Tide Gauge Data Research Articles

Towards reliable global allowances for sea level rise

Tide gauge data and information from tide, surge and ocean models have been used to calculate and validate the Gumbel scale parameters of extreme sea level distributions along the world coastline. The inclusion of ocean model information is found to result in significantly improved correspondence between observed and modelled scale parameters to that obtained using tide and surge model information alone. The scale parameters so obtained are shown to be consistent with findings reported previously, such as in assessments of the Intergovernmental Panel on Climate Change. However, the considerably improved provision of scale parameters along the coast means that coastal planners, and others concerned with impacts of sea level rise, can now undertake more complete investigations of the likely increase in sea level exceedance frequencies. In addition, coastal engineers will have access to more reliable estimates of the ‘sea level allowances’ needed to design defences for protecting coastal populations.

Derivation of MSL Riverbank Line from UAV DSM Data based on Tidal Slope Analysis

Deriving a riverbank line at a tidal river is complicated, since dynamic factors such as tide and slope topography influence its location. Hence, this study utilises the Unmanned Aerial Vehicle (UAV), geodetic, and tidal data to identify the exact riverbank line based on Mean Sea Level (MSL) value from UAV products. We carried out two epochs of UAV data collection during low and high tide conditions. Orthometric height (H) has been applied in UAV data processing to produce a high accuracy of orthophoto and Digital Surface Model (DSM). UAV-MSL derived riverbank lines have been identified from the UAV orthophoto image using supervised classification and feature extraction techniques. DSM model provides the topographical height in three-dimensional (3D) view and enables identifying low and high tide, MSL, and lowest astronomical tides (LAT) at the riverbank. The study found that UAV-MSL derived riverbank line was identified in between the MSL value (at 2.302 m) and chart datum (at 1.043 m). This study also identified the correlation model, R2 value at 0.9288 between UAV-MSL derived riverbank line with MSL-tide gauge data. This study provides an alternative for researchers to determine riverbank lines based on UAV derived MSL instead of using only tide gauge data.

Validation of Badan Informasi Geospasial Tide Model in the Sunda Strait Waters using Sebesi Island Tide Gauge Data

Badan Informasi Geospasial (BIG) has developed tide model using 128 tide gauges (TG) data in 2015. During the period of 2016-2020 BIG has established new 31 TG and these stations have not yet been included into the model, one of them was Sebesi TG located in Sunda Strait. The aim of this research was to validate the BIG tide model using latest data of Sebesi TG by comparing Harmonic Constituents (HC) and tide prediction. HC to be compared were M2, S2, N2, K2, K1, O1, P1, Q1 from BIG tide model and Sebesi TG data. Each HC were used to generate tide predictions for August 10-20, 2020 compared with Sebesi TG raw data. HC comparison results showed the maximum amplitude difference was S2(0.011 m) and minimum was Q1(0.001 m). The maximum phase difference was Q1(14.2170) and minimum was N2(1.4510). The RMS generated for tide prediction using BIG tide model HC compared to TG raw data was 17.788cm, while RMS generated for tide prediction using HC from Sebesi TG data compared to TG raw data was 17.707cm. The correlation between raw data and tide prediction using BIG tide model HC was 0.977 while the correlation between raw data and tide prediction using Sebesi TG HC was 0.980.

Sensitivity of Enhanced SRTM for Sea-Level Rise Variation on Egyptian Coasts

The accuracy of digital elevation models (DEMs) is very critical to planning adaptation strategies for coastal areas under changing scenarios of global climate and sea-level rise. This research assesses the accuracy of the interferometric DEM from the SRTM mission as it is one of the free available DEMs that have been widely used in many applications. The SRTM model is enhanced using the best fit global geoid model for Egypt instead of EGM96 and by applying a mathematical scale factor formula. This research aims to obtain a higher vertical accuracy of the SRTM model that achieves the requirements for coastal inundation studies with minimum field measurement data. The methodology approach consists of three phases. In the first phase, the enhancement scale factor formulas were derived using uniformly distributed ground control points (GCPs) at two different terrains data in Egypt. The second phase contains the evaluation and validation process. It was observed that the enhanced accuracy achieved ranged from 45% to 60 % based on the type of terrains. In the last phase, the sensitivity of these DEMs to sea-level projections using the recent available local tide gauge data was analyzed. It is recommended to investigate this method approach to determine the optimal size and distribution of reference ground control points needed to adjust various free open DEMs of low vertical accuracy for further studies and other applications.

