Paolo Ruti

Surface vector wind datasets from different assimilation systems and from scatterometers have been recently made available over the entire Mediterranean basin and for a large spectrum of spatial and temporal resolution. In this work, we compare wind vector analyses, derived from different routine assimilation systems and from blended products, to wind vectors obtained from QuikSCAT satellite sensor and to those directly measured by buoy-mounted anemometers. The analysis has been performed to verify the accuracy of the analyzed data, when the specific objective is the generation of surface winds field to force Mediterranean Sea simulations. The inter-comparison covers the period 2000-2005. Our analysis demonstrated that the spatial resolution of the data sets represents one of the main relevant sources of error in the analyzed wind fields, explaining the worst results of the reanalysis data and the relative accuracy of the ECMWF. This work also confirms the usefulness of blending QuikSCAT and reanalysis products, which could be used to force oceanic simulations. The blended data cover the period from July 1999 to present when QuikSCAT wind data are available. Before this period, blended products are not produced and different solutions to correct wind speed from routine assimilation systems have to be investigated. A simple empirical method to adjust the ERA40 wind speed product is then proposed. The analysis of the difference between the annual Mediterranean heat budget computed using the adjusted and the original ERA40 winds suggests that the impact of the correction is not negligible. Considering the year 2000, the annual average heat budget for the whole basin is modified from ˜ 34 W/m 2 to ˜ - 6 W/m 2.

MED-CORDEX is a coordinated action between CORDEX and HyMeX international programs. MED-CORDEX will make use of both regional atmospheric climate models and regional coupled systems. Strong air-sea interactions take place in the Mediterranean basin, motivating the use of coupled regional models. Two main phases have been envisaged within the MED-CORDEX action: performing state-of-the-art coupled runs feeding the next IPCC report (AR5) by the end of 2010; developing new experiments to test new components and improved schemes, based on the HyMeX field campaign outcomes (long term simulations). Here, we present a preliminary inter-comparison analysis of the ERAinterim simulations and of CIRCE coupled runs.

ABSTRACT We review the studies carried out during the African Monsoon Multidisciplinary Analysis (AMMA)-EU on the changes of interannual sea surface temperature (SST)–West African monsoon (WAM) covariability at multidecadal timescales, together with the influence of global warming (GW). The results obtained in the AMMA-EU suggest the importance of the background state, modulated by natural and anthropogenic variability, in the appearance of different interannual modes. The lack of reliability of current coupled models in giving a realistic assessment for WAM in the future is also stated. Copyright © 2011 Royal Meteorological Society

Rainfall over West Africa shows strong interannual variability related to changes in Sea Surface Temperature (SST). Nevertheless, this relationship seem to be non-stationary. A particular turning point is the decade of the 1970s, which witnessed a number of changes in the climatic system, including the climate shift of the late 1970s. The first aim of this study is to explore the change in the interannual variability of West African rainfall after this shift. The analysis indicates that the dipolar features of the rainfall variability over this region, related to changes in the Atlantic SST, disappear after this period. Also, the Pacific SST variability has a higher correlation with Guinean rainfall in the recent period. The results suggest that the current relationship between the Atlantic and Pacific El Niño phenomena is the principal responsible for these changes. A fundamental goal of climate research is the development of models simulating a realistic current climate. For this reason, the second aim of this work is to test the performance of Atmospheric General Circulation models in simulating rainfall variability over West Africa. The models have been run with observed SSTs for the common period 1957–1998 as part of an intercomparison exercise. The results show that the models are able to reproduce Guinean interannual variability, which is strongly related to SST variability in the Equatorial Atlantic. Nevertheless, problems in the simulation of the Sahelian interannual variability appear: not all models are able to reproduce the observed negative link between rainfall over the Sahel and El Niño-like anomalies in the Pacific, neither the positive correlation between Mediterranean SSTs and Sahelian rainfall.

Using both empirical and numerical ensemble approaches this study focuses on the Mediterranean/West African relationship in northern summer. Statistical analyses utilize skin temperature, sea surface temperature, in situ and satellite rainfall, outgoing longwave radiation (OLR) observations and reanalyzed data winds and specific humidity on isobaric surfaces. Numerical investigations are based on a large set of sensitivity experiments performed on four atmospheric general circulation models (AGCM): ARPEGE-Climat3, ECHAM4, LMDZ4 and UCLA7.3. Model outputs are compared to observations, discussed model by model and with an ensemble (multi-model) approach. As in previous studies the anomalous Mediterranean warm events are associated with specific impacts over the African monsoon region, i.e., a more intense monsoon, enhanced flux convergence and ascendances around the ITCZ, a strengthening of low level moisture advection and a more northward location of ascending motion in West Africa. The results show also new features (1) thermal variability observed in the two Mediterranean basins has unalike impacts, i.e. the western Mediterranean covaries with convection in Gulf of Guinea, while the eastern Mediterranean can be interpreted as Sahelian thermal-forcing; (2) although observations show symmetry between warming and cooling, modelling evidences only support the eastern warming influence; (3) anomalous East warm situations are associated with a more northward migration of the monsoon system accompanied by enhanced southwertely flow and weakened northeasterly climatological wind; (4) the multi-model response shows that anomalous East warm surface temperatures generate an enhancement of the overturning circulation in low and high levels, an increase in TEJ (Tropical Eeasterly Jet) and a decrease in AEJ (African Eeasterly Jet).

