scholarly journals On the Dynamics of Flows Induced by Topographic Ridges

2015 ◽  
Vol 45 (3) ◽  
pp. 927-940 ◽  
Author(s):  
Changheng Chen ◽  
Igor Kamenkovich ◽  
Pavel Berloff
Keyword(s):  
Potential Vorticity ◽  
Mesoscale Eddies ◽  
Dynamical Analysis ◽  
Far Field ◽  
Layer Potential ◽  
Zonal Jets ◽  
Vorticity Source ◽  
Quasigeostrophic Model ◽  
Stable Current

AbstractThis study describes a nonlocal mechanism for the generation of oceanic alternating jets by topographic ridges. The dynamics of these jets is examined using a baroclinic quasigeostrophic model configured with an isolated meridional ridge. The zonal topographic slopes of the ridge lead to the formation of a system of currents, consisting of mesoscale eddies, meridional currents over the ridge, and multiple zonal jets in the far field. Dynamical analysis shows that transient eddies are vital in sustaining the deep meridional currents over the ridge, which in turn play a key role in the upper-layer potential vorticity (PV) balance. The zonal jets in the rest of the domain owe their existence to the eddy forcing over the ridge but are maintained by the local Reynolds and form stress eddy forcing. The analysis further shows that a broad stable current that either becomes locally nonzonal or encounters a topographic ridge tends to become unstable. This instability provides a vorticity source and generates multiple zonal jets in the far field through a nonlocal mechanism.

scholarly journals On latency of multiple zonal jets in the oceans

2011 ◽  
Vol 686 ◽  
pp. 534-567 ◽  
Author(s):  
P. Berloff ◽  
S. Karabasov ◽  
J. T. Farrar ◽  
I. Kamenkovich
Keyword(s):  
Friction Coefficient ◽  
Mesoscale Eddies ◽  
Reynolds Numbers ◽  
Bottom Friction ◽  
Mean Flow ◽  
Flat Bottom ◽  
Zonal Jets ◽  
Multiple Jets ◽  
Dynamical Simulations ◽  
Quasigeostrophic Model

AbstractMost of the nearly zonal, multiple, alternating jets observed in the oceans are latent, that is, their amplitudes are weak relative to the ambient mesoscale eddies. Yet, relatively strong jets are often observed in dynamical simulations. To explore mechanisms controlling the degree of latency, we analyse solutions of an idealized, eddy-resolving and flat-bottom quasigeostrophic model, in which dynamically generated mesoscale eddies maintain and interact with a set of multiple zonal jets. We find that the degree of the latency is controlled primarily by the bottom friction: the larger the friction parameter, the more latent are the jets; and the degree of the latency is substantial for a realistic range of the oceanic bottom friction coefficient. This result not only provides a plausible explanation for the latency of the oceanic jets, but it may also be relevant to the prominent atmospheric multiple jets observed on giant gas planets, such as Jupiter. We hypothesize that these jets can be so strong because of the relative absence of the bottom friction. The mechanism controlling the latency in our solutions is understood in terms of the changes induced in the linear eigenmodes of the time–mean flow by varying the bottom friction coefficient; these changes, in turn, affect and modify the jets. Effects of large Reynolds numbers on the eddies, jets, and the latency are also discussed.

scholarly journals Piecewise potential vorticity inversion without far-field response?

2020 ◽  
Author(s):  
Joseph Egger ◽  
Klaus P. Hoinka
Keyword(s):  
Boundary Conditions ◽  
Potential Vorticity ◽  
Lower Layer ◽  
Far Field ◽  
Potential Vorticity Inversion ◽  
Field Response ◽  
Quasigeostrophic Model ◽  
Physical Impact ◽  
The Impact ◽  
Field Influence

