A biologist’s guide to assessing ocean currents: a review

Abstract

We review how ocean currents are measured (in both Eulerian and Lagrangian frameworks), how they are inferred from satellite observations, and how they are simulated in ocean general circulation models (OGCMs). We then consider the value of these 'direct' (in situ) and 'indirect' (inferred, simulated) approaches to biologists investigating current-induced drift of strong-swimming vertebrates as well as dispersion of small organisms in the open ocean. We subsequently describe 2 case studies. In the first, OGCM-simulated currents were compared with satellite-derived currents; analyses suggest that the 2 methods yield similar results, but that each has its own limitations and associated uncertainty. In the second analysis, numerical methods were tested using Lagrangian drifter buoys. Results indicated that currents simulated in OGCMs do not capture all details of buoy trajectories, but do successfully resolve most general aspects of current flows. We thus recommend that the errors and uncertainties in ocean current measurements, as well as limitations in spatial and temporal resolution of the surface current data, need to be considered in tracking studies that incorporate oceanographic data. Whenever possible, crossvalidation of the different methods (e.g. indirect estimates versus buoy trajectories) should be undertaken before a decision is reached about which technique is best for a specific application.

Orientation behaviour of leatherback sea turtles within the North Atlantic subtropical gyre

“…We subtracted an estimate of the surface current velocity from the observed turtle velocity at each location, following the methods described by Gaspar et al [46] and Fossette et al [47]. To calculate the surface current velocity, we extracted surface geostrophic and Ekman current data from the NOAA Environmental Research Division's Data Access Programme website.…”

Identification of genetically and oceanographically distinct blooms of jellyfish

“…Hence, it is important to note that our particle-tracking simulations do not reflect the actual numbers of jellyfish in the system, but provide estimates of relative dispersal connectivity strength. Furthermore, PTM should only be considered a 'best estimate' of jellyfish dispersal potential [39]. The capture distance (40 km to the centre of each destination area) provides an indication of relative rates of transport between sites, rather than absolute numbers of jellyfish being transported.…”

Active dispersal in loggerhead sea turtles ( Caretta caretta ) during the ‘lost years’

“…Because the ocean is a complex and dynamic environment, inferences about the directed movement of an individual animal within the open ocean require attention to the underlying physical processes that shape migratory pathways [8,12,13]. This is especially true for ocean currents, which have been shown to play key roles in sea turtle ecology [7,8].…”