During the 2010 Deepwater Horizon oil spill, an underestimated part of the oil was entrained into an intensified Loop Current Frontal Eddy (LCFE). These eddies, which are also known to play an essential role in the Loop Current eddy (LCE) shedding, are difficult to predict, and the dynamics involving their intensification are still not fully understood. The Loop Current (LC) and its strongest LCFEs were continuously tracked during 2009-2011 using sea surface height (SSH) from AVISO+. A mooring array provided complementary information about the internal structure of the LC-LCFE interaction. The intensification of the tracked LCFEs presented similar characteristics, independent of their location: a steep increase in kinetic energy, a corresponding negative increase in SSH, and an increase in its area. 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. The intensification of the front and the LCFEs is driven by the advection (nonlinear) term and the gradient pressure (linear) term in the momentum budget. Evidence of an inverse energy cascade suggests that LCFEs are extracting energy and mass from the submesoscale field to the zone of contact between the LC and the LCFE, strengthening the front and allowing the LCFEs to grow during periods of intensification. Understanding the physics driving the LCFE intensification is a key step to improve LC forecast models and to better predict LCE shedding events, as well as oil and particle transport around the LC.

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The Loop Current (LC) system has long been assumed to be close to geostrophic balance despite its strong flow and the development of large meanders and strong frontal eddies during unstable phases. The region between the LC meanders and its frontal eddies was shown to have high Rossby numbers indicating nonlinearity; however, the effect of the nonlinear term on the flow has not been studied so far. In this study, the ageostrophy of the LC meanders is assessed using a high-resolution numerical model and geostrophic velocities from altimetry. A formula to compute the radius of curvature of the flow from the velocity field is also presented. 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 force and the pressure-gradient force: LC meanders are in gradient-wind balance. The centrifugal force modulates the balance and modifies the flow speed, resulting in a subgeostrophic flow in the LC meander trough around the LCFE and supergeostrophic flow in the LC meander crest. The same pattern is found when correcting the geostrophic velocities from altimetry to account for the centrifugal force. The ageostrophic percentage in the cyclonic and anticyclonic meanders is 47% ± 1% and 78% ± 8% in the model and 31% ± 3% and 78% ± 29% in the altimetry dataset, respectively. Thus, the ageostrophic velocity is an important component of the LC flow and cannot be neglected when studying the LC system.

<p>Mesoscale eddies drive a large fraction of the variability in the ocean. Eddies with strong tangential velocity compared to their translation speed are able to stay coherent and travel long distances, carrying water mass properties, heat, nutrients, and particles around the ocean. The nonlinearity of these mesoscale features is greater for stronger flow and greater curvature, which, consequently, is associated with greater centrifugal force.</p><p>The Gulf of Mexico Loop Current (LC) system has long been assumed to be close to geostrophic balance despite its strong flow and the development of large meanders and strong frontal eddies during unstable phases. The region between the LC meanders and its frontal eddies was shown to have high Rossby numbers indicating nonlinearity; however, the effect of the nonlinear term on the flow has not been studied so far. In this study, the ageostrophy of the LC meanders is assessed using a high-resolution numerical model and geostrophic velocities from altimetry. The method used in this study can be applied in any region where the centrifugal force is important. A formula to compute the radius of curvature of the flow from the velocity field is also presented.</p><p>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 force and the pressure-gradient force: LC meanders are in gradient-wind balance. The centrifugal force modulates the balance and modifies the flow speed, resulting in a subgeostrophic flow in the LC meander trough around the frontal eddies and supergeostrophic flow in the LC meander crest. The same pattern is found when correcting the geostrophic velocities from altimetry to account for the centrifugal force. The ageostrophic percentage in the cyclonic and anticyclonic meanders is 47% ± 1% and 78% ± 8% in the model and 31% ± 3% and 78% ± 29% in the altimetry dataset, respectively. Thus, the ageostrophic velocity is an important component of the LC flow and cannot be neglected when studying the LC system.</p><p> </p><p> </p><p> </p>

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.