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Bellucci A.,Centro Euro Mediterraneo per i Cambiamenti Climatici | Gualdi S.,Instituto Nazionale Of Geosica E Vulcanologia | Navarra A.,Instituto Nazionale Of Geosica E Vulcanologia
Journal of Climate | Year: 2010

The double-intertropical convergence zone (DI) systematic error, affecting state-of-the-art coupled general circulation models (CGCMs), is examined in the multimodel Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) ensemble of simulations of the twentieth-century climate. The aim of this study is to quantify theDI error on precipitation in the tropical Pacic, with a specic focus on the relationship between the DI error and the representation of large-scale vertical circulation regimes in climate models. The DI rainfall signal is analyzed using a regime-sorting approach for the vertical circulation regimes. Through the use of this compositing technique, precipitation events are regime sorted based on the large-scale vertical motions, as represented by the midtropospheric Lagrangian pressure tendency v500 dynamical proxy. This methodology allows partition of the precipitation signal into deep and shallow convective components. Following the regime-sorting diagnosis, the total DI bias is split into an error affecting the magnitude of precipitation associated with individual convective events and an error affecting the frequency of occurrence of single convective regimes. It is shown that, despite the existing large intramodel differences, CGCMs can be ultimately grouped into a few homogenous clusters, each featuring a well-dened rainfall-vertical circulation relationship in the DI region. Three major behavioral clusters are identied within the AR4 models ensemble: two unimodal distributions, featuring maximum precipitation under subsidence and deep convection regimes, respectively, and one bimodal distribution, displaying both components. Extending this analysis to both coupled and uncoupled (atmosphere only) AR4 simulations reveals that the DI bias in CGCMs is mainly due to the overly frequent occurrence of deep convection regimes, whereas the error on rainfall magnitude associated with individual convective events is overall consistent with errors already present in the corresponding atmosphere stand-alone simulations. A critical parameter controlling the strength of the DI systematic error is identied in the model-dependent sea surface temperature (SST) threshold leading to the onset of deep convection (THR), combined with the average SST in the southeastern Pacic. The models featuring a THR that is systematically colder (warmer) than their mean surface temperature are more (less) prone to exhibit a spurious southern intertropical convergence zone. © 2010 American Meteorological Society.

Davini P.,Centro Euro Mediterraneo per i Cambiamenti Climatici | Davini P.,University of Venice | Cagnazzo C.,Centro Euro Mediterraneo per i Cambiamenti Climatici | Cagnazzo C.,CNR Institute of atmospheric Sciences and Climate | And 4 more authors.
Journal of Climate | Year: 2012

In this paper, Northern Hemisphere winter blocking is analyzed through the introduction of a set of new bidimensional diagnostics based on geopotential height that provide information about the occurrence, the duration, the intensity, and the wave breaking associated with the blocking. This analysis is performed with different reanalysis datasets in order to evaluate the sensitivity of the index and the diagnostics adopted. In this way, the authors are able to define a new category of blocking placed at low latitudes that is similar to midlatitude blocking in terms of the introduced diagnostics but is unable to divert or block the flow. Furthermore, over the Euro-Atlantic sector it is shown that it is possible to phenomenologically distinguish between high-latitude blocking occurring over Greenland, north of the jet stream and dominated by cyclonic wave breaking, and the traditional midlatitude blocking localized over Europe and driven by anticyclonic wave breaking. These latter events are uniformly present in a band ranging from the Azores up to Scandinavia. Interestingly, a similar distinction cannot be pointed out over the Pacific basin where the blocking activity is dominated by high-latitude blocking occurring over eastern Siberia. Finally, considering the large impact that blocking may have on the Northern Hemisphere, an analysis of the variability and the trend is carried out. This shows a significant increase of Atlantic low-latitude blocking frequency and an eastward displacement of the strongest blocking events over both the Atlantic and Pacific Oceans. © 2012 American Meteorological Society.

