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Observing and Quantifying Ocean Flow Properties Using Drifters with Drogues at Different Depths

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  • 1 a Physical Oceanography Department, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts
  • | 2 b SOCIB, Palma de Mallorca, Spain
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Abstract

This paper presents analyses of drifters with drogues at different depths—1, 10, 30, and 50 m—that were deployed in the Mediterranean Sea to investigate frontal subduction and upwelling. Drifter trajectories were used to estimate divergence, vorticity, vertical velocity, and finite-size Lyapunov exponents (FTLEs) and to investigate the balance of terms in the vorticity equation. The divergence and vorticity are O(f) and change sign along trajectories. Vertical velocity is O(1 mm s−1), increases with depth, indicates predominant upwelling with isolated downwelling events, and sometimes changes sign between 1 and 50 m. Vortex stretching is one of the significant terms, but not the only one, in the vorticity balance. Two-dimensional FTLEs are 2 × 10−5 s−1 after 1 day, 2 times as large as in a 400-m-resolution numerical model. Three-dimensional FTLEs are 50% larger than 2D FTLEs and are dominated by the vertical shear of horizontal velocity. Bootstrapping suggests uncertainty levels of ~10% of the time-mean absolute values for divergence and vorticity. Analysis of simulated drifters in a model suggests that drifter-based estimates of divergence and vorticity are close to the Eulerian model estimates, except when drifters get aligned into long filaments. Drifter-based vertical velocity is close to the Eulerian model estimates at 1 m but differs at deeper depths. The errors in the vertical velocity are largely due to the lateral separation between drifters at different depths and are partially due to only measuring at four depths. Overall, this paper demonstrates how drifters, heretofore restricted to 2D near-surface observations, can be used to learn about 3D flow properties throughout the upper layer of the water column.

Significance Statement

Drifting buoys, or drifters, are one of the oldest instruments in physical oceanography and have been used to study near-surface horizontal currents throughout the World Ocean. However, in flows with large vertical velocity or vertical shear, currents and transport properties change with depth, so conventional near-surface drifters are of limited use. This paper shows that a simultaneous release of drifters with drogues at different depths allows observing and quantifying both horizontal and vertical structure of the flow. Specifically, we present analyses of drifters with drogues at 1, 10, 30, and 50 m, which were deployed in the Mediterranean Sea in 2018. Drifter data are used to quantify such important flow properties as divergence, vorticity, vertical velocity, and finite-time Lyapunov exponents and to investigate the vorticity equation balance throughout the top 50 m of the water column.

© 2021 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Irina I. Rypina, irypina@whoi.edu

Abstract

This paper presents analyses of drifters with drogues at different depths—1, 10, 30, and 50 m—that were deployed in the Mediterranean Sea to investigate frontal subduction and upwelling. Drifter trajectories were used to estimate divergence, vorticity, vertical velocity, and finite-size Lyapunov exponents (FTLEs) and to investigate the balance of terms in the vorticity equation. The divergence and vorticity are O(f) and change sign along trajectories. Vertical velocity is O(1 mm s−1), increases with depth, indicates predominant upwelling with isolated downwelling events, and sometimes changes sign between 1 and 50 m. Vortex stretching is one of the significant terms, but not the only one, in the vorticity balance. Two-dimensional FTLEs are 2 × 10−5 s−1 after 1 day, 2 times as large as in a 400-m-resolution numerical model. Three-dimensional FTLEs are 50% larger than 2D FTLEs and are dominated by the vertical shear of horizontal velocity. Bootstrapping suggests uncertainty levels of ~10% of the time-mean absolute values for divergence and vorticity. Analysis of simulated drifters in a model suggests that drifter-based estimates of divergence and vorticity are close to the Eulerian model estimates, except when drifters get aligned into long filaments. Drifter-based vertical velocity is close to the Eulerian model estimates at 1 m but differs at deeper depths. The errors in the vertical velocity are largely due to the lateral separation between drifters at different depths and are partially due to only measuring at four depths. Overall, this paper demonstrates how drifters, heretofore restricted to 2D near-surface observations, can be used to learn about 3D flow properties throughout the upper layer of the water column.

Significance Statement

Drifting buoys, or drifters, are one of the oldest instruments in physical oceanography and have been used to study near-surface horizontal currents throughout the World Ocean. However, in flows with large vertical velocity or vertical shear, currents and transport properties change with depth, so conventional near-surface drifters are of limited use. This paper shows that a simultaneous release of drifters with drogues at different depths allows observing and quantifying both horizontal and vertical structure of the flow. Specifically, we present analyses of drifters with drogues at 1, 10, 30, and 50 m, which were deployed in the Mediterranean Sea in 2018. Drifter data are used to quantify such important flow properties as divergence, vorticity, vertical velocity, and finite-time Lyapunov exponents and to investigate the vorticity equation balance throughout the top 50 m of the water column.

© 2021 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Irina I. Rypina, irypina@whoi.edu
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