The role of local topography and external freshwater sources on the circulation and hydrographic structure of a proglacial fjord in Patagonia.

The observed retreat of the vast majority of glaciers in the Patagonian Ice Fields has been well documented, but the role that the large network on fjords and the adjacent South Pacific ocean is playing in this process has not. Here, the only existing multi-year oceanographic observations from a proglacial fjord in Patagonia are discussed. The site, adjacent to Jorge Montt Glacier (the northern-most glacier in the southern Patagonian Ice Fields) has been retreating for most of the XX century, with increased rates nearing 1 km/year in recent years. Seasonal oceanographic cruises from 2010 to 2015 are used to characterize the evolution of the freshwater plume generated by the glacier melt in tidal to interannual time-scales. High-resolution velocity fields were measured using long-term moorings and a novel, shipboard dual Acoustic Doppler Current Profiler system designed to resolve, simultaneously, the surface fresh layer and the deep, warm ocean inflow. Concurrent microstructure profiles were collected during the surveys to characterize the intensity and spatial structure of the mixing in the fjord. The results show that Jorge Montt (and, is suggested, many other Patagonian glaciers) are particularly sensitive to the variability of surface, not deep, water off the coast of Patagonia, which can reach up to 11 degC. This makes the Patagonian Ice Fields subject to one of, if not the most, warmest ocean forcing in the world. The resulting freshwater discharge from the glacier varies significantly in synoptic to interannual scales, and appears to be leaving the fjord system largely at depth due to the strong surface stratification caused by other sources of freshwater in the region. The role of external freshwater sources and the contraining effect of a shallow sill on the circulation and structure of the outgoing plume are discussed.