The assessment of temperature and salinity sampling strategies in the Mediterranean Sea: idealized and real cases

International audienceTemperature and salinity sampling strategies are studied and compared by means of the Observing System Simulation Experiment technique in order to assess their usefulness for data assimilation in the framework of the Mediterranean Forecasting System. Their impact in a Mediterranean General Circulation Model is quantified in numerical twin experiments via bivariate data assimilation of temperature and salinity profiles in summer and winter conditions, using the optimal interpolation algorithm implemented in the System for Ocean Forecasting and Analysis. The data impact is quantified by the error reduction in the assimilation run relative to the free run. The sampling strategies studied here include various combinations of temperature and salinity profiles collected along Volunteer Observing Ship (VOS) tracks, by Mediterranean Multi-sensor Moored Arrays (M3A), a Glider and ARGO floating profilers. Idealized sampling strategies involving VOS data allow to recognize the impact of individual tracks. As a result, the most effective tracks are those crossing regions characterized by high mesoscale variability and the presence of frontal structures between water masses. Sampling strategies adopted in summer?autumn 2004 and winter 2005 are studied to assess the impact of VOS and ARGO data in real conditions. The combination of all available data allows to achieve up to 30% error reductions. ARGO data produce a small impact when alone, but represent the only continuous coverage of the basin and are useful as a complement to VOS data sets. Localized data sets, as those obtained by M3As and the Glider, seem to have an almost negligible impact in the basin-scale assessment, and are expected to be more effective at regional scale

Observing System Simulation Experiments for the assessment of temperature sampling strategies in the Mediterranean Sea

International audienceFor the first time in the Mediterranean Sea various temperature sampling strategies are studied and compared to each other by means of the Observing System Simulation Experiment technique. Their usefulness in the framework of the Mediterranean Forecasting System (MFS) is assessed by quantifying their impact in a Mediterranean General Circulation Model in numerical twin experiments via univariate data assimilation of temperature profiles in summer and winter conditions. Data assimilation is performed by means of the optimal interpolation algorithm implemented in the SOFA (System for Ocean Forecasting and Analysis) code. The sampling strategies studied here include various combinations of eXpendable BathyThermograph (XBT) profiles collected along Volunteer Observing Ship (VOS) tracks, Airborne XBTs (AXBTs) and sea surface temperatures. The actual sampling strategy adopted in the MFS Pilot Project during the Targeted Operational Period (TOP, winter-spring 2000) is also studied. The data impact is quantified by the error reduction relative to the free run. The most effective sampling strategies determine 25?40% error reduction, depending on the season, the geographic area and the depth range. A qualitative relationship can be recognized in terms of the spread of information from the data positions, between basin circulation features and spatial patterns of the error reduction fields, as a function of different spatial and seasonal characteristics of the dynamics. The largest error reductions are observed when samplings are characterized by extensive spatial coverages, as in the cases of AXBTs and the combination of XBTs and surface temperatures. The sampling strategy adopted during the TOP is characterized by little impact, as a consequence of a sampling frequency that is too low. Key words. Oceanography: general (marginal and semi-enclosed seas; numerical modelling

Time variability of atmospheric and marine parameters over the Adriatic region

The time evolution of atmospheric and marine parameters over the Adriatic region is studied for the period 1946-1996 on different time-scales. On the interannual and interdecadal time-scales evidence is found of the inverted barometer effect on sea level and the strong connection between air and sea temperatures. By contrast, opposite relationships are found on longer (secular) time-scales, which might be explained as different results of global climatic fluctuations on the atmospheric and marine parameters involved. On the interannual time-scale a correlation is found between sea-level pressure gradient along the basin and the water inflow/outflow through the Otranto Channel, in terms of sea level and sea temperature

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

The sea level time series of Trieste, Molo Sartorio, Italy (1869–2021)

The sea level observations carried out at Trieste, Molo Sartorio, from 1869 to 2021 have been revised and updated. Information on the tide gauges and on the geodetic benchmarks on Molo Sartorio during that period have been collected. Basic quality checks have been applied. The hourly data for the 1917–1938 period, digitized from the original charts, have allowed us to build a time series of hourly sea level heights from 1905 to 2021. Gaps of up to 24 h have been filled by interpolation. The errors affecting the monthly and annual mean sea levels have been estimated. The availability of monthly and annual means prior to 1904 allowed us to build a mean sea level time series spanning 153 years, characterized by linear trends of an observed sea level of 1.36 ± 0.17 mm yr−1 and of an inverse-barometer-corrected sea level of 1.45 ± 0.13 mm yr−1. A significant acceleration of 0.008 ± 0.004 mm yr−2 was estimated from the inverse-barometer-corrected sea level time series. This data set represents the most up-to-date data set of sea level observations and ancillary information relative to the tide-gauge station of Trieste, Molo Sartorio. The data are available through SEANOE (https://doi.org/10.17882/62758, Raicich, 2022).</p

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

Lagrangian turbulence in the Adriatic Sea as computed from drifter data: effects of inhomogeneity and nonstationarity

The properties of mesoscale Lagrangian turbulence in the Adriatic Sea are studied from a drifter data set spanning 1990-1999, focusing on the role of inhomogeneity and nonstationarity. A preliminary study is performed on the dependence of the turbulent velocity statistics on bin averaging, and a preferential bin scale of 0.25 is chosen. Comparison with independent estimates obtained using an optimized spline technique confirms this choice. Three main regions are identified where the velocity statistics are approximately homogeneous: the two boundary currents, West (East) Adriatic Current, WAC (EAC), and the southern central gyre, CG. The CG region is found to be characterized by symmetric probability density function of velocity, approximately exponential autocorrelations and well defined integral quantities such as di usivity and time scale. The boundary regions, instead, are significantly asymmetric with skewness indicating preferential events in the direction of the mean flow. The autocorrelation in the along mean flow direction is characterized by two time scales, with a secondary exponential with slow decay time of 11-12 days particularly evident in the EAC region. Seasonal partitioning of the data shows that this secondary scale is especially prominent in the summer-fall season. Possible physical explanations for the secondary scale are discussed in terms of low frequency fluctuations of forcings and in terms of mean flow curvature inducing fluctuations in the particle trajectories. Consequences of the results for transport modelling in the Adriatic Sea are discussed.Comment: 45 pages, 18 figure

Real Time Marine Data Acquisition: The Coastal Oceanographic Observatory Network in the Adriatic sea.

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