Satellite‐tracked drifter measurement of inertial and semidiurnal currents in the northeast Pacific

Abstract

We examine current variability in the open northeast Pacific using 3‐hourly sampled trajectories from satellite‐tracked drifters drogued at depths of 15 and 120 m within the wind‐mixed layer and permanent pycnocline, respectively. Findings show the following. (1) Motions were dominated by inertial currents which accounted for 58% (35%) of the total variance measured by the shallow (deep) drifters. Semidiurnal motions accounted for roughly 10% of the variance at both depths. There were detectable currents at twice the inertial frequency (2f ≈ 3 cpd) and negligible diurnal currents. (2) Inertial currents attained root‐mean‐square (rms) speeds of around 50 cm/s in the mixed layer and 10 cm/s in the pycnocline. Speeds in the semidiurnal band were similar (≈10 cm/s) at both depths. (3) Inertial and semidiurnal currents were strongly clockwise rotary, while low‐frequency currents (<1 cpd) were primarily rectilinear. (4) Peak inertial frequencies ωf were at the local inertial frequency ƒ in the mixed layer (ωf/f ≈ 1.0006 ± 0.0022; n =26 estimates) but slightly greater frequencies in the pycnocline (ωf/ƒ ≈ 1.0128 ± 0.0055; n =21). (5) Currents in the inertial and low‐frequency bands were correlated vertically. In the inertial band, maximum horizontal coherency [γ2(ω; Δr) > 0.95] occurred for drifter separations Δr < 75 km. Horizontal coherency diminished for 75 < Δr < 425 km and became marginally significant (γ2 ≈ 0.4) for Δr > 500 km. Semidiurnal currents were uncorrelated beyond several tens of kilometers. (6) Inertial currents appear to be linked to the passage of atmospheric lows, and semidiurnal currents to the passage of remotely generated baroclinic tides. The spectral peak near 2f is indicative of nonlinear inertial wave interactions.