Cavicchia L.,Centro Euro Mediterraneo per i Cambiamenti Climatici | Cavicchia L.,Helmholtz Center Geesthacht | Cavicchia L.,University of Venice | von Storch H.,Helmholtz Center Geesthacht | von Storch H.,University of Hamburg
Climate Dynamics | Year: 2012

Medicanes, strong mesoscale cyclones with tropical-like features, develop occasionally over the Mediterranean Sea. Due to the scarcity of observations over sea and the coarse resolution of the long-term reanalysis datasets, it is difficult to study systematically the multidecadal statistics of sub-synoptic medicanes. Our goal is to assess the long-term variability and trends of medicanes, obtaining a long-term climatology through dynamical downscaling of the NCEP/NCAR reanalysis data. In this paper, we examine the robustness of this method and investigate the value added for the study of medicanes. To do so, we performed several climate mode simulations with a high resolution regional atmospheric model (CCLM) for a number of test cases described in the literature. We find that the medicanes are formed in the simulations, with deeper pressures and stronger winds than in the driving global NCEP reanalysis. The tracks are adequately reproduced. We conclude that our methodology is suitable for constructing multi-decadal statistics and scenarios of current and possible future medicane activities. © 2011 Springer-Verlag.

Davini P.,Centro Euro Mediterraneo per i Cambiamenti Climatici | Davini P.,University of Venice | Cagnazzo C.,Centro Euro Mediterraneo per i Cambiamenti Climatici | Cagnazzo C.,CNR Institute of atmospheric Sciences and Climate | And 2 more authors.
Geophysical Research Letters | Year: 2012

Through the adoption of a bidimensional blocking index based on geopotential height, it is shown that the blocking frequency over Greenland is not only a key element in describing the North Atlantic Oscillation (NAO) index, but it also operates as an essential element in modulating its pattern. When Greenland blocking is lower than average, the first Empirical Orthogonal Function (EOF) of the 500 hPa geopotential height over the Euro-Atlantic region no longer resembles the NAO. Moreover, the typical trimodal variability observed in the Atlantic eddy-driven jet stream is reduced to a bimodal variability. Consistent with this result, we link the eastward displacement of the NAO pattern observed in recent years to the decreasing frequency of Greenland Blocking. Considering the large bias seen in the simulated blocking frequency in climate models, such strong coupling might have important consequences in the analysis of the NAO in climate simulations. © 2012. American Geophysical Union.

Storto A.,Centro Euro Mediterraneo per i Cambiamenti Climatici | Dobricic S.,Centro Euro Mediterraneo per i Cambiamenti Climatici | Masina S.,Italian National Institute of Geophysics and Volcanology | di Pietro P.,Italian National Institute of Geophysics and Volcanology
Monthly Weather Review | Year: 2011

A global ocean three-dimensional variational data assimilation system was developed with the aim of assimilating along-track sea level anomaly observations, along with in situ observations from bathythermographs and conventional sea stations. All the available altimetric data within the period October 1992-January 2006 were used in this study. The sea level corrections were covariated with vertical profiles of temperature and salinity according to the bivariate definition of the background-error vertical covariances. Sea level anomaly observational error variance was carefully defined as a sum of instrumental, representativeness, observation operator, and mean dynamic topography error variances. The mean dynamic topography was computed from the model long-term mean sea surface height and adjusted through an optimal interpolation scheme to account for observation minus first-guess biases. Results show that the assimilation of sea level anomaly observations improves the model sea surface height skill scores as well as the subsurface temperature and salinity fields. Furthermore, the estimate of the tropical and subtropical surface circulation is clearly improved after assimilating altimetric data. Nonnegligible impacts of the mean dynamic topography used have also been found: compared to a gravimeter-based mean dynamic topography the use of the mean dynamic topography discussed in this paper improves both the consistency with sea level anomaly observations and the verification skill scores of temperature and salinity in the tropical regions. Furthermore, the use of a mean dynamic topography computed from the model long-term sea surface height mean without observation adjustments results in worsened verification skill scores and highlights the benefits of the current approach for deriving the mean dynamic topography. © 2011 American Meteorological Society.

