Lagrangian studies of deep ocean currents



It is now more than 10 years since the first experiment using subsurface drifters (so-called Sofar floats) took place in the Mid-Ocean Dynamics Experiment (Mode). Since then they have been applied in a series of exploratory studies culminating in the Polymode Local Dynamics Experiment (LDE), and more recently along 55°W in a study of the North Atlantic subtropical gyre in a region well removed from the dynamics of western boundary regions. These neutrally buoyant instruments, which can be ballasted to drift with the waters at pressures up to 2000 dbars (meters), are tracked acoustically over long periods of time (months to years). Conceptually the floats may be thought of as large molecules, fluid parcels whose pathways and speeds are explicitly known. The structure of their trajectories often yields surprisingly detailed information on the horizontal structure of the velocity field. When used in clusters they can tell us much about the mean field and the dispersive properties of the region. This article provides a brief retrospective o f what we have learned in the 10 years since their first application in Mode. We begin with a brief descripion of the Sofar float technology.