Venice as a paradigm of coastal flooding under multiple compound drivers

Full comprehension of the dynamics of hazardous sea levels is indispensable for assessing and managing coastal flood risk, especially under a changing climate. The 12 November 2019 devastating flood in the historical city of Venice (Italy) stimulated new investigations of the coastal flooding problem from different perspectives and timescales. Here Venice is used as a paradigm for coastal flood risk, due to the complexity of its flood dynamics facing those of many other locations worldwide. Spectral decomposition was applied to the long-term 1872-2019 sea-level time series in order to investigate the relative importance of different drivers of coastal flooding and their temporal changes. Moreover, a multivariate analysis via copulas provided statistical models indispensable for correctly understanding and reproducing the interactions between the variables at play. While storm surges are the main drivers of the most extreme events, tides and long-term forcings associated with planetary atmospheric waves and seasonal to inter-annual oscillations are predominant in determining recurrent nuisance flooding. The non-stationary analysis revealed a positive trend in the intensity of the non-tidal contribution to extreme sea levels in the last three decades, which, along with relative sea-level rise, contributed to an increase in the frequency of floods in Venice

Seasonal cycles of pH and carbonate system parameters in the southern Adriatic Sea during one year of VECTOR project

Within the VECTOR project (activity 8.1.2) the pH and other physical chemical parameters were acquired as seasonal time series from September 2007 to June 2008, at the AM1 station (in the centre of the Southern Adriatic Pit). The pH was measured by the spectrophotometric method (precision ? 0.003) and the results expressed on "total scale" at 25?C (pHT@25?C). In a few seasons also the total alkalinity (AT) was measured by potentiometric titration at 25?C (precision ? 3 Qmol/kg) and the results were checked against sea water certified as reference material (by dr. A.G. Dickson). The other derived parameters of the carbonate system (pCO2, DIC, lAr, lCa) were computed from pH, TA, salinity, temperature, SiO2, PO4 according to Lewis and Wallace 98. The pH seasonal variability was the highest in the upper layer (0-100 m), as clearly recognizable in fig 1a, b being the pH value mainly driven by biology during the productive seasons (from spring to late summer) or by mixing with deeper waters and exchange processes with atmosphere in winter. In the deeper layers (intermediate and bottom) the seasonal variability was lower but not negligible, probably driven by remineralization processes of dissolved and particulate organic matter locally produced, as suggested by Apparent Oxygen Utilization (AOU) and nitrate seasonal variabilities (fig. 1c, d, e, f). Generally, the highest differences of physical and biogeochemical properties can be observed in both the upper (0-100m ) and the intermediate (100-800 m) layers in September and June whereas during wintry season (January and February) variabilities were much lower. Through early to late summer season, the nutrients pH and dissolved inorganic carbon (TCO2) all suggest that both layers are strongly affected by biology (quite active primary production in the upper layer although in general the region has to be considered oligotrophic, and remineralisation processes in the intermediate layer). As confirmed by the good correlation with AOU and fluorescence. The vertical variabilities of such parameters are large, representing the 28 %, 0.4 %, -115 % of the total amount. Narrower changes can be observed passing from the intermediate to the bottom layer (800 - bottom) in January, February and June. A good correlation between changes of nutrients, pH, carbonate system and AOU is still observed, indicating the significant contribution of remineralisation processes to the final values. The physical and biogeochemical differences between the intermediate and the bottom layer further suggest that water masses of different origin filled these two layers. The persistence of inter layers variability through the year might suggest the absence of any abrupt change in the circulation scheme. The three forms of carbon dioxide in seawater (TCO2 aq, HCO3 -, CO3 = ) and the saturation states of calcite and aragonite were computed, from the experimental measures of pH and total alkalinity (reported in table 1) along the water column, in February June and October 2008. Values at surface show to be higher than the surface values of other oceanic regions, this is due to the higher alkalinity of the Mediterranean Sea, thus confirming peculiar characteristics of the carbonate system and the good saturation states of the Med Sea and southern Adriatic sea in particular

The carbonate system in the Gulf of Trieste: a two years time series at PALOMA station

