Blog entry by Buu-Lik Duong
Turbulence is generally described as an irregular and variable state of flow, in which swirling motions (eddies) of different sizes continuously interact. These eddies range from hundreds of kilometers to a few millimeters passing energy successively to smaller and smaller scales. At the tiniest scales, the so-called microscales, turbulent motion creates sharp gradients of velocity and tracers promoting conditions in which viscous friction becomes effective in transferring the turbulent kinetic energy (TKE) into heat. This process is known as ‘dissipation’. While it may look like small-scale chaos, turbulence has far-reaching consequences: A part of its TKE is used to mix water parcels across density surfaces, through an enhanced vertical transport of heat and salt. These changes of the density structure can shape ocean currents of different sizes – all the way up to the global-scale overturning circulation.
During the Meteor M212 expedition we study turbulence with a vertical microstructure probe (VMP) – a tethered profiling system designed to sink very gently through the water column. The characteristic white brushes on the tail section produce drag and help stabilise the falling speed (0.65 m/s) and the vertical position of the instrument (pitch). At the front of the VMP are highly sensitive probes that can measure the smallest vertical fluctuations in chlorophyll, turbidity, dissolved oxygen, velocity (shear) and temperature at scales of just a few centimeters, essentially catching turbulence in the act!
Secured by a rope and tethered to an electric winch, the VMP is lowered over the stern of the vessel (moving at a speed of 1 knot). As the external sensors are extremely fragile (contact with any solid object must by avoided) and sensitive to any kind of vibration, the deployment requires patience and teamwork: One person operates the winch, adjusting the release speed of the rope. Another person leads it from the spool and makes sure there is always enough slack rope so that the VMP can truly free-fall even if the rope gets jammed or wedged on the winch. A third person keeps track of the cast protocol and of the clock as the instrument’s depth is estimated by its descending time. Once the aimed depth is reached, the instrument is recovered using the winch and directly redeployed. After an ensemble of typically three deployments, the VMP is brought back on deck.
Early in the cruise, we deployed the VMP twice to get a first glimpse of turbulence while approaching the working area of Newfoundland. Then, an approaching raging storm foiled with the expedition’s plan, station work was postponed, and any microstructure work needed to be put on the sideline to regain the lost time. Days slipped by, then weeks – all without the VMP. Approaching the end of the expedition, “MSS” is finally written on the station plan – not only for one or two, but nine locations along a northwestward section at Grand Banks! Our excitement got even surpassed by exploring a new uprising kit consisting of an orange flotation collar, weight-release system, and ballast collar (see photo below), allowing us to pull the VMP to a desired depth and then release it to perform an inverted profile. By doing so, the instrument can acquire near-surface measurements of turbulence. “Does this kit work from a large vessel in the open sea?” was the yet unanswered question. Our reply: “Yes!” Helping hands from all scientists and excellent manoeuvring of the RV Meteor by the nautical officers contributed to a successful acquisition of multiple uprising profiles.
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