Abstract

The surface winds from the moored buoy data set available from the National Data Buoy Center are examined for the occurrence of inertial range oscillations in the surface winds by employing a novel joint time frequency analysis of the two-component vector winds. This data set was chosen for its high time resolution (1-h sampling) that enabled the full characterization of these inertial oscillations, its at least partially global coverage, and its record length, which is needed to extract the signals from the background noise. The observation system is comprised of over 200 sites with measurements as far back as 1972. Because of the large amount of data, many harmonics of the diurnal atmospheric tidal signature in the surface winds (24, 12, 8, and 3 h) were found in high-precision power spectra. These tidal oscillations are found to be important contributors to the inertial range oscillation at latitudes of 30°, where the largest inertial range oscillations are found. By turning to local (in time) spectral analysis using the S-transform, considerable energy was also found in the inertial frequency ranges at other latitudes, typically in the form of very large (>10 m s—1), yet short-lived (of the order of a few days) events. The surface wind rotation direction in near inertial frequency bands favors resonant inertial oscillations at all latitudes, and in every season of the year.

Highlights

  • A ubiquitous feature of upper ocean currents are oscillations near the local inertial frequency with an anticyclonic sense of rotation (Webster, 1968)

  • There is strong evidence from both observations (Anderson et al, 1983) and modeling efforts (Fu, 1981; Poulain, 1990; Kudryavtsev, 1994; Pereira and Mascarenhas, 1994; Levine and Zervakis, 1995) that these oscillations in the ocean can be forced by the surface wind field

  • Inertial oscillations can be generated by the sudden onset of a steady wind blowing over an ocean mixed layer (Pollard and Millard, 1970) and play a major role in deepening the layer (Pollard et al, 1973)

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Summary

Introduction

A ubiquitous feature of upper ocean currents are oscillations near the local inertial frequency with an anticyclonic (i.e. clockwise in the Northern Hemisphere) sense of rotation (Webster, 1968). Center (NDBC) moored buoy data set (Section 2) are examined for variability at near inertial frequencies. Ocean tidal motion can advect the buoy surface structure across the watch circle in a periodic way that might confound detection of surface wind variability on local inertial time-scales. For a diurnal tidal current, a typical watch circle diameter might be transited every 12 h, leading to an erroneous apparent wind velocity of about 0.01 m s−1 This error is not significant with respect to the reported wind speed accuracies noted above. In the event of a passing atmospheric front, the buoy surface structure might be advected across the watch circle diameter, in the direction of propagation of the frontal system, on time-scales of the order of 1 h. While the direction of propagation will lead to consistent underestimates of the wind speed, the amplitude is again below the accuracy resolution of the data

Geographic distribution
Power spectral estimation
Analysis of rotating vector time series
Joint time–frequency analysis of the two-component wind
Latitude dependence of the resonant inertial oscillations
Seasonal variations of the inertial range
Discussion and conclusion
Distribution of inertial range oscillation amplitudes
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