No abstract

We present the first densely-sampled hydrographic survey of the Amundsen Sea Polynya (ASP) region, including a detailed characterization of its freshwater distributions. Multiple components contribute to the freshwater budget, including precipitation, sea ice melt, basal ice shelf melt, and iceberg melt, from local and non-local sources. We used stable oxygen isotope ratios in seawater (d 18 O) to distinguish quantitatively the contributions from sea ice and meteoric-derived sources. Meteoric fractions were high throughout the winter mixed layer (WML), with maximum values of 2-3% (±0.5%). Because the ASP region is characterized by deep WMLs, column inventories of total meteoric water were also high, ranging from 10-13 m (±2 m) adjacent to the Dotson Ice Shelf (DIS) and in the deep trough to 7-9 m (±2 m) in shallower areas. These inventories are at least twice those reported for continental shelf waters near the western Antarctic Peninsula. Sea ice melt fractions were mostly negative, indicating net (annual) sea ice formation, consistent with this area being an active polynya. Independently determined fractions of subsurface glacial meltwater (as one component of the total meteoric inventory) had maximum values of 1-2% (±0.5%), with highest and shallowest maximum values at the DIS outflow (80-90 m) and in iceberg-stirred waters (150-200 m). In addition to these upwelling sites, contributions of subsurface glacial meltwater could be traced at depth along the ~ 27.6 isopycnal, from which it mixes into the WML through various processes. Our results suggest a quasi-continuous supply of melt-laden iron-enriched seawater to the euphotic zone of the ASP and help to explain why the ASP is Antarctica's most biologically productive polynya per unit area.

The circulation over the Ross Sea continental shelf facilitates the exchange between the Southern Ocean and the Ross Ice Shelf cavity. Here transport and mixing processes control the access of oceanic heat from the Southern Ocean to the ice shelf base, the formation of sea ice, and the production of High Salinity Shelf Water (HSSW) in polynyas and hence the subsequent formation of Antarctic Bottom Water. A climatological ocean‐ice shelf coupled model of the Ross Sea Sector including the cavity, with prescribed sea ice fluxes, was used to examine the details of currents and their seasonal variability over the continental shelf. A system of two cyclonic and three anticyclonic persistent circulation features has been identified. Transports steadily increase throughout winter, with individual currents carrying up to 2 Sv, nearly doubling their minimum. The seasonal modulation is driven by lateral differences in density and subsequent baroclinic pressure gradients, induced through dense shelf water formation in the Ross Sea and the Terra Nova Bay Polynas. Wind plays a minor role in ocean momentum variability. Sensitivity experiments suggest a weakening of transports with increasing wind stress. Horizontal density variations at the ocean surface are smoothed by the wind. The source of momentum in the cavity is the gravity‐driven bottom flow of HSSW, produced in the Ross Sea Polynya as part of the thermohaline overturning circulation. Tracer experiments suggest that HSSW forming in the Terra Nova Bay Polynya has no cavity access, but instead is the main contributor to Antarctic Bottom Water formed in the northwest slope.