Lagrangian coherence and source of water of Loop Current Frontal Eddies in the Gulf of Mexico

TL;DR

This study investigates the water sources, coherence duration, and transport capabilities of Loop Current Frontal Eddies in the Gulf of Mexico, revealing their role in water exchange and passive tracer transport during the 2010 oil spill.

Contribution

It provides new insights into the formation, Lagrangian coherence duration, and water source origins of LCFEs using altimetry and high-resolution modeling.

Findings

LCFEs can remain coherent for up to 25 days at the surface and 29 days at depth.

Water in LCFEs mainly originates from the outer band of the Loop Current front.

LCFEs can facilitate cross-shelf exchange of water, nutrients, and tracers.

Abstract

Loop Current Frontal Eddies (LCFEs) are known to intensify and assist in the Loop Current (LC) eddy shedding. These eddies can also modify the circulation in the eastern Gulf of Mexico (GoM) by attracting water and passive tracers such as chlorophyll and pollutants to the LC-LCFE front. During the 2010 Deepwater Horizon oil spill, part of the oil was entrained not only in the LC-LCFE front but also inside an LCFE, where it remained for weeks. This study assesses the ability of the LCFEs to transport water and passive tracers without exchange with the exterior (i.e., Lagrangian coherence) using altimetry and a high-resolution model. The following open questions are answered: (1) How long can the LCFEs remain Lagrangian coherent at and below the surface? (2) What is the source of water for the formation of LCFEs? (3) Can the formation of LCFEs attract shelf water? The results show that LCFEs are composed of waters originating from the outer band of the LC front, the region north of the LC, and the shelf, and potentially drive cross-shelf exchange of particles, water properties, and nutrients. At depth (~180 m), most LCFE water comes from the outer band of the LC front in the form of smaller frontal eddies. Once formed, LCFEs can transport water and passive tracers in their interior without exchange with the exterior for weeks: these eddies remained Lagrangian coherent for up to 25 days in the altimetry dataset and 18 days at the surface and 29 days at depth (~180 m) in the simulation. LCFE can remain Lagrangian coherent up to a depth of ~560 m. Additional analyses confirm that the LCFE involved in the oil spill formed from water near the oil rig location. Temperature-salinity diagrams show that LCFEs are composed of GoM water as opposed to LC water. Thus, LCFE formation modify the surrounding circulation and the transport of oil and other passive tracers in the eastern GoM.