Navarra, Antonio

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 16xCO2 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.

Recent results indicate that climate predictions require models which can simulate accurately natural circulation regimes and their associated variability. The main purpose of this study is to investigate whether (and how) a coupled model can simulate the real world weather regimes. A 200-year control integration of a coupled GCM (the «SINTEX model») is considered. The output analysed consists of monthly mean values of Northern Hemisphere extended winter (November to April) 500-hPa geopotential heights. An Empirical Orthogonal Function (EOF) analysis is first applied in order to define a reduced phase space based on the leading modes of variability. Therefore the principal component PDF in the reduced phase space spanned by two leading EOFs is computed. Based on a PDF analysis in the phase space spanned by the leading EOF1 and REOF2, substantial evidence of the nongaussian regime structure of the SINTEX northern winter circulation is found. The model Probability Density Function (PDF) exhibits three maxima. The 500-hPa height geographical patterns of these density maxima are strongly reminiscent of well-documented Northern Hemisphere weather regimes. This result indicates that the SINTEX model can not only simulate the non-gaussian structure of the climatic attractor, but is also able to reproduce the natural modes of variability of the system.

This study investigates the possible changes that the greenhouse global warming might generate in the character- istics of the tropical cyclones (TCs). The analysis has been performed using climate scenario simulations carried out with a fully coupled high–resolution global general circulation model (INGV-SXG) with a T106 atmospheric resolution. The capability of the model to reproduce a reasonably realistic TC climatology has been assessed by comparing the model results from a simulation of the XX Century with observations. The model appears to be able to simulate tropical cyclone-like vortices with many features similar to the observed TCs. The simulated TC activity exhibits realistic geographical distribution, seasonal modulation and interannual variability, suggesting that the model is able to reproduce the major basic mechanisms that link the TC occurrence with the large scale circulation. The results from the climate scenarios reveal a substantial general reduction of the TC frequency when the atmospheric CO2 concentration is doubled and quadrupled. The reduction appears particularly evident for the tropical north west Pacific (NWP) and north Atlantic (ATL). In the NWP the weaker TC activity seems to be associated with a reduced amount of convective instabilities. In the ATL region the weaker TC activity seems to be due to both the increased stability of the atmosphere and a stronger vertical wind shear. Despite the generally reduced TC activity, there is evidence of increased rainfall associated with the simulated cyclones. Using the new fully coupled CMCC model (CMCC_MED), with a T159 atmospheric resolution, we found a significant modulation of the Ocean Heat Transport (OHT) induced by the TC activity. Thus the possible changes that greenhouse induced global warming during 21st century might generate in the characteristics of the TC-induced OHT have been analyzed.

The Indian Ocean Dipole Mode (IODM) is examined by comparing the characteristics of oceanic and atmospheric circulations, heat budgets, and possible mechanisms of IODM between El Nino and non-El Nino years. ERA-40 reanalysis data, Reynold SST, and ocean analysis from Modular Ocean Model with the assimilation of the temperature profile from World Ocean Dataset 1998 are used to form three-year composites of IODM during El Nino (72, 82, 97) and non-El Nino (61, 67, 94) years. In El Nino years, two off-equatorial, anti-cyclonic circulations develop as a Rossby-wave response to the increased pressure over the Indian Ocean. The resultant winds from easterlies to northeasterlies (from southerlies to southeasterlies) in the northwestern (southeastern) tropical Indian Ocean warms (cools) the mixed layer temperature by inducing an anomalous zonal (meridional and vertical) component in the ocean current that advects the basic-state mixed layer temperature. In non-El Nino years, a monsoon-like flow induces winds from westerlies to southwesterlies (from southerlies to southeasterlies) in the northwestern (southeastern) Indian Ocean. As a result, the cold advection by the anomalous eastward current (northward current) in the northwestern (southeastern) tropical Indian Ocean becomes dominant in non-El Nino years. In addition, the anomalous winds in these regions are the same sign as the climatological monthly mean winds. Hence the anomalous latent and sensible heat fluxes further contribute to the decrease of SST in the northwestern and the southeastern Indian Ocean. Consequently, the cooling of the eastern tropical Indian Ocean rather than the warming of western tropical Indian Ocean becomes the major feature of the IODM during non-El Nino years.

