Glacier meltwater transport and mixing in Jorge Montt fjord, Patagonia

Abstract:

Quantifying the meltwater discharge from retreating glaciers, and understanding the ocean processes that influence that discharge is a key step to improve projections of sea level rise. Here, oceanographic observations collected near Jorge Montt, a rapidly retreating glacier in the Southern Patagonian Ice field, are used to (i) quantify the seasonal changes in the freshwater discharge from the glacier and the relative contribution of meltwater and runoff to it; (ii) to understand the relative role that un- or underresolved circulation processes, including tides, have on estimates of the freshwater discharge components; and (iii) to characterize the mixing processes which help explain the spatial distribution of the freshwater discharge components along the fjord. In order to quantify the freshwater discharge from the glacier, we conducted tidally-resolving, cross-channel surveys of current velocity and hydrographic profiles during summer 2013 and winter 2014. The current velocity observations were collected using a 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. Additionally, along-shelf fjord surveys of water properties were conducted to provide context for the cross-channel surveys. In the context of recent idealized theoretical and numerical model work, the results highlight the importance of including ambient stratification as well the far field temperature and subsurface runoff as key parameters to understand the structure and magnitude of the freshwater outflow. They also show that freshwater (and therefore, meltwater) discharge estimates can have large biases or errors when using observations that are not tidally-resolving or that exclude the often relatively thin, fresh surface layer present in these systems.