Improving the accuracy of barotropic and internal tides embedded in a high-resolution global ocean circulation model of MITgcm

In recent years, high-resolution global ocean circulation models have begun to include astronomical tidal forcing to concurrently simulate ocean circulation, barotropic and internal tides. The inclusion of tides is of great significance for understanding three-dimensional (3-D) ocean mixing and developing internal tide prediction. However, due to uncertainty in topography and damping terms, improper representation of the self-attraction and loading term, barotropic tidal estimate cannot match that in the data-constrained barotropic tidal model. In this study, the TPXO8 data are assimilated to optimize the tides embedded in a global 9-km 3-D general circulation model of MITgcm as follows. For each time step of the MITgcm, the previous 25-hour-averaged sea surface height (SSH) is subtracted from the instantaneous SSH to obtain the MITgcm surface tidal elevation. Then, the difference of surface tidal elevation between the MITgcm and TPXO8 together with a damping coefficient acts as an additional term for the right-hand side of the SSH equation in order to correct the SSH field. Results show that the global M2 (K1) error of the corrected surface tidal elevation relative to the TPXO8 reaches 2.12 cm (0.89 cm). Compared with tide gauge data, the accuracy of surface tidal elevation after using the correction scheme is almost the same as that of TPXO8 for all areas except for some continental shelf and coastal areas. The internal tidal signals are subsequently improved compared to the along-track altimetry and TAO observations. Compared to previous tidal assimilation scheme, the implementation of the correction scheme used in this study is simple and more suitable for realistic applications.

Sea Surface Heights and Marine Gravity Determined from SARAL/AltiKa Ka-band Altimeter Over South China Sea

SARAL/AltiKA (SRL) is the first altimetry satellite with a Ka-band altimeter. To validate the advantages of the Ka-band altimeter over traditional Ku-band altimeters in marine geodetic applications, a comprehensive analysis is carried out over the South China Sea (SCS) (0–30° N, 105–125° E) from three aspects, namely the influence of load on waveforms, the precision of sea surface heights (SSHs), and the precision of altimeter-derived marine gravity field. Coastal waveforms of SRL, Jason-2, and CryoSat-2 are separately compared with corresponding ocean-type waveforms. The radius of coastal influence on SSHs of SRL/exact repeat mission (SRL/ERM) is the smallest, being about 3 km. Crossover discrepancies, global mean sea surface models, and tide gauge data are used to assess the precision of altimetric SSHs. Compared with the SSH precision of Ku-band Jason-2/ERM, the SSH precision of Ka-band SRL/ERM is 4.6% higher over the SCS and 10% higher in offshore areas. Gridded gravity anomalies are derived from measurements of SRL/drifting phase (SRL/DP) and CryoSat-2 through the inverse Vening-Meinesz formula, respectively. According to the assessment by shipborne gravity data and global marine gravity models, the precision of SRL/DP-derived gravity is higher than that of CryoSat-2-derived gravity over the SCS, especially in offshore areas. In some cycles, ground tracks of SRL/ERM have large drifting of more than 10 km from nominal tracks. The results show that the Ka-band altimeter of SRL has better precision in SSHs and marine gravity recovery than the Ku-band altimeter over the SCS, particularly in offshore areas.

