Eddy Trains and Striations in Quasigeostrophic Simulations and the Ocean

Abstract

This study explores the relationship between coherent eddies and zonally elongated striations. The investigation involves an analysis of two baroclinic quasigeostrophic models of a zonal and double-gyre flow and a set of altimetry sea level anomaly data in the North Pacific. Striations are defined by either spatiotemporal filtering or empirical orthogonal functions (EOFs), with both approaches leading to consistent results. Coherent eddies, identified here by the modified Okubo-Weiss parameter, tend to propagate along well-defined paths, thus forming ''eddy trains'' that coincide with striations. The striations and eddy trains tend to drift away from the intergyre boundary at the same speed in both the model and observations. The EOF analysis further confirms that these striations in model simulations and altimetry are not an artifact of temporal averaging of random, spatially uncorrelated vortices. This study suggests instead that eddies organize into eddy trains, which manifest themselves as striations in low-pass filtered data and EOF modes.

Striations in Marginal Seas and the Mediterranean Sea

“…This result is less robust for JS, as DOA1 decreases again for T > 80 months. The time scales are shorter than those reported by C. Chen et al (2016), so in general, 10 years is appropriate for identifying striations in marginal seas and the MS.…”

“…Time averaging is usually the first step in detecting striations (Buckingham & Cornillon, ; C. Chen et al, ; Maximenko et al, , ). To choose a suitable averaging time scale, C. Chen et al () defined a parameter based on the degree of anisotropy and the ratio of area mean zonal velocity to a reference velocity. Their result shows that stationary striations can be identified by choosing a time scale longer than 9 years.…”

“…Time averaging is usually the first step in detecting striations (Buckingham & Cornillon, 2013;C. Chen et al, 2016;Maximenko et al, 2005Maximenko et al, , 2008.…”

“…They were first thought to be a kinematic artifact of time averaging of westward-propagating eddies (Schlax & Chelton, 2008), but Melnichenko et al (2010) examined the generation of striations in the eastern boundary areas of the Pacific using potential vorticity diagnostics and reported that eddy fluxes contribute to the striations' time-mean potential vorticity budget. In other studies, striations were inferred to be independent flow components embedded in an idealized eddy-resolving quasi-geostrophic model (Berloff et al, 2011), while others concluded that striations are organized by eddy movement in the North Pacific (Buckingham & Cornillon, 2013;C. Chen et al, 2016) or result from the instabilities extending from an unstable eastern boundary current (J.…”

Zonally Elongated Transient Flows: Phenomenology and Sensitivity Analysis

“…On the other hand, even randomly distributed westward propagating vortices can appear as zonally elongated flow patterns in time‐averaged fields (Schlax & Chelton, ). Although Buckingham and Cornillon () and Chen et al () argue that randomly distributed propagating eddies cannot explain the entire signal associated with anisotropic flow patterns, the time‐averaging does not seem to be a reliable tool for identifying ZELTs.…”