Saturday, March 11, 2017

Looking at Data, Episode 1

After approximately five days of surveying our present area, following the floats and buoy, we’ve been able to gather some really interesting preliminary data. Today we’re going to talk about one particular characteristic of the float data that has caught our attention. As mentioned previously, each of the floats is freely drifting with the currents, profiling from approximately 200 meters depth up to the surface. On the way up, the floats gather temperature, conductivity, pressure, and velocity measurements. The conductivity is then converted into a value for salinity, or how “salty” the water is.  

In much of the world’s ocean, some portion of the top of the ocean gets thoroughly mixed by atmospheric forcing, and so up to some depth, the temperature and salinity remain relatively constant. This portion of the ocean is referred to as the mixed layer and this description is most true for regions or times with regular strong mixing events (such as a storm) without a lot of heat or freshwater input. For example, in the North Atlantic Ocean in winter, where there are incredibly strong storms with relatively weak shortwave radiation from the sun, the mixed layer can be several hundred meters deep. However, in regions where there is a daily cycle of intense solar radiation or extreme rain events under low wind conditions, there is very little mixing and so the ocean can be substantially stratified all the way up to the surface because substantial changes to the temperature or salinity are occurring at the surface that can’t be mixed down.
Depth profile of temperature (red), salinity (blue), and density (green). Notice the twists and turns in each profile between 0m and 50m, suggesting substantial changes in each quantity. 

 Climatological (average) data for the region we are currently in suggests that there should be a winter mixed layer on the order of 100 meters deep. However, the float data we have gathered thus far shows substantial changes in both temperature and salinity beginning from the near-surface. The profile shown here illustrates that (temperature is in red, salinity in blue, and density of the water in green). If there was a well-defined mixed layer, we would see the top section of each of these profiles almost completely vertical and uniform. What we actually see here is that the top 50 meters has variations in all three variables. If you look carefully, you can see that between roughly 50 and 125 meters there is a somewhat uniform section (or at least less variation), which could be the remnants of a previous mixed layer on top of which other effects have been superimposed.

Determining the exact cause of this pattern of surface stratification is difficult, and will certainly require more in depth analysis of our data from multiple sources. Increased temperature at the surface (which appears regularly in different profiles) could be due to strong solar radiation effects (which penetrate some distance into the water column) that are not being fully mixed in, for example. It seems unlikely that this is happening in this case because the near-surface temperature doesn’t show the exponential decay associated with radiation (because the floats typically stop measuring around 10 meters depth, we can’t rule it out yet). Another possibility is that the float is moving between different water masses as it profiles due to differences in horizontal velocity at different depths. A third option is that these variations could also be due to interleaving of pre-existing parcels of water from the type of restratification event we are looking for. At this early stage, it is hard to know for sure.

One step in the float retrieval process. Making sure the instruments at the top don't hit the side of the ship is of paramount importance. (Photo credit: Rosalind Echols)
Today we’re picking up the buoy and floats and heading north. 70 and sunny has been a lovely respite from winter weather, but in order to find the kinds of mixing events that will allow us to observe restratification processes in real time, we need to go in search of a front near some more interesting weather.

Bringing the buoy alongside the ship, while protecting the sensors at the top. (Photo credit: Rosalind Echols)

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