An Analysis of Total Water Storage Changes Obtained from GRACE FO Observations over the Venezia Islands Area Supported with Additional Data

The Venezia Islands are a very special area from the hydrological point of view due to its water mass changes. Regular floods results in the need for the regular monitoring of water mass changes. For this purpose, a Gravity Recovery and Climate Experiment mission (GRACE) can be used as a source of data. The aim of the paper is to compare the latest results of the new GRACE FO observations. The comparisons were carried out all over Venezia Island using the L3 level, RL06 release data obtained with spherical harmonics degree and order extension of up to 120, by the three most important computational centres: JPL, GFZ, CSR. Results are compared to an average month values of precipitation and evapotranspiration and tide gauge data in the nearby area. Based on the research, no dependence between TWS and evapotranspiration and evapotranspiration change were found.

Risk Assessment of Coastal Flooding under Different Inundation Situations in Southwest of Taiwan (Tainan City)

The Pacific island countries are particularly vulnerable to the effects of global warming including more frequent and intense natural disasters. Seawater inundation, one of the most serious disasters, could damage human property and life. Regional sea level rise, highest astronomic tide, vertical land motions, and extreme sea level could result in episodic, recurrent, or permanent coastal inundation. Therefore, assessing potential flooding areas is a critical task for coastal management plans. In this study, a simulation of the static flooding situation in the southwest coast of Taiwan (Tainan city) at the end of this century was conducted by using a combination of the Taiwan Digital Elevation Model (DEM), regional sea level changes reconstructed by tide gauge and altimetry data, vertical land deformation derived from leveling and GPS data, and ocean tide models. In addition, the extreme sea level situation, which typically results from high water on a spring tide and a storm surge, was also evaluated by the joint probability method using tide gauge records. To analyze the possible static flood risk and avoid overestimation of inundation areas, a region-based image segmentation method was employed in the estimated future topographic data to generate the flood risk map. In addition, an extreme sea level situation, which typically results from high water on a spring tide and a storm surge, was also evaluated by the joint probability method using tide gauge records. Results showed that the range of inundation depth around the Tainan area is 0–8 m with a mean value of 4 m. In addition, most of the inundation areas are agricultural land use (60% of total inundation area of Tainan), and two important international wetlands, 88.5% of Zengwun Estuary Wetlands and 99.5% of Sihcao Wetlands (the important Black-faced Spoonbills Refuge) will disappear under the combined situation. The risk assessment of flooding areas is potentially useful for coastal ocean and land management to develop appropriate adaptation policies for preventing disasters resulting from global climate change.

Adjoint estimation and interpretation of spatially varying bottom friction coefficients of the [formula omitted] tide for a tidal model in the Bohai, Yellow and East China Seas with multi-mission satellite observations

In order to estimate the spatially varying bottom friction coefficients (BFCs) in the Bohai, Yellow and East China Seas (BYECS), multi-mission satellite observations from TOPEX/Poseidon, Jason-1 and Jason-2 are assimilated into a two-dimensional tidal model with an adjoint method. This work aims to: (1) study the characteristics of spatially varying BFCs in BYECS; and (2) find out the possible sources and mechanisms of spatially varying BFCs. In the numerical experiment, the M2 tide is simulated and the spatially varying BFCs in BYECS are estimated by assimilating observations from tidal gauge stations and multi-mission altimeters. The simulated M2 tide matches well with the tide gauge data, indicating the reasonability of present methodology of spatially varying BFCs. The characteristics and possible mechanisms of the spatially varying BFCs in BYECS are concluded and discussed, and the main conclusions include: (1) the values of BFCs in shallow water are usually larger than those in deep water, which can reach the order of 10−2 in shallow areas; (2) the values of BFCs generally decrease with the strengthened depth-averaged tidal current velocity in BYECS; (3) the high-value BFC areas near the coasts always appear in the downstream sides of headlands or peninsulas, which are possibly caused by the propagation of tidal waves; (4) the high-value BFC area along Jiangsu coast is affected by the bottom morphology of radially distributed tidal sand ridges, which might enhance the form drag; (5) the high-value BFC areas in the middle of the South Yellow Sea and Fujian coast correspond to the muddy areas, where the mixture of different sediment types changes the bottom roughness.

