Given our capability to ballast RAFOS floats to drift on a specific isopycnal surface in the main thermocline, an obvious application that came up was to extend this skill to measure stretching vorticity or local stratification at the float. The approach was very simple: change the volume of the float ever so slightly so it would float up and down to measure the depth of neighboring isopycnal surfaces to get their vertical separation.
To do this Jim Fontaine and Skip Carter developed a small motor-driven volume changer (vocha) that under microprocessor control would repeatedly first increase, and then symmetrically decrease the volume of the float by the same amount, causing the float to first equilibrate on a shallower surface and then on a deeper surface before returning to its resting surface. We learned a lot from this first study.
The vocha and the microprocessor control worked as planned. The lower panel of the figure shows how the float first ascends then then descends toward neighboring isopycnal surfaces. The plot shows that we didn’t allow enough time for the float to equilibrate before starting the next phase of the measurement cycle. In retrospect, we should have given each step more than enough time and then based on these first results decide how much to shorten the steps. Above all the time to get to the lower level was much too short. The reason for cycling through the up-down steps as quickly as possible was to minimize the effect of vertical shear: to get the float back to where it would have been had it not made these vertical excursions. During the 5-day study the float crossed the Gulf Stream to warmer waters such that the isopycnals deepened by about 250 m.
The upper panel shows temperature. The parking (or middle) temperature increased from 8 to 8.5°C, but the main change is between hours 60 and 80 so I interpret this as a drift into water with slightly different T/S characteristics on that isopycnal. This didn’t happen for the top and bottom temperatures. During these ~200 m excursions the temperature ranged from 10.5 to 7°C indicating a 3.5/200 = 0.0175°C/m stratification.
When this study was done, in 1988, the bandwidth for data transmission was limited. Today, with Iridium, the float can collect and transmit vastly larger volumes of data. One can imagine equipping the float with other sensors to zoom in on local property variations such as light level, oxygen, local backscatter, even photographs!
This paper gives a detailed report on that study.
Rossby, T., J. Fontaine and E.L. Carter, Jr. The f/h float-measuring stretching vorticity directly. Deep Sea Res., 41, 975-992, 1994.