Trajectories were integrated back in time for 40 days, from July 19 to June 10; in one experiment using the 2018 velocity from HyCOM GOM10.04 expt. 32.5, and in another experiment using the  2003–2014 HyCOM climatological velocity described in \citet{Duran_2018}.  The almost 7 thousand trajectories in each experiment were computed using a fourth-order Runge-Kutta with cubic interpolation, a combination that has been shown to be efficient and accurate when integrating discrete data \citep*{Nordam2020}. The 2018 velocity was a two-dimensional field at 5 meters depth, while the climatological velocity is a surface field. 

Climatological Lagrangian Coherent Structures

Climatological Lagrangian Coherent Structures (cLCS) were developed in   \citet{Duran_2018} to identify persistent or recurrent Lagrangian transport patterns. The instantaneous trajectories in this paper are compared with the Gulf of Mexico cLCS computed in \citet{Duran_2018}.  The code for cLCS is described and freely available \citep{j2019};  the climatological velocity (with which the results of \citealt{Duran_2018}  can be replicated) is also available at that repository.