Sea Level Fusion of Satellite Altimetry and Tide Gauge Data by Deep Learning in the Mediterranean Sea

Satellite altimetry and tide gauges are the two main techniques used to measure sea level. Due to the limitations of satellite altimetry, a high-quality unified sea level model from coast to open ocean has traditionally been difficult to achieve. This study proposes a fusion approach of altimetry and tide gauge data based on a deep belief network (DBN) method. Taking the Mediterranean Sea as the case study area, a progressive three-step experiment was designed to compare the fused sea level anomalies from the DBN method with those from the inverse distance weighted (IDW) method, the kriging (KRG) method and the curvature continuous splines in tension (CCS) method for different cases. The results show that the fusion precision varies with the methods and the input measurements. The precision of the DBN method is better than that of the other three methods in most schemes and is reduced by approximately 20% when the limited altimetry along-track data and in-situ tide gauge data are used. In addition, the distribution of satellite altimetry data and tide gauge data has a large effect on the other three methods but less impact on the DBN model. Furthermore, the sea level anomalies in the Mediterranean Sea with a spatial resolution of 0.25° × 0.25° generated by the DBN model contain more spatial distribution information than others, which means the DBN can be applied as a more feasible and robust way to fuse these two kinds of sea levels.

A Comprehensive Estuarine Hydrodynamics-Salinity Study: Impact of Morphologic Changes on Ria de Aveiro (Atlantic Coast of Portugal)

Shallow coastal lagoons driven by tidal processes are extremely dynamic environments prone to continuous natural and anthropogenic pressures. The hydrodynamics of these systems deeply depends on the effect of local morphology on the tidal propagation, so their permanent evolution constantly changes tidal dependent processes. For this reason, the present work aims to review the main characteristics of Ria de Aveiro hydrodynamics, a shallow lagoon located at the Atlantic Coast of Portugal, evaluating its evolution over the last 30 years (between 1987 and 2020) and investigating the main morphological changes in its origin. For this purpose, a comparative analysis is performed to determine the main process, including the observed hydrodynamic changes: Deepening of the inlet channel or of the main lagoon channels. To achieve these goals, the authors explored a remarkable database including bathymetric, tide gauge, and salinity data from 1987 until the present. This analysis is completed by the exploitation of a hydrodynamical model (Delft3D), validated against field data. Several simulations were performed to analyse changes in tidal propagation along the lagoon channels (considering the main semi-diurnal constituent M2), tidal asymmetry, tidal currents, tidal prism, and salinity patterns. The results show that the general deepening of the lagoon observed between 1987 and 2020 led to important changes in the lagoon hydrodynamics, namely the increase/decrease of the M2 constituent amplitude/phase, as well as the increase of tidal currents and salt intrusion within the entire lagoon, with the changes being amplified towards the head of the main channels. Although the inlet deepening partially contributed to the modifications found, the results revealed that the deepening of the main lagoon channels had the most significant contribution to the changes observed during the last 30 years.

The X-TRACK/ALES multi-mission processing system: New advances in altimetry towards the coast