AbstractGiven a flow domain D with subdomains D1 and D2, piecewise potential vorticity inversion (PPVI) inverts a potential vorticity (PV) anomaly in D2 and assumes vanishing PV in D1 where boundary conditions must be taken into account. It is a widely held view that the PV anomaly exerts a far-field influence on D1 which is revealed by PPVI. Tests of this assertion are conducted using a simple quasigeostrophic model where an upper layer D2 contains a PV anomaly and D1 is the layer underneath. This anomaly is inverted. Any downward physical impact of PV in D2 must also be represented in the results of a downward piecewise density inversion (PDI) based on the hydrostatic relation and the density in D2 as following from PPVI. There is no doubt about the impact of the mass in D2 on the flow in the lower layer D1. Thus results of PPVI and PDI have to agree closely. First, PPVI is applied to a locally confined PV-anomaly in D2. There is no far-field ’response’ in D1 if stationarity is imposed. Modifications of boundary conditions lead to “induced” flows in D1 but the results of PPVI and PDI differ widely. This leads to a simple proof that there is no physical far-field influence of PV-anomalies in D2. Wave patterns of the streamfunction restricted to D2 are prescribed in a second series of tests. The related PV-anomalies are obtained by differentiation and are also confined to D2 in this case. This approach illustrates the basic procedure to derive PV-fields from observations which excludes a far-field response.

Vortex Dipole Formation by Baroclinic Instability of Boundary Currents

2007 ◽  
Vol 37 (6) ◽  
pp. 1661-1677 ◽  
Author(s):  
L. Chérubin ◽  
X. Carton ◽  
D. G. Dritschel
Keyword(s):  
Potential Vorticity ◽  
Baroclinic Instability ◽  
Point Vortex ◽  
Wave Dispersion ◽  
Finite Amplitude ◽  
Layer Potential ◽  
Weakly Nonlinear ◽  
In Situ Data ◽  
Vortex Dipole ◽  
Quasigeostrophic Model

Abstract In situ data of the Mediterranean Water undercurrents and eddies south of Portugal indicate that the undercurrents have a tubelike structure in potential vorticity and that dipole formation can occur when the lower undercurrent extends seaward below an offshore upper countercurrent. A two-layer quasigeostrophic model is used to determine the dynamical conditions under which dipole formation is possible. With piecewise-constant potential vorticity, the flow exhibits two linear modes of instability comparable to those found in the Phillips model with topography. Weakly nonlinear analysis and fully nonlinear simulations of the flow evolution agree on the regimes of either finite-amplitude perturbation saturation, corresponding to filamentation, or amplification, corresponding to vortex or dipole formation. This latter regime is more specifically studied: vortex dipole formation and ejection from the coast is obtained for long waves, with opposite-signed but similar amplitude layer potential vorticities. A simple point vortex model reproduces this phenomenon under the same conditions. It is then shown that dipole formation occurs for minimal wave dispersion, and hence for weak horizontal velocity shears. As observed at sea, dipoles are formed when the lower potential vorticity core extends seaward below a countercurrent.

scholarly journals Radiating Instability of a Meridional Boundary Current

2008 ◽  
Vol 38 (10) ◽  
pp. 2294-2307 ◽  
Author(s):  
Hristina G. Hristova ◽  
Joseph Pedlosky ◽  
Michael A. Spall
Keyword(s):  
Western Boundary ◽  
Far Field ◽  
Boundary Current ◽  
Important Conclusion ◽  
Western Boundary Currents ◽  
Horizontal Structure ◽  
Boundary Currents ◽  
Zonal Jets ◽  
Eastern Boundary ◽  
Eastern Boundary Current

Abstract A linear stability analysis of a meridional boundary current on the beta plane is presented. The boundary current is idealized as a constant-speed meridional jet adjacent to a semi-infinite motionless far field. The far-field region can be situated either on the eastern or the western side of the jet, representing a western or an eastern boundary current, respectively. It is found that when unstable, the meridional boundary current generates temporally growing propagating waves that transport energy away from the locally unstable region toward the neutral far field. This is the so-called radiating instability and is found in both barotropic and two-layer baroclinic configurations. A second but important conclusion concerns the differences in the stability properties of eastern and western boundary currents. An eastern boundary current supports a greater number of radiating modes over a wider range of meridional wavenumbers. It generates waves with amplitude envelopes that decay slowly with distance from the current. The radiating waves tend to have an asymmetrical horizontal structure—they are much longer in the zonal direction than in the meridional, a consequence of which is that unstable eastern boundary currents, unlike western boundary currents, have the potential to act as a source of zonal jets for the interior of the ocean.

