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Air–sea heat fluxes are essential climate variables, required for understanding air–sea interactions, local, regional and global climate, the hydrological cycle and atmospheric and oceanic circulation. In situ measurements of fluxes over the ocean are sparse and model reanalysis and satellite data can provide estimates at different scales. The accuracy of such estimates is therefore essential to obtain a reliable description of the occurring phenomena and changes. In this work, air–sea radiative fluxes derived from the SEVIRI sensor onboard the MSG satellite and from ERA5 reanalysis have been compared to direct high quality measurements performed over a complete annual cycle at the ENEA oceanographic observatory, near the island of Lampedusa in the Central Mediterranean Sea. Our analysis reveals that satellite derived products overestimate in situ direct observations of the downwelling short-wave (bias of 6.1 W/m2) and longwave (bias of 6.6 W/m2) irradiances. ERA5 reanalysis data show a negligible positive bias (+1.0 W/m2) for the shortwave irradiance and a large negative bias (−17 W/m2) for the longwave irradiance with respect to in situ observations. ERA5 meteorological variables, which are needed to calculate the air–sea heat flux using bulk formulae, have been compared with in situ measurements made at the oceanographic observatory. The two meteorological datasets show a very good agreement, with some underestimate of the wind speed by ERA5 for high wind conditions. We investigated the impact of different determinations of heat fluxes on the near surface sea temperature (1 m depth), as determined by calculations with a one-dimensional numerical model, the General Ocean Turbulence Model (GOTM). The sensitivity of the model to the different forcing was measured in terms of differences with respect to in situ temperature measurements made during the period under investigation. All simulations reproduced the true seasonal cycle and all high frequency variabilities. The best results on the overall seasonal cycle were obtained when using meteorological variables in the bulk formulae formulations used by the model itself. The derived overall annual net heat flux values were between +1.6 and 40.4 W/m2, depending on the used dataset. The large variability obtained with different datasets suggests that current determinations of the heat flux components and, in particular, of the longwave irradiance, need to be improved. The ENEA oceanographic observatory provides a complete, long-term, high resolution time series of high quality in situ observations. In the future, more similar sites worldwide will be needed for model and satellite validations and to improve the determination of the air–sea exchange and the understanding of related processes.

The heat contained in the ocean (OHC) dominates the Earth’s energy budget and hence represents a fundamental parameter for understanding climate changes. However, paucity of observational data hampers our knowledge on OHC variability, particularly in abyssal areas. Here, we analyze water characteristics, observed during the last three decades in the abyssal Ionian Sea (Eastern Mediterranean), where two competing convective sources of bottom water exist. We find a heat storage of ~1.6 W/m2 – twice that assessed globally in the same period – exceptionally well-spread throughout the local abyssal layers. Such an OHC accumulation stems from progressive warming and salinification of the Eastern Mediterranean, producing warmer near-bottom waters. We analyze a new process that involves convectively-generated waters reaching the abyss as well as the triggering of a diapycnal mixing due to rough bathymetry, which brings to a warming and thickening of the bottom layer, also influencing water-column potential vorticity. This may affect the prevailing circulation, altering the local cyclonic/anticyclonic long-term variability and hence precondition future water-masses formation and the redistribution of heat along the entire water-column.

In this article, the authors describe an innovative multimodel system developed within the Climate Change and Impact Research: The Mediterranean Environment (CIRCE) European Union (EU) Sixth Framework Programme (FP6) project and used to produce simulations of the Mediterranean Sea regional climate. The models include high-resolution Mediterranean Sea components, which allow assessment of the role of the basin and in particular of the air–sea feedbacks in the climate of the region. The models have been integrated from 1951 to 2050, using observed radiative forcings during the first half of the simulation period and the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emissions Scenarios (SRES) A1B scenario during the second half. The projections show a substantial warming (about 1.5°–2°C) and a significant decrease of precipitation (about 5%) in the region for the scenario period. However, locally the changes might be even larger. In the same period, the projected surface net heat loss decreases, leading to a weaker cooling of the Mediterranean Sea by the atmosphere, whereas the water budget appears to increase, leading the basin to lose more water through its surface than in the past. Overall, these results are consistent with the findings of previous scenario simulations, such as the Prediction of Regional Scenarios and Uncertainties for Defining European Climate Change Risks and Effects (PRUDENCE), Ensemble-Based Predictions of Climate Changes and Their Impacts (ENSEMBLES), and phase 3 of the Coupled Model Intercomparison Project (CMIP3). The agreement suggests that these findings are robust to substantial changes in the configuration of the models used to make the simulations. Finally, the models produce a 2021–50 mean steric sea level rise that ranges between +7 and +12 cm, with respect to the period of reference.