In the context of the ESA Climate Change Initiative project, a new coastal sea level altimetry product has been developed in order to support advances in coastal sea level variability studies. Measurements from Jason-1,2&3 missions have been retracked with the Adaptive Leading Edge Subwaveform (ALES) Retracker and then ingested in the X-TRACK software with the best possible set of altimetry corrections. These two coastal altimetry processing approaches, previously successfully validated and applied to coastal sea level research, are combined here for the first time in order to derive a 16-year-long (June 2002 to May 2018), high-resolution (20-Hz), along-track sea level dataset in six regions: Northeast Atlantic, Mediterranean Sea, West Africa, North Indian Ocean, Southeast Asia and Australia. The study demonstrates that this new coastal sea level product called X-TRACK/ALES is able to extend the spatial coverage of sea level altimetry data ~3.5 km in the land direction, when compared to the X-TRACK 1-Hz dataset. We also observe a large improvement in coastal sea level data availability from Jason-1 to Jason-3, with data at 3.6 km, 1.9 km and 0.9 km to the coast on average, for Jason-1, Jason-2 and Jason-3, respectively. When combining measurements from Jason-1 to Jason-3, we reach a distance of 1.2–4 km to the coast. When compared to tide gauge data, the accuracy of the new altimetry near-shore sea level estimations also improves. In terms of correlations with a large set of independent tide gauge observations selected in the six regions, we obtain an average value of 0.77. We also show that it is now possible to derive from the X-TRACK/ALES product an estimation of the ocean current variability up to 5 km to the coast. This new altimetry dataset, freely available, will provide a valuable contribution of altimetry in coastal marine research community.

Analyzing the Effects of Short-Term Persistence and Shift in Sea Level Records along the US Coast

Is sea level change affected by the presence of autocorrelation and abrupt shift? This question reflects the importance of trend and shift detection analysis in sea level. The primary factor driving the global sea level rise is often related to climate change. The current study investigates the changes in sea level along the US coast. The sea level records of 59 tide gauge data were used to evaluate the trend, shift, and persistence using non-parametric statistical tests. Mann-Kendall and Pettitt’s tests were utilized to estimate gradual trends and abrupt shifts, respectively. The study also assessed the presence of autocorrelation in sea level records and its effect on both trend and shift was examined along the US coast. The presence of short-term persistence was found in 57 stations and the trend significance of most stations was not changed at a 95% confidence level. Total of 25 stations showed increasing shift between 1990–2000 that was evaluated from annual sea level records. Results from the current study may contribute to understanding sea level variability across the contiguous US. This study extends an elaborative understanding of sea level trends and shifts which might be useful for water managers.

The decadal sea level variability observed in the Indian Ocean tide gauge records and its association with global climate modes

Decadal sea level variability of the Indian Ocean is studied using long-term tide gauge observations of 17 stations. The existence of close association between Indian Ocean sea level and the global decadal climate modes, such as Pacific Decadal Oscillation (PDO) and/or Atlantic Multidecadal Oscillation (AMO) is revealed in most of the tide gauge data sets. An anti-phase sea level evolution with PDO index is identified in the regions such as west coast of India, east coast of India and the western coast of Australia. Moreover, the PDO related sea level variability is found to be stronger at the tide gauge stations along the west coast of India compared to that along the east coast of India, ruling out the remote forcing from the equatorial Indian Ocean. On the other hand, the tide gauge station at Mumbai with more than 100 years of data shows an in-phase relationship with AMO index on the decadal timescales, whereas such relationship is very weak at the tide gauge station Fremantle where similar long-term data is available. The analysis of sea level pressure (SLP), rainfall and winds over the tropical Indian Ocean indicates existence of strong relationship with AMO especially over the Arabian Sea region, strongly suggesting that the variability observed at the tide gauge station Mumbai may have larger spatial signatures. During the positive phase of AMO, the Arabian Sea region is characterized by large fall in SLP and a strong cross equatorial flow towards the Indian land mass, which are primarily responsible for the positive sea level anomalies (SLAs) at the Mumbai tide gauge station and the adjacent regions and positive rainfall over India.

