The Density-Driven Winter Intensification of the Ross Sea Circulation

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 2Sv, 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. Plain Language Summary The cavity beneath an ice shelf (Antarctica's massive floating glaciers) contains some of the most remote and poorly sampled waters on Earth. Yet these waters control the melting and stability of ice shelves and their ability to prevent the accelerated discharge of Antarctica's grounded ice sheets into the ocean. Such catastrophic events would cause global sea level rise. Here computer simulations are used to link together sparse observations and understand the present-day behavior of the ocean beneath the Ross Ice Shelf, the largest ice shelf on Earth. We focus on the seasonal patterns and drivers of ocean currents in the Ross Sea which transport heat from deep in the Southern Ocean to the Ross Ice Shelf. The simulations show that currents are driven by horizontal differences in density induced by rapid winter sea ice formation on a wind-swept, partially open, ocean. Yet in regions of more complete sea ice cover stronger winds reduce the circulation by mixing water masses of different density and smoothing out density differences. Slower currents would carry less heat into the Ross Ice Shelf cavity, reducing melting there. Establishing the circulation's indirect wind dependency adds an important detail in predicting the future of the Ross Ice Shelf.