Annual and semiannual cycles of midlatitude near-surface temperature and tropospheric baroclinicity: reanalysis data and AOGCM simulations

Abstract. Seasonal variability in near-surface air temperature and baroclinicity from the ECMWF ERA-Interim (ERAI) reanalysis and six coupled atmosphere–ocean general circulation models (AOGCMs) participating in the Coupled Model Intercomparison Project phase 3 and 5 (CMIP3 and CMIP5) are examined. In particular, the annual and semiannual cycles of hemispherically averaged fields are studied using spectral analysis. The aim is to assess the ability of coupled general circulation models to properly reproduce the observed amplitude and phase of these cycles, and investigate the relationship between near-surface temperature and baroclinicity (coherency and relative phase) in such frequency bands. The overall results of power spectra agree in displaying a statistically significant peak at the annual frequency in the zonally averaged fields of both hemispheres. The semiannual peak, instead, shows less power and in the NH seems to have a more regional character, as is observed in the North Pacific Ocean region. Results of bivariate analysis for such a region and Southern Hemisphere midlatitudes show some discrepancies between ERAI and model data, as well as among models, especially for the semiannual frequency. Specifically, (i) the coherency at the annual and semiannual frequency observed in the reanalysis data is well represented by models in both hemispheres, and (ii) at the annual frequency, estimates of the relative phase between near-surface temperature and baroclinicity are bounded between about ±15° around an average value of 220° (i.e., approximately 1-month phase shift), while at the semiannual frequency model phases show a wider dispersion in both hemispheres with larger errors in the estimates, denoting increased uncertainty and some disagreement among models. The most recent CMIP climate models (CMIP5) show several improvements when compared with CMIP3, but a degree of discrepancy still persists though masked by the large errors characterizing the semiannual frequency. These findings contribute to better characterizing the cyclic response of current global atmosphere–ocean models to the external (solar) forcing that is of interest for seasonal forecasts

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

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.

Spatial patterns and temporal variability of drought in Western Iran

An analysis of drought in western Iran from 1966 to 2000 is presented using monthly precipitation data observed at 140 gauges uniformly distributed over the area. Drought conditions have been assessed by means of the Standardized Precipitation Index (SPI). To study the long-term drought variability the principal component analysis was applied to the SPI field computed on 12-month time scale. The analysis shows that applying an orthogonal rotation to the first two principal component patterns, two distinct sub-regions having different climatic variability may be identified. Results have been compared to those obtained for the largescale using re-analysis data suggesting a satisfactory agreement. Furthermore, the extension of the large-scale analysis to a longer period (1948–2007) shows that the spatial patterns and the associated time variability of drought are subjected to noticeable changes. Finally, the relationship between hydrological droughts in the two sub-regions and El Niño Southern Oscillation events has been investigated finding that there is not clear evidence for a link between the two phenomen

Response Properties of Human Amygdala Subregions: Evidence Based on Functional MRI Combined with Probabilistic Anatomical Maps

The human amygdala is thought to play a pivotal role in the processing of emotionally significant sensory information. The major subdivisions of the human amygdala—the laterobasal group (LB), the superficial group (SF), and the centromedial group (CM)—have been anatomically delineated, but the functional response properties of these amygdala subregions in humans are still unclear. We combined functional MRI with cyto-architectonically defined probabilistic maps to analyze the response characteristics of amygdala subregions in subjects presented with auditory stimuli. We found positive auditory stimulation-related signal changes predominantly in probabilistically defined LB, and negative responses predominantly in SF and CM. In the left amygdala, mean response magnitude in the core area of LB with 90–100% assignment probability was significantly larger than in the core areas of SF and CM. These differences were observed for pleasant and unpleasant stimuli. Our findings reveal that the probabilistically defined anatomical subregions of the human amygdala show distinctive fMRI response patterns. The stronger auditory responses in LB as compared with SF and CM may reflect a predominance of auditory inputs to human LB, similar to many animal species in which the majority of sensory, including auditory, afferents project to this subdivision of the amygdala. Our study indicates that the intrinsic functional differentiation of the human amygdala may be probed using fMRI combined with probabilistic anatomical maps

