Alfonso Sutera

Spatial Variability of Drought: An Analysis of the SPI in Sicily

An analysis of drought in Sicily from 1926 to 1996 is presented.In identifying drought over the region, both the NCEP/NCARreanalysis precipitation data and those observed in 43 gauges,located quite uniformly over the territory of the Island, areused. Drought occurrence is estimated by means of theStandardized Precipitation Index (SPI). To study long-termdrought variability, a Principal Component Analysis was appliedto the SPI field.Results suggest that the entire Island is characterised by adrought variability with a multi-year fluctuations and atendency towards drier periods from the seventies onward. Apreliminary comparison between results obtained using themeteorological large-scale analysis and that derived from actualobservations on the ground shows a general good agreement,although further efforts are needed to get a better downscalingof the large-scale precipitation fields. Furthermore, byapplying orthogonal rotations to the principal componentpatterns, it has been found that three distinct areas havingcoherent climatic variability may be identified.Finally, the sensitivity of the SPI values on the calibrationperiod is also investigated.

Fifty years of precipitation: Some spatially remote teleconnnections

In the present paper the authors analyse the drought occurrence over the European region by using the NCEP/NCAR reanalysis precipitation rates covering the period from 1948 to 2000. The drought assessment is based on the Standardized Precipitation Index (SPI), which has been proposed as an indicator of drought condition. At variance with other fields derived from precipitation, the SPI is, by construction, a Gaussian field. Thus, the understanding of its covariance structure exhausts the study of the associated density distribution. A method allowing a factorisation of a multivariate Gaussian distribution is the one known as Principal Component Analysis (PCA) or Kauman-Loeve decomposition. Therefore, a PCA is used to study the main spatial patterns and the time variability of drought first over Europe and then over the Northern Hemisphere. The analysis reveals a downward trend for the index over most of central Europe and the Mediterraneanbasin, implying an overall decrease of precipitation in the above mentioned regions. Moreover, the scores associated with the PCA covariance decomposition, besides the aforementioned trend, show few long-term periodicities.Similar drought analyses have been performed by considering the Palmer Drought Severity Index (PDSI).A preliminary comparison between the SPI and PDSI obtained by using the previously discussed data set is presented. It is shown that the indices compare favourably in assessing drought variability. Finally, when the SPI analysis is extended to the Northern Hemisphere some interesting spatially remote teleconnnections linking the Tropical Pacific with the European area are shown.

Drought variability and its climatic implications

Abstract 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.

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.

Tropospheric Double Jets, Meridional Cells, and Eddies: A Case Study and Idealized Simulations

The observed low-frequency variability of the zonally averaged atmospheric circulation in the winter hemisphere is found to be amenable to an interpretation where the subtropical jet is flanked by a secondary midlatitude one. Observations also suggest that the link between the stratosphere and the troposphere modulates the variability of the tropospheric double-jet structure. Moreover, the summer hemisphere is characterized by a strong midlatitude jet sided by an intermittent subtropical one and easterly winds in the stratosphere. This work addresses the question about the role of eddies in generating and maintaining these key features of the general circulation by means of a simplified general circulation model. Model solutions for different parameter settings and external radiative forcings in the stratosphere are studied with and without eddies active on the system. The following main findings are noted. 1) Eddy dynamics alone, through the baroclinic instability processes in an atmosphere subjected to radiative forcing and dissipation, may account for the observed meridional variance of the tropospheric jets. 2) The Hadley cell can extend to the pole overlying the Ferrel cell, a feature supported by observations in the summer hemisphere. 3) The meridional temperature gradient reversal in the summer stratosphere contributes to the observed lowfrequency variability introducing an intermittent formation of a subtropical jet and the occurrence of easterlies in the tropical stratosphere. 4) Poleward propagation of the zonal wind anomaly is, when it occurs, related to the activity of synoptic eddies.

SPI Modes of Drought Spatial and Temporal Variability in Portugal: Comparing Observations, PT02 and GPCC Gridded Datasets

Regional drought modes in Portugal are identified applying the Principal Component Analysis (PCA) and Varimax rotation to the Standardized Precipitation Index (SPI) computed on various time scales using the three precipitation datasets covering the period 1950–2003: (i) The observation dataset composed of 193 rain-gauges distributed almost uniformly over the country, (ii) the PT02 high-resolution gridded dataset provided by the Portuguese Meteorological Institute, and (iii) the GPCC dataset with 0.5° spatial resolution. Results suggest that the three datasets well agree in identifying the principal drought modes, i.e. two sub-regions in northern and southern Portugal with independent climate variability. The two sub-regions appear stable when the SPI time scale is varied from 3- to 24-month, and the associated rotated principal component scores (RPCs) do not show any statistically significant linear trend. The degree of similarity between the rotated loadings or REOFs of different SPI time scales for the three used datasets was examined through the congruence coefficients, whose results show a good agreement between the three datasets in capturing the main Portuguese sub-regions. A third spatial mode in central-eastern Portugal was identified for SPI-24 in PT02, with the associated RPC characterized by a statistically significant downward trend. The stability of the identified sub-regions as a function of studied time period was also evaluated applying the same methodologies to a set of three different time windows and it was found that the southern sub-region is very stable but the northern and central-eastern sub-regions are very sensitive to the selected time window.

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.

A MINIMAL BAROCLINIC MODEL FOR THE STATISTICAL PROPERTIES OF LOW-FREQUENCY VARIABILITY

Abstract A recent analysis of atmospheric observations has shown evidence of bimodality in the statistical distribution of wave amplitude in the ultralong (zonal wavenumber group 2–4), low frequency (period >5 days). Similar analysis of the zonal wind and its average shear shows no clear sign of bimodality. Both are characterized by a very variance (≅ 1 m s−1) and the associated kinetic energy fluctuations are not sufficient to account for the variations in wave amplitudes. Global energetic analysis confirms this finding. maintenance of the waves is dominated by baroclinic processes. On the other hand, from a theoretical point of view, barotropic models for wave generation and maintenance be brought into agreement with observed statistics by introducing nonlinear bending of the stationary resonant response to topographic modulation allowing different values of the equilibrium amplitude to correspond to the same value of the zonal flow. However, because of aforementioned energetic difficulties, in a barotr...

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 OutflowWater 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.