Estimating Lagrangian Trajectories with an ADCP

Lagrangian trajectories may be estimated with an Acoustic Doppler Current Profiler (ADCP) as an alternative to tracking parcels with floats. By using the depth of isopycnals from a conductivity–temperature–depth (CTD) instrument and the velocities at various depths from the ADCP, velocities may be mapped on isopycnals. These isopycnal velocities may be integrated with respect to time to determine Lagrangian trajectories. This method of tracking water parcels with CTD and ADCP was implemented in two stages—a real-time tracking concurrent with a survey of the vicinity and a refinement of the tracking based on data from the vicinity. Trajectories of isopycnal parcels in a kinematic model and floats in Gulf Stream meanders were estimated by this method; these estimated trajectories were compared with computed trajectories in the model and with acoustically determined trajectories of the floats in the meanders, respectively. The comparison shows that the differential GPS renders ADCP data accurate enough to estimate Lagrangian trajectories over timescales of a few days. This method can be used as an alternative to tracking isopycnal parcels with devices such as the isopycnal Swallow float. A distinct advantage of this method over shipboard tracking of floats is that the density and velocity measurements in the vicinity of an isopycnal parcel can be used to determine the Lagrangian trajectories of other parcels distributed in the horizontal and vertical.