Friday, March 24, 2017

Looking at Data, Episode 3 (also: Suprises!)

One of the joys of science is the opportunity to be surprised. We are approaching the world with curiosity and questions, and looking for ways to make sense of what we observe. As much as we may understand about the universe from centuries of careful study, every year scientists collectively discover unexpected and exciting new things, and this kind of discovery is what motivates us to spend a month in an anything-but-stationary floating lab. It also requires a degree of humility and open-mindedness that can be difficult at times, yet is essential to the successful execution of an experiment.

During our second deployment, we eventually reached the point where we had the buoy with its chain of dangling instruments, 15 EM-APEX floats, and the ship towing SWIMS all surveying simultaneously, in addition to periodic CTD casts from the ship and a continuously running ADCP mounted on the ship. This may seem like an extraordinary amount of equipment to be directed towards the same small area, but because each instrument has its own particular set of advantages and limitations, the way we learn the most about what is happening is by utilizing them all simultaneously.
The final step in deploying the buoy is lifting the surface structure with the A-frame while all the subsurface instruments hang in the water below. Definitely a team effort. (Photo credit: RE)
Both the buoy and the floats are characterized as “Lagrangian”, meaning that they are intended to drift with a particular water mass. With the buoy, the X-wings on the chain are intended to facilitate this, and therefore the expectation is that the buoy will nominally follow the water mass at the depth of the X-wings. The floats are a little more difficult to predict, because they are continuously moving up and down, passing through shear (different velocities at different depths), and so they may not be tracking one specific water parcel. (This is one of the things we also need to keep in mind when looking at the data: differences in the density measured by a float may be due to temporal changes, spatial changes, or simply the fact that the float could be entering and exiting different chunks of water). In any event, observing how the movement of these two platforms evolves is always a matter of great curiosity.
Bluer values of salinity represent fresher water, while redder
values represent saltier water.
Over the course of approximately 7 days several things happened which surprised us all. By about halfway through, the floats had moved in a vaguely semi-circular pattern and we started joking that it would be funny if they turned again and ended up back where they started. Well, they did. Almost exactly. In fact, the full pattern (as seen in the images) describes a shape that looks somewhat like a carabiner used in rock climbing. The figures included here show a couple different properties measured by the floats, all for a depth near 20 meters: temperature, salinity, and horizontal velocity. Additional work will be necessary to refine these data, but even in a rough form changes in velocity (indicated by the length of the arrows), temperature, and salinity are visible, as well as the fact that they came back to the start!
Notice the warm-ish water in the upper left corner and
the much colder water to the right. 

Even more surprising, the buoy also followed almost the same track. At one point, it continued to head south on its own and we almost started taking bets about whether it would actually keep going that way and have to be rescued or turn and follow the floats. (It’s hard not to anthropomorphize in these moments. Will the buoy say, “Hey guys, wait for me!” and start going north?) To our surprise, it took an abrupt left turn and started following the floats north again. What must be happening with the velocity field for this to happen? (Having the buoy so closely track the position of the floats is incredibly informative, since it has both subsurface ADCPs that measure currents but also has a suite of meteorological instruments mounted on top, which allow very localized measurements of rain, wind, and heat fluxes.)
In looking at the velocity, it almost looks as though it goes in phases: alternating faster and slower periods. 

In the grand scheme of things, these are fairly small surprises, but they point to the truth that unexpected things can happen any time in science. Hopefully, we will find even more interesting features in the data as we dive deeper.

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