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The Mediterranean is a climate change hot spot: it is warming 20% faster than the rest of the globe's oceans. Despite the importance of monitoring such dramatic changes and their consequences, the processes and the mechanisms involved in deep-sea variability are still unclear, due to a lack of long-term observations above 2000 m of depth. The availability of seafloor time series data and full-depth CTD profiles collected in the Ionian Sea allowed studying processes connecting the deep variability with the whole water column. The analysis of the in-situ seafloor time series showed a near-inertial peak in the current kinetic energy spectrum in the bottom layer, which hints at the presence of local vorticity. Moreover, the analysis of the CTD profiles revealed for the first time in this area the presence of variability at tidal periodicity in the deep layers (below 2000m), suggesting a connection with what was observed in the surface and subsurface layers. A unique opportunity to study and validate this mechanism was offered by the adjustment of the water stratification before and after the Eastern Mediterranean Transient, the major climate event occurred at the beginning of the 90s when warmer and saltier Aegean waters replaced colder and fresher Adriatic deep waters in the bottom layers of the Ionian basin. Data collected between 1999 and 2003 depict a stable water mass in the Ionian deep layers, identified as the Ionian Abyssal Water (IAW). The presence of the IAW layer could be a key condition for catching such variability in the deep. This allows studying the role that the stratification can have on the rapid propagation of the perturbation in depth, and also how the relative layer thicknesses and different densities can trigger the instability transport throughout the water column. The observed mean structure of the Ionian Sea stratification suggests that a 4-layer scheme should be enough to have a realistic yet straightforward theoretical representation. To study how much and under which conditions a potential vorticity input can propagate, a quasi-geostrophic equation has been considered, with 4 coupled layers of arbitrary thickness and density, simulated with a custom-designed algorithm. The relative stability of the coupled layers, and their response to external forcing, is crucial to understand how vorticity can propagate through the water column down/up and to/from the deepest layers. This case study aims to give more insight into how energy stored by the deep-sea layers can be released along the entire water column. This will enable a better parametrization of the deep processes also contributing to future Mediterranean climate numerical models.

The warming trend is already well known for the Mediterranean region, which is considered a climate hotspot warming 20 % faster than the global average. Every year, the Mediterranean Sea reaches new records for seawater warming, and year after year, this heat is penetrating deeper and deeper into the sea. New temporal and spatial evidence of this thermal penetration were depicted in the Tyrrhenian Sea thanks to a twenty-year continuous XBT monitoring. This work aims at dealing with the Tyrrhenian Sea sub-basin dynamics and processes. In particular, the mechanisms responsible for penetration of warming signal down to the deep layers (1800 m). What can hinder or exacerbate this spread and what areas are mostly affected by mechanisms of propagation and why?It’s well known that the seafloor's uneven topography and bottom roughness influences ocean circulation in two basic ways: first, it steers local vorticity flows; second, it provides barriers that prevent deep waters from mixing, except within deep passageways and straits that connect ocean basins or in hydraulically controlled overflow regions. The ways in which the warm signal entering from the south spreads rapidly northward affecting the entire Tyrrhenian Sea basin will be depicted, also considering its sub-basin peculiarities, such as features of the wind-driven surface circulation, strong stratification, and related mixing processes along the entire water column as well as its variability.

A joint effort between the Copernicus Climate Change Service (C3S) and the Group for High Resolution Sea Surface Temperature (GHRSST) has been dedicated to an intercomparison study of eight global gap-free sea surface temperature (SST) products to assess their accurate representation of the SST relevant to climate analysis. In general, all SST products show consistent spatial patterns and temporal variability during the overlapping time period (2003–18). The main differences between each product are located in the western boundary current and Antarctic Circumpolar Current regions. Linear trends display consistent SST spatial patterns among all products and exhibit a strong warming trend from 2012 to 2018 with the Pacific Ocean basin as the main contributor. The SST discrepancy between all SST products is very small compared to the significant warming trend. Spatial power spectral density shows that the interpolation into 1° spatial resolution has negligible impacts on our results. The global mean SST time series reveals larger differences among all SST products during the early period of the satellite era (1982–2002) when there were fewer observations, indicating that the observation frequency is the main constraint of the SST climatology. The maturity matrix scores, which present the maturity of each product in terms of documentation, storage, and dissemination but not the scientific quality, demonstrate that ESA-CCI and OSTIA SST are well documented for users’ convenience. Improvements could be made for MGDSST and BoM SST. Finally, we have recommended that these SST products can be used for fundamental climate applications and climate studies (e.g., El Niño).