Decadal Seismicity Prior to Great Earthquakes at Subduction Zones: Roles of Major Asperities and Low-Coupling Zones

Decadal forerunning seismic activity of magnitude Mw ≥ 5.0 is mapped for all 45 mainshocks of Mw 7.7 to 9.1 at subduction zones of the world from 1993 to mid 2020. The zones of high slip in nearly all great earthquakes were nearly quiescent beforehand and are identified as the sites of great asperities and zones of strong seismic coupling. Much forerunning activity occurred at smaller asperities along the peripheries of the rupture zones of many great and giant mainshocks. Those sizes of great asperities as ascertained from forerunning activity generally agree with the areas of high seismic slip as determined by others from geodetic and tide-gauge data and finite-source seismic modeling. Asperities are strong, well-coupled portions of plate interfaces. Different patterns of forerunning activity on time scales of about 5 to 45 years are attributed to either the sizes and spacing of asperities (or lack of). This permits many great asperities to be mapped decades before they rupture in great and giant shocks. Several poorly coupled subduction zones such as Java, Lesser Sunda, Marianas, Tonga and Kermadec are characterized by few great thrust earthquakes and little, in any forerunning activity. Rupture zones of many great and giant earthquakes are bordered either along strike, updip, or downdip by zones of lower plate coupling. Several bordering regions were sites of forerunning activity, aftershocks, and slow-slip events. The detection of forerunning and precursory activities of various kinds should be sought on the peripheries of great asperities as well as within zones of high co-seismic slip.

The ambiguous sea level rise at Brest’s 212 yearlong record elucidated

Abstract The tide gauge record at Brest, France, along Eastern part of Atlantic coast is one of the longest records in Europe spanning 212 years (1807–2019). Analyzing these records has important ramifications in assessing anthropogenic impact of climate change at local and regional scales during this period. All the previous studies that analyzed Brest’s tide gauge record have used vaguely defined quadratics models and did not incorporate the effect of sea level variations at various frequencies, which confounded the presence or absence of a plausible uniform acceleration. Here, we entertained two competing kinematic models; one with a uniform acceleration representing 212 years of monthly averaged tide gauge data, the other is a two-phase trend model (Phase I is 93 years long and Phase II is 119 years long). Both models include statistically significant (α = 0.05) common periodic effects, and sub and super harmonics of luni-solar origin for representing monthly averaged sea level anomalies observed at Brest. The least squares statistics for both models’ solutions cannot distinguish one model over the other, like earlier studies. However, the assessment of Phase I segment of the records disclosed the absence of a statistically significant trend and a uniform acceleration during this period. This outcome eliminates conclusively the occurrence of a uniform acceleration during the entire 212-year data span of the tide gauge record at Brest, favoring the two-phase trend model as a sound alternative.

Sea-Level Change at the Brest (France) Tide Gauge and the Markowitz Component of Earth's Rotation

Le Mouel, J.-L.; Lopes, F., and Courtillot, V., 2021. Sea-level change at the Brest (France) tide gauge and the Markowitz component of Earth's rotation. Journal of Coastal Research, 37(4), 683–690. Coconut Creek (Florida), ISSN 0749-0208.This paper centers on singular spectrum analysis (SSA) of variations in sea level in Brest (France) and in Earth's axis of rotation. Brest tide gauge data (recording the Brest sea level [BSL]) are available from 1807 onward. The main features of the BSL curve are common to most stations in the Northern Hemisphere; hence, the Brest curve has recorded a gross Earth datum. Pole positions (coordinate m2), another gross Earth datum, are from the International Earth Rotation and Reference System Service (1845–2019). The first SSA components of both series, i.e. the trends, are similar, with a major acceleration event near 1900 and sea level lagging pole motion by ∼5 to 10 years. SSA components with periods of 1, 5.4, and ∼11 years are common to the two series. An important feature is a 0.5-year component that is present in sea level but absent from pole motion. The remarkable similarity of the two trends and their phase lag suggests a causal relationship opposite the one that is generally accepted.

