Intensification of Loop Current Frontal Eddies and their Interactions with the Loop Current and Surrounding Flow

Abstract

Loop Current Frontal Eddies (LCFEs) are cold-core vortices located in the vicinity of the Loop Current (LC) and are known to intensify and play an essential role in the LC shedding. The amplification of the LCFEs also affects the local circulation. During the 2010 Deepwater Horizon oil spill, part of the oil was entrained around and inside an intensified LCFE. The goal of this research is to characterize the LCFE intensification and understand its effects on the LC and surrounding flow. Firstly, the LC-LCFE interaction was investigated using altimetry and a mooring array. The intensification of the observed LCFEs shows similar characteristics over time, independent of their location: a steep increase in kinetic energy, a corresponding decrease in SSH, and an increase in size. LCFE intensification is dependent on the distance from the LC front. As the LCFE grows, the flow at the interface with the LC becomes stronger and deeper, and the horizontal density gradient between the features increases. Further intensification of the LC front and the LCFEs is suggested to be driven by the advection (nonlinear) term and the pressure-gradient (linear) term in the momentum budget. Secondly, the ageostrophy of the LC meanders during LCFE intensification is assessed using HYCOM velocity and geostrophic velocity from altimetry. The results indicate that during strong meandering, especially before and during LC shedding and in the presence of frontal eddies, the centrifugal force becomes as important as the Coriolis and the pressure-gradient forces, i.e., the LC meanders are in gradient-wind balance. Finally, the ability of LCFEs to transport particles without exchange with the exterior (i.e., material coherence) is investigated. The results show that the frontal eddies can remain coherent for up to 20 days at the surface and up to 25 days at deeper layers. Particles inside the frontal eddies were tracked backward in time and showed that the material coherence of the eddies builds up from Gulf water and can drive cross-shelf exchange of particles, water properties, and nutrients.