On the mid-latitude tropopause height and the orographic-baroclinic adjustment theory

In the extratropics the analysis of the time–space structure of the dynamical tropopause shows a marked signature of nonpropagating, low-frequency (time-scale >10 d), ultra-long (zonal wavenumber <5) waves. This suggests the extension of theories relating the tropopause height to the baroclinic adjustment to the orographic-baroclinic disturbances, generally operating in the low-frequency domain. Such an extension is here proposed. By analysing Eady modes in a Boussinesq atmosphere, it has been found that the form-drag instability must be accounted for in an extended theory of baroclinic neutralization. The produced unstable standing waves carry a poleward large amount of heat at planetary scale for most of the external parameter settings and their spatial structure strongly resembles the observed winter mid-latitude eddy fields. Furthermore, we show how a simple representation of the stratosphere affects the tropopause neutralization requirements

Noise in multiple sclerosis: unwanted and necessary

As our knowledge about the etiology of multiple sclerosis (MS) increases, deterministic paradigms appear insufficient to describe the pathogenesis of the disease, and the impression is that stochastic phenomena (i.e. random events not necessarily resulting in disease in all individuals) may contribute to the development of MS. However, sources and mechanisms of stochastic behavior have not been investigated and there is no proposed framework to incorporate nondeterministic processes into disease biology. In this report, we will first describe analogies between physics of nonlinear systems and cell biology, showing how small-scale random perturbations can impact on large-scale phenomena, including cell function. We will then review growing and solid evidence showing that stochastic gene expression (or gene expression “noise”) can be a driver of phenotypic variation. Moreover, we will describe new methods that open unprecedented opportunities for the study of such phenomena in patients and the impact of this information on our understanding of MS course and therapy

Thermohaline circulation sensitivity to intermediate‐level anomalies

A two-dimensional Boussinesq ocean model has been used to investigate the effect ofintermediate-level thermal and saline anomalies on the known multiple equilibria structure ofthe thermohaline circulation. These anomalies are taken as a crude representation of theMediterranean outflow in the Atlantic Ocean. The associated perturbation drives the systemtowards an overturning which resembles the present average Atlantic thermohaline circulation.The sensitivity to the depth at which the anomaly is placed is also investigated. We found thatnear-surface anomalies are more efficient in affecting the structure of the equilibria.DOI: 10.1034/j.1600-0870.2002.01284.

North Atlantic MOC variability and the Mediterranean Outflow: A box-model study

A simple box-model is used to investigate the effect of intermediate level heat/freshwater fluxes on the variability of the oceanic meridional overturning circulation. The model includes a simple representation of the spreading of the Mediterranean Outflow Water in the North Atlantic. We identify an internal advective feedback affecting the amplitude of the thermohaline oscillations. When a salinity gradient is maintained in the ocean interior the oscillations are amplified. Instead, if the intermediate level fluxes are spread in the ocean deep layers, the model variability is reduced. We suggest that this mechanism may be relevant for climate variability on interdecadal timescales

Formula for a baroclinic adjustment theory of climate

Recently, a theory relating baroclinic neutrality and midlatitudes tropopause height has beenproposed. However, GCM results have shown that the dependence of the theory on externalparameters is not consistent with that displayed by these numerical experiments. In the presentpaper we suggest an analytic formula for baroclinic adjustment to the neutrality of Eady wavesthrough tropopause modification. This formula extends considerably the abovementionedtheory by taking into account both a simple representation of the stratosphere and the topography.These modifications alter the tropopause condition for a baroclinically neutral stateand its sensitivity to the external parameters. In particular, the topography introduces a dependenceon the tropospheric vertical wind shear of the neutrality condition. This feature is notpresent in other models that assume a background state with a zero potential vorticity gradientin the troposphere.We show, furthermore, that the modified neutrality condition has sensitivitiesthat may resemble those displayed by GCM simulations, with respect to the parameters definingthe background flow.DOI: 10.1034/j.1600-0870.2002.00296.

On stochastic perturbation of simple blocking models.

Discusses some problems connected with the stochastic dynamics generated by the effect of small random perturbations acting on prototype equations governing large-scale flows. We focus on the features of the process which occur when the main instability is the result of orographic forcing on simple barotropic motion. In particular, we study a three-mode truncation of barotropic flow over topography when a stochastic white forcing acts on the system

On stochastic perturbation and long-term climate behaviour.

A very simple energy-balance equation for the mean surface temperature of the planet (Budyko-type model) is perturbed by a stochastic term, simulating the numerous 'weather' processes not considered in the deterministic feed-back term. In the first instance, a symmetrical deterministic process with 3 equilibrium states is considered.-from Autho

ON THE PROBABILITY DENSITY DISTRIBUTION OF PLANETARY-SCALE ATMOSPHERIC WAVE AMPLITUDE

The statistical properties of a measure of planetary-scale wave activity are investigated in a 16 winter NMC dataset. The probability density distribution of the wavenumber 2 to 4 amplitude is found to be bimodal, confirming earlier results from a smaller dataset. Partitioning the data based on the density estimation reveals 2 statistical flow regimes in physical space. One corresponds to an amplified planetary-scale wave pattern and the other to a predominantly zonal flow. Both regimes exhibit a baroclinic vertical structure but the difference between them is equivalent barotropic. These differences extend through the depth of the troposphere and appear to be of hemispheric exten