What do we measure and how?
MSS being lifted over
MSS deployed overboard
MSS shear probes
The ocean is vertically layered in density with the lightest water at the top and the densest water at the bottom. We call this ‘stratification’ and the usual scenario one finds in the ocean is for the warmest water to be found at the surface and the coldest water at the seabed as warm water is lighter than cold. However, the density of seawater is also dependent on salinity, with more salty water obviously denser than fresh water as it contains all the water plus lots of salt.
A key issue in the ocean is that water, and more importantly what’s in it, cannot easily move vertically across the stratification, from heavy water to light and vice versa. Of particular interest is the mechanism by which nutrient rich water from beneath the thermocline reaches the surface and triggers plankton blooms. The thermocline is the layer at about 100 metres depth during JR311 across which temperature drops, and density increases, rapidly. It represents strong stratification and the nutrients can’t normally get across it into the sunlit surface layers.
One mechanism by which they may do so however is by eroding that thermocline by turbulent mixing. Turbulence mixes together the water that forms the thermocline, reducing the rapid increase in density with depth and therefore the stratification. The deep water that contains the nutrients is then able to move vertically into the surface layers where the combination of nutrients and light enables photosynthesis, the essential building block of the marine ecosystem. Conversely, the erosion of the thermocline by turbulence allows the downward penetration of heat, which is generally transferred to the ocean by sunlight at the surface. Without the downwards transport of heat, the ocean circulation would be very different so a good understanding of where, when and how the turbulence necessary for achieving this happens is critical to our understanding of the climate system.
During JR311 we measured turbulence frequently with the ISW Microstructure Sensing System, MSS-90. The MSS is a freely falling, loosely tethered probe that falls vertically downwards from the surface to a depth we choose. Data is transmitted back to the ship along the wire that trails behind the MSS and by which we recover it. Whilst the instrument houses a number of sensors to measure temperature, conductivity, oxygen, chlorophyll and turbidity, the important sensors are those that measure the very small scale velocity fluctuations that define turbulence. These are motions occurring on scales of millimetres to centimetres that evolve on timescales of seconds. To measure these rapidly fluctuating but comparatively weak signals requires very delicate and specialised sensors. The MSS is equipped with two identical sensors (called shear probes)to measure the turbulence. They are similar to aircraft wings in that they are designed to generate lift as they experience flow past them. In this case however, the lift caused by the small-scale currents associated with turbulence pushes the probes to one side and changes the voltage measured by a piezo-ceramic beam inside the probe. The bigger the change in voltage, the stronger the turbulence.