We compare 45 years of the reanalyses of National Center for Environmental Prediction-National Center for Atmospheric Research and European Center for Mid-Range Weather Forecast in terms of their representation of the mid-latitude winter atmospheric variability for the overlapping time frame 1957-2002. We adopt the classical approach of computing the Hayashi spectra of the 500 hPa geopotential height fields and we introduce an ad hoc integral measure of the variability observed in the Northern Hemisphere on different spectral subdomains. Discrepancies are found especially in the pre-satellite years of the records in the high frequency-high wavenumber propagating waves. This implies that in the pre-satellite period the two datasets have a different representation of the baroclinic available energy conversion processes. Minor differences are also found in the description of low frequency-low wavenumber standing waves. We observe a positive impact of the satellite data on the representation of wave activity over the oceanic sectors in the period starting from 1979, in particular on the description of high frequency variability. Since in the pre-satellite period the assimilated data are more scarce, predominately over the oceans, and of lower quality than found later on, they provide a weaker constraint to the model dynamics. Therefore, the resulting discrepancies in the reanalysis products may be mainly attributed to differences in the models’ behaviour.

We present an atmosphere–ocean regional climate model for the Mediterranean basin, called the PROTHEUS system, composed by the regional climate model RegCM3 as the atmospheric component and by a regional configuration of the MITgcm model as the oceanic component. The model is applied to an area encompassing the Mediterranean Sea and compared to a stand-alone version of its atmospheric component. An assessment of the model performances is done by using available observational datasets. Despite a persistent bias, the PROTHEUS system is able to capture the inter-annual variability of seasonal sea surface temperature (SST) and also the fine scale spatio-temporal evolution of observed SST anomalies, with spatial correlation as high as 0.7 during summer. The close inspection of a 10-day strong wind event during the summer of 2000 proves the capability of the PROTHEUS system to correctly describe the daily evolution of SST under strong air–sea interaction conditions. As a consequence of the model’s skill in reproducing observed SST and wind fields, we expect a reliable estimation of air–sea fluxes. The model skill in reproducing climatological land surface fields is in line with that of state of the art regional climate models.

In this work, we study the mean tropopause structure from the National Center for Environmental Prediction National Center for Atmospheric Research reanalysis in the framework of baroclinic adjustment theories, focusing on the impact of baroclinic eddies on the mean tropopause height. In order to measure the effects of such perturbations, we introduce an appropriate global index that selects events of high baroclinic activity and allows us to distinguish the phases of growth and decay of baroclinic waves. We then composite the tropopause mean structure before and after baroclinic events, finding that baroclinic disturbances cause the zonally averaged midlatitude winter tropopause height to rise. Our results establish the importance of baroclinic adjustment processes for midlatitude tropopause dynamics.

The nature of intraseasonal and interannual variability of African easterly waves (AEWs) in IPCC–CMIP3 global simulations is investigated in comparison with 40-year NCEP and ERA40 products. AEWs are a major source of atmospheric variability over the Sahel and particularly over West Africa. An accurate representation of AEWs dynamics is therefore an important precondition to skilled climate predictions and seasonal forecasts for this area. We describe the synoptic features of these disturbances and we illustrate a statistical link, at interannual timescale, between Sea Surface Temperature and AEWs activity. At intraseasonal time scale, the models exhibit a wide variety of behaviours in reproducing the synoptic features of the disturbances, namely AEWs amplitude and pattern. It is possible to classify the models into two groups, one localizing intense variability well inside the continental area, and the other exhibiting a weaker variance mostly placed over West Africa. Concerning the inter-annual variability, we point out a statistical link between SST anomalies and AEWs activity that reveals a strong influence of the ENSO events on the variability of the disturbances over the Guinean coasts. Warm/cold ENSO events occur in conjunction with suppressed/enhanced AEWs through upper tropospheric teleconnection bridge. Only two model running at significantly different vertical and horizontal resolution (INM and INGV-SGX) are able to reproduce this mechanism in accordance to what observed in the global reanalysis systems. Our result introduces non-negligible caveats with respect to the ability of most of CMIP3 models in obtaining reliable description of AEWs.