This study investigates the Tropical Cyclone (TC) effect on the northern hemisphere Ocean Heat Transport (OHT) and the possible changes that greenhouse induced global warming might generate in the characteristics of the TC-induced OHT (TCiOHT). The analysis has been performed using 20C3M (20th Century) and A1B (21st Century) IPCC scenario climate simulations obtained running a fully coupled high-resolution global general circulation model named CMCC_MED. The Atmospheric model component has a T159 horizontal resolution and 31 vertical levels. The Ocean model component has a horizontal resolution ranging from 2 degrees to 0.5 degrees near the equator and 31 vertical levels. The capability of the model to reproduce a reasonably realistic TC climatology has been assessed by comparing the model results from the simulation of the 20th Century with observations. TC detection method has been implemented thanks to the TC-MIP project. The model appears to be able to simulate tropical cyclone-like vortices with many features similar to the observed TCs. The simulated TC activity exhibits realistic structure, geographical distribution and interannual variability, suggesting that the model is able to reproduce the major basic mechanisms that link the TC activity with the large scale circulation. The TC-induced ocean cooling is well represented and the resulting column-integrated ocean heating makes the poleward OHT larger in the subtropics and decreases the poleward heat transport out of the deep tropics. This effect, investigated looking at the 100 most intense Northern Hemisphere TCs, is strongly correlated to the TC-induced momentum flux at the surface of the ocean: the winds associated to the TCs significantly weaken the trade winds in the 5-18N latitude belt and reinforce them in the 18-30N band. TCs frequency and intensity appear to be substantially stationary through the whole 1950-2069 period. The effect of the TCs on the OHT is overall less pronounced in the 21st century when compared to the 20th century.

This paper describes a set of sensitivity experiments to several formulations of orography. Three sets are considered: a "Standard" orography consisting of an envelope orography produced originally for the ECMWF model, a"Navy" orography directly from the US Navy data and a "Scripps" orography based on the data set originally compiled several years ago at Scripps. The last two are mean orographies which do not use the envelope enhancement. A new filtering technique for handling the problem of Gibbs oscillations in spectral models has been used to produce the "Navy" and "Scripps" orographies, resulting in smoother fields than the "Standard" orography. The sensitivity experiments show that orography is still an important factor in controlling the model performance even in this class of models that use a semi-lagrangian formulation for water vapour, that in principle should be less sensitive to Gibbs oscillations than the Eulerian formulation. The largest impact can be seen in the stationary waves (asymmetric part of the geopotential at 500 mb) where the differences in total height and spatial pattern generate up to 60 m differences, and in the surface fields where the Gibbs removal procedure is successful in alleviating the appearance of unrealistic oscillations over the ocean. These results indicate that Gibbs oscillations also need to be treated in this class of models. The best overall result is obtained using the "Navy" data set, that achieves a good compromise between amplitude of the stationary waves and smoothness of the surface fields.

Seasonal rainfall in the Caribbean Basin is known to be modulated by sea surface temperature anomalies (SSTAs) in the Atlantic and Pacific Oceans, and particularly those in the Equatorial Pacific and Atlantic and the Tropical North Atlantic. However, little is known about how these major oceans influence the seasonal precipitation of individual small island states within the region as climate variability at the island-scale may di er from the Caribbean as a whole. Correlation and composite analyses were determined using monthly rainfall data for the southernmost island of the Caribbean, Trinidad, and an extended area of global SSTAs. In addition to the subregions that are known to modulate Caribbean rainfall, our analyses show that sea surface temperatures (SSTs) located in the subtropical South Pacific, the South Atlantic, and the Gulf of Mexico also have weak (r2 < 0.5) yet significant influences on the islands’ early rainy season (ERS) and late rainy season (LRS) precipitation. Composite maps confirm that the South Pacific, South Atlantic, and the Gulf of Mexico show significant SSTAs in December–January–February (DJF) and March–April–May (MAM) prior to the ERS and the LRS. Statistical models for seasonal forecasting of rainfall at the island scale could be improved by using the SSTAs of the Pacific and Atlantic subregions identified in this study.