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.

This study aims to evaluate to what extent atmospheric, land and ocean related datasets in the Climate Data Store are suitable for performing studies on the Arctic freshwater cycle and the interaction with the North Atlantic. The Arctic freshwater cycle is analyzed on the mean, seasonal cycle, and the trend of the atmospheric terms, runoff, ocean liquid and sea-ice freshwater storage over Arctic Ocean (AO) and transport through the Fram Strait (FS), Bering Strait (BS), Barents Sea branch (BSB) and Canadian Arctic Archipelago (CAA).It is found that (1) the annual mean freshwater input to the AO is dominated by the river runoff (38%), inflow through BS (30%), and net precipitation (24%) and the total freshwater export from the AO is dominated by the outflow through the FS (53%) and CAA (34%). Though the net precipitation over ocean, runoff from drainage basin and seawater and sea-ice freshwater transport through the BS are close to other studies, the much lower annual mean ocean freshwater exports from the FS and CAA contribute to the imbalance of the AO freshwater cycle based on ORAS5 reanalysis data. (2) The precipitation and total water column over the ocean and land are largest in summer, while the evaporation is smallest over ocean and largest over land in summer. The total runoff in June is largest and is modulated by the snow melting though the net precipitation is the smallest. AO liquid freshwater storage increases from May to September with a peak value in September. The ocean liquid freshwater imports from the BS and exports from CAA show much larger values in summer, while the sea-ice freshwater exports in summer is strongest for the CAA but weakest for the FS. The weakest sea-ice freshwater export from the FS is consistent with other studies though the values are much smaller. (3) Both the precipitation and evaporation over the AO increased significantly, while over land only the evaporation increased and the net precipitation decreased during both 1979-2018 and 1990-2018. The moisture convergence over land increased significantly during 1979-2018 and the total water volume over the ocean and land has also increased. The annual mean runoff decreased during 1979-2018 and is much improved with a lower trend from ERA5-land outputs than ERA5. The annual mean AO freshwater storage as sea ice decreased, while the annual mean ocean liquid freshwater storage increased during both 1979-2018 and 1990-2018. It is indicated that (1) the usage of ERA5 reanalysis data is recommended for the atmospheric freshwater cycle, and ERA5-land data for runoff, while freshwater transport from the FS and CAA are not well represented on ORAS5 reanalysis data. (2) The trends of AO liquid and sea-ice freshwater transport are very sensitive to the chosen period and quite uncertain. Extreme care must be exercised when using ORAS5 data to study the AO freshwater transport. (3) The use of ORAS5 ocean products is not recommended before 1990, as some adjustment seems to occur during the 1979-1990 period.

The City of Venice and the surrounding lagoonal ecosystem are highly vulnerable to variations in relative sea level. In the past ~150 years, this was characterized by a secular linear trend of about 2.5 mm/year resulting from the combined contributions of vertical land movement and sea-level rise. This literature review reassesses and synthesizes the progress achieved in understanding, estimating and predicting the individual contributions to local relative sea level, with focus on the most recent publications. The current best estimate of historical sea-level rise in Venice, based on tide-gauge data after removal of subsidence effects, is 1.23 ± 0.13 mm/year (period from 1872 to 2019). Subsidence thus contributed to about half of the observed relative sea-level rise over the same period. A higher – yet more uncertain – rate of sea-level rise is observed during recent decades, estimated from tide-gauge data to be about 2.76 ± 1.75 mm/year in the period 1993–2019 for the climatic component alone. An unresolved issue is the contrast between the observational capacity of tide gauges and satellite altimetry, with the latter tool not covering the Venice Lagoon. Water mass exchanges through the Gibraltar Strait currently constitute a source of substantial uncertainty for estimating future deviations of the Mediterranean mean sea-level trend from the global-mean value. Subsidence and regional atmospheric and oceanic circulation mechanisms can deviate Venetian relative sea-level trends from the global mean values for several decades. Regional processes will likely continue to determine significant interannual and interdecadal variability of Venetian sea level with magnitude comparable to that observed in the past, as well as non-negligible differential trends. Our estimate of the likely range of mean sea-level rise in Venice by 2100 due to climate change is presently estimated between 11 and 110 centimetres. An improbable yet possible high-end scenario linked to strong ice-sheet melting yields about 170 centimetres of mean sea-level rise in Venice by 2100. Projections of natural and human induced vertical land motions are currently not available, but historical evidence demonstrates that they can produce a significant contribution to the relative sea-level rise in Venice, further increasing the hazard posed by climatically-induced sea-level changes.

