Using Acoustic Travel Time to Monitor the Heat Variability of Glacial Fjords

Monitoring the heat content variability of glacial fjords is crucial to understanding the effects of oceanic forcing on marine-terminating glaciers. A pressure-sensor-equipped inverted echo sounder (PIES) was deployed midfjord in Sermilik Fjord in southeast Greenland from August 2011 to September 2012 alongside a moored array of instruments recording temperature, conductivity, and velocity. Historical hydrography is used to quantify the relationship between acoustic travel time and the vertically averaged heat content, and a new method is developed for filtering acoustic return echoes in an ice-influenced environment. We show that PIES measurements, combined with a knowledge of the fjord's two-layer density structure, can be used to reconstruct the thickness and temperature of the inflowing water. Additionally, we find that fjord-shelf exchange events are identifiable in the travel time record implying the PIES can be used to monitor fjord circulation. Finally, we show that PIES data can be combined with moored temperature records to derive the heat content of the upper layer of the fjord where moored instruments are at great risk of being damaged by transiting icebergs.