Dispersion of a tracer in the deep Gulf of Mexico

Abstract

A 25 km streak of CF3SF5 was released on an isopycnal surface approximately 1100 m deep, and 150 m above the bottom, along the continental slope of the northern Gulf of Mexico, to study stirring and mixing of a passive tracer. The location and depth of the release were near those of the deep hydrocarbon plume resulting from the 2010 Deepwater Horizon oil well rupture. The tracer was sampled between 5 and 12 days after release, and again 4 and 12 months after release. The tracer moved along the slope at first but gradually moved into the interior of the Gulf. Diapycnal spreading of the patch during the first 4 months was much faster than it was between 4 and 12 months, indicating that mixing was greatly enhanced over the slope. The rate of lateral homogenization of the tracer was much greater than observed in similar experiments in the open ocean, again possibly enhanced near the slope. Maximum concentrations found in the surveys had fallen by factors of 104, 107, and 108, at 1 week, 4 months, and 12 months, respectively, compared with those estimated for the initial tracer streak. A regional ocean model was used to simulate the tracer field and help interpret its dispersion and temporal evolution. Model‐data comparisons show that the model simulation was able to replicate statistics of the observed tracer distribution that would be important in assessing the impact of oil releases in the middepth Gulf.

Heat Content Anomaly and Decay of Warm‐Core Rings: the Case of the Gulf of Mexico

“…They are however significantly larger than Ledwell et al, (2008)'s estimates in Sargasso sea mode water eddies (20 m 2 /s). Although extensive field work and numerical experiments have recently provided important information on the dispersion of tracers in the deep GoM (Ledwell et al, 2016), to the best of our knowledge, no estimate of lateral diffusivity is available for LCRs so that a strict comparison of our results with previous work is not possible. It is, however, important to note that the coarse resolution of the 10.1029/2019GL085600 altimetry data might lead to an overestimate of K h : the radial LHC gradient, which controls the denominator of equation (13), is considerably smoothed on the altimetry grid.…”

Spatial Variation of Diapycnal Diffusivity Estimated From Seismic Imaging of Internal Wave Field, Gulf of Mexico

“…Although a considerable amount of research has been carried out on the large‐scale flow of water masses within the Gulf of Mexico, relatively little effort has focused on small‐scale mixing processes. Ledwell et al () carried out a tracer‐release experiment at a depth of ∼1,100 m in order to investigate spreading and dissipation of hydrocarbon fluid following an accidental spillage. They estimate a diapycnal diffusivity of order m 2 s −1 across the continental slope of the northern Gulf of Mexico.…”

Temporal variability of chlorophyll distribution in the Gulf of Mexico: bio-optical data from profiling floats

“…This flux is generally considered proportional to the nitrate vertical gradient (S N ) through the relationship F N = K z · S N (with K z the diffusion coefficient; Okubo, 1971). The latest estimate of K z for the interior GOM is 0.15 × 10 −4 m 2 s −1 (Ledwell et al, 2016), which is similar to what is observed in the open ocean (e.g., Ledwell et al, 1998). Thus, considering K z constant, a steeper nitracline in CG (p<0.05, Fig.…”