Persistent transport barrier on the West Florida Shelf

Abstract

[1] Analysis of drifter trajectories in the Gulf of Mexico has revealed the existence of a region on the southern portion of the West Florida Shelf (WFS) that is not visited by drifters that are released outside of the region. This socalled ''forbidden zone'' (FZ) suggests the existence of a persistent cross-shelf transport barrier on the southern portion of the WFS. In this letter a year-long record of surface currents produced by a Hybrid-Coordinate Ocean Model simulation of the WFS is used to compute Lagrangian coherent structures (LCSs), which reveal the presence of a persistent cross-shelf transport barrier in approximately the same location as the boundary of the FZ. The location of the cross-shelf transport barrier undergoes a seasonal oscillation, being closer to the coast in the summer than in the winter. A month-long record of surface currents inferred from high-frequency (HF) radar measurements in a roughly 60 km  80 km region on the WFS off Tampa Bay is also used to compute LCSs, and these also reveal the presence of transient transport barriers. While the HF-radarderived transport barriers cannot be unambiguously linked to the boundary of the FZ, this analysis does demonstrate the feasibility of monitoring transport barriers on the WFS using a HF-radar-based measurement system. The implications of a persistent cross-shelf transport barrier on the WFS for the development of harmful algal blooms on the shoreward side of the barrier are considered.

Near-Surface Transport Pathways in the North Atlantic Ocean: Looking for Throughput from the Subtropical to the Subpolar Gyre

“…Nearly all past oceanographic applications of Lagrangian DS analysis have been to idealized models or to realistic circulations of limited scale, such as Monterey Bay (Coulliette et al 2007;Shadden et al 2009), the Philippine Archipelago (Rypina et al 2010), the Mediterranean Sea (d 'Ovidio et al 2004;d'Ovidio et al 2009), the Gulf of Mexico (Olascoaga et al 2006;Kuznetsov et al 2002), and the Adriatic Sea (Rypina et al 2009). The validity of the approximation T L /T E ( 1 is rarely discussed in these studies, though some involve analysis of coherent gyres or eddies whose basic geometry remains intact for lengthy periods.…”

Relative dispersion from a high-resolution coastal model of the Adriatic Sea

“…Like Richardson's original time-distance graphs (Richardson, 1926), the FSLE provides a measure of dispersion as a function of the spatial scale and serves to isolate the different dispersion regimes corresponding to the different scales of the oceanic flows. Spatial distributions of the FSLE and related measures such as finite-time Lyapunov exponents have recently been used to define so-called Lagrangian coherent structures (Shadden ct al., 2005) and to map transport barriers and mixing regimes in a number of oceanic applications (Abraham and Bowen, 2002;d'Ovidio et al, 2004;Waugh et al, 2006;Olascoaga et al, 2006).…”

Lagrangian structure of flows in the Chesapeake Bay: challenges and perspectives on the analysis of estuarine flows

“…be the only case where such an approximation is adequate since, due to the imposed boundary conditions, the free surface in shallow water ocean models is, in principle, a material surface (see (7b)). We note that the Lagrangian transport in surface flows, obtained both from shallow water ocean models and high-frequency radar measurements, was analyzed numerous times in the past using the 2-D assumptions (see, for example, Lekien et al, 2005b;Olascoaga et al, 2006Olascoaga et al, , 2008 and led to some insightful results.…”

“…Many of these Lagrangian structures were successfully identified in geophysical flows as oceanic currents, fronts and eddies (e.g. Samelson and Wiggins, 2006;Rogerson et al, 1999;Mancho et al, 2008;Lekien et al, 2005a;Olascoaga et al, 2006;Mathur et al, 2007). These evolving structures are difficult to cross for fluid particles and they can be rather persistent.…”