We present preliminary results and insights from the analysis of the ensemble of Oceanic Reanalysis System 5 (ORAS5), produced by the European Center for Medium Weather Forecast (ECMWF), which reconstruct ocean’s past history from 1979 to 2018, with monthly means temporal and spatial resolution of 0.25° and 75 vertical levels.We focused on the AMOC, which can be considered as one of the main drivers of the Earth’s Climate System, and we observed that the strength at 26.5°N presents a shift in the mean of about 5 Sverdrup in the period 1995-2000 which can be considered as a climate tipping point. We aim to investigate the causes of this reduction and propose three mechanisms responsible for the observed AMOC volume transport reduction: the Gulf Stream Separation path, changes of the Mediterranean Outflow Water (MOW) and the North Atlantic Deep Water (NADW) formation processes in the Labrador Sea respectively. The Gulf Stream Separation path is investigated by visualizing the barotropic stream function averaged over two periods, before and after the 1995-2000. In particular it is possible to detect a shift in the direction of the barotropic currents, which is enhanced further by seasonal climatology analysis. In the first period (greater volume transport), patterns are more intense, and the Gulf Stream reach higher latitudes, allowing for a more vigorous deep water formation in the Labrador Sea than in the second period. Moreover, we observe the AMOC volume transport reduction at 26.5°N accompanied with a reduction in the heat fluxes over the Labrador Sea. We think this reduction of heat fluxes has a cascade effect on horizontal averages for temperature, salinity, and potential density profiles, which are manifestations of less deep water production in the Labrador Sea, that can ultimately drive the AMOC weakening. Finally, the Mediterranean Sea has experienced, in the last decades, a general warming trend, in particular of deep water temperatures since the mid-1980s. It is well known that this warming induce a large variability in the hydrological characteristics of the MOW becoming more likely one key factor driving the AMOC variability observed in ORAS5. In fact, there’s a larger ensemble spread in both the temperature and salinity climatological profiles at 40°N, i. e. in correspondence of the Gibraltar Strait and Gulf of Cadiz.This analysis highlights the high sensitivity of the MOW to perturbations producing the different ensemble members of ORAS5.Our hypothesis is that the nonlinear interaction between these three mechanisms could have a complex feedback on the AMOC variability. In conclusion, our preliminary results brought out the relevance of the deep water formation process in the Labrador Sea, the MOW and the Gulf Stream path as the main sources of the AMOC variability and stability. Besides, our analysis points out the need for further studies, e. g. increasing resolution at the Straits (like Gibraltar Strait), investigating correlations with the variability of the subpolar gyre and developing conceptual studies, using Intermediate Complexity Models interpreted under the lens of Dynamical System Theory and Statistical Mechanics.

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.

Microbial processes involved in water biogeochemistry are linked to physical and chemical variability of water masses. While the occurrence of hydrological changes in the Eastern Mediterranean Sea has been documented, its effects on microbial dynamics and processes have been poorly investigated. An interdisciplinary survey was carried out in November 2011 to depict recent scenarios in the Ionian Mediterranean Sea. This study provides a snapshot of the hydrology and biogeochemistry of this basin, with particular emphasis on the abyssal waters, where two cores of the Eastern Mediterranean Deep Water, both of Adriatic origin, were found. The first core, located at about 3000 m, overlies the second one, which has a recent origin. Both water masses have Adriatic origin and the upper one is older than the lower one. A novel approach has been adopted to relate changes in microbial abundance and metabolism to new thermohaline and biogeochemical findings. A close relationship between surface processes driven by air-sea interaction, microbial variables, physical and chemical parameters (nutrients, dissolved organic carbon and oxygen) was observed. Being intimately coupled to DOC patterns, the microbial respiratory activity was likely the most responsive microbial parameter to hydrological changes, confirming this variable as a sentinel of environmental changes.

