Emboldened by the smoothness of glider calibration casts on the 5th and 6th of May and hurried by weather reports of a looming storm, we took advantage of the close proximity of the gliders to perform the remaining two glider calibrations back-to-back on the morning of 7 May. SG143's CTD calibration cast took place first, at a distance of ~200m from the glider and with SG143 spending a total of 18 minutes on the surface. It was a little tricky because 143 has been diving faster than the other gliders, and popped up after its medium-length set-up dive half an hour early -- during breakfast. Then SG141 was late to surface, but it didn't cover much ground in its set-up dive, so we were fairly well in place when it finally came up for it calibration. We began the last CTD cast once we made visual contact with SG141 at ~300m, and it spent a little over half an hour on the surface.

The first, most troublesome issue these measurements resolve is the apparent offsets in temperature readings between the gliders:

SG140 reads ~1.623 degrees Celsius below, and SG141 reads ~1.721 degrees above. Note that the measurements taken by the CTD are closest in time and space to the descending portion of the glider's dive -- by the time the glider beings its ascent, it is nearly 3 hours and 3 kilometers away, and when it surfaces it is typically 6 hours and 6 kilometers away, which could be on the other side of a front seeing an entirely different water mass, or might be seeing early-afternoon thermal stratification at the surface. Still, at these scales we expect variability on the order of half a degree Celsius in temperature, not one and a half.

The other issue that is being resolved is the question of the "blips" in chlorophyll fluorescence that occur below 200m.

Unlike the gliders, which sample more infrequently at depth in order to conserve energy, the CTD profiles continuously. Here, the blips appear as spikes in both chlorophyll fluorescence and backscatter:

Originally, it was thought that the blips might be a result of interference between the two fluorometers on SGs 141, 142 and 143. It is still possible that there is some interference which heightens the effect, so more analysis there won't hurt. But those blips in both fluorescence and backscatter occur on SG140, where there is only one fluorometer, which is certainly promising:

The SG140 dive illustrated above ended 3 hours before and ~8km NE of the ship's location during the CTD cast. We suspect that the "blips" or "spikes" at depth are sinking phytoplankton, possibly nutrient-starved aggregates. Katherine Richardson and her graduate student will likely be able to tell what those spikes are from the water samples that are currently being taken.