A 90-m-Tall Neutrally Buoyant Instrument Chain for Measuring Ocean Fine Structure in a Quasi-Lagrangian Reference Frame

Abstract Measurements from Lagrangian floats have enabled many advances in our understanding of small-scale ocean flows and their resulting turbulence. However, they lack the vertical span needed to fully contextualize the flow. As a compromise, a 90-m-tall, neutrally buoyant drifting instrumented system was designed and tested. An adaptive buoyancy engine ensures the system stays within a preset depth range while dangling a densely instrumented thermistor chain and an upward-looking 300-kHz ADCP. Once the system reaches its target depth, the buoyancy engine shuts down. The system then follows isopycnals, drifting laterally and vertically with the mean flow averaged over its vertical extent. This reference frame offers a unique view of the evolving shear and strain fields. Preliminary observations and interpretations demonstrate the system’s usefulness and provide a detailed view of near-inertial and high-frequency internal waves and their estimated mixing. Significance Statement We have built and tested a new instrument consisting of a Doppler current profiler inside a neutrally buoyant float and a dangling vertical string of closely spaced temperature sensors. The system allows investigation of the small-scale breaking waves that lead to ocean turbulence while drifting with the waves as they are swept along with the background flow. Understanding the details of how these motions lead to ocean mixing is important to improving climate simulations.