Page 60 - Spring2020

Arctic Acoustic Oceanography
Figure 7. The geometry of the 2019–2020 Coordinated Arctic Acoustic Thermometry Experiment (CAATEX). The acoustic transceivers are located at SIO1 and NERSC1 (blue). There are four vertical receiving arrays: SIO2, SIO3, NERSC2, and NERSC3 (orange). The mooring at NERSC4 (green) has conventional oceanographic instrumentation to measure temperature, salinity, and currents. All of the NERSC moorings are in the Nansen Basin south of the Gakkel Ridge; ice conditions prevented deployment further north. The sea ice concentration on October 31, 2019 is from the Advanced Microwave Scanning Radiometer 2 (AMSR2) dataset provided by the University of Bremen (Spreen et al., 2008). Available at acousticstoday.org/sea-ice.
Twenty years later, the joint US-Norwegian Coordinated Arctic Acoustic Thermometry Experiment (CAATEX) was deployed in late summer 2019 to repeat the basin-scale mea-
surements made during the TAP and ACOUS experiments, allowing present-day basin-scale heat content to be compared with that in the 1990s. Six moorings were installed, providing both basin-scale and shorter range measurements (Figure 7). Two moorings (one each in the eastern and western Arctic) are transceivers. Each has a 35-Hz acoustic source at 60-meter depth (Figure 8) and a vertical receiving array extending down below the source. Four moorings have only vertical receiving arrays. The sources are programmed to transmit every three days. The moorings are scheduled to be recovered in late summer 2020.
Fram Strait
Fram Strait, with a width of nearly 400 kilometers and a sill depth of ~2,600 meters, is the only deepwater connection
between the Arctic and the rest of the oceans in the world (Figure 9). The northbound West Spitzbergen Current on the east side of the Strait transports relatively warm and salty Atlantic water into the Arctic, forming the Atlantic Layer. The southbound East Greenland Current on the west side of the Strait transports sea ice and relatively cold and fresh polar water out of the Arctic. Significant recirculation of the Atlantic water and intense mesoscale variability in the center of the Strait make it difficult to accurately measure ocean transports through the Strait.
A series of tomographic experiments have been conducted in Fram Strait to determine whether or not integral acous- tic measurements, when combined with other data and ocean models, can provide improved estimates of the transports of the inflowing and outflowing water masses.
A preliminary engineering test in 2008–2009 as part of the Developing Arctic Modeling and Observing Capabilities
Figure 8. An ultralow-frequency source assembly (f0 = 35 Hz; Δf = 4 Hz) on the US Coast Guard Cutter (CGC) Healy during deployment in the Canada Basin during September 2019. The source transducer, developed by GeoSpectrum Technology, Inc., is mounted at the bottom of the frame. It is approximately 1 meter in diameter and 0.2 meter thick. Above it are high-pressure gas bottles for the pressure- compensation system needed to keep the internal gas pressure equal to the external water pressure. The pressure compensation system itself as well as the source controller, power amplifier, and batteries are on the opposite side of the frame. Photo by Lee Freitag, Woods Hole Oceanographic Institution, Woods Hole, MA.
60 | Acoustics Today | Spring 2020