Statistical properties of the surface velocity field in the northern Gulf of Mexico sampled by GLAD drifters

Abstract

The Grand LAgrangian Deployment (GLAD) used multiscale sampling and GPS technology to observe time series of drifter positions with initial drifter separation of O(100 m) to O(10 km), and nominal 5 min sampling, during the summer and fall of 2012 in the northern Gulf of Mexico. Histograms of the velocity field and its statistical parameters are non‐Gaussian; most are multimodal. The dominant periods for the surface velocity field are 1–2 days due to inertial oscillations, tides, and the sea breeze; 5–6 days due to wind forcing and submesoscale eddies; 9–10 days and two weeks or longer periods due to wind forcing and mesoscale variability, including the period of eddy rotation. The temporal e‐folding scales of a fitted drifter velocity autocorrelation function are bimodal with time scales, 0.25–0.50 days and 0.9–1.4 days, and are the same order as the temporal e‐folding scales of observed winds from nearby moored National Data Buoy Center stations. The Lagrangian integral time scales increase from coastal values of 8 h to offshore values of approximately 2 days with peak values of 3–4 days. The velocity variance is large, O(1) m2/normals2, the surface velocity statistics are more anisotropic, and increased dispersion is observed at flow bifurcations. Horizontal diffusivity estimates are O(103) m2/s in coastal regions with weaker flow to O(105) m2/s in flow bifurcations, a strong jet, and during the passage of Hurricane Isaac. The Gulf of Mexico surface velocity statistics sampled by the GLAD drifters are a strong function of the feature sampled, topography, and wind forcing.

Anisotropy and Inhomogeneity in Drifter Dispersion

“…Especially S1 drifters experience significant oscillations in their RMS pair separations. These reflect the near‐inertial oscillations dominating almost all S1 trajectories, as seen in Figure a and reported by Mariano et al (). At the 100‐m scale, the three deployments show consistent statistics, except that Z pairs linger for an extra 1.5 days at the O (1)‐km separation.…”

Submesoscale evolution of surface drifter triads in the Gulf of Mexico

“…Drifter trajectories during the first 6 days show the different fate of the two launches (Figure a). Strong inertial oscillations are visible in the trajectories, as expected due to shallow mixed layers in the summer season [ Mariano et al , ]. The triplet gyration radius, R , increases in time (Figure b) in a similar way for S 1 and S 2 for values up to R ≈3 km.…”

Impacts of Convergence on Structure Functions from Surface Drifters in the Gulf of Mexico

“…High-resolution models have been the prime exploratory tool of these scales (Luo et al 2016a; Barkan et al 2017a,b;Choi et al 2017), to include those utilizing data-assimilation (Carrier et al 2014b), but direct observational evaluation of the statistics of submesoscale phenomena are more challenging to obtain and analyze. Their utility has led to several releases of surface drifters throughout the GoM in the last decades, from which previous work has made estimations of velocity statistics-sometimes without distinguishing between Eulerian and semi-Lagrangian sampling in interpretation-in the submesoscale range (Poje et al 2014;Beron-Vera and LaCasce 2016;Mariano et al 2016;Balwada et al 2016;Sansón et al 2017). We revisit some of the conclusions made in those papers by exploiting releases of simulated surface drifters (Choi et al 2017) mimicking the design of the Grand LAgrangian Deployment [GLAD, (Poje et al 2014)] and LAgrangian Submesoscale ExpeRiment [LASER, (D'Asaro et al 2018)].…”