A regional atmosphere–ocean coupled model has been used to estimate sea level rise in the Mediterranean basin under present and future conditions. A present climate simulation has been forced by ERA40 reanalysis covering the period 1958–2001. Moreover a simulation has been forced by the global coupled model ECHAM5-MPIOM under present climate conditions for the period 1951–2000. Two other 50-year simulations have been performed under the SRESA1B scenario for the twenty-first century and differ only in temperature and salinity profiles used to relax the ocean model in the Atlantic buffer zone. The present climate simulation has been verified in terms of temperature, salinity and sea level against observed data, showing good performances both in mean values and variability over the whole Mediterranean Sea and over different sub-basins. The future scenario simulations show that the steric sea level averaged over the entire basin rises of about 2 or 7 cm in 50 years depending on the Atlantic boundary conditions. The difference of about 1 °C and 0.5 psu in the upper layers of the Atlantic sea reflects mainly on the halosteric component that contributes negatively to the sea level rise, when fresher and colder boundary conditions are used in the Atlantic buffer zone, and positively in the other case. The impact of the boundary conditions is not uniform in the basin and is particularly strong in some easternmost regions.

Two sea surface temperature (SST) time series, the Extended Reconstructed SST version 3 (ERSST.v3) and the Hadley Centre Sea Ice and Sea Surface Temperature dataset (HadISST), are used to investigate SST multidecadal variability in the Mediterranean Sea and to explore possible connections with other regions of the global ocean. The consistency between these two time series and the original International Comprehensive Ocean–Atmosphere Dataset version 2.5 (ICOADS 2.5) over the Mediterranean Sea is investigated, evaluating differences from monthly to multidecadal scales. From annual to longer time scales, the two time series consistently describe the same trends and multidecadal oscillations and agree with Mediterranean ICOADS SSTs. At monthly time scales the two time series are less consistent with each other because of the evident annual cycle that characterizes their difference. The subsequent analysis of the Mediterranean annual SST time series, based on lagged-correlation analysis, multitaper method (MTM), and singular spectral analysis (SSA), revealed the presence of a significant oscillation with a period of about 70 yr, very close to that of the Atlantic multidecadal oscillation (AMO). An extension of the analysis to other World Ocean regions confirmed that the predominance of this multidecadal signal with respect to longer period trends is a unique feature of the Mediterranean and North Atlantic Ocean, where it reaches its maximum at subpolar latitudes. Signatures of multidecadal oscillations are also found in the global SST time series after removing centennial and longer-term components. The analysis also reveals that Mediterranean SST and North Atlantic indices are significantly correlated and coherent for periods longer than about 40 yr. For time scales in the range 40–55 yr the coherence between the Mediterranean and subpolar gyre temperatures is higher than the coherence between the Mediterranean SST and North Atlantic Oscillation (NAO) or AMO. Finally, the results of the analysis are discussed in the light of possible climate mechanisms that can couple the Mediterranean Sea with the North Atlantic and the Global Ocean.

2 Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA), Rome, Italy ABSTRACT: We investigated the impact of land-cover changes on regional climate over the Euro- Mediterranean area. For this purpose, we compared a control simulation, which employs the present- day vegetation cover, with 2 different potential vegetation distributions performed with the regional climate model RegCM3 for the time period 1981-2000. The simulation results show how land-cover changes are able to modify both the thermodynamics and dynamics of the lower troposphere in central Europe, particularly during summer. Within some of the regions where land-cover change occurred, we found significant changes in heat fluxes, while weak changes were found in surface temperature and precipitation fields. Changes in vegetation composition modify the probability density function of the 2 m temperature, and hence land-cover changes may have a strong impact on extreme events.

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.

An eddy-permitting model of 1 8 ° resolution is implemented covering the whole Mediterranean Sea. Within this grid a 1 24 ° resolution model of the Strait of Gibraltar is embedded. The two grids belong to different models that are coupled through an external parallel driver. The robustness of the adopted grid refinement procedure is tested on a multi-decadal integration simulating the present climate. The good agreement found between the model circulation and most of the available observations confirms both the robustness and effectiveness of the two-way grid refinement technique. The effects produced on the Mediterranean circulation by the grid refinement are investigated through the comparison of two simulations differing only in the presence of the grid refinement. Even though the main characteristics of the thermohaline circulation appear similar in the two simulations, some quantitative and qualitative differences are observed: the main differences found in the Strait of Gibraltar propagate into the whole basin, have an impact on the water column stratification, and consequently on the convection events.

