Hyperbolicity in temperature and flow fields during the formation of a Loop Current ring

Abstract

Abstract. Loop Current rings (LCRs) are among the largest mesoscale eddies in the world ocean. They arise when bulges formed by the Loop Current in the Gulf of Mexico close off. The LCR formation process may take several weeks, and there may be several separations and reattachments before final separation occurs. It is well established that this period is characterized by a persistent saddle point in the sea surface height field, as seen in both model and satellite data. We present here a detailed study of this saddle region during the formation of Eddy Franklin in 2010, over multiple days and at several depths. Using a data-assimilating Gulf of Mexico implementation of the HYbrid Coordinate Ocean Model (HYCOM), we compare the vertical structure of the currents and temperature fields on 5 and 10 June 2010. Finite-time Lyapunov exponents (FTLE) are computed from the surface down to 200 m to estimate the location of relevant transport barriers. Several new features of the saddle region associated with LCR formation are revealed: the ridges in the FTLE fields are shown to be excellent surrogates for the manifolds delineating the material flow structures with only slight degradation at depth. The intersection of the ridges representing stable and unstable manifolds drops nearly vertically through the water column at both times; remarkably, the material boundary shapes are maintained even as they are advected. Moreover, velocity stagnation points and saddle points in the temperature field are consistently found near the intersections at all depths, and their geographic positions are also nearly constant with depth.

Lagrangian descriptors and the assessment of the predictive capacity of oceanic data sets

“…On the first day of our study, 11 June, the Loop Current is pinched off forming a large anticyclonic Loop as reported by Sulman et al (2013). Consistently, this scenario is observed both from AVISO and HYCOM data as observed from Fig.…”

“…A similar depth choice for this model is used, for instance, in Sulman et al (2013) or for CUPOM (Colorado University Princeton Ocean Model) in Branicki and Kirwan (2010). In the latter work the authors show how the resulting Lagrangian structure is close to 2-D surfaces that extend nearly vertically into the water column, thus confirming that the choice of a deeper layer just affects the noise on the observed structures.…”

“…The Yucatán-Florida current system is in this region, also known as the Loop Current (Forristall et al, 1992;AndradeCanto et al, 2013;Sulman et al, 2013). This current enters the gulf through the Yucatán channel, then circulates clockwise and exits the gulf through the Straits of Florida, flowing eventually into the Gulf Stream.…”

“…Specific details can be found at http://hycom.org/ dataserver/goml0pt04. The output of this model has been previously used to study transport processes in the Gulf of Mexico by Olascoaga (2010), Mezic et al (2010), andSulman et al (2013).…”

Lagrangian analysis of the vertical structure of eddies simulated in the Japan Basin of the Japan/East Sea

“…Two-dimensional Lagrangian approach has been successfully applied for studying horizontal transport and mixing in numerically-generated and altimetric velocity fields in different basins, from bays (Lekien et al, 2005;Lipphardt et al, 2006;Gildor et al, 2009;Prants et al, 2013b) and seas (Abraham and Bowen, 2002;d'Ovidio et al, 2004;Schneider et al, 2005;Mancho et al, 2008;Prants et al, 2013a) to the ocean scale (Beron-Vera et al, 2008;Waugh and Abraham, 2008;Prants, 2013;Prants et al, 2014c). Lagrangian methods are especially suitable because they allow to delineate eddy's boundaries (Prants et al, 2011a;Beron-Vera et al, 2013;Prants et al, 2014c), to visualize transport barriers and corridors along which the core of an eddy is exchanged by water with its surrounding (Sulman et al, 2013;Prants et al, 2013b,a) and to quantify that transport (Miller et al, 1997).…”