Eddy Backscatter and Counter-Rotating Gyre Anomalies of Midlatitude Ocean Dynamics

Igor Shevchenko,Pavel Berloff

doi:10.3390/fluids1030028

Abstract

This work concerns how two competing mechanisms—eddy backscatter and counter-rotating gyre anomalies—influence the midlatitude ocean dynamics, as described by the eddy-resolving quasi-geostrophic (QG) model of wind-driven gyres. We analyzed dynamical balances and effects of different eddy forcing components, as well as their dependencies on increasing vertical resolution and decreasing eddy viscosity and found that the eastward jet and its adjacent recirculation zones are maintained mostly by the eddy forcing via the eddy backscatter mechanism, whereas the time-mean eddy-forcing component plays not only direct jet-supporting but also indirect jet-inhibiting role. The latter is achieved by inducing zonally elongated anticyclonic/cyclonic Counter-rotating Gyre Anomaly (CGA) in the subpolar/subtropical gyre. The indirect effect of CGAs on the eastward jet is found to be moderate relative to the dominant eddy backscatter mechanism. We also found that the higher the vertical baroclinic mode, the weaker its backscatter role and the stronger its CGA-driving role. Although the barotropic and first baroclinic modes are the most efficient ones in maintaining the backscatter, the higher, up to the fifth baroclinic modes also have significant but reverse impact that reduces the backscatter.

Highlights

Dynamics of the large-scale ocean circulation is tackled from various perspectives ranging from theoretical analyses of light, process-oriented problems to extremely large computations of comprehensive ocean general circulation models (OGCMs) (e.g., [1,2,3])
A rare study of the influence of vertical resolution on the eastward jet extension is [11], where the authors were focused on the eddy-permitting three- and six-layer QG double gyres and concluded that the third and higher baroclinic modes play a catalytic role resulting in the nonlinear amplification of the eastward jet
We focused on how eddies affect the eastward jet extension of the western boundary currents and the interior gyres

Summary

Introduction

Dynamics of the large-scale ocean circulation is tackled from various perspectives ranging from theoretical analyses of light, process-oriented problems to extremely large computations of comprehensive ocean general circulation models (OGCMs) (e.g., [1,2,3]). A rare study of the influence of vertical resolution on the eastward jet extension is [11], where the authors were focused on the eddy-permitting three- and six-layer QG double gyres and concluded that the third and higher baroclinic modes play a catalytic role resulting in the nonlinear amplification of the eastward jet. Another series of works [12,13] studied more idealized, horizontally homogeneous, eddy-resolving QG dynamics and found that for the ocean-like stratification the kinetic energy is transferred from high baroclinic modes through the first baroclinic mode to the barotropic one. QG model with focus on the eastward jet extension of the western boundary currents and on the interior gyres

Double-Gyre Model

Analyses of the Double-Gyre Solutions

Eddy Backscatter and Vertical Modes

Counter-Rotating Gyre Anomalies

Findings

Conclusions