Objective Detection of Oceanic Eddies and the Agulhas Leakage

Abstract

Mesoscale oceanic eddies are routinely detected from instantaneous velocities derived from satellite altimetry data. While simple to implement, this approach often gives spurious results and hides true material transport. Here it is shown how geodesic transport theory, a recently developed technique from nonlinear dynamical systems, uncovers eddies objectively. Applying this theory to altimetry-derived velocities in the South Atlantic reveals, for the first time, Agulhas rings that preserve their material coherence for several months, while ring candidates yielded by other approaches tend to disperse or leak within weeks. These findings suggest that available velocity-based estimates for the Agulhas leakage, as well as for its impact on ocean circulation and climate, need revision.

Eddy Trains and Striations in Quasigeostrophic Simulations and the Ocean

“…However, it is still possible that more realistic flows, such as our double-gyre flow and real ocean circulations, contain vortices that straddle zonal jets and thus only partly contribute to striations. The estimate of eddy contribution to the striations depends on the method of identifying eddy boundaries, which is a challenging task (e.g., Beron-Vera et al 2013;Wu 2014;Faghmous et al 2015;Wang et al 2015). For example, Buckingham and Cornillon (2013) showed that eddies detected by a contour identification method account for 30%-70% of the velocity variance of striations.…”

“…Figure 2a shows a cross section of an anticyclonic Gaussian eddy, and the corresponding E 2 parameter; this parameter is nonzero only within the vortex core, which overlaps the vortex part circled by the extremum of the rotational speed. This E 2 parameter is based on rotation and fluid deformation, unlike some other techniques that emphasize eddy material transport (e.g., Beron-Vera et al 2013;Wang et al 2015). A comparison of different methods on identifying mesoscale eddies can be found in Faghmous et al (2015).…”

Imprint of Southern Ocean eddies on chlorophyll

“…Indeed, observations of eddies carrying the signature of their origin in their cores support the trapping effect (Bernard et al, 2007;Ansorge et al, 2009;Lehahn et al, 2011), and so does the modeling study by Early et al (2011). Yet, likely only few eddies are truly efficient in their trapping (Beron-Vera et al, 2013;Haller, 2015). They tend to continuously exchange some fluid with their surroundings, i.e., their trapping is limited by them being permeable.…”

“…Eddies tend to not trap perfectly but continuously leak and entrain waters ambient to their cores (Beron-Vera et al, 2013;Wang et al, 2015;Haller, 2015). The importance of such lateral entrainment or permeable trapping is supported 5 by the pronounced monopole contributions to the shapes of the local δChl imprint (Figures 5,6).…”

Benchmarking the mesoscale variability in global ocean eddy-permitting numerical systems

“…Despite the implementation of this cutoff, OW has been proved to be rather affected by noise generated in W-fields that increases falsely detected eddies (Souza et al 2011a;D'Ovidio et al 2009). To overcome this issue, some novel approaches based on flow geometry characteristics or additional physical constraints were developed (e.g., Ari Sadarjoen and Post 2000;Turiel et al 2007;Chaigneau et al 2008;Nencioli et al 2010;Ubelmann and Fu 2011;Beron-Vera et al 2013;Halo et al 2014). These methods include additional empirical parameters to identify the correct structures, complicating the eddy identification procedure.…”