Multi-Month Dissipation Estimates Using Microstructure from Autonomous Underwater Gliders

Abstract

Ocean turbulence is inherently episodic and patchy. It is the pri- mary mechanism that transforms water mass properties and drives the exchanges of heat, freshwater, and momentum across the water column. Given its episodic nature, capturing the net impact of turbulence via direct measurements requires sustained observations over extended temporal and/or broad spatial scales. Seagliders, autonomous platforms that steer through the water column by controlling pitch, roll, and buoyancy, glide smoothly from the ocean surface typically to 1,000 m depth and back while collecting profiles of temperature, salinity, and other oceanic vari- ables. Mission duration depends largely on ambient stratification and profile depth; some Seaglider missions have profiled continuously for more than nine months while others operate from ships for shorter periods. Based on measurements collected using a fast accelerometer logger during a mission in the Kuroshio, a strong Pacific western boundary current, Seagliders show very little vibra- tion during most of their profile. Platform accelerations are comparable to free-falling microstructure instruments and do not affect microstructure measurements. Similar results have been obtained from Slocum gliders (Wolk et al., 2009). Speeds through the water are about 0.3–0.4 m s¯¹, predominantly in the horizontal direction, allowing a glider to cover distances of about 20 km per day while occupying regular patterns. Their speed and minimal vibration as they move through the water make gliders desirable platforms for carrying turbulence sensors.