scholarly journals Spatial Growth of Perturbations in a Turbulent Baroclinic Jet

2011 ◽  
Vol 68 (11) ◽  
pp. 2731-2741
Author(s):  
Rahul B. Mahajan ◽  
Gregory J. Hakim
Keyword(s):  
Prediction Models ◽  
Weather Prediction ◽  
Storm Track ◽  
Spreading Rate ◽  
Forecast Errors ◽  
Far Field ◽  
Quasigeostrophic Model ◽  
Spreading Rates ◽  
Analysis Errors ◽  
Numerical Weather Prediction Models

Abstract The spatial spreading of infinitesimal disturbances superposed on a turbulent baroclinic jet is explored. This configuration is representative of analysis errors in an idealized midlatitude storm track and the insight gained may be helpful to understand the spreading of forecast errors in numerical weather prediction models. This problem is explored through numerical experiments of a turbulent baroclinic jet that is perturbed with spatially localized disturbances. Solutions from a quasigeostrophic model for the disturbance fields are compared with those for a passive tracer to determine whether disturbances propagate faster than the basic-state flow. Results show that the disturbance spreading rate is sensitive to the structure of the initial disturbance. Disturbances that are localized in potential vorticity (PV) have far-field winds that allow the disturbance to travel downstream faster than disturbances that are initially localized in geopotential, which have no far-field wind. Near the jet, the spread of the disturbance field is observed to exceed the tracer field for PV-localized disturbances, but not for the geopotential-localized disturbances. Spreading rates faster than the flow for geopotential-localized disturbances are found to occur only for disturbances located off the jet axis. These results are compared with those for zonal and time-independent jets to qualitatively assess the effects of transience and nonlinearity. This comparison suggests that the average properties of localized perturbations to the turbulent jet can be decomposed into a superposition of dynamics associated with a time-independent parallel flow plus a “diffusion” process.

scholarly journals On Spectral Analysis of Mesoscale Eddies. Part I: Linear Analysis

2013 ◽  
Vol 43 (12) ◽  
pp. 2505-2527 ◽  
Author(s):  
P. Berloff ◽  
I. Kamenkovich
Keyword(s):  
Reynolds Number ◽  
Spectral Analysis ◽  
Large Scale ◽  
Normal Modes ◽  
Mesoscale Eddies ◽  
Linear Analysis ◽  
Vertical Shear ◽  
Systematic Analysis ◽  
Linear Dynamics ◽  
Zonal Jets

Abstract This study aims to understand the ocean’s circulation, which is characterized by the presence of multiple alternating zonal jets and transient mesoscale eddies, by systematic analysis of the underlying linear dynamics of this system. For this purpose, properties of the linear normal modes such as growth rates, dispersion, spatial structure, and nonlinear self-interactions are explored for a hierarchy of idealized, vertically, and horizontally sheared flows with increasing complexity. The authors find that large-scale background vertical shear, alternating multiple zonal jets, bottom friction, and the Reynolds number have important effects on these modes. This study hypothesizes that when these effects are taken into account, the linear results can be used to predict many properties of nonlinear mesoscale eddies. This hypothesis is confirmed in Part II of this paper.

scholarly journals Dynamics of the Upper Oceanic Layers in Terms of Surface Quasigeostrophy Theory

2006 ◽  
Vol 36 (2) ◽  
pp. 165-176 ◽  
Author(s):  
G. Lapeyre ◽  
P. Klein
Keyword(s):  
Potential Vorticity ◽  
Surface Density ◽  
Three Dimensional ◽  
Surface Boundary ◽  
Strongly Correlated ◽  
Basin Scale ◽  
Surface Boundary Condition ◽  
Balanced Flow ◽  
Circumpolar Current ◽  
Quasigeostrophic Model

