Comparing the Currents Measured by CARTHE, CODE and SVP Drifters as a Function of Wind and Wave Conditions in the Southwestern Mediterranean Sea

Abstract

Instruments drifting at the ocean surface are quasi-Lagrangian, that is, they do not follow exactly the near-surface ocean currents. The currents measured by three commonly-used drifters (CARTHE, CODE and SVP) are compared in a wide range of sea state conditions (winds up to 17 m/s and significant wave height up to 3 m). Nearly collocated and simultaneous drifter measurements in the southwestern Mediterranean reveal that the CARTHE and CODE drifters measure the currents in the first meter below the surface in approximately the same way. When compared to SVP drogued at 15 m nominal depth, the CODE and CARTHE currents are essentially downwind (and down-wave), with a typical speed of 0.5–1% of the wind speed. However, there is a large scatter in velocity differences between CODE/CARTHE and SVP for all wind and sea state conditions encountered, principally due to vertical and horizontal shears not related to the wind. For the CODE drifter with wind speed larger than 10 m/s and significant wave height larger than 1 m, about 30–40% of this difference can be explained by Stokes drift.

On the Structure and Kinematics of an Algerian Eddy in the Southwestern Mediterranean Sea

“…This is a good agreement, given the two different natures and horizontal resolutions of the velocity estimates. During that period, the ERA5 winds had speeds of 5-10 m/s, which would correspond to about 10 cm/s difference between currents measured by CARTHE and CODE drifters and those, mainly geostrophic, deeper in the surface mixed layer [26]. This agreement proves that the AE is essentially in geostrophic balance, which is expected since the local Rossby number is less than one (see vorticity estimates below).…”

“…If compared to drogued SVP drifters, CARTHE and CODE drifters move essentially downwind by 0.5-1% of the wind speed [26]. The differences between the CARTHE-CODE and SVP currents are due to direct wind-driven slippage, rectification by surface waves, Stokes drift and the shear of mainly ageostrophic currents between the surface and 15 m depth.…”

“…On 16 June, ERA5 winds were blowing from the E with a speed of 6 m/s. If we assume a correction of ~6 cm/s towards the E (1% of wind speed [26]) and apply it to the CARTHE and CODE velocities, regression results improve slightly (RMSE of 10.6 instead of 10.9 cm/s; see Table 1 and Figure 8 right panel) but the difference is not statistically significant. This reveals that, for this case, corrections do not improve the mismatch between geostrophic and drifter velocities.…”

Inertial Oscillations and Frontal Processes in an Alboran Sea Jet: Effects on Divergence and Vertical Transport

“…SVPs are standard drifters of the Global Drifter Program (Centurioni, 2018), they comprise a surface float linked to a drogue spanning the layer from 12 to 18 m depth with observed water‐following capabilities centered around 15 m depth. While CARTHEs sample near‐surface dynamics, including direct atmospheric forcing, wind‐waves interaction, surface Ekman currents, Stokes drift and Langmuir circulation, SVPs are less influenced by near‐surface processes (Poulain et al., 2022).…”

“…The data focus on an intense 1‐day survey (Section 2) sampling a submesoscale jet in the eastern side of an anticyclone, under the effect of variable winds with a significant across front component (Poulain et al., 2022). We start from the analysis of near surface divergence observations (Section 3) from drifters following the front for approximately 20 hr, complemented by divergence from currents measured along three cross‐front transects.…”