This study investigates the variations of the cyclonic activity in the Mediterranean region that would be produced by doubling the CO 2 atmospheric content. The analysis is based on the SLP (sea-level pressure) fields produced by two 30 yr long time slice experiments of the ECHAM-4 model at T106 resolution, carried out by DMI, simulating the present and doubled CO 2 scenarios. The cyclonic activity in the Mediterranean region is similar in the 2 climate scenarios. The present climate is characterized with a slightly, but statistically significant, higher overall number of cyclones. The doubled CO 2 simulation is characterized with more extreme weather events, but the difference between the 2 scenarios is hardly significant. No variation in the regions of formation of the cyclones was clearly identified. An, admittedly small, number of cyclones of both scenarios was simulated using a limited area model (LAM) with 0.25°resolution. These simulations do not suggest that an increased model resolution should add new major findings to the results of this study, but the possibility that a climate change signal is not evident because of the coarse T106 model resolution remains open for further investigations. However, this study does not show a large change in the regime of the cyclones in the Mediterranean region due to the atmospheric CO 2 doubling. KEY WORDS: Cyclones · Regional scenarios · Climate change · CO 2 doubling · Mediterranean region · Extreme eventsResale or republication not permitted without written consent of the publisher Clim Res 22: 147-159, 2002 atmosphere general circulation model has shown a decreased total number of storms in the northern hemisphere and a tendency towards deeper low centers (Carnell & Senior 1998). These changes were, however, not generally statistically significant. A different analysis, based on the identification of cyclonic vorticity centers and applied to the results of a different model (the CSIRO9 general circulation model, including a slab ocean with prescribed heat fluxes), has shown a decrease of 10 to 15% in cyclonic activity (Sinclair & Watterson 1999). The changes in the cyclonic activity over the North Atlantic and Europe have been evaluated using a 240 yr long run of the ECHAM/OPYC3 coupled ocean-atmosphere general circulation model with transient greenhouse forcing (Knippertz et al. 2000). The analysis has shown a north-eastward shift of the cyclone activity accompanied by a decrease in the number of cyclones and an increase in deep cyclones. Therefore, these studies agree that there is reduced cyclonic activity in a doubled CO 2 climate. Marginally lower values of the pressure minima in the cyclone centers have also been suggested, but it has been pointed out that this result might be produced by a decreased mean SLP (sealevel pressure) and is not a necessary indication of more intense cyclones (Sinclair & Watterson 1999). All these analysis were carried out with coarse resolution datasets: 2.5°latitude × 3.25°longitude for the Hadley Centre model; 3....

The diapycnal diffusivity in the ocean is one of the least known parameters in current climate models. Measurements of this diffusivity are sparse and insufficient for compiling a global map. Inferences from inverse methods and energy budget calculations suggest as much as a factor of 5 difference in the global mean value of the diapycnal diffusivity. Yet, the climate is extremely sensitive to the diapycnal diffusivity. In this paper the sensitivity of the current climate to the diapycnal diffusivity is studied, focusing on the changes occurring in the ocean circulation. To this end, a coupled model with a three-dimensional ocean with idealized geometry is used.The results show that, at equilibrium, the strength of the thermohaline circulation in the North Atlantic scales with the 0.44 power of the diapycnal diffusivity, in contrast to the theoretical value based on scaling arguments for uncoupled models of 2/3. On the other hand, the strength of the circulation in the South Pacific scales with the 0.63 power of the diapycnal diffusivity in closer accordance with the theoretical value.The vertical heat balance in the global ocean is controlled by, in the downward direction, (i) advection and (ii) diapycnal diffusion; in the upward direction, (iii) isopycnal diffusion and (iv) parameterized mesoscale eddy [Gent-McWilliams (GM)] advection. The size of the latter three fluxes increases with diapycnal diffusivity, because the thickness of the thermocline also increases with diapycnal diffusivity leading to greater isopycnal slopes at high latitudes, and hence, enhanced isopycnal diffusion and GM advection. Thus larger diapycnal diffusion is compensated for by changes in isopycnal diffusion and GM advection. Little change is found for the advective flux because of compensation between downward and upward advection.Sensitivity results are presented for the hysteresis curve of the thermohaline circulation. The stability of the climate system to slow freshwater perturbations is reduced as a consequence of a smaller diapycnal diffusivity. This result is consistent with the findings of two-dimensional climate models. However, contrary to the results of these studies, a common threshold for the shutdown of the thermohaline circulation is not found in this model.

The sensitivity of the ocean’s climate to the diapycnal diffusivity in the ocean is studied for a global warming scenario in which CO2 increases by 1% yr−1 for 75 yr. The thermohaline circulation slows down for about 100 yr and recovers afterward, for any value of the diapycnal diffusivity. The rates of slowdown and of recovery, as well as the percentage recovery of the circulation at the end of 1000-yr integrations, are variable, but a direct relation with the diapycnal diffusivity cannot be found. At year 70 (when CO2 has doubled) an increase of the diapycnal diffusivity from 0.1 to 1.0 cm2 s−1 leads to a decrease in surface air temperature of about 0.4 K and an increase in sea level rise of about 4 cm. The steric height gradient is divided into thermal component and haline component. It appears that, in the first 60 yr of simulated global warming, temperature variations dominate the salinity ones in weakly diffusive models, whereas the opposite occurs in strongly diffusive models. The analysis of the vertical heat balance reveals that deep-ocean heat uptake is due to reduced upward isopycnal diffusive flux and parameterized-eddy advective flux. Surface warming, induced by enhanced CO2 in the atmosphere, leads to a reduction of the isopycnal slope, which translates into a reduction of the above fluxes. The amount of reduction is directly related to the magnitude of the isopycnal diffusive flux and parameterized-eddy advective flux at equilibrium. These latter fluxes depend on the thickness of the thermocline at equilibrium and hence on the diapycnal diffusion. Thus, the increase of deep-ocean heat uptake with diapycnal diffusivity is an indirect effect that the latter parameter has on the isopycnal diffusion and parameterized-eddy advection.

Abstract. The speed of Atlantic surface depressions, occurred during the autumn and winter seasons and that lead to intense precipitation over Italy from 1951 to 2000, was investigated. Italy was divided into 5 regions as documented in previous climatological studies (based on Principal Component Analysis). Intense precipitation events were selected on the basis of in situ rain gauge data and clustered according to the region that they hit. For each intense precipitation event we tried to identify an associated surface depression and we tracked it, within a large domain covering the Mediterranean and Atlantic regions, from its formation to cyclolysis in order to estimate its speed. "Depression speeds" were estimated with 6-h resolution and clustered into slow and non-slow classes by means of a threshold, coinciding with the first quartile of speed distribution and depression centre speeds were associated with their positions. Slow speeds occurring over an area including Italy and the western Mediterranean basin showed frequencies higher than 25%, for all the Italian regions but one. The probability of obtaining by chance the observed more than 25% success rate was estimated by means of a binomial distribution. The statistical reliability of the result is confirmed for only one region. For Italy as a whole, results were confirmed at 95% confidence level. Stability of the statistical inference, with respect to errors in estimating depression speed and changes in the threshold of slow depressions, was analysed and essentially confirmed the previous results.

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