Hindcasting Surface Drift Trajectories in the St. Lawrence Estuary Using Operational Numerical Modeling vs. Measurements of Surface Currents From High-Frequency Radars

Cedric P Chavanne, Sandy O Gregorio, Tamkpanka Tamtare and Dany Dumont, University of Quebec at Rimouski UQAR, ISMER, Rimouski, QC, Canada
Lagrangian drift forecasts are required for tracking floating particles and substances in the ocean such as microplastic debris and oil droplets. At the ocean surface, the drift of floating particles is influenced by both the surface Eulerian currents and the wave-induced Stokes drift. However, the latter is generally not represented in most operational ocean models, and estimating numerically the drift of floating particles therefore requires ocean circulation models coupled to a wave model to include the Stokes drift contribution. Alternatively, measurements of surface currents by high-frequency radars (HFRs) have been used to hindcast surface drift trajectories, but a debate remained as to whether or not the Stokes drift was already included in the HFRs measurements. Recently, it has been confirmed experimentally that HFRs measure the so-called filtered surface Stokes drift, which is numerically very close to the surface Stokes drift, in addition to near-surface Eulerian currents, making HFRs a very appropriate instrument to hindcast surface drift trajectories.

Our goal is to compare the hindcast skills of surface drift trajectories estimated using operational numerical modeling vs. measurements of surface currents from HFRs, against trajectory observations from surface drifters deployed in the Lower St. Lawrence Estuary (eastern Canada) in 2014 and 2015. The surface drifters were designed to be comparable to small debris floating at the ocean surface with a minimum windage. Four HFRs are combined to map hourly surface currents : two CODARs (Coastal Ocean Dynamics Applications Radars) on the south shore, and two WERAs (WavE RAdars) on the north shore of the estuary. The operational numerical modeling system includes a 5-km resolution coupled ice-ocean circulation model over the Gulf of St. Lawrence (GSL5km), and a regional spectral wave model (WAVEWATCH III).