Multiple zonal jets on the polar beta plane

Geophysical Research Letters

2012

[1] Geophysical turbulence is strongly influenced by the variation of the Coriolis force with latitude (b-effect): when this effect is significant an anisotropic inverse cascade is developed since energy is preferentially transferred towards zonal modes. The consequent emergence of flow structures along the zonal direction can strongly impact turbulent transport and modify meridional scalar diffusion. In this letter we investigate zonal and meridional diffusion in laboratory experiments of turbulence affected by a b-effect. The degree of anisotropy and the flow characteristic scales have been quantified according to a Lagrangian approach based on the reconstruction of tracer trajectories, i.e., in analogy with oceanic drifters data, and on the Finite-Scale Lyapunov Exponent (FSLE) analysis. An experimental confirmation of a recently introduced scaling law for the meridional diffusion in the marginally zonostrophic regime is also presented. Turbulence and Diffusion in Flows With a b-Effect[2] Large scale planetary and terrestrial circulation are studied under the paradigm of the geostrophic turbulence introduced by Charney [1971] to model a turbulent, rotating, stably stratified fluid in near geostrophic balance. This model, based on the three-dimensional inviscid conservation equation for the potential vorticity, presents several analogies with purely 2D turbulence, e.g., the conservation of two quadratic invariants. Nevertheless, important differences exist [Read, 2001]. Here, among these differences, we will focus on turbulence modification by the meridional variation of the Coriolis parameter, i.e., the so-called b-effect, defined approximately as f = f 0 + by, where f 0 is twice the rotation angular velocity W and y is the latitude coordinate. Rhines considered this problem in the context of unforced barotropic flows [Rhines, 1975] demonstrating that the b-effect induces the anisotropization of the upward energy transfer preferentially directing it towards zonal modes. The emergence of the flow anisotropy was explained in terms of a competition between nonlinear and b terms and anisotropy of the dispersion relation. The associated characteristic scale separating the regions of wave vector space where either b or nonlinear effects respectively dominate is known as the Rhines wavenumber, n R , and can be expressed in terms of the root mean square of the velocity and b. Similar ideas can be applied to geostrophic turbulence where the baroclinic instability takes the role of the small-scale forcing and the non-linear interaction between eddies and the mean flow plays an important role in flow anisotropization; nevertheless, for small Burger numbers, the inverse cascade develops in the barotropic mode due to the barotropization mechanism [Salmon, 1980;Rhines, 1994;Read, 2001;Galperin et al., 2010].[3] The concept of the cascade arrest was introduced by Rhines [1975] for unforced flows and recently revisited by Sukoriansky et al. [2007] in the context of small-scale forced, large-scale damped, barotropic vorticit...

Section: Turbulence and Diffusion In Flows With A β‐Effectmentioning

confidence: 99%

On the influence of a β‐effect on Lagrangian diffusion

Lacorata

Geophysical Research Letters

2012

“…The formation of multiple jets was observed also in experiments with a spatially localized heater (Fig. 5) [39]. [39].…”

Section: The β-Plane Turbulencementioning

confidence: 66%

Zonal Jets in Rotating Shallow Water Turbulence

Nitto

Cenedese

2013

EARTH

During the last three decades, the appearance of multiple zonal jets in planetary atmospheres and in the Earth's oceans has widely studied. Evidences of this phenomenon were recovered in numerical simulations [1], laboratory experiments [2][3][4] and in field measurements of giant planets' atmosphere [5]. Recent studies have revealed the presence of zonation also in the Earth's oceans; in fact, zonal jets were recovered in the outputs of Oceanic General Circulation Models-GCMs [6] and from satellite altimetry observations [7]. In previous works [3-4], we have investigated the impact of several experimental parameters on jets organization both in decaying and forced regimes. This work shows new results in the context of continuously forced flows obtained performing experiments in a bigger domain. The experimental set-up consists of a rotating tank where the initial distribution of vorticity is generated via the Lorentz force in an electromagnetic cell and the latitudinal variation of the Coriolis parameter is simulated by the parabolic profile assumed by the free surface of the rotating fluid. The velocity fields were measured using an image analysis technique. The flow is characterized in terms of zonal and radial flow pattern, flow variability and jet scales.

“…The existence of a banded structure of alternating zonal jets is recognized as one of the most important phenomena related to atmospheric and oceanic circulation. In this context, the dynamic's underlying jet formation is still debated and several mechanisms have been identified as a possible cause of zonation: the redirection of the inverse energy cascade into zonal modes due to turbulence-wave interaction (Galperin et al 2004;Nadiga 2006;Galperin et al 2010), potential vorticity mixing (Dritschel and McIntyre 2008), baroclinic instability and baroclinic eddy-mean flow interactions (Kaspi and Flierl 2007), and b-plume perturbation (Afanasyev et al 2011;Slavin and Afanasyev 2012;Wang et al 2012). Oceanic jets are, in fact, difficult to detect because they are narrow (18-38 of latitude) and weaker than the surrounding mesoscale eddies; also, they have a higher time variability compared to planetary jets.…”

Section: Introductionmentioning

confidence: 99%

Anisotropic Lagrangian Dispersion in Rotating Flows with a β Effect

Journal of Physical Oceanography

Lacorata

Nitto

2014

A detailed analysis of Lagrangian tracer dispersion is performed on datasets obtained from laboratory experiments that simulate rotating turbulence in the presence of a b effect. Compatible with the limitations of the experimental apparatus, a relatively wide range of the zonostrophy index R b , a parameter used to characterize flow regimes in b-plane turbulence, is explored. The considered range spans from values ;O(10 21 ), for which the flow is nearly isotropic, to values ;O(1), corresponding to the so-called transitional range in which the flow gradually leaves the friction-dominated regime to enter the full zonostrophic regime. The degree of anistropy and the characteristic scales of the flow have been estimated by means of a Lagrangian approach based on the reconstruction of tracer trajectories and on the measure of the finite-scale Lyapunov exponents (FSLE). The FSLE analysis allows one to identify the regimes of two-particle dispersion and to relate them to the physical parameters of the system. Moreover, a Lagrangian anisotropy index (LAI) is introduced and defined in terms of the FSLE zonal and radial components, in order to describe the onset of anisotropy and to check if it is consistent with the theoretical predictions. It is remarkable that the finite-scale dispersion rates are very sensitive to the degree of anistropy of turbulence, more so than other indicators defined in terms of Eulerian quantities. Furthermore, they offer an effective diagnostic tool of the degree of anisotropy that can be used even prior to attaining a fully developed regime of zonostrophic turbulence.