Turbulence and Mixing in a Shallow Shelf Sea From Underwater Gliders

Abstract

The seasonal thermocline in shallow shelf seas acts as a natural barrier for boundary‐generated turbulence, damping scalar transport to the upper regions of the water column and controlling primary production to a certain extent. To better understand turbulence and mixing conditions within the thermocline, two unique 12 and 17 day data sets with continuous measurements of the dissipation rate of turbulent kinetic energy (ɛ) collected by autonomous underwater gliders under stratified to well‐mixed conditions are presented. A highly intermittent ɛ signal was observed in the stratified thermocline region, which was mainly characterized by quiescent flow (turbulent activity index below 7). The rate of diapycnal mixing remained relatively constant for the majority of the time with peaks of higher fluxes that were responsible for much of the increase in bottom mixed layer temperature. The water column stayed predominantly strongly stratified, with a bulk Richardson number across the thermocline well above 2. A positive relationship between the intensity of turbulence, shear, and stratification was found. The trend between turbulence levels and the bulk Richardson number was relatively weak but suggests that ɛ increases as the bulk Richardson number approaches 1. The results also highlight the interpretation difficulties in both quantifying turbulent thermocline fluxes as well as the responsible mechanisms.

Measuring the Dissipation Rate of Turbulent Kinetic Energy in Strongly Stratified, Low‐Energy Environments: A Case Study From the Arctic Ocean

“…The platform for our microstructure measurements was the 1,000‐m‐rated Slocum G2 ocean glider Comet , one of the gliders also used by Schultze et al (). The glider samples autonomously in a vertical sawtooth pattern, surfacing at predetermined intervals to establish a satellite link in order to update its position estimate, send low‐resolution flight and hydrography data, and receive updated sampling instructions from an onshore pilot.…”

“…In contrast to this approach, we follow the method of Schultze et al () and use the steady state model of Merckelbach et al () to obtain the angle of attack but then use the measured pitch and pressure to estimate U dynamically using where W is the glider's vertical velocity estimated from the measured rate‐of‐change of pressure. We found that this quasi‐dynamic estimate of U leads to more consistent results between profiles of ε from upcasts and downcasts.…”

“…The 4‐s length of the subsegments passed to the FFT sets the scale for the largest wavelengths (smallest wavenumbers) included in the shear spectrum Φ( k ); it is identical to the FFT length chosen by Fer et al () and Schultze et al (). Given the average glider speeds in Table , a typical FFT calculation includes along‐path wavelengths as large as 164 cm on upcasts and 100 cm on downcasts, resolving the low‐wavenumber transition between the inertial and viscous ranges of the turbulence spectrum (Lueck et al, ).…”

“…In contrast to this approach, we follow the method of Schultze et al (2017) and use the steady state model of Merckelbach et al (2010) to obtain the angle of attack but then use the measured pitch and pressure to estimate U dynamically using Note. is the measured pitch, the estimated angle of attack, the estimated glide angle, and U the estimated speed through water.…”

OceanGliders: A Component of the Integrated GOOS

“…can have a great impact on the ocean circulation/ecosystem state and evolution through their particular structures and transport mechanisms. Other fine scale processes are clearly involved in the ocean mixing, like microturbulence (Fer et al, 2014;Palmer et al, 2015;Schultze et al, 2017) or frontogenesis, filamentation due to stirring or symmetric instability (Figure 11 and Ruiz et al, 2012;Thompson et al, 2014Thompson et al, , 2016Thomsen et al, 2016;Pietri et al, 2013;Brannigan et al, 2017;Buffett et al, 2017;Du Plessis et al, 2017;Pascual et al, 2017;Kolodziejczyk et al, 2018) that can lead to significant vertical velocities and fluxes. However, the extent and variability of their impact over long periods of time still needs to be assessed.…”