Oceanographic Variability in Cumberland Bay, South Georgia, and Its Implications for Glacier Retreat.

South Georgia is a heavily glaciated sub‐Antarctic island in the Southern Ocean. Cumberland Bay is the largest fjord on the island, split into two arms, each with a large marine‐terminating glacier at the head. Although these glaciers have shown markedly different retreat rates over the past century, the underlying drivers of such differential retreat are not yet understood. This study uses observations and a new high‐resolution oceanographic model to characterize oceanographic variability in Cumberland Bay and to explore its influence on glacier retreat. While observations indicate a strong seasonal cycle in temperature and salinity, they reveal no clear hydrographic differences that could explain the differential glacier retreat. Model simulations suggest the subglacial outflow plume dynamics and fjord circulation are sensitive to the bathymetry adjacent to the glacier, though this does not provide persuasive reasoning for the asymmetric glacier retreat. The addition of a postulated shallow inner sill in one fjord arm, however, significantly changes the water properties in the resultant inner basin by blocking the intrusion of colder, higher salinity waters at depth. This increase in temperature could significantly increase submarine melting, which is proposed as a possible contribution to the different rates of glacier retreat observed in the two fjord arms. This study represents the first detailed description of the oceanographic variability of a sub‐Antarctic island fjord, highlighting the sensitivity of fjord oceanography to bathymetry. Notably, in fjords systems where temperature decreases with depth, the presence of a shallow sill has the potential to accelerate glacier retreat. Plain Language Summary: Cumberland Bay, a fjord on the sub‐Antarctic island of South Georgia, is split into two arms, each with a large marine‐terminating glacier. One of these glaciers is retreating much faster than the other, possibly due to differences in oceanography between the arms. Here, we investigate how the oceanography in Cumberland Bay varies seasonally and with the seabed depth by using oceanographic data and numerical ocean simulations. We find that the properties of buoyant plumes, which arise from meltwater entering the ocean from beneath the glacier, are sensitive to the seabed depth near the glaciers, resulting in strong differences in ocean flow between the fjord arms. Assuming higher ocean temperatures increase glacier melting, we find that the presence of a near‐glacier shallow sill likely increases melting by blocking deeper, colder waters and trapping warmer surface waters close to the glacier. As a shallow sill is likely to be present near the rapidly retreating glacier only, this result offers a persuasive explanation for the observed glacier retreat. Understanding the variability in oceanography and glacier retreat is important as they directly impact the marine ecosystem at South Georgia by influencing the availability of nutrients for primary production and food availability for higher predators. Key Points: Observational data and a new high‐resolution model are combined to describe oceanographic variability in Cumberland Bay, South GeorgiaWe show that the properties of buoyancy‐driven outflow plumes are sensitive to bathymetry and drive spatial differences between fjord armsWe highlight that the presence of a postulated inner sill may be a key factor in the observed rapid glacier retreat [ABSTRACT FROM AUTHOR]