We analyze the stability of an idealized model of the Atlantic meridional overturning circulation by adopting a two-dimensional Boussinesq model. We define a two-dimensional parameter space descriptive of the freshwater forcing of the system and determine a bounded region on which multiple equilibria are realized. We also analyze the dependence of the response of the system to finite amplitude perturbations in the boundary conditions with different rates of change. It is possible to define a robust separation between slow and fast regimes of forcing. The critical rate of increase is similar to the ratio of the typical intensity of the hydrological cycle to the advective time scale of the system. If the change of the forcing is faster (slower) than the critical rate, the system responds as to instantaneous (adiabatic) changes of the same amplitude. Since the advective time scale is proportional to the square root of the vertical diffusivity, our analysis supports the conjecture that the efficiency of vertical mixing might be critical in determining the response of the ocean circulation to transient climatic changes.

We reconsider the problem of the stability of the thermohaline circulation as described by a two-dimensional Boussinesq model with mixed boundary conditions. We determine how the stability properties of the system depend on the intensity of the hydrological cycle. We define a two-dimensional parameters' space descriptive of the hydrology of the system and determine, by considering suitable quasi-static perturbations, a bounded region where multiple equilibria of the system are realized. We then focus on how the response of the system to finite-amplitude surface freshwater forcings depends on their rate of increase. We show that it is possible to define a robust separation between slow and fast regimes of forcing. Such separation is obtained by singling out an estimate of the critical growth rate for the anomalous forcing, which can be related to the characteristic advective time scale of the system.

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.

In this chapter we show results from an innovative multi-model system used to produce climate simulations with a realistic representation of the Mediterranean Sea. The models (hereafter simply referred to as the “CIRCE models”) are a set of five coupled climate models composed by a high-resolution Mediterranean Sea coupled with a relatively high-resolution atmospheric component and a global ocean, which allow, for the first time, to explore and assess the role of the Mediterranean Sea and its complex, small-scale dynamics in the climate of the region. In particular, they make it possible to investigate the influence that local air-sea feedbacks might exert on the mechanisms responsible for climate variability and change in the European continent, Middle East and Northern Africa. In many regards, they represent a new and innovative approach to the problem of regionalization of climate projections in the Mediterranean region.

Planton, Serge et al., Capítulo de: LIONELLO, Piero (ed.). The Climate of the Mediterranean Region: From the Past to Future. London: Elsevier, 2012

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.

[1] The ability of a three-dimensional sigma coordinate free surface model to reproduce the interfacial layer between the Atlantic and Mediterranean waters in the Strait of Gibraltar is verified. The model uses a coastal-following curvilinear orthogonal grid that includes the Gulf of Cadiz and the Alboran Sea, with very high resolution in the strait (less than 500 m). It is forced by imposing the two major semidiurnal tidal components (M2 and S2) along the Atlantic and the Mediterranean open boundaries. The main experiment covers an entire spring-neap cycle (fortnight period). Results show that the model is able to reproduce a thick interfacial layer that carries a substantial fraction of the total transport. The interfacial thickness is strongly modulated by tidal forcing that enhances the vertical exchanges between layers. In order to take into account the thick interfacial layer, a three-layer approximation is used to study the hydraulics. Results show that the exchange regime is intermittently maximal due to cyclically controlled regions over Camarinal Sill and within Tarifa Narrows. The analysis has been repeated in a two-layer framework to evaluate the impact of the introduction of an interfacial layer on the calculation of hydraulics. In the two-layer approximation, there is not any controlled region extending on the whole cross section, so the exchange is always submaximal. Concluding, the two-layer approximation underestimates hydraulic controls in the strait respect to the three-layer approach.

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.