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.DOI: 10.1034/j.1600-0870.2002.01284.x

Critical Infrastructures (CI) are technological systems (encompassing telecommunication and electrical networks, gas and water pipelines, roads and railways) at the heart of citizen’s life. CI protection, issued to guarantee their physical integrity and the continuity of the services they deliver (at the highest possible Quality of Service), is one of the major concern of public authorities and of private operators, whose economic results strictly depend on the way they are able to accomplish this task . Critical Infrastructure Protection (CIP) is thus a major issue of nations as the impact of CIs malfunctioning or, even, their outage might have dramatic and costly consequences for humans and human activities (1; 2). EU has recently issued a directive to member states in order to increase the level of protection to their CIs which, in a EU-wide scale, should be considered as unique, trans-national bodies, as they do not end at national borders but constitute an unique, large system covering all the EU area (3). Activities on CI protection attempt to encompass all possible causes of faults in complex networks: from those produced by deliberate human attacks to those occurring in normal operation conditions up to those resulting from dramatic events of geological or meteorologic origin. Although much effort has been devoted in realizing new strategies to reduce the risks of occurrence of events leading to the fault of CI elements, a further technological activity is related to the study of possible strategies to be used for predicting and mitigating the effects produced by CI crisis scenarios. To this aim, it is evident that a detailed knowledge of what is going to happen might enormously help in preparing healing or mitigation strategies in due time, thus reducing the overall impact of crises, both in social and economic terms. CIP issues are difficult to be analyzed as one must consider the presence of interdependence effects among different CIs. A service reduction (or a complete outage) on the electrical system, for instance, has strong repercussions on other infrastructures which are (more or less) tightly related to the electrical system. In an electrical outage case, for instance, also vehicular traffic might have consequences as petrol pumps need electrical power to deliver petrol; pay tolls do need electrical current to establish credit card transactions. As such, also Risk Analysis and Crisis Scenario Evaluation in Critical Infrastructures Protection

One of the key processes of the thermohaline circulation is the water mass transformation caused by the external forcing and the continuing renewal and stratification of the water column. In this context the marginal seas, such as the Mediterranean Sea, the Arctic Sea, the Greenland Sea, etc., play a fundamental role by supplying the global ocean with dense water. In particular, the Mediterranean Water (MW) is one of the intermediate-type water masses observed in the North Atlantic. It consists of a mixture of Levantine Intermediate Water (LIW) and deep waters produced within the Mediterranean basin and flows into the Atlantic Ocean through the narrow Strait of Gibraltar. The Gulf of Cadiz is the transition sub-basin where the pure MW undergoes, for the first time, a strong mixing and entrainment with the Atlantic water. The MW lies between 700 and 1400 m depth characterized, albeit density compensated, by significant salt and temperature anomalies, with Δ S ∼1–2 and Δ T ∼2–3 °C with respect to the overlying North Atlantic Central water. In the present work new temperature and salinity observations have shown seasonal, interannual and decadal variations. Changes in water properties of MW outflow were found, with an average value of 0.16 °C/decade and 0.05 in salinity per decade over the last 50 years (1948–1999). Even though the data set is not regularly distributed in time, it seems to be clear that the layer thickness ventilated by MW increases almost regularly in time, but with an evident acceleration in the last three decades: about 1000 m thickness and with salinity ranging between 36.0 and 37.0. In order to explain these variability and trends, some possible mechanisms are taken into consideration, in particular those related to the variability of Mediterranean thermohaline circulation and the physics of the Gibraltar Strait.

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 Outflow Water 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.

We investigate the spreading of passive tracers in closed basins. If the characteristic length scale of the Eulerian velocities is not very small compared with the size of the basin the usual diffusion coefficient does not give any relevant information about the mechanism of spreading. We introduce a finite size characteristic time $\tau(\delta)$ which describes the diffusive process at scale $\delta$. When $\delta$ is small compared with the typical length of the velocity field one has $\tau(\delta) \sim \lambda^{-1}$, where $\lambda$ is the maximum Lyapunov exponent of the Lagrangian motion. At large $\delta$ the behavior of $\tau(\delta)$ depends on the details of the system, in particular the presence of boundaries, and in this limit we have found a universal behavior for a large class of system under rather general hypothesis. The method of working at fixed scale $\delta$ makes more physical sense than the traditional way of looking at the relative diffusion at fixed delay times. This technique is displayed in a series of numerical experiments in simple flows., Comment: 14 pages RevTeX, 9 PS figures, in press on Physics of Fluids