In the framework of VECTOR project (activity 6.2.2), pH, Total Alkalinity (AT) and physical/chemical parameters were acquired on a monthly basis since January 2008, in the water column at the PALOMA site (Advanced Oceanic Laboratory PlatforM for the Adriatic sea, Gulf of Trieste, 25m depth ). The pH was measured by the spectrophotometric method (precision ? 0.003) and the results expressed on "total scale" at 25?C (pHT@25?C). AT was measured by potentiometric titration at 25?C (precision ? 3 Qmol/kg) and the results were checked against sea water certified as reference material. The other parameters of the carbonate system (pCO2, DIC, CO3 =, lAr, lCa) were computed from pH, AT, salinity, temperature, SiO2, PO4. To our knowledge this is the first time series of these parameters collected in the North Adriatic Sea. These data allowed an initial identification of roles played by biological ad physical factors in controlling the carbonate system dynamics and the pH annual cycle. During the stratified period (April to September), CO2 uptake by primary producers in the upper layer (DO sat > 100 %, Fig 1) determined the highest annual values of pHT@25?C in both years (Fig 1). By contrast, remineralization processes generally prevailed in the deeper waters undersaturated of oxygen (DO down to 48%, Fig 1) and the minima annual values of pHT@25?C were reached. From January to March of both years the water column was homogeneous and cold, reaching the lowest annual temperatures (down to 8.8 ?C). The pHT@25?C values were generally low and constant and the oxygen saturation was around 100 %. These characteristics indicated that biological processes were playing a minor role in determining the observed values of pHT@25?C while physical factors as temperature induced CO2 solubilization were more important. AT concentrations (median value 2633 Qmol/kg) were higher than in open Mediterranean sea (~ 2600 Qmol/Kg ) due to the inflow of rivers with a carbonatic drainage basin. AT variability was mainly modulated by riverine inputs with variable AT concentrations and by the occurrence of strong remineralization processes in the bottom layer (Aug.- Nov. 2008, up to 2658 Qmol/kg, S=37.5) as shown by the relationship with AOU. The seasonal evolution of in situ pCO2 was deeply influenced by the variations of temperature that modulated not only CO2 solubility but also the chemical equilibria between carbonate species. Despite the production processes in the upper water column during summer, pCO2 values were higher than 400 Qatm on the whole water column, from July to December 2008 and from August to October 2009. During these months the Gulf of Trieste was thus acting as a potential CO2 source. In contrast, from January to June of both years, pCO2 values were always lower than 400 Qatm and the Gulf was a CO2 sink (up to -19.0 mmol C m-2 d-1, on 14 Jan 2009) especially during high wind events. An exception to this trend were the high pCO2 value (up to 606 Qatm) observed in April 2009 and May 2008, in surface low salinity waters (S down to 27.6 psu), which were ascribed to the ventilation of CO2 from supersaturated riverine waters

Extreme floods of Venice: characteristics, dynamics, past and future evolution (review article)

Floods in the Venice city centre result from the superposition of several factors: astronomical tides; seiches; and atmospherically forced fluctuations, which include storm surges, meteotsunamis, and surges caused by atmospheric planetary waves. All these factors can contribute to positive water height anomalies individually and can increase the probability of extreme events when they act constructively. The largest extreme water heights are mostly caused by the storm surges produced by the sirocco winds, leading to a characteristic seasonal cycle, with the largest and most frequent events occurring from November to March. Storm surges can be produced by cyclones whose centres are located either north or south of the Alps. Historically, the most intense events have been produced by cyclogenesis in the western Mediterranean, to the west of the main cyclogenetic area of the Mediterranean region in the Gulf of Genoa. Only a small fraction of the inter-annual variability in extreme water heights is described by fluctuations in the dominant patterns of atmospheric circulation variability over the Euro-Atlantic sector. Therefore, decadal fluctuations in water height extremes remain largely unexplained. In particular, the effect of the 11-year solar cycle does not appear to be steadily present if more than 100 years of observations are considered. The historic increase in the frequency of floods since the mid-19th century is explained by relative mean sea level rise. Analogously, future regional relative mean sea level rise will be the most important driver of increasing duration and intensity of Venice floods through this century, overcompensating for the small projected decrease in marine storminess. The future increase in extreme water heights covers a wide range, largely reflecting the highly uncertain mass contributions to future mean sea level rise from the melting of Antarctica and Greenland ice sheets, especially towards the end of the century. For a high-emission scenario (RCP8.5), the magnitude of 1-in-100-year water height values at the northern Adriatic coast is projected to increase by 26–35 cm by 2050 and by 53–171 cm by 2100 with respect to the present value and is subject to continued increase thereafter. For a moderate-emission scenario (RCP4.5), these values are 12–17 cm by 2050 and 24–56 cm by 2100. Local subsidence (which is not included in these estimates) will further contribute to the future increase in extreme water heights. This analysis shows the need for adaptive long-term planning of coastal defences using flexible solutions that are appropriate across the large range of plausible future water height extremes

