The year 1954 is an important one in oceanography. That was when John Swallow started his work with drifting subsurface floats. As the float drifted at depth it sent out acoustic pings when enabled the ship to keep track of its motions. Today we might call these isobaric floats for they drifted at constant depth. It would be more than 3 decades before the isopycnal float became a reality (see my post Aug 17, 2023). We have so much about oceanic movements with these instruments for they reveal is marvelous detail what the ocean is doing – be it motion along fronts, in planetary waves, or coherent vortices, say. But there are situations along coasts where on- and offshore winds cause vertical diapycnal motions. How might we be able to visualize these?
This was the challenge my colleagues Dave Hebert and Mark Prater undertook together with Jim Fontaine. Imagine for a moment you want to study internal waves with a float drifting at constant depth. You equip the float with slanted vanes so it will rotate as water moves up and down past the float. The rotation is monitored thanks to the earth’s magnetic field. Doug Webb built such a float already in the 1960s. But if you were to put vanes on a float riding on a density surface, what we call an isopycnal float, it would not rotate as it rides up and down in the internal wave field. Now imagine deploying the float at some depth on an isopycnal surface near a coast where the winds are causing the surface waters to drift offshore. These waters must be replaced by waters drifting inshore at depth, so our float drifts in toward the coast. At some point the water will be forced upward and mix with overlying less dense water (coming from farther inshore, say). But the float can’t follow through since it is isopycnal. This was the challenge.
They equipped the float with slanted vanes so it would rotate in the presence of vertical motion. It was equipped with a sensor to measure its rotation in the earth’s magnetic field. They tested and calibrated the rotation vs. vertical motion in a deep tank here on the GSO campus. The float was also equipped with a ‘vocha’ (May 16, 2024), a small piston/cylinder arrangement that would change the volume and hence density of the float. That is the key to the diapycnal float. Now, if the float senses rotation, meaning there is a vertical flow past it, the float changes its density to counter the tendency to rotate. The ’vocha’ in effect subtracts or adds just the amount of volume so that its density matched that of the water. This is the inshore-upwelling-offshore 2-dimensional view of what is happening in the coastal zone.
While these in- and offshore motions are taking place there is also a strong drift along the coast. That is where the beauty of these floats comes in; they reveal movement in all three dimensions, as it upwells it will drift offshore far from where it drifted inshore. They had hoped to use this new RAFOS float design to study upwelling along the Oregon coast, but, sadly, the plans to do so were not funded.