Abstract In this study, the relation between the interior and the surface dynamics for nonlinear baroclinically unstable flows is examined using the concepts of potential vorticity. First, it is demonstrated that baroclinic unstable flows present the property that the potential vorticity mesoscale and submesoscale anomalies in the ocean interior are strongly correlated to the surface density anomalies. Then, using the invertibility of potential vorticity, the dynamics are decomposed in terms of a solution forced by the three-dimensional (3D) potential vorticity and a solution forced by the surface boundary condition in density. It is found that, in the upper oceanic layers, the balanced flow induced only by potential vorticity is strongly anticorrelated with that induced only by the surface density with a dominance of the latter. The major consequence is that the 3D balanced motions can be determined from only the surface density and the characteristics of the basin-scale stratification by solving an elliptic equation. These properties allow for the possibility to reconstruct the 3D balanced velocity field of the upper layers from just the knowledge of the surface density by using a simpler model, that is, an “effective” surface quasigeostrophic model. All these results are validated through the examination of a primitive equation simulation reproducing the dynamics of the Antarctic Circumpolar Current.

scholarly journals Role of Eddies in the Maintenance of Multiple Jets Embedded in Eastward and Westward Baroclinic Shears

Fluids ◽  
2018 ◽  
Vol 3 (4) ◽  
pp. 91 ◽  
Author(s):  
Hemant Khatri ◽  
Pavel Berloff
Keyword(s):  
Large Scale ◽  
Bottom Layer ◽  
Relative Vorticity ◽  
Global Ocean ◽  
Dynamic Interactions ◽  
Zonal Jets ◽  
Eddy Dynamics ◽  
Quasigeostrophic Model ◽  
The Impact

Multiple zonal jets observed in many parts of the global ocean are often embedded in large-scale eastward and westward vertically sheared background flows. Properties of the jets and ambient eddies, as well as their dynamic interactions, are found to be different between eastward and westward shears. However, the impact of these differences on overall eddy dynamics remains poorly understood and is the main subject of this study. The roles of eddy relative vorticity and buoyancy fluxes in the maintenance of oceanic zonal jets are studied in a two-layer quasigeostrophic model. Both eastward and westward uniform, zonal vertically sheared cases are considered in the study. It is shown that, despite the differences in eddy structure and local characteristics, the fundamental dynamics are essentially the same in both cases: the relative-vorticity fluxes force the jets in the entire fluid column, and the eddy-buoyancy fluxes transfer momentum from the top to the bottom layer, where it is balanced by bottom friction. It is also observed that the jets gain more energy via Reynolds stress work in the layer having a positive gradient in the background potential vorticity, and this is qualitatively explained by a simple reasoning based on Rossby wave group velocity.

Tropopause Evolution in a Rapidly Intensifying Tropical Cyclone: A Static Stability Budget Analysis in an Idealized Axisymmetric Framework

2019 ◽  
Vol 76 (1) ◽  
pp. 209-229 ◽  
Author(s):  
Patrick Duran ◽  
John Molinari
Keyword(s):  
Potential Vorticity ◽  
Lower Stratosphere ◽  
Potential Temperature ◽  
Static Stability ◽  
Layer Potential ◽  
Upper Troposphere ◽  
Budget Analysis ◽  
Upper Level ◽  
Cold Point ◽  
Differential Advection

Abstract Upper-level static stability (N2) variations can influence the evolution of the transverse circulation and potential vorticity in intensifying tropical cyclones (TCs). This paper examines these variations during the rapid intensification (RI) of a simulated TC. Over the eye, N2 near the tropopause decreases and the cold-point tropopause rises by up to 4 km at the storm center. Outside of the eye, N2 increases considerably just above the cold-point tropopause and the tropopause remains near its initial level. A budget analysis reveals that the advection terms, which include differential advection of potential temperature θ and direct advection of N2, are important throughout the upper troposphere and lower stratosphere. These terms are particularly pronounced within the eye, where they destabilize the layer near and above the cold-point tropopause. Outside of the eye, a radial–vertical circulation develops during RI, with strong outflow below the tropopause and weak inflow above. Differential advection of θ near the outflow jet provides forcing for stabilization below the outflow maximum and destabilization above. Turbulence induced by vertical wind shear on the flanks of the outflow maximum also modifies the vertical stability profile. Meanwhile, radiative cooling tendencies at the top of the cirrus canopy generally act to destabilize the upper troposphere and stabilize the lower stratosphere. The results suggest that turbulence and radiation, alongside differential advection, play fundamental roles in the upper-level N2 evolution of TCs. These N2 tendencies could have implications for both the TC diurnal cycle and the tropopause-layer potential vorticity evolution in TCs.

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