Article

Measuring global mean sea level variations using TOPEX/POSEIDON altimeter data

January 1995
Journal of Geophysical Research Atmospheres 100(C12):25135-25151

Authors:

University of Colorado Boulder

The analysis of historical tide gauge records generally indicates that sea level has risen at a rate of about 2 mm/yr during the last 100 yr. The prospect of measuring variations in global mean sea level has been assessed using approximately 2.5 yr of satellite altimeter data from the TOPEX/POSEIDON (T/P) mission. The global mean sea level variations measured by T/P every 10 days have an rms of 6 mm (4 mm after detrending), some of which is shown to be correlated with sea surface temperature variations. The rate of change of global mean sea level derived from 2.5 yr of data is ±5.8 mm/yr with a scatter of 0.7 mm/yr. Currently, it is impossible to accurately estimate the error in the measured rate of sea level rise, since little is known about the long-term behaviour of the measurement errors at the millimeter level. In addition, there is evidence from the sea surface temperature record that the measured rate of sea level rise is associated with a relatively short-term (interannual) variation unrelated to the long-term signal expected from global warming. Nevertheless, these results suggest that T/P is achieving the necessary repeatability to measure global sea level variations caused by climate change -from Author

Time‐Space Sampling‐Related Uncertainties of Altimetric Global Mean Sea Level Estimates

Article

Full-text available

Nov 2019

Uncertainty measures for global mean sea level (GMSL) estimates are quantified, resulting from limited (in space and time) along‐track altimetric sampling of the global sea level field by altimetric satellite missions. To estimate such sampling‐related uncertainty, sea surface height (SSH) fields simulated by the high‐resolution STORM/NCEP ocean circulation model were subsampled along altimeter tracks for the period 1993–2010 and subsequently processed into global SSH averages using similar techniques to those used by six groups worldwide. Results show that the underlying satellite space‐time sampling has a substantial impact on the accuracy of GMSL estimates. This uncertainty originates primarily from data missing over sea ice‐covered regions; omitted data from shallow seas also contributes. Uncertainties in GMSL estimates result both from interpolation techniques required to fill data gaps such as missing tracks, and the choice of the mean sea surface required to estimate SSH anomalies. Cumulative effects lead to errors in GMSL estimates from ∼0.8 to ∼3.2 mm (root‐mean‐square, RMS), depending on the underlying details of the estimation method. Results suggest that sampling limitations in meridional direction are a fundamental constraint on the accuracy level reachable for any altimetric GMSL estimate, resulting in a systematic Gaussian uncertainty of about 1.2 mm (RMS), 50% of which occurs on monthly time scales, while some fraction occurs on time scales of several years. In all cases, a significant fraction of the error results from a mass exchange between the global ocean and sea ice‐covered polar regions. Contributions from data processing details are measurable but less significant.

Les variations actuelles du niveau de la mer : Observations et causes.

Article

Nov 2005

A. Lombard

Whereas sea level has changed little over the last 2000 years, it has risen at a rate of about 2 mm/year during the 20th century. This unexpected sea level rise has been attributed to the anthropogenic global warming, recorded over several decades. Sea level variations have been measured globally and precisely for about 12 years due to satellite altimeter missions Topex/Poseidon and Jason-1. These observations indicate a global mean sea level rise of about 3 mm/year since 1993, a value significantly larger than observed during previous decades. Recent observations have allowed us to quantify the various climatic factors contributing to observed sea level change : thermal expansion of sea water due to ocean warming, melting of mountain glaciers and ice sheets, and changes in the land water reservoirs. A water budget based on these new observations allows us to partly explain the observed sea level rise. In particular, we show that the thermal expansion explains only 25% of the secular sea level rise as recorded by tide-gauges over the last 50 years, while it contributes about 50% of sea level rise observed over the last decade. Meanwhile, recent studies show that glacier and ice sheet melting could contribute the equivalent of 1mm/year in sea level rise over the last decade. In addition, the high regional variability of sea level trends revealed by satellite altimetry is mainly due to thermal expansion. There is also an important decadal spatio-temporal variability in the ocean thermal expansion over the last 50 years, which seems to be controlled by natural climate fluctuations. We question for the first time the link between the decadal fluctuations in the ocean thermal expansion and in the land reservoirs, and indeed their climatic contribution to sea level change. Finally a preliminary analysis of GRACE spatial gravimetric observations over the oceans allows us to estimate the seasonal variations in mean sea level due to ocean water mass balance variations.

Environmental Effects of Increased Atmospheric Carbon Dioxide

Article

Full-text available

Sep 1999
Energ Environ

A review of the literature concerning the environmental consequences of increased levels of atmospheric carbon dioxide leads to the conclusion that increases during the 20th century have produced no deleterious effects upon global climate or temperature. Increased carbon dioxide has, however, markedly increased plant growth rates as inferred from numerous laboratory and field experiments. There is no clear evidence, nor unique attribution, of the global effects of anthropogenic CO2 on climate. Meaningful integrated assessments of the environmental impacts of anthropogenic CO2 are not yet possible because model estimates of global and regional climate changes on interannual, decadal and centennial time scales remain highly uncertain.

