An Undulating Towed Array (UTA) for high-resolution observations of the upper ocean

David Mark Spencer, Daisuke Hasegawa, Takahiro Tanaka, Zhen LU, Yuji Okazaki and Takeshi Okunishi, Tohoku National Fisheries Research Institute, Japan Fisheries Research and Education Agency, Shiogama, Japan
Abstract:
The upper ocean is a highly dynamic environment comprising complex physical, chemical and biological interactions. Physical oceanographic features such as eddies, boundary currents, and associated fronts can promote high biological activity and generate ecosystem hotspots. Better understanding smaller-scale dynamics of these features and how they affect species distributions will greatly benefit fishery and conservation management. Here we developed a system to detect small-scale (1–100 m) oceanographic structure in the top 300 m of the water column. The UTA system is an array of oceanographic instruments attached to a cable that is winched up and down to collect vertical profiles while the ship moves steadily along a transect at 3-3.5 kts. The current system is made up of JFE-Advantech instruments – 3x multi-sensor RINKO-profilers, 5x DEFI2-CT sensors, 3x DEFI2-D50HG (pressure) sensors – as well as a Marport real-time depth monitor and a V-Wing underwater depressor at the end of the array. We tested this system in the northwest Pacific Ocean at the Tsugaru Warm Current (TWC) and cold Oyashio Current (OY) confluence zone. The series of instruments produced a high-resolution 2D temperature map and resolved fine-scale horizontal and vertical oceanographic features, particularly where the TWC and OY water masses converged. The RINKO profilers were also able to resolve detailed characteristics of important biological parameters, including chl a and DO. However, the slow response time of the DEFI CT thermistor proved problematic for the UTA system when calculating salinity. Future development will involve the use of additional RINKO-profilers and other sensors (e.g. nitrate) to detect high-resolution changes in other parameters that will be comparable to the resolution of the 2D temperature map. Future application will see UTA coupled with a multi-frequency echosounder to relate these parameters to species distributions (e.g. krill and sardines).