Tumbling along like a tumbling tumbleweed

Ok, so the title might just have outed my love for all things Lebowski, but hopefully I will be able to make a link, irregardless of how tenuous it might be, between tumbleweeds and and the subject of this post, drifters.

First things first, we have to remind ourselves what is a drifter, and how does it differ from a float. First, drifters float and floats sink, yes, this is correct, floats sink, drifters don’t. Drifters, or more precisely, passive Lagrangian drifters (PLD), are the kind we deploy during NAAMES in the North Atlantic. These are basically big bobbers which are “anchored” to the surface ocean by an amalgamation of fabric (at times brightly colored in pink camouflage), metal and plastic that is called a “holy sock drogue.” Maybe a picture will help, with the visual aid of me in the holy sock drogue.

Ok, so what do these drifters do? Think of them as a breadcrumb trail that we drop and leave behind the boat. We drive this big ship through the ocean and choose what might seem to be random spots to stop and sample. The truth is these spots are not random. They are the result of the integration of lots of satellite observations, and at times, data we get from the floats (that sink), all which help us decide where we want to sample. Once we arrive “on station”, we drop three of these drifters. I’ll go into details as to why we drop three of them later, but for now, think of this as the breadcrumbs we drop to mark the water mass where we have sampled. Since the ocean is always moving, the drifter moves along with the surface layer of the ocean and allow us to follow them in time in order to stay with the same “water mass” that we initially sampled. During the NAAMES project, we also have a big airplane that samples not only the air, but also shoots a laser into the ocean to measure phytoplankton from the surface to about 50 m. Using the drifters, the plane can sample the same water mass that we sampled with the ship a few days prior.

So why drop three of the $3,000 drifters at once you may ask? Well, these drifters allow us to measure something that is actually really hard to measure in the ocean, that is diffusion. Diffusion is basically how fast do things spread apart, and how does this “rate of spreading” change based on where we dropped the drifters. Together with other members of the (Sub)mesoscale Group, we will use the rate at which these drifters spread in time to estimate how mesoscale eddies influence the diffusion of the oceans surface waters. To do this, we get hourly updates on the positions of all the drifter and compute the evolution of the area between the three drifters over time. All this fun stuff won’t get analyzed for some time as we need to allow enough time for all the drifters to spread apart.

In addition to dumping drifters on station, Ali and I have designed a few diffusion experiments. We deployed (fancy word for dumped) triplicate drifters: outside of eddies, on the edge of eddies, at the point in the eddy where currents are at a maximum, and at the eddy center. This will allow us to compute how eddies influence diffusion and compare how the impact of these eddy varies between cyclones and anticyclones (i.e., the cold and warm core eddies).

But enough with all this science, we also have fun. It’s cool to deploy drifters as it’s a neat feeling to put out an instrument that talks with a satellite every hour for the next few years! For example, we deployed drifters in the middle of a warm core ring, which is a really big and strong eddy that is shed from the Gulf Stream (see some details about our warm core ring project here). One of these drifter stayed in the ring for a few days making larger loops around the eddy. In addition, superimposed on the big loops, were small little loops that are the result of what we call inertial motions. Basically, because the Earth rotates, surface water in the ocean oscillates in little circles all day, every day. After a few days, the drifter was spit out of the eddy and started heading clear across the Gulf Steam. The resulting drifter track was a very pretty pattern reminiscent of a rose.

To wrap up, drifters drift, just as the proverbial tumbleweed, allowing us to follow a mass of water to sample with the ship and plane. Sometimes the patterns made by the drifters are really pretty, like our rose.

Written by Peter Gaube