1,720,994 research outputs found
Teleconnections between Indian monsoon and Sahel rainfall and the Mediterranean
No full textThe teleconnections with Indian monsoon and Sahel rainfall indices are investigated here on an interannual time scale in terms of meteorological and marine dynamics over the Mediterranean area. Sea-level pressure from gridded data sets and from individual stations, together with sea-level data from stations all around the Mediterranean coastlines, are used. In summer (July-August-September, JAS) the sea-level pressure field over the eastern Mediterranean anticorrelates with the Indian monsoon index (correlation coefficient C = -0.5 on average). A Mediterranean pressure index (MPI), defined as the standardized difference between sea-level atmospheric pressure at Mersa Matruh (southeastern Mediterranean) and Marseille (northwestern Mediterranean) stations, anticorrelates with Indian monsoon index even more (C = -0.68). The MPI is proportional to the mean geostrophic surface flow field across an imaginary line joining the two stations and turns out to be significantly correlated with the meridional wind component over the eastern Mediterranean, known as the low-level Etesian wind regime. This wind regime represents the inflow surface field into the African inter-tropical convergence zone and, therefore, has an association with the Indian monsoon regime. The ocean response, evident by sea-level anomalies at coastal stations, shows a maximum anticorrelation with Indian monsoon index in late summer and autumn (September-October-November, SON). The Sahel index anticorrelates with sea-level pressure, with the maximum absolute value in June-July-August. This may be interpreted as a tendency of the Mediterranean sea-level pressure anomalies to precede those of Sahel precipitation, which is characterized by maximum rainfall in July-September. The MPI anticorrelates with Sahel index during and before JAS, indicating that the Etesian wind regime intensity is connected to Sahel rainfall. The sea level again anticorrelates with the Sahel index, with the maximum absolute value in SON, as for the sea-level-Indian monsoon correlation. © 2003 Royal Meteorological Society
Tide gauge data archaeology provides natural subsidence rates along the coasts of the Po Plain and of the Veneto-Friuli Plain, Italy
No full textIn Northern Italy, natural subsidence affects the Po and Veneto-Friuli Plains. Anthropogenic activities which started during the 1930s enhanced the natural rates considerably. Information on land lowering can be obtained not only by geodetic or geological data, but also analysing and comparing sea-level time-series of neighbouring tide gauges. In the Northern Adriatic, several tide gauge stations were operational before the onset of the anthropogenic activities. We analysed data spanning the period 1873-1922 from Marina di Ravenna, Venice and Trieste, in Italy. The 1897-1922 data of Pula, Croatia, were also considered for the analysis, but this time-series was finally discarded because it was too short. Trieste, located in a relatively stable area, is characterized by a sea-level rate of 1.21 ± 0.35 mm yr-1 (1875-1922) that can be assumed to be a reliable estimate of the local sea-level rise during the period of interest. We compared the rate observed at Trieste with those obtained at Marina di Ravenna, 3.09 ± 0.31 mm yr-1 (1873-1922), and Venice, 2.05 ± 0.22 mm yr-1 (1873-1922). This comparison shows that the natural subsidence rate decreases from Marina di Ravenna to Venice and Trieste, turning out to be 1.88 ± 0.47 and 0.84 ± 0.41 mm yr-1 at Marina di Ravenna and Venice, respectively
Interannual variability of gps heights and environmental parameters over europe and the mediterranean area
Full text linkVertical 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
Detection of discontinuities in GPS coordinates time series through the Sequential t-Test Analysis of regime Shifts (STARS)
No full textThe time series of GPS coordinates, in general, are affected by the superposition of different long- and short- period signals which shall be identified and their physical nature needs to be understood. Examples are long-period
tectonic variations, effects of climatic origin and short- period seasonal fluctuations. In addition to these variations, discontinuities are commonly observed that impair the estimate of trends. These abrupt shifts can be originated by changes of the station equipment and/or the reference system, by the occurrence of earthquakes and, likely, by a number of unforeseen events. The magnitude of these jumps shall be estimated and the series corrected accordingly. There exists several methods allowing to detect discontinuities in the time series and to estimate their relevant magnitude. Typically, these adopt hypothesis-driven analyses, meaning, for example, that the detection of a jump resulting from a documented event (e.g. instrumentation change, earthquakes etc.) is accomplished through the a priori knowledge of the event timing. We have studied time series of GPS coordinates characterized by several jumps due to different reasons by means of a method known as Sequential t-Test Analysis of Regime Shift (STARS) developed by Rodionov (Geophys. Res. Lett., 31, L09204, 2004). This technique does not make any a priori assumption on the time series. A discontinuity or jump is detected, and its magnitude is estimated, when the time series mean value exhibits a significant change. Since the approach belongs to the sequential/exploratory type of statistical methods, jumps are detected as soon as they appear in the series. Two parameters are required, namely a cut-off length, which allows selecting the fluctuations time scales, and a significance level, used in the statistical t-test upon which the STARS algorithm is based. We have investigated the coordinates time series of several stations located in northeastern Italy, including those of the BOLG and MSEL Euref stations
Rescue of the 1873–1922 high and low waters of the Porto Corsini/Marina di Ravenna (northern Adriatic, Italy) tide gauge
No full textThe historical data corresponding to the period 1873–1922 of the Porto Corsini (northern Adriatic, Italy) tide gauge have been retrieved from the archive of the Italian Istituto Geografico Militare, in Florence (http://www.igmi.org). The data consist of daily high and low waters, which were recorded in six handwritten volumes. The 1873–1896 observations were, until now, totally unknown since they were never published, while, for the remaining years, only monthly mean values were available. We digitized and quality controlled the rescued data. Searching in different archives, we also found detailed information on the three sites occupied by the tide gauge within the Porto Corsini/Marina di Ravenna harbor. In addition, the rescue of the leveling data and of the datum definition made it possible to realize a homogeneous time series of monthly mean sea-level values, which now spans 144 years. Sea-level time series exceeding a century are rare, and they are most important because they provide key information for studying climate change-related processes. The Porto Corsini/Marina di Ravenna area is affected by natural subsidence, much enhanced by anthropogenic activities during the period 1940–1980, in particular. Using leveling, GPS and InSAR data we have modeled the subsidence behavior over the 1873–2016 period. After removing the subsidence from the series of monthly mean values, we estimated a long-period linear sea-level rise of + 1.25 ± 0.16 mm/year