Publisher Summary This chapter discusses that the North Atlantic plus the Mediterranean Sea is viewed as a unique system whose internal dynamics, regulated by the exchanges at the Strait of Gibraltar, is still rather unknown. It discusses numerical modeling results as a complement to the data analysis. Three issues are discusses in the chapter related to the dynamics of such a system. It focuses on the variability of water mass transformation processes inside the Mediterranean Sea with special attention to the time necessary for water masses formed within the Mediterranean Sea to spread into the North Atlantic. The chapter also discusses the physics of the interface between the two sub-systems, which is the exchange at the Strait of Gibraltar. Using results from a hierarchy of numerical ocean model, the chapter reviews the spreading of Mediterranean Outflow Water (MOW) in the North Atlantic and its possible relation to the variability of the meridional overturning circulation of the global ocean.

Publisher Summary This chapter presents a description of the major features of the large-scale Mediterranean Sea circulation and the most important changes that have been observed in the physical oceanography of the basin. It describes the basic circulation of the Mediterranean Sea that has long been recognized to be that of a concentration basin. The transformation of the inflowing Atlantic water to outflowing Mediterranean water is made through a thermohaline cell that involves the whole basin and leads to the formation of the Levantine Intermediate Water (LIW). The chapter discusses the oceanographic changes within the Mediterranean Basin concentrating mainly on the internal processes of the basin in relation to the regional forcing. It focuses on the large-scale thermohaline circulation of the Mediterranean and examines sub-basin processes when these are directly linked with deep water formation. It discusses two other key factors that are linked to the oceanic circulation, sea level variations and the wind–wave field.

[1] A three-dimensional sigma coordinate free surface model is used to investigate the semidiurnal tidal exchange through the Strait of Gibraltar. The model makes use of a coastal-following, curvilinear orthogonal grid that includes the Gulf of Cadiz and the Alboran Sea, with very high resolution in the strait (

[1] Aspects of the circulation of the Sicily channel region are studied with a high-resolution, primitive equation, sigma coordinate model. This work is an attempt to study the process governing the purely baroclinic dynamics and its sensitivity to seasonal stratification, thus no atmospheric forcing is used. Two numerical experiments are analyzed using, as initial conditions, diagnostic velocity fields derived from the winter and summer climatological hydrologies. The quasi-steady state is characterized by circulation patterns, in the depth range of the Modified Atlantic Water (MAW) and the Levantine Intermediate Water (LIW), strongly controlled by mesoscale eddies and topography. Seasonal difference is found in the surface current which exhibits the major variability on the African side, downstream of Adventure Bank and the Ionian Slope where the absolute maximum occurs. The intermediate circulation instead does not show relevant seasonal differences. LIW enters through the two passages south of Malta Plateau and preferentially flows along the Sicilian shelf edge exiting over the northern sill. Its pathway is strongly controlled by subsurface cyclonic and anticyclonic cells. The results are in good agreement with observations. An energy diagnostics is applied to model results and sheds important light on dynamical characteristics of the mesoscale eddy field and indicates seasonal differences in the energetic exchange with the mean flow. The analysis of the mean (MKE) and eddy (EKE) kinetic energy, the transfer terms between energy compartments and the eddy momentum flux divergence indicates how eddies are strongly energetic in summer and exchange energy more efficiently with the mean flow. In the quasi-steady state MAW volume transport and MKE and EKE fluctuations are strongly correlated. This result suggests that mesoscale motions can control up to 40% of volume transport variability.

OGCM's numerical experiments for climatic studies can be performed only after testing their capability to well represent the today ocean circulation. In order to improve the model results we assimilated satellite SST (from NOAA-AVHRR sensors) by means of a nudging method. Daily 1987 - 1992 NASA Pathfinder SST have been interpolated in time and space by means of an objective analysis algorithm to fill gaps in the time series. These data were analyzed to investigate the main variability of the Mediterranean SST field. The 1988 data was selected to be assimilated in the OGCM. To relate more closely the assimilation of the SST data to the physics of the air-sea interaction, here we propose a new parameterization of the surface relaxation time. Actually the use of realistic surface forcing strongly contribute to reproduce the correct hydrological values of the Mediterranean water masses, the right energetic basin budgets and the mesoscale features of the Mediterranean circulation.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