The Sea-Level Changes in Hong Kong From Tide-Gauge Records and Remote Sensing Observations Over the Last Seven Decades

The importance of studying the sea-level change (SLC) in Hong Kong (HK) is emphasized by factors related to high population density, intensive urban, and industrial development, particularly along the coast that involves a considerable landreclamation. To address this issue, we investigate the spatiotemporal characteristics of SLC in the HK territories by analyzing tide-gauge (TG) records collected from 1954 to 2019 and satellite-altimetry (SA) data from 1993 to 2019. The application of the ocean-tide and inverted barometer corrections to TG data substantially decreased uncertaintie by achieving a submillimeter accuracy. The SLC rates detected at six TG stations vary significantly even within this relatively small coastline. According to our estimates, the annual rates vary between 0.32 ± 0.51 and 4.19 ± 0.46 mm/yr from 1997 to 2019. These large differences are related to different patterns of ocean currents and the freshwater discharge in the Pearl River Estuary. The empirical orthogonal function analysis confirms the expected increasing rising in SLC with two anomalous periods during 2002-2003 and 2015-2016 that are likely attributed to the El Niño-Southern Oscillation (ENSO). SLCs are positively correlated with the ENSO index and ENSO precedes several months (normally 5 months) the interannual SLC. The analysis of the GPS trends reveals a prevailing subsidence in the HK territories, which aggravates the relative SLC estimates. Between 1997 and 2019, the absolute sea level raised at the annual rate of 3.17 ± 1.56 mm/yr (when using the GPS+TG) and 2.88 ± 0.59 mm/yr (when using SA observations).

Sea-Level Change along the Emilia-Romagna Coast from Tide Gauge and Satellite Altimetry

Coastal flooding and retreat are markedly enhanced by sea-level rise. Thus, it is crucial to determine the sea-level variation at the local scale to support coastal hazard assessment and related management policies. In this work we focus on sea-level change along the Emilia-Romagna coast, a highly urbanized, 130 km-long belt facing the northern Adriatic Sea, by analyzing data from three tide gauges (with data records in the last 25–10 years) and related closest grid points from CMEMS monthly gridded satellite altimetry. The results reveal that the rate of sea-level rise observed by altimetry is coherent along the coast (2.8 ± 0.5 mm/year) for the period 1993–2019 and that a negative acceleration of −0.3 ± 0.1 mm/year is present, in contrast with the global scale. Rates resulting from tide gauge time series analysis diverge from these values mainly as a consequence of a large and heterogeneous rate of subsidence in the region. Over the common timespan, altimetry and tide gauge data show very high correlation, although their comparison suffers from the short overlapping period between the two data sets. Nevertheless, their combined use allows assessment of the recent (last 25 years) sea-level change along the Emilia-Romagna coast and to discuss the role of different interacting processes in the determination of the local sea level.

Point wise Determination of Vertical Deformation at southern Coastal Areas of Caspian Sea by Combination of Satellite Altimetry and Tide Gauge Observations

In this paper, a combination of satellite altimetry (SA) and tide-gauge (TG) observations is used to determine the pattern of vertical deformation at southern coasts of Caspian Sea. Satellite altimetry measures the variations of sea water surface with a respect to a reference ellipsoid, while tide gauge data gives the sea level relative to a particular ground station; consequently, the difference between these two observations will give the vertical motion at the ground station. However, altimeter satellites still have problems at shallow waters or coastal areas which leads to inaccurate and corrupted sea surface height at tide gauge stations. To minimize this imprecision, two strategies are applied for the selection of the suitable SA points near the TG stations. The First one is to choose those close passes to the TG stations, provided that the SA data is not corrupted (range observation is available). The second strategy is to extrapolate the SA points at TG stations using Spatio- Temporal Kriging method. In both point of views, use is made of re-tracking methods to improve the ranges of SA observations near the coast. Amplitudes of tidal components are removed from both SA and TG observations to derive the residual tide free signals. Afterward, a linear trend is fitted to the residual signals in the sense of least square; the difference between these two, supplies the vertical deformation of the ground station. The results illustrate that the average vertical crustal motion at Anzali, Noshahr and Neka are -1.387, -1.903 and -3.14 mm/year, respectively.