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Amanda K. O’Rourke and Geoffrey K. Vallis

Abstract

The eddy-driven and subtropical jets are two dynamically distinct features of the midlatitude upper-troposphere circulation that are often merged into a single zonal wind maximum. Nonetheless, the potential for a distinct double-jet state in the atmosphere exists, particularly in the winter hemisphere, and presents a unique zonal-mean flow with two waveguides and an interjet region with a weakened potential vorticity gradient upon which Rossby waves may be generated, propagate, reflect, and break.

The authors investigate the interaction of two groups of atmospheric waves—those with wavelengths longer and shorter than the deformation radius—within a double-jet mean flow in an idealized atmospheric model. Patterns of eddy momentum flux convergence for long and short waves differ greatly. Short waves behave following classic baroclinic instability theory such that their eddy momentum flux convergence is centered at the eddy-driven jet core. Long waves, on the other hand, reveal strong eddy momentum flux convergence along the poleward flank of the eddy-driven jet and within the interjet region. This pattern is enhanced when two jets are present in the zonal-mean zonal wind.

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Amanda K. O'Rourke and Geoffrey K. Vallis

Abstract

The feedback between planetary-scale eddies and analogs of the midlatitude eddy-driven jet and the subtropical jet is investigated in a barotropic β-plane model. In the model the subtropical jet is generated by a relaxation process and the eddy-driven jet by an imposed wavemaker. A minimum zonal phase speed bound is proposed in addition to the established upper bound, where the zonal phase speed of waves must be less than that of the zonal mean zonal flow. Cospectral analysis of eddy momentum flux convergence shows that eddy activity is generally restricted by these phase speed bounds.

The wavenumber-dependent minimum phase speed represents a turning line for meridionally propagating waves. By varying the separation distance between the relaxation and stirring regions, it is found that a sustained, double-jet state is achieved when either a critical or turning latitude forms in the interjet region. The interjet turning latitude filters eddies by zonal wavenumber such that shorter waves tend to be reflected off of the relaxed jet and are confined to the eddy-driven jet. The interjet region is transparent to long waves that act to blend the jets and may be associated with barotropic instability. The eddy-driven and relaxed jets tend to merge owing to the propagation of these long waves through the relaxed jet waveguide.

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Amanda K. O’Rourke, Brian K. Arbic, and Stephen M. Griffies

Abstract

Low-frequency variability at the ocean surface can be excited both by atmospheric forcing, such as in exchanges of heat and momentum, and by the intrinsic nonlinear transfer of energy between mesoscale ocean eddies. Recent studies have shown that nonlinear eddy interactions can excite an energy transfer from high to low frequencies analogous to the transfer of energy from high to low wavenumbers (small to large spatial scales) in quasi-two-dimensional turbulence. As the spatial inverse cascade is driven by oceanic eddies, the process of energy exchange across frequencies may be sensitive to ocean model resolution. Here a cross-spectrum diagnostic is applied to the oceanic component in a hierarchy of fully coupled ocean–atmosphere models to address the transfer of ocean surface kinetic energy between high and low frequencies. The cross-spectral diagnostic allows for a comparison of the relative contributions of coupled atmospheric forcing through wind stress and the intrinsic advection to low-frequency ocean surface kinetic energy. Diagnostics of energy flux and transfer within the frequency domain are compared between three coupled models with ocean model horizontal resolutions of 1°, 1/4°, and 1/10° to address the importance of resolving eddies in the driving of energy to low frequencies in coupled models.

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Nicholas J. Lutsko, Isaac M. Held, Pablo Zurita-Gotor, and Amanda K. O’Rourke

Abstract

In Earth’s atmosphere eddy momentum fluxes (EMFs) are largest in the upper troposphere, but EMFs in the lower troposphere, although modest in amplitude, have an intriguing structure. To document this structure, the EMFs in the lower tropospheres of a two-layer quasigeostrophic model, a primitive equation model, and the Southern Hemisphere of a reanalysis dataset are investigated. The lower-tropospheric EMFs are very similar in the cores of the jets in both models and the reanalysis data, with EMF divergence (opposing the upper-tropospheric convergence) due to relatively long waves with slow eastward phase speeds and EMF divergence (as in the upper troposphere) due to shorter waves with faster eastward phase speeds.

As the two-layer model is able to capture the EMF divergence by long waves, a qualitative picture of the underlying dynamics is proposed that relies on the negative potential vorticity gradient in the lower layer of the model. Eddies excited by baroclinic instability mix efficiently through a wide region in the lower layer, centered on the latitude of maximum westerlies and encompassing the lower-layer critical latitudes. Near these critical latitudes, the mixing is enhanced, resulting in increased EMF convergence, with compensating EMF divergence in the center of the jet. The EMF convergence at faster phase speeds is due to deep eddies that propagate on the upper-tropospheric potential vorticity gradient.

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