Floats off the Starboard Bow!

On Tuesday, we deployed 9 EM-APEX profiling floats in a tightly spaced array, all within a 0.4 nautical mile square box. Since then, they’ve been drifting freely as they profile up and down to 200 meters roughly every 90 minutes. These floats have temperature, conductivity, and pressure sensors on them, as well as electrodes that allow us to measure velocity. Roughly half of them also have microstructure instruments, which allow us to measure very small fluctuations in temperature to interpret fine scale motions. Each time they surface, they send back their data and position via satellites so we can monitor them. Since deployment, they’ve drifted apart along a north-south line and are now spaced out across about 4 nautical miles.

Float and ship paths over time. The red triangle that looks like the ship ran it over is probably the float we saw with the spotlight at 3am.

We’ve been following the floats with the ship, moving around and between them as they drift, gathering additional data using our towed instrument. You can see the paths of the floats in the image here (green dots and lines), as well as the ship track (red line). It’s really important to keep track of the most recent float positions relative to the float track to make sure we don’t run them over. Given how tiny the floats are in the vast ocean, this might seem like an unlikely event. Using the spotlights from the bridge at 3 o’clock this morning, however, we spotted two that had surfaced and were reporting to the satellites, so it’s not as improbable as one might think (when preparing the floats pre-cruise, we taped the tops with highly reflective tape, so they show up spectacularly with the spotlight). The good news is that we are gathering extensive data about the region surrounding the floats, and have so far managed to dodge all of the ones that have surfaced near the ship.

Highly-reflective tape on the casing of the microsrtucture instrument. (Photo credit: Rosalind Echols)

 The path the floats are following results from a combination of the mean current of the ocean as they profile, which seems to be generally toward the southeast at the moment, and what are termed “inertial motions”, which is an oscillatory motion that occurs due to the rotation of the earth. We see this show up as periods of speeding up and slowing down in the float positions.