An EOF and SVD analysis of interannual variability of GPS coordinates, environmental parameters and space gravity data
No full textEleven years, 1999-2010, of GPS positions of stations located in Europe and the Mediterranean were analyzed together with time series of atmospheric pressure, terrestrial water storage, and surface mass anomalies provided by the GRACE gravity mission. The Empirical Orthogonal Functions (EOF) technique was applied to single variables data series detrended and de-seasoned to identify the main modes of interannual variability in both time and space. For the GPS coordinates, the analysis has revealed the presence of significant interannual fluctuations which were also identified in the time series of the environmental parameters and of the space gravity data. A prominent feature common to the main modes of the time components of all variables is a slope inversion occurring around 2003-2004. During 2003 most of the European continent was affected by a severe drought. A second type of analysis was performed by applying the Singular Value Decomposition (SVD) approach to a few series of variable pairs to study the coupled space/time variability. This mathematical tool is used to examine possible cause-effect physical relationships between variables pairs. The analysis shows, for example, for the atmospheric pressure and the up coordinate that the first two SVDs account for about 97% of the total covariance. The clear anticorrelation observed between the two SVD1 spatial patterns represents the vertical crustal displacements associated with variations in atmospheric pressure. East-West (EW) and North-South (NS) gradients of the environmental parameters and of the GRACE surface mass anomalies were computed and compared to the EOFs of the east and north GPS coordinates respectively. Significant correlations were found, in particular, between the EW atmospheric pressure gradient and the east EOFs and between the GRACE EW and NS gradients and the east and north EOFs respectively. Significant correlations were also identified between the Multivariate ENSO Index (MEI) and the EOFs of GPS coordinates
Observing and assessing non-tidal ocean loading using ocean, continuous GPS and gravity data in the Adriatic area
No full textThe effect of nontidal ocean loading (NTOL) is
observed in the height series of four permanent GPS
stations in the northern Adriatic. A validation of the
ECCO model is performed by comparing model estimates
of sea-level anomalies from tide-gauges with TOPEX/
POSEIDON data, and ECCO model estimates of bottom
pressure with those derived from temperature and salinity
observations. The amplitudes of theECCO sea-level anomaly
are found to be 1.4 times smaller than observations; bottom
pressure is 2 times smaller. Using a Green’s functions
approach to determine elastic deformations, the ECCO ocean
bottom pressure is used to estimate surface displacements at
the GPS sites. Model results were compared with the height
series and with the observed NTOL effect. The height series
and the predicted NTOL are highly correlated at all four
stations. The analysis performed on superconducting
gravimeter data at the Medicina station also shows high
correlation
Climate-related vertical ground movements measured by GPS and gravity in Northeastern Italy
No full textHydrological mass variations are known to play a major role in the seasonal height and gravity variability. On
longer time scales, climate-related variations of GPS height and gravity have not yet been clearly identified mainly because of the limited temporal extent of most of the continuous series and for the lack of information on spatial and temporal variability of groundwater storage. Continuously recording GPS stations with relatively long time series are available in northeastern Italy. This area is affected by both natural and human-induced subsidence;
the natural rate decreases from the southern part, where even exceeds 2 mm/yr towards the north. At local scale,
anthropogenic effects may enhance significantly the natural tendency. Additionally, the coastal areas are threatened by rising sea level. We have studied GPS height, gravity and hydrological time series by using the Empirical Orthogonal Functions and Singular Value Decomposition analyses. Significant common patterns in the spatial and temporal variability of these parameters have been identified. In particular, hydrology-induced variations are clearly observable starting 2002 in the southern part of the Po Plain for the longest time series, and from 2005 over the whole area. Observing and modeling long- and short-period signals in the height and gravity time series allows to better understand and quantify subsidence. This knowledge is important because of the relevant societal impacts of this phenomenon
Severe marine storms in the Northern Adriatic: Characteristics and trends
No full textThis paper discusses present characteristics and trends of marine storminess in the Northern Adriatic. It merges oceanographic and meteorological aspects, by considering storm surges, wind waves and the atmospheric cyclones that cause them. The paper describes the dynamics of these three processes, the different role of south-easterly (Sirocco) and easterly (Bora) wind regimes. The specific characteristics of cyclones producing severe marine storms in terms of location where cyclogenesis occurs, trajectories and intensity, are compared with those of generic cyclones crossing northern Italy. It is shown that cyclones producing high waves and surges have different characteristics and their lists overlap only partially. However, both high wave and surge events have a similar annual cycle, with maximum activity in November and hardly any event in summer (June-July-August). The trends of severe high wave and surge events are discussed (various thresholds are considered) and they are shown to be consistent. Timeseries, which show large interannual variability and very little overall tendencies on multi-decadal time scale, are suggestive of progressively milder storms during the second half of the 20th century
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