Variability in the Distribution of Fast Ice and the Sub-ice Platelet Layer Near McMurdo Ice Shelf

Variability in the volume of supercooled Ice Shelf Water outflow in McMurdo Sound is reflected in the thickness and distribution of fast ice and the sub-ice platelet layer beneath. Ground-based electromagnetic induction and drill hole surveys of the distribution and thickness of ice shelf-influenced fast ice and the sub-ice platelet layer in McMurdo Sound were carried out in late spring of 2011, 2013, 2016, and 2017. In 2011 and 2017, thicker sub-ice platelet layers of up to 7.5 and 6 m were observed, respectively. Fast ice formation throughout the winters of 2011 and 2017 was influenced by a higher occurrence of strong southerly wind events and resultant activity of the Ross Sea Polynya. In contrast, lower wind conditions in 2016 led to largely undisturbed sea ice growth and anomalously extensive fast ice coverage. A thinner sub-ice platelet layer of up to 4 m was observed in 2016. In 2011 and 2017, substantial and variable sub-ice platelet layers were detected in a region of exchange of water masses between the Ross Sea and the McMurdo-Ross ice shelf cavity, which were not observed in 2013 and 2016. We hypothesize that a higher frequency of strong southerly wind events, resultant polynya activity, and High Salinity Shelf Water production over winter accelerates circulation and increases melting in the proximal shallow McMurdo Ice Shelf and the deeper Ross Ice Shelf regions of the conjoined cavity. The outflow of supercooled Ice Shelf Water and sub-ice platelet layer formation in McMurdo Sound are consequently promoted. Plain Language Summary Ocean-driven basal melting of ice shelves forms cold and fresh Ice Shelf Water (ISW) which can crystallize into platelet ice. Sea ice growth near an ice shelf is enhanced as the upper ocean is cooled and stabilized by ISW or by freezing of platelet ice into the sea ice. An unconsolidated mass of platelet ice called a sub-ice platelet layer (SPL) can form beneath. To investigate variability in the influence of ISW, we measured sea ice and SPL thicknesses near McMurdo Ice Shelf in the Ross Sea, Antarctica with drill hole and geophysical surveys. Thicker SPLs were observed in years subject to a higher occurrence of strong offshore winds over winter as the sea ice was forming. Strong winds blow sea ice offshore and form regions of open water called polynyas. New ice is produced intensively within polynyas and the rejected salts form a very dense and saline water mass called High Salinity Shelf Water which can circulate into ice shelf cavities and drive basal melting and ISW formation at depth. We hypothesize that a higher occurrence of strong offshore winds results in more polynya activity, High Salinity Shelf Water production, and ISW formation which outflows into McMurdo Sound producing thicker SPLs. Key Points Ice shelf-influenced sea ice and the sub-ice platelet layer were assessed with electromagnetic induction and drill hole thickness surveys The sub-ice platelet layer was thicker in years with more strong southerly wind events and polynya activity over winter Ice Shelf Water outflow and sub-ice platelet layer thickness in McMurdo Sound are connected to polynya-driven HSSW circulation over winter