Seasonal Evolution of Ocean Heat Supply and Freshwater Discharge From a Rapidly Retreating Tidewater Glacier: Jorge Montt, Patagonia

Abstract Proglacial fjords are critical conduits for the exchange of heat and freshwater between the ocean and land ice, and their dynamics heavily modulate the rate of retreat of tidewater glaciers. Here the magnitude, spatial structure, and seasonal evolution of the ocean forcing on the proglacial fjord off a rapidly retreating glacier (Jorge Montt, Patagonia) are characterized using data from multiple shipboard surveys from 2010 to 2016, a multiyear mooring array, and a weather station. The melting of the glacier is forced by relatively warm (8–11°C) waters entering the fjord over a shallow (45 m deep) sill ≈20 km from the terminus. This warm subsurface water is renewed every summer from the surface layer of Gulf of Penas., is transported along the 100‐km long Baker Channel, and reaches the proglacial fjord in the late austral summer to early fall, where it remains relatively warm until late austral spring. Analysis of the freshwater fractions in the fjord reveals that the total freshwater content has a strong seasonal signal with maximum fractional values in the summer, mostly explained by the seasonality of subglacial discharge from the glacier. The submarine melting fraction has no strong seasonal signal, but is a first‐order contributor to the freshwater content of the fjord in winter. This seasonal evolution is consistent with idealized theories of meltwater production, and suggests that the seasonal supply of heat is critical to sustain a high mean temperature for melting, but does not directly control the melting rate variability.