On the determination of atmospheric water vapor content from ground-based GPS

Atmospheric water vapor has a fundamental role in radiative transfer, atmospheric dynamics, and the hydrological cycle. Despite the considerable progress in the Earth’s observing system, it is still inadequately characterized because of its high spatial and temporal variability and the lack of long-term homogeneous observational data. The availability of long records of high-temporal resolution ground-based GPS measurements of total column water vapor (TCWV) offers a unique opportunity for a variety of atmospheric research studies. The potential of such measurements that are not yet fully exploited are here presented and discussed. In particular, the following issues are outlined: (i) the characteristics of high-temporal resolution TCWV data (i.e., time behavior, spectral features, long-term memory), (ii) their usefulness for hydrological applications at basin/regional level (i.e., monitoring weather extremes, estimates of water balance components), and (iii) their use in climatological studies (i.e., long-term trends, computation of precipitation efficiency and monitoring meteorological wet/dry spells). For these purposes, sample stations in U.S. and South America from the SuomiNet and IGS networks, which have rather continuous measurements for the last decade and a few missing values, have been considered and data analyzed. Comparisons with reanalysis products have been carried out and are here illustrated. Novel findings have been the subject of four publications in the international scientific journals; they are here presented and discussed. The overall results suggest that for a comprehensive monitoring of a region, a GPS network that encloses the area of concern equipped with ground meteorological sensors is suitable and desirable. GPS observations, jointly with other meteorological data, can provide an accurate estimate of the imbalance between evapotranspiration and precipitation that is of interest for drought assessment, and of the terrestrial water storage rate of change that is known to be difficult to measure. The expected (positive) impact of ground-based GPS data assimilation into numerical weather prediction models on the short-term forecasts skill should be operationally verified, as well as the advantages of having estimates of precipitation efficiency and data with long-term memory features

Regime dependent instability as a transition mechanism in large-scale atmospheric flow

Two apparently distinct approaches to studying the observed low frequency variability in the atmosphere have evolved over the past few years. One approach invokes multiple, recurrent flow regimes to explain the observed variability. An alternative approach involves the Linear stability properties of the climatological mean flow. In the present study, these approaches are merged to understand the different stability properties of the basic states associated with different flow regimes. In particular, the different zonally asymmetric components of the regime basic states lead to differing stability properties that in rum may explain the transition mechanism between regimes. This is particularly true for the transition between an amplified atmospheric planetary wave flow regime and a zonal regime. The observed transition streamfunction anomaly pattern compares very well to the most unstable stationary eigenmode of a linear stability calculation for both the barotropic and two-level baroclinic case studied. However, the growth rate of the barotropic case is too slow compared to observations and it is quite sensitive to the dissipation rate and the resolution of the calculation. In the baroclinic case, the same eigenmode appears but with a faster growth rate and more structural stability. Within the constraints of a two-layer model, the effect of baroclinicity is to remove the dependence on dissipation rate of the growth rate of the most unstable barotropic mode, allowing East growth without sensitivity to chosen parameters. The most unstable stationary baroclinic eigenmode strongly resembles the anomaly pattern of the observed transition. The energetics of the growing mode involves extraction of energy from the zonally asymmetric flow in agreement with observations. Experiments with greatly increased dissipation reveal very little sensitivity of the growth rate of this stationary eigenmode to the rate of dissipation. Alternatively, the eigenmodes for the opposite transition considered, from the zonal to amplified wave regime, are different from this former case in terms of structure, growth rate and energetics. Therefore, we conclude that the linear stability properties of the atmospheric flow are a function of the amplitude of the zonal asymmetries in the antecedent regime basic state, and that the dynamics of the transitions between regimes might be understood within the context of the linear instability properties of the regime basic states

Drought variability and its climatic implications

In the present paper, the climatic variability over several time scales, from the year up to millennia is discussed. After reviewing some modelling efforts of the latter, it will be shown evidence that periods ranging from the year up to the century contain a great deal of variability that differs from that produced by a red noise process. The analysis is based on 50 years of precipitation data derived from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis. After transforming the data into a Gaussian index, known as the standardized precipitation index (SPI), the time evolution of this index has been studied, first over the globe and then zooming over Europe. It has been found that significative trends can be detected in the data. The technique here employed is a standard principal component analysis (PCA), which, for the case of the Gaussian field under study, exhausts the knowledge of the multivariate probability density function field. On these grounds some speculations on the interaction between drought occurrence and the North Atlantic Ocean circulation will be offered. (C) 2003 Elsevier B.V. All rights reserved