of the Chukchi Sea shelf

Summer and fall hydrographic sections in the northeastern Chukchi Sea frequently capture 5–20 m thick intrapycnocline lenses or horizontal plumes of warm, moderately salty summer Bering Sea Water flowing northward from Bering Strait. These features occur within the shallow (~20 m depth) pycnocline separating cold, dilute, surface meltwater from near-freezing, salty, winter-formed waters beneath the pycnocline. An idealized numerical model suggests that the features arise from eddies and meanders generated by instability of the surface front separating meltwater from Bering Sea Water. Warm Bering Sea Water is transported across the front and into the pycnocline by the cross-frontal velocities associated with the instabilities. The accompanying lateral eddy heat fluxes may be important both in summer for promoting ice melt and in fall by delaying the onset of ice formation over portions of this shelf. Lateral heat flux magnitudes depend upon the stratification of the Bering Sea Water.

Annual sea‐air CO2 fluxes in the Bering Sea: Insights from new autumn and winter observations of a seasonally ice‐covered continental shelf

High-resolution data collected from several programs have greatly increased the spatiotemporal resolution of pCO2(sw) data in the Bering Sea, and provided the first autumn and winter observations. Using data from 2008 to 2012, monthly climatologies of sea-air CO2 fluxes for the Bering Sea shelf area from April to December were calculated, and contributions of physical and biological processes to observed monthly sea-air pCO2 gradients (?pCO2) were investigated. Net efflux of CO2 was observed during November, December, and April, despite the impact of sea surface cooling on ?pCO2. Although the Bering Sea was believed to be a moderate to strong atmospheric CO2 sink, we found that autumn and winter CO2 effluxes balanced 65% of spring and summer CO2 uptake. Ice cover reduced sea-air CO2 fluxes in December, April, and May. Our estimate for ice-cover corrected fluxes suggests the mechanical inhibition of CO2 flux by sea-ice cover has only a small impact on the annual scale (<2%). An important data gap still exists for January to March, the period of peak ice cover and the highest expected retardation of the fluxes. By interpolating between December and April using assumptions of the described autumn and winter conditions, we estimate the Bering Sea shelf area is an annual CO2 sink of ?6.8 Tg C yr?1. With changing climate, we expect warming sea surface temperatures, reduced ice cover, and greater wind speeds with enhanced gas exchange to decrease the size of this CO2 sink by augmenting conditions favorable for greater wintertime outgassing.