Search for: All records

Abstract Westward-propagating Caribbean Current eddies modify the volume-integrated potential vorticity (PV) balance in the western Caribbean Sea, influencing the circulation of the Panamá-Colombia Gyre (PCG) and of coastal currents hundreds of kilometers to the south of the eddies’ mean trajectory. Using 22 years of output from the Hybrid Coordinate Ocean Model, we apply a volume-integrated eddy phase-averaged 1.5 layer PV balance, showing that PV fluxes into the PCG region are balanced by frictional PV dissipation represented by linear drag along the coastline. Coastal currents in the PCG region vary by a factor of two in phase with the passage of a Caribbean Current eddy over the 116 day average eddy period. Flow separation at the Isthmus of Panamá results in a vortex shed from Darien Gulf, which slows the coastal currents in the gyre region from their maximum during eddy events. An annual ensemble average PV balance in the gyre region shows that the mean PV influx to this region is higher from August to October. Correspondingly, the range of coastal currents in the gyre region over an eddy event is modestly influenced by the PV influx magnitude. Eddy influenced reversals in the coastal current can occur between November and July at Bocas del Toro and year-round at Colón. Such coastal current reversals are important for alonghshore transport of larvae, freshwater, and chemical tracers. 

We use salinity observations from drifters and moorings at the Quinault River mouth to investigate mixing and stratification in a surf-zone-trapped river plume. We quantify mixing based on the rate of change of salinity DS/Dt in the drifters’ quasi-Lagrangian reference frame. We estimate a constant value of the vertical eddy diffusivity of salt of Kz=(2.2 +/- 0.6) x 10^-3 m^2 s^-1, based on the relationship between vertically integrated DS/Dt and stratification, with values as high as 1 x 10^-2 m^2 s^-1 when stratification is low. Mixing, quantified as DS/Dt, is directly correlated to surf-zone stratification, and is therefore modulated by changes in stratification caused by tidal variability in freshwater volume flux. High DS/Dt is observed when the near-surface stratification is high and salinity gradients are collocated with wave-breaking turbulence. We observe a transition from low stratification and low DS/Dt at low tidal stage to high stratification and high DS/Dt at high tidal stage. Observed wave-breaking turbulence does not change significantly with stratification, tidal stage, or offshore wave height; as a result, we observe no relationship between plume mixing and offshore wave height for the range of conditions sampled. Thus, plume mixing in the surf zone is altered by changes in stratification; these are due to tidal variability in freshwater flux from the river and not wave conditions, presumably because depth-limited wave breaking causes sufficient turbulence for mixing to occur during all observed conditions.