Wind Effects on Drogued and Undrogued Drifters in the Eastern Mediterranean

Abstract

The wind effects on drogued and undrogued drifters are assessed using Coastal Ocean Dynamics Experiment (CODE) and Surface Velocity Program (SVP) drifter datasets and ECMWF wind products in the eastern Mediterranean. Complex and real linear regression models are used to estimate the relative slip of undrogued SVP drifters and to extract the wind-driven currents from the drifter velocities. The frequency response of the wind-driven currents is studied using cross-spectral analysis. By comparing the velocities of cotemporal and nearly collocated undrogued and drogued SVP drifters, it appears that undrogued SVP drifters have a general downwind slippage of about 1% of the wind speed. Time-lagged complex correlations and cross-spectral results show that the wind response is almost simultaneous. The velocities of SVP drifters drogued to 15 m are poorly correlated with the winds (R 2 ' 3%): wind-driven currents have a magnitude of 0.7% of the wind speed and are 278-428 to the right of the wind. For undrogued SVP drifters, the correlation with the winds increases to R 2 ' 22% and the angle between winds and currents decreases to 178-208. The magnitude of the wind-driven currents is about 2% of the wind speed. For CODE designs, wind-driven currents are 1% of the wind speed at an angle of about 288 to the right of the wind (R 2 ' 8%). Spectral and cospectral analyses reveal that the drifters sampled more anticyclonic than cyclonic motions. The inner coherence spectra show that wind and currents are more correlated at temporal scales spanning 3-10 days. They also confirm that the wind response is quasi-simultaneous and that currents are generally to the right of the wind.

Remote sensing of ocean surface currents: a review of what is being observed and what is being assimilated

“…Although the determination of upper wind and wave-driven currents provided by the above equation may not be accurate (see for example Rascle and Ardhuin, 2009), observations do see, in general, differences between different types of drifters (Rio et al, 2014). Interestingly, these differences are very small in the Mediterranean (Poulain et al, 2009(Poulain et al, , 2012. Although the slab model has vertically homogeneous velocities, the inclusion of the Coriolis-Stokes induces vertical variations of the velocity field since, in general d S is smaller than the mixed-layer depth (H in Eq.…”

“…where B and θ are constants to be fitted with observed velocities (Ralph and Niiler, 1999;Rio and Hernandez, 2003;Poulain et al, 2009;Chiswell, 2016). As a consequence, the resulting velocities derived from satellite wind measurements will be representative of the motion at the depth of measurements.…”

“…The angle θ can be derived from the observation of drifter trajectories. It has been found to be within the range 20-60 • for the global ocean and the eastern Mediterranean sea (Rio and Hernandez, 2003;Poulain et al, 2009) using surface velocity program (SVP) drifter trajectories that represent the motion of a 10 m layer centered at 15 m deep (see Lumpkin et al, 2017, and references therein). Moreover, Poulain et al (2009) found very small differences in the direction between the SVP and CODE (drogued at ∼ 1m Lumpkin et al, 2017) buoys in the Mediterranean sea when fitting the model given by Eq.…”

Validation of FOAM near-surface ocean current forecasts using Lagrangian drifting buoys

“…If the drogue should become detached, the surface buoy will be subject to increased wind slip as well as other effects such as Stokes drift and shear effects from wind-driven surface currents. All of these factors increase the downwind slippage for an undrogued buoy to approximately 1 % of the wind speed; Poulain et al (2009), based on experiments carried out in the in the Eastern Mediterranean, calculated a minimum of 7 cm s −1 slip in 10 m s −1 winds for SVP drifters. Most SVP drifters are tracked by the Argos Data Collection and location system on the NOAA polar-orbiting satellites, and the reported locations are accurate to approximately 1km.…”

Remote Sensing With TDMF Radar: Some Preliminary Results

“…First, all the platforms have measurement errors. Second, radars have limited radial velocity resolution (a few cm/s) due to frequency resolution, while instruments have slippage errors of few cm/s [17]. Third, HF radar provides time averages data over CITs of typical 10 minutes, while ADCPs or drifters provide instantaneous information.…”