Cherchi A.,Italian National Institute of Geophysics and Volcanology | Alessandri A.,Centro Euro Mediterraneo per i Cambiamenti Climatici | Masina S.,Italian National Institute of Geophysics and Volcanology | Navarra A.,Italian National Institute of Geophysics and Volcanology
Climate Dynamics | Year: 2011

Increased atmospheric carbon dioxide concentration provided warmer atmospheric temperature and higher atmospheric water vapor content, but not necessarily more precipitation. A set of experiments performed with a state-of-the-art coupled general circulation model forced with increased atmospheric CO2 concentration (2, 4 and 16 times the present-day mean value) were analyzed and compared with a control experiment to evaluate the effect of increased CO2 levels on monsoons. Generally, the monsoon precipitation responses to CO2 forcing are largest if extreme concentrations of carbon dioxide are used, but they are not necessarly proportional to the forcing applied. In fact, despite a common response in terms of an atmospheric water vapor increase to the atmospheric warming, two out of the six monsoons studied simulate less or equal summer mean precipitation in the 16×CO2 experiment compared to the intermediate sensitivity experiments. The precipitation differences between CO2 sensitivity experiments and CTRL have been investigated specifying the contribution of thermodynamic and purely dynamic processes. As a general rule, the differences depending on the atmospheric moisture content changes (thermodynamic component) are large and positive, and they tend to be damped by the dynamic component associated with the changes in the vertical velocity. However, differences are observed among monsoons in terms of the role played by other terms (like moisture advection and evaporation) in shaping the precipitation changes in warmer climates. The precipitation increase, even if weak, occurs despite a weakening of the mean circulation in the monsoon regions ("precipitation-wind paradox"). In particular, the tropical east-west Walker circulation is reduced, as found from velocity potential analysis. The meridional component of the monsoon circulation is changed as well, with larger (smaller) meridional (vertical) scales. © 2010 Springer-Verlag.

Storto A.,Centro Euro Mediterraneo per i Cambiamenti Climatici | Masina S.,Centro Euro Mediterraneo per i Cambiamenti Climatici | Masina S.,Italian National Institute of Geophysics and Volcanology | Dobricic S.,Centro Euro Mediterraneo per i Cambiamenti Climatici
Quarterly Journal of the Royal Meteorological Society | Year: 2013

This article explores an ensemble strategy for evaluating the impact of different observing networks. The impact is represented by the relative ensemble spread increase, in model space, of data-denial ensemble simulations with respect to an 'all-observation' ensemble experiment, evaluated independently for each observing network. The forecast-error covariance intercomparison reduces to the ensemble spread intercomparison; thus, the method can be applied to any assimilation system and requires only the proper construction of an ensemble system, although the impact assessment results depend on the specific configuration of the investigated analysis system. Our approach allows us to determine the impact of the observing networks in model space (unlike Observing System Experiments) and for different forecast ranges of the ocean general circulation model. No tangent-linear and adjoint coding of the ocean model are required. The method is applied for demonstration to a large-scale global ocean variational analysis system. The ensemble members are generated by (i) perturbing the observations within the 3D-Var assimilation scheme, (ii) perturbing the surface forcing, and (iii) stochastically perturbing the ocean model parametrisation tendencies. The impact is calculated for CTDs, XBTs, moorings, Argo, sea-level anomaly observations and sea-surface temperature measurements from space-borne microwave instruments within the three-year period from January 2003 to December 2005. It turns out, on the global scale, that altimetry exhibits the largest impact on near-surface temperature and sea-surface height. In contrast, deep-ocean impacts are led by the Argo float network. As expected, space-borne observations (sea-level anomaly and sea-surface temperature observations) increase their impact in the Southern Ocean, due to the lack of a robust network of in situ observations. The results of the impact on the salinity indicate the great importance of Argo floats, especially in the northern Extratropics. © 2013 Royal Meteorological Society.

