Extracting quasi-steady Lagrangian transport patterns from the ocean circulation: An application to the Gulf of Mexico

Abstract

We construct a climatology of Lagrangian coherent structures (LCSs)—the concealed skeleton that shapes transport—with a twelve-year-long data-assimilative simulation of the sea-surface circulation in the Gulf of Mexico (GoM). Computed as time-mean Cauchy–Green strain tensorlines of the climatological velocity, the climatological LCSs (cLCSs) unveil recurrent Lagrangian circulation patterns. The cLCSs strongly constrain the ensemble-mean Lagrangian circulation of the instantaneous model velocity, showing that a climatological velocity can preserve meaningful transport information. The quasi-steady transport patterns revealed by the cLCSs agree well with aspects of the GoM circulation described in several previous observational and numerical studies. For example, the cLCSs identify regions of persistent isolation, and suggest that coastal regions previously identified as high-risk for pollution impact are regions of maximal attraction. We also show that cLCSs are remarkably accurate at identifying transport patterns observed during the Deepwater Horizon and Ixtoc oil spills, and during the Grand LAgrangian Deployment (GLAD) experiment. Thus it is shown that computing cLCSs is an efficient and meaningful way of synthesizing vast amounts of Lagrangian information. The cLCS method confirms previous GoM studies, and contributes to our understanding by revealing the persistent nature of the dynamics and kinematics treated therein.

Persistent Lagrangian Transport Patterns in the Northwestern Gulf of Mexico

“…The cLCS computation, as developed by Duran et al (2017), involves performing suitable averages in two subsequent steps. In the first step we construct a daily climatological flow.…”

“…LCS studies therefore typically focus on casestudies or scenarios which verify observations with LCS maps (e.g., comparisons of LCS maps to satellite imagery of chlorophyll concentrations, sea surface temperature, or oil spill observations (Gough et al 2016;Olascoaga;Haller 2012;Olascoaga et al 2013)). Recently, 5 climatological LCSs, or cLCSs, computed from a numerical model simulation have been shown to effectively identify persistent surface transport patterns (Duran et al 2017). Here we deviate from traditional LCS study practices by extracting LCSs from daily climatological surface current velocities constructed from a multi-year model simulation to compute cLCSs following Duran et al (2017).…”

“…Recently, 5 climatological LCSs, or cLCSs, computed from a numerical model simulation have been shown to effectively identify persistent surface transport patterns (Duran et al 2017). Here we deviate from traditional LCS study practices by extracting LCSs from daily climatological surface current velocities constructed from a multi-year model simulation to compute cLCSs following Duran et al (2017). There are two primary objectives to this study: 1) provide a guideline for interpreting the cLCSs and 2) provide maps of persistent sea surface Lagrangian transport patterns in the NWGoM.…”

The Implications of Oil Exploration off the Gulf Coast of Florida

“…In other words, the western coastline of Florida may not be particularly susceptible to oiling. The construction of Lagrangian coherent structures (LCS) from twelve-year-long ocean surface circulation data confirms the absence of squeezelines from the shore to about the 50 m isobath off Florida's west shelf [46] making the 50 m isobath an important boundary. From the shore to the 50 m isobath one can expect very little ocean current activity, meaning that oil within this area is likely to stagnate or move very slowly.…”

“…Florida is situated in a unique geographical position in relation to ocean currents. Recent work suggests that Florida's western shelf is isolated from cross-shelf "squeezelines" or current velocity fields that tend to attract nearby particle trajectories [46]. In other words, the western coastline of Florida may not be particularly susceptible to oiling.…”