Bi-stability of the Filchner-Ronne Ice Shelf Cavity Circulation and Basal Melt Rates

The Filchner-Ronne Ice Shelf (FRIS) is Antarctica's largest floating glacier. Circulation and water mass transformation within the FRIS cavity create precursors to Antarctic bottom water, which in turn closes the global overturning circulation. The FRIS currently maintains relatively low levels of melt, but previous studies using ocean models forced with atmospheric states derived from future climate projections have indicated that warm water intrusions may induce an order-of-magnitude increase in basal melt during the coming century. In these studies, a return to present day climate does not return the FRIS melt rates to present day levels. However, it remains unclear what properties of the near-Antarctic climate control the access of warm water to the FRIS cavity.

In this study we develop a regional model of the southern Weddell Sea to explore FRIS circulation under a wide range of atmospheric forcing regimes. Experiments with modified initial cavity conditions reveal that the cavity circulation is bi-stable: for the same atmospheric state, the FRIS cavity can stabilize in either a ``warm’’ or ``cold’’ state, with an order of magnitude difference in basal melt rates between the two. We then investigate the wind-driven sensitivity of the FRIS cavity to idealized climate changes, which reveal that relatively modest perturbations to the katabatic winds can shift the FRIS cavity between the ``warm’’ and ``cold’’ states. Our experiments further indicate that the ``warm’’ and ``cold’’ FRIS states occur when modified Circumpolar Deep Water (mCDW) and High Salinity Shelf Water (HSSW) occupy the cavity, respectively. We pose a conceptual model in which the FRIS cavity state is determined by whether mCDW or HSSW is denser, and thus floods the cavity; these states are bi-stable because there is a feedback between the basal melt rate and the salinity of HSSW. Our findings imply that Antarctica’s major ice shelves may experience rapid changes in melt if certain climatic thresholds are exceeded, and that such changes would strongly inhibit a return to present-day melt rates. Alternatively, existing cold, dense water masses within ice shelf cavities could buffer against future intrusions of warm water and acceleration of Antarctic mass loss.