The Dynamical Structure of a Warm Core Ring as Inferred from Glider Observations and Along-Track Altimetry

Abstract

This study investigates the vertical structure of the dynamical properties of a warm-core ring in the Gulf of Mexico (Loop Current ring) using glider observations. We introduce a new method to correct the glider’s along-track coordinate, which is, in general, biased by the unsteady relative movements of the glider and the eddy, yielding large errors on horizontal derivatives. Here, we take advantage of the synopticity of satellite along-track altimetry to apply corrections on the glider’s position by matching in situ steric height with satellite-measured sea surface height. This relocation method allows recovering the eddy’s azimuthal symmetry, precisely estimating the rotation axis position, and computing reliable horizontal derivatives. It is shown to be particularly appropriate to compute the eddy’s cyclo-geostrophic velocity, relative vorticity, and shear strain, which are otherwise out of reach when using the glider’s raw traveled distance as a horizontal coordinate. The Ertel potential vorticity (PV) structure of the warm core ring is studied in details, and we show that the PV anomaly is entirely controlled by vortex stretching. Sign reversal of the PV gradient across the water column suggests that the ring might be baroclinically unstable. The PV gradient is also largely controlled by gradients of the vortex stretching term. We also show that the ring’s total energy partition is strongly skewed, with available potential energy being 3 times larger than kinetic energy. The possible impact of this energy partition on the Loop Current rings longevity is also discussed.

Reconstructing the Three-Dimensional Structure of Loop Current Rings from Satellite Altimetry and in situ Data using the Gravest Empirical Modes Method

“…Consistent with previous ship and glider observations of individual LCRs [7,27], the typical LCR is characterized by a warm temperature anomaly with weaker stratification, and a double core salinity structure, with a fresher anomaly near the surface and a high salinity anomaly between 150 and 300 m. Although typical LCRs are large eddies, we found that the gradient-wind-balanced velocity was significantly larger than the geostrophic velocity (≈ +20% between the rotation axis and the maximum velocity radius), similar to Meunier et al's [30] recent observation in LCR Poseidon. This results in an increased relative vorticity, reaching half of the Coriolis frequency, showing that average LCRs (medium size) are significantly non linear eddies with a Rossby radius of 0.5.…”

“…The average LCR's PV structure consists of a bowl of low PV deflecting the main pycnocline's high PV strip downwards, and is mostly controlled by density stratification. It should be pointed out that the average LCR computed here has a weaker PV anomaly, with a lesser vertical extension, than the recent observations of Meunier et al [30] of LCR Poseidon. We stress this is related to the exceptionally thick thermostadt observed in Poseidon, while the work is focused on describing an average LCR.…”

“…However, the homogeneity of temperature in the upper part of the eddy's core (thermostadt) is not faithfully reproduced by the GEM method, which exhibits a slightly more stratified structure. Note that LCR Poseidon was an uncommon LCR with an exceptionally thick thermostadt [27,30], so we hypothesize that this difference is in part related to the exceptional nature of Poseidon. The salinity sections, on the other hand are not subject to this bias, and the double core structure, consisting of a salinity maximum between 200 and 350 m, and an homogeneous salinity minimum above, is well reproduced by the GEM reconstruction.…”

“…It was equipped with a sea bird CT-sail CTD probe, and had mean vertical and horizontal resolution of 2 m and 2 km, respectively. More details on the glider observations can be found in Meunier et al [27], [28], [29], and [30].…”

“…This results in an under (over) estimation of distances and an over (under) estimation of velocity in section 1 (2). This bias was discussed in detail by Meunier et al [27] and a correction method was proposed by Meunier et al [30]. However, for validation of the GEM fields, we chose to use the uncorrected glider along track coordinate to keep the analysis as straightforward as possible.…”