In the ocean, the water column is characterized by many parameters such as density, temperature, salinity, number of particles or chlorophyll, pressure… All these parameters are modulated by local currents. Therefore, it is necessary to collect data at certain fixed positions to study the properties of the water column. For this purpose, two specific measuring devices have been deployed at common locations to collect information on the water column.
Vertical Microstructure Profiler (VMP):
The VMP is composed of several probes to realize high frequency measurement (64 Hz) of the pressure, shear and temperature in the water column. It is inserted vertically into the water column with the probes pointing downwards. A profile consists of a round trip of the VMP between 0 and 1000 m depth. It is introduced “by hand”: several people take turns to lower and raise the device with the help of a rope. This exercise is more physical than it looks!
High frequency measurements allows to see very small perturbations of the physical properties and with some processing to retrieve the turbulent kinetic energy and energy dissipation rate. These turbulent scales notably produce local density inversions, then not in agreement with the broad hydrostatic equilibrium (in which case density should only increase downwards). The higher the sampling frequency, the smaller the resolved scales and turbulence. Depending on the kinetic energy input, 64 Hz usually enables to see as small as viscous dissipation scales.
CTD (Conductivity, Temperature, Depth):
In addition to the VMP measurements, CTD measurements are carried out. As the VMP, they allow to record the pressure, temperature or salinity in the water column and a profile consists of a round trip of the device in the water column. The CTD can go deeper than the VMP and is also equipped with bottles (up to 12) dedicated to taking water samples at specific depths. These will provide additional information on particle and chlorophyll content as a function of depth. Here an ADCP is attached to the CTD probe and also allows to measure the ocean currents along the cast.