SATELLITE-TRACKED DRIFTER MEASUREMENT OF INERTIAL AND SEMIDIURNAL CURRENTS IN THE NORTHEAST PACIFIC

We examine current variability in the open northeast Pacific using 3-h ourly sampled trajectories from satellite-tracked drifters drogued at depths of 15 and 120 m within the wind-mixed layer and permanent pycno cline, respectively. Findings show the following. (1) Motions were dom inated 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 approximate to 3 cpd) and negligible diurnal currents. (2) Inertial currents attaine d root-mean-square (rms) speeds of around 50 cm/s in the mixed layer a nd 10 cm/s in the pycnocline. Speeds in the semidiurnal band were simi lar (approximate to 10 cm/s) at both depths. (3) Inertial and semidiur nal currents were strongly clockwise rotary, while low-frequency curre nts (<1 cpd) were primarily rectilinear. (4) Peak inertial frequencies of were at the local inertial frequency f in the mixed layer (w(f)/f approximate to 1.0006 +/- 0.0022; n = 26 estimates) but slightly great er frequencies in the pycnocline (w(f)/f approximate to 1.0128 +/- 0.0 055; n = 21). (5) Currents in the inertial and low-frequency bands wer e correlated vertically. In the inertial band, maximum horizontal cohe rency [gamma(2)(omega; Delta r) > 0.95] occurred for drifter separatio ns Delta r < 75 km. Horizontal coherency diminished for 75 < Delta r < 425 km and became marginally significant (gamma(2) approximate to 0.4 ) for Delta r > 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 pa ssage of remotely generated baroclinic tides. The spectral peak near 2 f is indicative of nonlinear inertial wave interactions.