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

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.

Automated summary

This study compares the currents measured by three different drifters in various sea state conditions. The results show that the CARTHE and CODE drifters measure currents in a similar way, while the currents measured by these drifters are mainly downwind and down-wave compared to the SVP drifter. However, there is a significant scatter in velocity differences between CODE/CARTHE and SVP, mainly due to vertical and horizontal shears not related to the wind. Additionally, about 30-40% of the differences in current measurements by the CODE drifter can be explained by Stokes drift when wind speed and significant wave height exceed certain thresholds.