Spada G.,Urbino University | Colleoni F.,Centro Euro Mediterraneo per i Cambiamenti Climatici | Ruggieri G.,Urbino University
Tectonophysics | Year: 2011

On time scales from decades to centuries, continental cryospheric forcing in response to climate change constitutes a major source of isostatic disequilibrium that may influence future regional sea level variations. Current vertical displacements and gravity field variations are often estimated neglecting rheological effects and thus assuming a fully elastic response of the Earth. In this study, we adopt a more general point of view, aiming at describing ongoing surface movements resulting from recent glacial instabilities, also taking into account the effects associated with shallow upper mantle and crustal rheologies. Our computations are based on the Post-Widder Laplace inversion formula, which permits the straightforward computation of load-deformation coefficients for steady state and transient rheologies up to very high harmonic degrees. Using a surface load with a simple geometry and time history, we compare the classical elastic solutions to those obtained considering the rheological response of the shallow upper mantle. While at the center of the ice sheet rheology only magnifies the elastic response, the pattern and time history of vertical displacement at the ice sheet margins show a greater complexity, mainly due to the development of lateral forebulges whose shape and amplitude are particularly sensitive to the rheology of the shallow upper mantle. In this region, assuming an elastic rheology is generally appropriate on a century time scale, but significant deviations from a purely elastic response (both sign and amplitude) are observed at longer time scales or when a low viscosity zone with Maxwell rheology is taken into account. © 2010 Elsevier B.V.

Wang G.,VU University Amsterdam | Dolman A.J.,VU University Amsterdam | Alessandri A.,Centro Euro Mediterraneo per i Cambiamenti Climatici
Hydrology and Earth System Sciences | Year: 2011

Recent summer heat waves in Europe were found to be preceded by precipitation deficits in winter. Numerical studies suggest that these phenomena are dynamically linked by land-atmosphere interactions. However, there exists as yet no complete observational evidence that connects summer climate variability to winter precipitation and the relevant circulation patterns. In this paper, we investigate the functional responses of summer mean and maximum temperature (June-August, Tmean and Tmax) as well as soil moisture proxied by the self-calibrating Palmer drought severity index (scPDSI) to preceding winter precipitation (January- March, PJFM) for the period 1901-2005. All the analyzed summer fields show distinctive responses to PJFM over the Mediterranean. We estimate that 10∼15% of the interannual variability of Tmax and Tmean over the Mediterranean is statistically forced by PJFM. For the scPDSI this amounts to 10∼25%. Further analysis shows that these responses are highly correlated to the North Atlantic Oscillation (NAO) regime over the Mediterranean. We suggest that NAO modulates European summer temperature by controlling winter precipitation that initializes the moisture states that subsequently interact with temperature. This picture of relations between European summer climate and NAO as well as winter precipitation suggests potential for improved seasonal prediction of summer climate for particular extreme events. © 2011 Author(s).

Dobricic S.,Centro Euro Mediterraneo per i Cambiamenti Climatici
Monthly Weather Review | Year: 2013

The sequential variational (SVAR)method minimizes the weakly constrained four-dimensional cost function by splitting it into a set of smaller cost functions. This study shows howit is possible to applySVARin practice by reducing the computational effort required by the algorithm. A major finding of the study is that, instead of using tangent linear and adjoint models, it is possible to estimate the largest eigenvalues and the corresponding eigenvectors of the evolution of the background error covariances only by applying successive nonlinear model integrations. Anothermajor finding is that the impact of future observations on previous state estimatesmay be obtained in an accurate and numerically stable way by using suitably defined cost functions and control space transformations without any additional model integrations. The new method is applied in a realistic data assimilation experiment with a primitive equations ocean model. © 2013 American Meteorological Society.

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