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

water cycle We use CMIP3 multi-model simulations to show how individual hydroclimatic changes will concur to determine even greater alterations of 21st century Mediterranean water cycle characteristics, with contrasting behavior over land and sea. By 2070-2099, the average of the models predicts a 20% decrease in land surface water availability and a 24% increase in the loss of fresh water over the Mediterranean Sea due to precipitation reduction and warming-enhanced evaporation, with a remarkably high consensus among analyzed models. The projected decrease in river runoff from the surrounding land will further exacerbate the increase in Mediterranean Sea fresh water deficit. 20th century simulations indicate that the 'transition' toward drier conditions has already started to occur and has accelerated around the turn of the century towards the larger rates projected for the 21st century. These tendencies are supported by observational evidence of century-long negative trends in regionally averaged precipitation, PDSI and discharge from numerous rivers; and are consistent with reported increases in Mediterranean sea water salinity.

This paper describes the seasonal characteristics of the Mediterranean Sea general circulation as simulated by a primitive equation general circulation model. The forcing is composed of climatological monthly mean atmospheric parameters, which are used to compute the heat and momentum budgets at the air-sea interface of the model. This allows heat fluxes to be determined by a realistic air-sea interaction physics. The Strait of Gibraltar is open, and the model resolution is in the horizontal and 19 levels in the vertical. The results show the large seasonal cycle of the circulation and its transient characteristics. The heat budget at the surface is characterized by lateral boundary intensifications occurring in downwelling and up welling areas of the basin. The general circulation is composed of subbasin gyres, and cyclonic motion dominates the northern and anticyclonic motion the southern part of the basin. The Atlantic stream which enters from Gibraltar and assumes the form of different boundary current subsystems is a coherent structure at the surface. At depth it appears as current segments and jets around a vigorous gyre system. The seasonal variability is manifested not only by a change in amplitude and location of the gyres but also by the appearance of seasonally recurrent gyres in different parts of the basin. Distinct westward propagation of these gyres occurs, together with amplitude changes. For the first time a Mersa-Matruh Gyre is successfully simulated due to the introduction of our heat fluxes at the air-sea interface. The seasonal thermocline is formed each summer, and a deep winter mixed layer is produced in the region of Levantine intermediate water formation. Deep water renewal does not occur, probably due to the climatological forcing used.

A nonlinear model of generation of internal «solitary» marine waves is discussed: when a large surface wave—for instance a superficial wave or a bore of tidal origin—passes over a submarine mountain or crosses a strait, packets of internal waves may often be detected. We study this phenomenon taking into account the effect of the air-sea surface; we show that the phenomenon can be schematized, in an approximate but realistic way, by using the solutions of an inhomogeneous KdV equation. The forcing term depends on the air-sea surface elevation and on the bottom topography. We then apply our model to the marine currents, of tidal origin, flowing through the straits of Gibraltar.

By applying a perturbation technique to the Euler equation for a two-layer fluid we show that the evolution of the interface is described by a Korteweg-de Vries equation. This model is applied to describe the tide generation of internal waves in the Straits of Gibraltar.

Moored temperature signals, tide gauge data and CTD profiles measured on the Sicilian continental shelf are studied. EOF decomposition and cross spectral analysis of the moored temperature data and analysis of the CTD profiles provide information on the hydrological characteristics in the area. Power and line spectral analysis of the moored temperature signals are discussed; major results are that (1) a tidally generated, topographically trapped baroclinic wave with diurnal period dominates the dynamics at all depths below the surface mixed layer. The analysis of the tide gauge data allows for evaluating the role of the barotropic tide. (2) Contrary to what usually happens in the Mediterranean Sea, the semidiurnal tide is always weaker than the diurnal component. This behavior is ascribed to the presence of an amphidromic point of the M 2 tide in the vicinity of the thermistor chain. (3) A weak inertial wave component, possibly important in the seasonal evolution of the average stratification, is recognizable in the line spectra; and (4) the power spectra of temperature oscillations display a power law behaviorP(ω) ≈ ω -1.8 in the frequency range of internal wave activity.

We study a process of thermocline erosion measured in the Channel of Sicily in the period 16 November–December 1985. Evidence is presented that the transition from Summer to Winter stratification has been an impulsive process. We show that the erosion of the seasonal thermocline and the deepening of the surface mixed layer were stimulated by the action of two strong atmospheric perturbations. Possible relations between mixing events and internal wave breaking are discussed.