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Binson Joseph and Bernard Legras

Abstract

Maximum stretching lines in the lower stratosphere around the Antarctic polar vortex are diagnosed using a method based on finite-size Lyapunov exponents. By analogy with the mathematical results known for simple dynamical systems, these curves are identified as stable and unstable manifolds of the underlying hyperbolic structure of the flow. For the first time, the exchange mechanism associated with lobe dynamics is characterized using atmospheric analyzed winds. The tangling manifolds form a stochastic layer around the vortex. It is found that fluid is not only expelled from this layer toward the surf zone but also is injected inward from the surf zone, through a process similar to the turnstile mechanism in lobe dynamics. The vortex edge, defined as the location of the maximum gradient in potential vorticity or tracer, is found to be the southward (poleward) envelope of this stochastic layer. Exchanges with the inside of the vortex are therefore largely decoupled from those, possibly intense, exchanges between the stochastic layer and the surf zone. It is stressed that using the kinematic boundary defined by the hyperbolic points and the manifolds as an operational definition of vortex boundary is not only unpractical but also leads to spurious estimates of exchanges. The authors anticipate that more accurate dynamical systems tools are needed to analyze stratospheric transport in terms of lobe dynamics.

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Robert Vautard and Bernard Legras

Abstract

We present a new statistical-dynamical approach to the concept of weather regimes, including the effect of tralisients, without any assumption other than scale separation. The method is applied to a quasi-geostrophic channel model without topography and forced by a local baroclinic jet. Baroclinic perturbations grow and decay along a storm track which is linked with a maximum of low-frequency variability towards its exit, in agreement with the observations.

The weather regimes are searched within the subspace spanned by the large scales only. They are identified through the resolution of a stationary problem in which the feedback of the transients is included as an ensemble average over analogs of the large-scale flow. In this way, the feedback is a continuous function of the large-scale flow only, and the system of equations is closed, taking into account the whole coupling. The solution is obtained using a nonlinear optimization technique.

Several regimes are identified corresponding to zonal and blocking situations. The blocking flow is characterized by a well-marked barotropic dipole at the end of the storm track of synoptic perturbations. The feedback term is shown to act positively in both cases though there are major differences between zonal and blocking regimes. In particular we show that the dipole of the blocking flow is essentially maintained against dissipation by the small-scale fluxes. It is shown that full nonlinearity is required to explain the observed behavior.

The efficiency of the method in this simple case allows us to discuss its extension to a more ambitious diagnostic of regimes in atmospheric observations as well as GCM simulations.

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Robert Vautard, Bernard Legras, and Michel Déqué

Abstract

The forcing of low-frequency variability by synoptic transient traveling perturbations is investigated within a quasi-geostrophic channel forced by a localized baroclinic jet. Spontaneously generated baroclinic perturbations grow and decay along a storm track; at the end of the track a maximum of low-frequency variability is obtained, in agreement with atmospheric observations. The structure of low-frequency variability is studied with a combination of statistical methods, using a multivariate red noise model as a random reference. We show that the anomalies are preferentially linked with local stationary structures or long-wave vacillations according to their location and their sign.

A systematic study of persistence is conducted with a criterion based on rms of the streamfunction variations. The interesting quantity is the probability of persistence which shows a very inhomogeneous distribution in phase space and several separated maxima. The composites based on these maxima exhibit the characters of zonal and blocking regimes. The transient feedback has a positive role in extending the jet downstream but the primary effect is the maintenance of the blocking circulation.

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Emily Shuckburgh, Francesco d’Ovidio, and Bernard Legras

Abstract

The Lyapunov diffusivity is used to investigate local isentropic mixing events in the upper troposphere–lower stratosphere (UTLS) region. The diagnostic highlights the seasonal cycle of the longitudinally varying mixing properties, in particular those associated with the monsoon circulations and the westerly ducts in the subtropics. The results are broadly consistent with studies of Rossby wave–breaking frequencies. The mixing structure is shown to be modulated by modes of atmospheric variability. El Niño–Southern Oscillation (ENSO) is found to strongly influence the mixing structure throughout the tropics and subtropics. The subtropical jet is associated with longitudinal bands of mixing minima (isentropic mixing barriers) separated by synoptic-scale regions of strong mixing activity. The greatest ENSO modulation in December–February is confined to the Pacific sector, where the barriers associated with the subtropical jets extend farther into the eastern Pacific, and in the western Pacific a barrier is found at the equator during the positive phase. During June–August, the influence is seen to extend beyond the Pacific region, with the barrier at the subtropical jet in the Southern Hemisphere increasing in strength at all longitudes and with an increase in strength (and isolation of) monsoon-related mixing over Asia and North America. The local influence of the North Atlantic Oscillation in wintertime is investigated. During the positive phase, a double-barrier structure is associated with the subtropical jet, the northern branch crosses the Atlantic toward Scandinavia, and the southern branch tends toward North Africa. The Antarctic Oscillation is shown to influence whether the subvortex region is isolated from midlatitudes.

