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

Pierre-Marie Poulain,Tamay Özgökmen,Luca Centurioni

doi:10.3390/s22010353

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.

Highlights

Academic Editor: Jiachen YangSince the advent of satellite positioning and data telemetry in the late 1970s, instrumented buoys freely-drifting at the ocean surface have become increasingly popular to measure meteo-marine properties near the air–sea interface
The downwind velocity difference mostly increases with wind speed for wind speed larger than 8 m/s velocity difference mostly increases with wind speed for wind speed larger than 8 m/s during CALYPSO 2019
CARTHE drifters measure the currents in the first meter below the sea surface in approximately the same way, and they can be combined to calculate ocean surface velocity statistics and dynamics

Summary

Introduction

Since the advent of satellite positioning and data telemetry in the late 1970s, instrumented buoys freely-drifting at the ocean surface have become increasingly popular to measure meteo-marine properties near the air–sea interface. Drogued drifting buoys, hereafter referred to as drifters, do not exactly follow the water around them, and the estimation of horizontal velocities from their displacements is prone to systematic errors. This is especially true in high sea conditions, when winds, surface waves and vertical shear in the upper water column can produce significant slippage of the drifters. Be inferred from surface divergence using the continuity equation (e.g., [8])

Objectives

Methods

Results

Discussion

Conclusion