Assimilation of range-and-depth-averaged sound speed in Fram Strait

Acoustic thermometry measurements of ocean sound speed were used to improve state estimates of ocean temperature in Fram Strait. This is the first time that large-scale acoustic measurements have been assimilated into an ocean model in the Arctic. From September 2010 to July 2012 the Acoustic Technology for Observing the Interior of the Arctic Ocean (ACOBAR) experiment measured acoustic travel times between Greenland and Spitsbergen. These acoustic tomography measurements were taken along 167-301 km long sections between 3 bottom-mounted moorings. The measurements were inverted to yield time series of range-and-depth-averaged ocean sound speed for 0-1000 m ocean depth.

The ocean sound speed time series was assimilated into a regional numerical ocean model using the Massachusetts Institute of Technology General Circulation Model-Estimating the Circulation and Climate of the Ocean four-dimensional variational (MITgcm-ECCO 4DVAR) assimilation system. The data assimilation improved the range-and-depth-averaged ocean temperatures at the independent 78°50’N oceanographic mooring section in Fram Strait (0-1000 m depth). The RMS error of the ocean state estimate (0.21°C) was comparable to the uncertainty of the interpolated mooring section (0.23°C). The lack of depth information in the assimilated ocean sound speed measurements caused an increased temperature bias at shallow depths (0-200 m). The temporal correlations with the mooring section were not improved because short-term variations in the mooring measurements and the ocean state estimate did not coincide in time. This was likely due to the small-scale eddying and non-linearity of the ocean circulation in Fram Strait. Furthermore, the horizontal resolution of the state estimate (4.5 km) was eddy-permitting, rather than eddy resolving. Therefore, the state estimate could not represent the full ocean dynamics of the region. This study demonstrates the usefulness of large-scale acoustic measurements for improving ocean state estimates at high latitudes.