The development of the INGV (Istituto Nazionale di Geofisica e Vulcanologia)-CMCC (Centro Euro-Mediterraneo per i Cambiamenti Climatici) Seasonal Prediction System (SPS) is documented. In this SPS the ocean initial conditions estimation includes a Reduced Order Optimal Interpolation procedure for the assimilation of temperature and salinity profiles at the global scale. Nine member ensemble forecasts have been produced for the period 1991-2003 for two starting dates per year in order to assess the impact of the subsurface assimilation in the ocean for initialization. Comparing the results with control simulations (i.e.: without assimilation of subsurface profiles during ocean initialization), we showed that the improved ocean initialization increases the skill in the prediction of tropical Pacific SSTs in our system for boreal winter forecasts. Considering the forecast of the El Ni˜no 1997-1998, the data assimilation in the ocean initial conditions leads to a considerable improvement in the representation of its onset and development. Our results indicate a better prediction of global scale surface climate anomalies for the forecasts started in November, probably due to the improvement in the tropical Pacific. For boreal winter, in both tropics and extra tropics, we show significant increases in the capability of the system to discriminate above normal and below normal temperature anomalies.

This work presents a methodology to study the interannual variability associated with summertime months in which extremely hot temperatures are frequent. Daily time series of maximum and minimum temperature fields (T max and T min, respectively) are used to define indexes of extreme months based on the number of days crossing thresholds. An empirical orthogonal function (EOF) analysis is applied to the monthly indexes. EOF loadings give information about the geographical areas where the number of days per month with extreme temperatures has the largest variability. Correlations between the EOF principal components and the time series of other fields allow plotting maps highlighting the anomalies in the large scale circulation and in the SSTs that are associated with the occurrence of extreme events. The methodology is used to construct the “climatology” of the extremely hot summertime months over Europe. In terms of both interannual and intraseasonal variability, there are three regions in which the frequency of the extremely hot days per month homogeneously varies: north-west Europe, Euro-Mediterranean and Eurasia region. Although extremes over those regions occur during the whole summer (June to August), the anomalous climatic conditions associated with frequent heatwaves present some intraseasonal variability. Extreme climate events over the north-west Europe and Eurasia are typically related to the occurrence of blocking situations. The intraseasonal variability of those patterns is related to the amplitude of the blocking, the relative location of the action centre and the wavetrain of anomalies downstream or upstream of the blocking. During June and July, blocking situations which give extremely hot climate conditions over north-west Europe are also associated with cold conditions over the eastern Mediterranean sector. The Euro-Mediterranean region is a transition area in which extratropical and tropical systems compete, influencing the occurrence of climate events: blockings tend to be related to extremely hot months during June while baroclinic anomalies dominate the variability of the climate events in July and August. We highlight that our method could be easily applied to other regions of the world, to other fields as well as to model outputs to assess, e.g. the potential change of extreme climate events in a warmer climate.

In recent years increasing interest has been put on the role that intense Tropical Cyclones can play in the climate system. The following study is aimed at highlighting the effects of strong Tropical Cyclones over the Tropical Atlantic on the mean climate. Their composite effect on the surface winds is made apparent by a wide cyclonic perturbation that affects a large portion of the Atlantic tropical Ocean. Teleconnection patterns, which are visible in the Sea Level Pressure anomalies associated with this Tropical Composite Cyclone, appear to link the activity of the hurricanes to the Arctic Ocean. A significant negative correlation between the energy dissipated by hurricanes in the Tropical atmosphere and the sea ice cover along the Transpolar Drift Stream path, has also been found.