We present a quantitative analysis of the water content in the atmosphere, soil and ocean over the Mediterreanean catchment area. The Regional Earth System developed by ENEA-ICTP, the PROTHEUS system, is an optimal modelling tool for this purpose as it explicitly accounts for the various components of the hydrological cycle and their interactions. In particular, the PROTHEUS system provides a reliable description of high resolution sea surface temperature and wind fields over the ocean, in close agreement to observations thereby providing a reliable description of air-seas fluxes (particularly the latent heat flux). In this analysis, all the terms of hydrological cycle are computed for different simulations performed by an implemented version of PROTHEUS with interactive river runoff. To assess model performances we compare a control simulation driven by ERA40 with observational datasets. The same model configuration is adopted to perform a 1951-2050 simulation, driven at the lateral boundaries by ECHAM5-MPIOM global simulation included in the IPCC-AR4,. The modelling tools presented in this work will also contribute to the Med-CORDEX activities.

In this work, we study the mean tropopause structure from the National Center for Environmental Prediction–National Center for Atmospheric Research reanalysis in the framework of baroclinic adjustment theories, focusing on the impact of baroclinic eddies on the mean tropopause height. In order to measure the effects of such perturbations, we introduce an appropriate global index that selects events of high baroclinic activity and allows us to distinguish the phases of growth and decay of baroclinic waves. We then composite the tropopause mean structure before and after baroclinic events, finding that baroclinic disturbances cause the zonally averaged midlatitude winter tropopause height to rise. Our results establish the importance of baroclinic adjustment processes for midlatitude tropopause dynamics.

A regional atmosphere-ocean coupled model (the Protheus system) has been used to estimate sea level rise in the Mediterranean basin. A present climate simulation has been forced by ERA40 covering the period 1958-2001. Another simulation has been forced by the coupled model ECHAM5-MPIOM for the period 1951-2000 and under the scenario SRES A1B for the period 2001-2050. The present climate simulation has been verified in terms of temperature and salinity against observed data, showing good performances both in the mean values and in the variability. Halosteric and thermosteric components have been computed and the total steric sea level has been compared with satellite data. The comparison with altimeter data has been done for the whole Mediterranean and for sub-basins, the capability of the system to reproduce the inter-annual variability of the sea level has been verified. Data from the scenario simulation have been used to evaluate long term trends for the XX and XXI centuries.

We compare 45 years of the reanalyses of National Center for Environmental Prediction–National Center for Atmospheric Research and European Center for Mid-Range Weather Forecast in terms of their representation of the mid-latitude winter atmospheric variability for the overlapping time frame 1957–2002. We adopt the classical approach of computing the Hayashi spectra of the 500 hPa geopotential height fields and we introduce an ad hoc integral measure of the variability observed in the Northern Hemisphere on different spectral subdomains. Discrepancies are found especially in the pre-satellite years of the records in the high frequency-high wavenumber propagating waves. This implies that in the pre-satellite period the two datasets have a different representation of the baroclinic available energy conversion processes. Minor differences are also found in the description of low frequency–low wavenumber standing waves. We observe a positive impact of the satellite data on the representation of wave activity over the oceanic sectors in the period starting from 1979, in particular on the description of high frequency variability. Since in the pre-satellite period the assimilated data are more scarce, predominately over the oceans, and of lower quality than found later on, they provide a weaker constraint to the model dynamics. Therefore, the resulting discrepancies in the reanalysis products may be mainly attributed to differences in the models’ behaviour.

In this study we compare the representation of the southern hemisphere midlatitude winter variability in the NCEP-NCAR and ERA40 reanalyses. We use the classical Hayashi spectral technique, recently applied to compare the description of the atmospheric variability in the northern hemisphere on different spectral sub-domains. We test the agreement of the two reanalysis systems in the representation of the atmospheric activity. In the southern hemisphere, even in the satellite period, the assimilated data are relatively scarce, predominately over the oceans, and they provide a weaker constraint to the model dynamics. We find relevant discrepancies in the description of the variability at different spatial and temporal scales. ERA40 is generally characterised by a larger variance, especially in the high frequency spectral region. In the pre-satellite period the discrepancies between the two reanalyses are large and randomly distributed while after the 1979 the discrepancies are systematic. Moreover, a sudden jump in the VTPR period (1973-1978) is observed, mostly in the ERA40 reanalysis. Our results suggest that today we do not have a well-defined picture of the properties of the winter mid-latitude variability in the southern hemisphere to be used in the evaluation of the realism of climate models and demand for an intercomparison study for the assessment of the self-consistency of the IPCC models in the representation of the analysed properties.