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Francesco d’Ovidio, Emily Shuckburgh, and Bernard Legras

Abstract

A new diagnostic (the “Lyapunov diffusivity”) is presented that has the ability to quantify isentropic mixing in diffusion units and detects local mixing events by describing latitude–longitude variability. It is a hybrid diagnostic, combining the tracer-based effective diffusivity with the particle-based Lyapunov exponent calculation. Isentropic mixing on the 350-K surface shows that there is significant longitudinal variation to the strength of mixing at the northern subtropical jet, with a strong mixing barrier over Asia and the western Pacific, a weaker mixing barrier over the western Atlantic, and active mixing regions at the jet exits over the eastern Pacific and Atlantic.

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Paul-Antoine Michelangeli, Robert Vautard, and Bernard Legras

Abstract

Two different definitions of midlatitude weather regimes are compared. The first seeks recurrent atmospheric patterns. The second seeks quasi-stationary patterns, whose average tendency vanishes. Recurrent patterns are identified by cluster analysis, and quasi-stationary patterns are identified by solving a nonlinear equilibration equation. Both methods are applied on the same dataset: the NMC final analyses of 700-hPa geopotential heights covering 44 winters. The analysis is performed separately over the Atlantic and Pacific sectors.

The two methods give the same number of weather regimes—four over the Atlantic sector and three over the Pacific sector. However, the patterns differ significantly. The investigation of the tendency, or drift, of the clusters shows that recurrent flows have a systematic slow evolution, explaining this difference. The patterns are in agreement with the ones obtained from previous studies, but their number differs.

The cluster analysis algorithm used here is a partitioning algorithm, which agglomerates data around randomly chosen seeds and iteratively finds the partition that minimizes the variance within clusters, given a prescribed number of clusters. The authors develop a classifiability index, based on the correlation between the cluster centroids obtained from different initial pullings. By comparing the classifiability index of observations with that obtained from a multivariate noise model, an objective definition of the number of clusters present in the data is given. Although the classifiability index is maximal by prescribing two clusters in both sectors, it only differs significantly from that obtained with the noise model using four Atlantic clusters and three Pacific clusters. The partitioning clustering method turns out to give more statistically stable clusters than hierarchical clustering schemes.

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Guglielmo Lacorata, Erik Aurell, Bernard Legras, and Angelo Vulpiani

Abstract

The EOLE experiment is revisited to study turbulent processes in the lower stratosphere circulation from a Lagrangian viewpoint and to resolve a discrepancy on the slope of the atmospheric energy spectrum between the work of Morel and Larchevêque and recent studies using aircraft data. Relative dispersion of balloon pairs is studied by calculating the finite-scale Lyapunov exponent, an exit-time-based technique that is particularly efficient in cases in which processes with different spatial scales are interfering. The main goal is to reconciliate the EOLE dataset with recent studies supporting a k −5/3 energy spectrum in the 100–1000-km range. The results also show exponential separation at smaller scales, with a characteristic time of order 1 day, and agree with the standard diffusion of about 107 m2 s−1 at large scales. A remaining question is the origin of a k −5/3 spectrum in the mesoscale range between 100 and 1000 km.

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Annarita Mariotti, Carlos R. Mechoso, Bernard Legras, and Vincent Daniel

Abstract

The ozone evolution in the lower stratosphere of the Southern Hemisphere during the period 5–10 August 1994 is analyzed. The analysis focuses on the ozone “collar” (the band of maximum values in ozone mixing ratio around the Antarctic ozone “hole” at these altitudes) and the development of “collar filaments.” Ozone mixing ratios provided by the Microwave Limb Sounder (MLS) on board the Upper Atmosphere Research Satellite and by an ER-2 aircraft participating in the Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft campaign are compared with values at corresponding locations in high-resolution isentropic maps obtained by using the numerical scheme of “contour advection with surgery” (CAS).

The CAS reconstructed ozone maps provide a view of the way in which air masses are exported from the outskirts of the collar to form the “tongues” of higher mixing ratios observed at lower latitudes on MLS synoptic maps. There is an overall consistency between the datasets insofar as the collar location is concerned. This location seems to be primarily defined by the local properties of the flow. Nevertheless the CAS reconstructed collar tends to become weaker than that depicted by MLS data. By means of radiative calculation estimates, it is argued that diabatic descent may be responsible for maintaining the ozone concentration approximately constant in the collar while filaments isentropically disperse collarlike mixing ratios from this region toward lower latitudes.

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