The Gulf of Mexico (GoM) is home to some of the most energetic eddies in the ocean. They detach from the Loop-Current and drift through the basin, transporting large amounts of heat and salt. These eddies, known as Loop Current rings (LCRs) have a crucial role in the GoM’s dynamics and in the weather of the eastern US, and this role is largely conditioned by their longevity and decay properties. Here, we use an empirical method to estimate the energy evolution of all LCRs detached since 1993. We found that, contrary to the commonly accepted idea that LCRs conserve their energy as they drift through the GoM and decay suddenly against the western platform, LCRs’ energy decays faster in the eastern basin, and they typically lose three-quarter of their energy before encountering the continental shelf. We also show that wind-current feedback largely contributes to the energy decay and conversion.

Biological connectivity studies are crucial for explaining the structure of marine populations of species with a pelagic larval stage. Numerical modelling is a powerful tool for evaluating marine dispersal pathways of planktonic organisms. The high-resolution numerical circulation model HYCOM (HYbrid Coordinate Ocean Model) has been used to examine fish connectivity patterns in the Gulf of Mexico (GoM), but without biological validation. We examined the connectivity of fish larvae caught in the northwestern GoM by coupling ichthyoplankton surveys with numerical modelling and particle backtracking experiments. Fish larvae were collected with 200 m oblique bongo tows along two parallel transects extending from the edge of the shelf to deep waters of the Perdido region in the northwestern GoM (24°N to 26°N and 94.5°W to 97°W) during four cruises (June and October 2016, April and November 2017). Larvae of coastal and shelf-spawning species were used to infer offshore transport. In order to explore their dispersal pathways, the real-time HYCOM 1/25° model hourly output with the Navy Coupled Ocean Data Assimilation (NCODA) was used to simulate Lagrangian trajectories for each cruise and transect. Particles were seeded at sampling stations and the circulation model was run backward in time to infer larval origin. Patterns of spatial distribution and abundance of the coastal and shelf larvae caught in oceanic stations among cruises exhibited a reasonable agreement with the results of modelling exercises, and indicate the shelves of Tamaulipas and Texas were the main source of larvae to Perdido’s deepwater region. Our results suggest the combined use of ichthyoplankton surveys and ocean circulation models can yield insight into the dispersal pathways of larvae of neritic fish species to deep waters regions in the GoM. Likewise, these results imply that fish larval distributions are a useful tool for evaluating the reliability of using 2-D velocity fields from circulation models to infer larval transport at time scales of several weeks.

The Deep Water Horizon oil spill has dramatically impacted the Gulf of Mexico from the seafloor to the surface. While dispersion of contaminants at the surface has been extensively studied, little is known about deep water dispersion properties. This study describes the results of the Deep Water Dispersion Experiment, which consisted in the release of surface drifters and RAFOS floats drifting at 300 and 1500 dbar in the Gulf of Mexico. We show that surface diffusivity is elevated, and decreases with depth. The separation dependence of relative diffusivity follows a Richardson law at all depths. Time dependence of dispersion suggests a Richardson regime near the surface and a mixed Richardson/ballistic regime in depth at scales of [10-100 km]. Finite Scale Lyapunov Exponents and pair separation Kurtosis show the existence of a Lundgren regime at scales smaller than the Rossby radius near the surface, and at smaller scales in depth.