Coastal Sea Level Monitoring in the Mediterranean and Black Seas

Spanning over a century, a traditional way to monitor sea level variability by tide gauges is – in combination with modern observational techniques like satellite altimetry – an inevitable ingredient in sea level studies over the climate scales and in coastal seas. The development of the instrumentation, remote data acquisition, processing and archiving in last decades allowed for extending the applications towards a variety of users and coastal hazard managers. The Mediterranean and Black50 seas are an example for such a transition – while having a long tradition for sea level observations with several records spanning over a century, the number of modern tide gauge stations are growing rapidly, with data available both in real-time and as a research product at different time resolutions. As no comprehensive survey of the tide gauge networks has been carried out recently in these basins, the aim of this paper is to map the existing coastal sea level monitoring infrastructures and the respective data availability. The survey encompasses description of major monitoring networks in the Mediterranean and Black55 seas and their characteristics, including the type of sea level sensors, measuring resolutions, data availability and existence of ancillary measurements, altogether collecting information about 236 presently operational tide gauge stations. The availability of the Mediterranean and Black seas sea level data in the global and European sea level repositories has been also screened and classified following their sampling interval and level of quality-check, pointing to the necessity of harmonization of the data available with different metadata and series at different repositories. Finally, an assessment of the networks’ capabilities60 for their usage in different sea level applications has been done, with recommendations that might mitigate the bottlenecks and assure further development of the networks in a coordinated way, being that more necessary in the era of the human-induced climate changes and the sea level ris

Annual mean-sea-level atmospheric pressure from 1841 to 2018 at Trieste, Italy

A time series of mean-sea-level atmospheric pressure was built from observations performed in Trieste from 1 January 1841 to 31 December 2018. Data until 1877 come from direct measurements with mercury barometers, from 1878 onwards barograph records are also available. The main time series consists of mean daily values, computed from 24 hourly data, when possible, or otherwise adjusted to 24-hr means. The time series was homogenized on the basis of the available metadata, reducing pressure to 0° C and to mean sea level. Monthly and annual means were also computed from the daily values

Interannual Variability of GPS Heights and Environmental Parameters over Europe and the Mediterranean Area

Vertical deformations of the Earth’s surface result from a host of geophysical and geological processes. Identification and assessment of the induced signals is key to addressing outstanding scientific questions, such as those related to the role played by the changing climate on height variations. This study, focused on the European and Mediterranean area, analyzed the GPS height time series of 114 well-distributed stations with the aim of identifying spatially coherent signals likely related to variations of environmental parameters, such as atmospheric surface pressure (SP) and terrestrial water storage (TWS). Linear trends and seasonality were removed from all the time series before applying the principal component analysis (PCA) to identify the main patterns of the space/time interannual variability. Coherent height variations on timescales of about 5 and 10 years were identified by the first and second mode, respectively. They were explained by invoking loading of the crust. Single-value decomposition (SVD) was used to study the coupled interannual space/time variability between the variable pairs GPS height–SP and GPS height–TWS. A decadal timescale was identified that related height and TWS variations. Features common to the height series and to those of a few climate indices—namely, the Arctic Oscillation (AO), the North Atlantic Oscillation (NAO), the East Atlantic (EA), and the multivariate El Niño Southern Oscillation (ENSO) index (MEI)—were also investigated. We found significant correlations only with the MEI. The first height PCA mode of variability, showing a nearly 5-year fluctuation, was anticorrelated (−0.23) with MEI. The second mode, characterized by a decadal fluctuation, was well correlated (+0.58) with MEI; the spatial distribution of the correlation revealed, for Europe and the Mediterranean area, height decrease till 2015, followed by increase, while Scandinavian and Baltic countries showed the opposite behavior

L’esperienza di ISMAR-CNR in Adriatico, Un approccio a lungo termine per la comprensione dei cambiamenti climatici

Variability of the oceanographic structure in the Adriaticb Sea and the its relation to climate change: the activity of ISMAR-CNR. Over the last few decades, ISMAR-CNR carried out several oceanographic studies in the Adriatic sea using research vessels, buoys, moored instrumentations and satellite remote sensing for earth exploration and climate change interpretation. The present paper describe the activities of ISMAR-CNR finalized to the answering of key scientific questions and defines the drivers for sea-level rise, eutrophication, hypoxia, and mucilage. In particular, the importance of long term research for spatial and temporal observations of semienclosed marine basins as a key environment for the earth system as a whole is highlited