All four gliders are working well. We have just completed two calibration CTD casts on the R/V Knorr, one each between 9-10am local time (UTC) on the mornings of May 5th and 6th. We lucked out and got a bonus on the calibration cast for SG142: there's a tantalizing feature down at 300m that illustrates nicely the physical processes that contribute to the carbon pump.

The calibration casts required coordination between Craig Lee in Seattle, who was responsible for setting the glider to be calibrated in a series of sequentially shorter dives, and myself on the Knorr for organizing the ship-side effort and piloting from the onset of the last pre-calibration dive to the successful resumption of its diving mission. We tried using the ship's VoIP phone through WHOI -- it cut out precisely when we said anything important. Email turned out to be the best method of communication between ship and land.

The piloting part here in the North Atlantic was easy, thanks to the great setup by Craig! The hard part was predicting the glider's next surfacing location and time, communicating that to the bridge, and then getting the ship downwind of the glider and far enough away that we wouldn't hit it while still being close enough to make visual contact in a few minutes. I was hoping for 200-300m to make a good calibration. On SG140's penulatimate surfacing, the ship was 7km away, not the half-nautical mile I'd intended, but with 45 minutes' warning I was able to get a better estimate for the final surfacing and the Mate had enough time to get there. We were on station and steamed slowly forward for a few minutes -- it took 10 minutes from the initial GPS fix to making visual contact and then we moved the ship a little closer. In the end, SG140 spent about 41 minutes on the surface, during which time it drifted so that its calibration dive started 564m away from the ship.

That night I stayed up and fixed a couple bugs in my code. We shut off the ship's thrusters during the glider's surfacings as a safety precaution, and that turned out to be a good idea: for SG142's penultimate pre-calibration surfacing, the ship was only 900m away; for the final surfacing, the ship was 200m away with the glider right off the bow. The captain spotted it in the first couple of minutes and SG140 was able to resume diving, having spent only 17 minutes on the surface.

On the way to SG140's second-to-last pre-calibration surfacing, the ship crossed a front, marked by a half-degree drop in surface temperature. A very interesting feature showed up in the calibration cast:

After the calibration cast, we took more water samples around that 300m feature, and found a healthy phytoplankton community composed of species that were not found at the surface. Because the phytoplankton at depth still had fat chloroplasts, we suspect that they had been in the surface water exposed to sunlight fairly recently. The increased oxygen content of the water also suggests that it was subducted from the surface to that depth -- probably at the front we crossed. The big question now is whether that feature can be seen by SG142 with its custom, sensitive fluorometer.