High‐frequency response of wind‐driven currents measured by drifting buoys and altimetry over the world ocean

Abstract

[1] We investigate the wind-driven current response over the world ocean using SVP drifting buoys, ERS-1 and ERS-2, and TOPEX altimetry data and ECMWF wind stress fields. Wind-driven ageostrophic currents are estimated from drifting buoys by removing altimetry deduced geostrophic currents from observed drifting velocities. Removing the geostrophic signal enhances the coherence with wind stress at periods longer than 10 days. For superinertial periods lower than 20 days, a two-parameter (i.e., angle and amplitude) model is fitted to the isolated ageostrophic signal to study the upper ocean response to surface wind stress in that frequency band. The model, which can explain up to 30% of the variance in some areas, indicates that the currents spiral to the right (left) of the wind with depth in the Northern (Southern) Hemisphere, in agreement with the Ekman theory. The absolute value of the angle parameter is found to exhibit significant spatial and seasonal variability. The vertical eddy viscosity and the Ekman layer depth as deduced from Ekman theory are compared to previous results. Their seasonal variability is also studied. Model results are then used to compute the mean and standard deviation of ageostrophic currents in 5°boxes over the world ocean. The equatorial divergence and the subpolar and subtropical convergence zones are well reproduced.INDEX TERMS: 4508 Oceanography: Physical: Coriolis effects; 4512 Oceanography: Physical: Currents; 4572 Oceanography: Physical: Upper ocean processes; KEYWORDS: altimetry, drifting buoys, Ekman current's modeling, rotary spectra, Ekman depth, eddy viscosity Citation: Rio, M.-H., and F. Hernandez, High-frequency response of wind-driven currents measured by drifting buoys and altimetry over the world ocean,

Surface Circulation at the Tip of the Antarctic Peninsula from Drifters

“…Both of these components were found to be small with peak values consistently around 1 cm s 21 , which is less than the expected drogue slip. The work of Rio and Hernandez (2003) confirms that Ekman velocities are small near the tip of the Antarctic Peninsula, although there is not coverage FIG. 7.…”

Cyclonic and anticyclonic motion in the upper ocean

“…This large scale pattern can be seen in the global zonally averaged distribution (Figure 2, right), with the anticyclonic band enhanced in the Southern hemisphere most likely because of Agulhas rings (as suggested also by the results in Figure 3, bottom, and Figure 4, bottom). The presence of an anticyclonic band has been previously noticed in drifter rotary spectra [Rio and Hernandez, 2003;Elipot, 2006], while the cyclonic band has not been detected before to the authors' knowledge.…”

“…[15] For cyclonic motion, the coherence with the wind is significantly smaller than it is for anticyclonic motion, and no definite wind polarity has been detected [Rio and Hernandez, 2003]. Also the persistence of the looping trajectories (Figure 4, top) does not suggest Ekman dynamics.…”

“…For the anticyclonic band, cross-spectral analysis [Rio and Hernandez, 2003] has shown significant coherence between drifter velocity and the wind, which itself has anticyclonic polarity. Coherences are not high (typically below 0.5), but they show maxima in the same areas where we see the highest concentration of small-scale features, i.e.…”

Satellite Altimetry and Key Observations: What We've Learned, and What's Possible with New Technologies

“…Surface drifter data are one way to measure total velocity, and are a valuable complement to altimeter data. Drifters have proved themselves as a useful measure of the time-invariant dynamic ocean topography ( [53], [54], [55] and [37], also Sect. 1.2 ii above), allowing detailed assessment of eddy mean flow interactions (e.g.…”