Regional Sea Level Changes in the East China Sea from 1993 to 2020 Based on Satellite Altimetry

Article

Full-text available

Sep 2024

A comprehensive analysis was carried out to investigate the driving factors and influencing mechanisms of spatiotemporal variation of sea level at multiple scales in the East China Sea (ECS) via satellite altimetry datasets from 1993 to 2020. Based on the altimetry grid data processed by the local mean decomposition method, the spatiotemporal changes of ECS sea level are analyzed from the multi-scale perspective in terms of multi-year, seasonal, interannual, and multi-modal scales. The results revealed that the ECS regional mean sea level change rate is 3.41 ± 0.58 mm/year over the 28-year period. On the seasonal scale, the regional mean sea level change rates are 3.45 ± 0.66 mm/year, 3.35 ± 0.60 mm/year, 3.39 ± 0.71 mm/year, and 3.57 ± 0.75 mm/year, for the four seasons (i.e., spring, summer, autumn, and winter) respectively. The spatial distribution analysis showed that ECS sea level changes are most pronounced in coastal areas. The northeast sea area of Taiwan and the edge of the East China Sea shelf are important areas of mesoscale eddy activity, which have an important impact on regional sea level change. The ECS seasonal sea level change is mainly affected by monsoons, precipitation, and temperature changes. The spatial distribution analysis indicated that the impact factors, including seawater thermal expansion, monsoons, ENSO, and the Kuroshio Current, dominated the ECS seasonal sea level change. Additionally, the ENSO and Kuroshio Current collectively affect the spatial distribution characteristics. Additionally, the empirical orthogonal function was employed to analyze the three modes of ECS regional sea level change, with the first three modes contributing 26.37%, 12.32%, and 10.47%, respectively. Spatially, the first mode mainly corresponds to ENSO index, whereas the second and third modes are linked to seasonal factors, and exhibit antiphase effects. The analyzed correlations between the ECS sea level change and southern oscillation index (SOI), revealed the consistent spatial characteristics between the regions affected by ENSO and those by the Kuroshio Current.

Satellite altimetry: The technology and its application in geodesy

Article

Full-text available

Dec 2017
GEOD LIST

Radar satellite altimetry is a method that enables obtaining the global high-precision sea level data related to desired geocentric reference frame. Satellite altimeter observations enable efficient solving of the main geodetic tasks addressed by the definition of geodesy, which include measuring the Earth’s size and shape as well as the determination of the Earth’s gravitational field. The measurements obtained by satellite altimeters are direct indicator of the Earth’s response to climate change and other geophysical phenomena. The obtained data are used for determination of the height systems over the land and the sea, bathymetric map making, estimation of vertical land motion etc. This paper presents the basic concept of satellite altimeter technology along with principal principles of altimeter data processing. A special attention was paid to the review and analysis of the altimeter data corrections due to signal propagation through the atmosphere and the influence of geophysical phenomena on the emitted signal. Further, the study reviews in detail the development of technology through three phases, analyzing the contribution of each and describing the guidelines for future technology development. Finally, the study shows the products derived from satellite altimetry currently available for utilization in geodetic applications such as the models of gravity field, bathymetry, sea level trends, and others.

Satelitska altimetrija: tehnologija i primjena u geodeziji (Satellite altimetry: The technology and its application in geodesy)

Article

Full-text available

Dec 2017
GEOD LIST

Radarska satelitska altimetrija metoda je prikupljanja globalnih visoko-preciznih podataka o razini mora u odnosu na odabrani geocentrični referentni okvir. Opažanja satelitskom altimetrijom omogućuju određivanje oblika i veličine Zemlje te računanje Zemljina polja ubrzanja sile teže što su i osnovne geodetske zadaće zadane definicijom geodezije. Mjerenja promjene srednje razine mora prikupljena satelitskom altimetrijom služe kao izravni pokazatelj djelovanja klimatskih promjena i drugih geofizičkih procesa na Zemlji, a primijenjena su i za definiranje visinskih sustava na kopnu i moru, izradu karata dubina, procjenu vertikalnih gibanja obalnih područja i sl. U ovom radu detaljno je prikazan osnovni koncept tehnologije uz primijenjene principe obrade radarskih opažanja. Poseban je naglasak na prikazu i analizi korekcija altimetrijskih opažanja potrebnih zbog propagacije signala kroz atmosferu te utjecaja geofizičkih fenomena vodenih površina na odaslani signal. Rad detaljno prikazuje razvoj tehnologije kroz tri faze uz analizu doprinosa svake te opisuje smjernice budućeg razvoja tehnologije. Naposljetku, prikazani su proizvodi satelitske altimetrije dostupni za primjenu u geodetske svrhe kao što su modeli anomalija ubrzanja sile teže, dubina, trendova srednje promjene razine mora i drugi. Ključne riječi: satelitska altimetrija, promjena razine mora, obalna altimetrija, prostorni podaci. ABSTRACT. Radar satellite altimetry is a method that enables obtaining the global high-precision sea level data related to desired geocentric reference frame. Satellite altimeter observations enable efficient solving of the main geodetic tasks addressed by the definition of geodesy, which include measuring the Earth’s size and shape as well as the determination of the Earth’s gravitational field. The measurements obtained by satellite altimeters are direct indicator of the Earth’s response to climate change and other geophysical phenomena. The obtained data are used for determination of the height systems over the land and the sea, bathymetric map making, estimation of vertical land motion etc. This paper presents the basic concept of satellite altimeter technology along with principal principles of altimeter data processing. A special attention was paid to the review and analysis of the altimeter data corrections due to signal propagation through the atmosphere and the influence of geophysical phenomena on the emitted signal. Further, the study reviews in detail the development of technology through three phases, analyzing the contribution of each and describing the guidelines for future technology development. Finally, the study shows the products derived from satellite altimetry currently available for utilization in geodetic applications such as the models of gravity field, bathymetry, sea level trends, and others.

The estimation of the Adriatic Sea level variability and changes based on multi-satellite altimetry data

Thesis

Apr 2017

Marijan Grgic

The measurements of sea level change are a fundamental variable of the Earth system that indicates Earth's response to climate change and other geophysical phenomena. Changes in the height of the ocean are described through the relative and absolute sea level change depending on the geodetic reference the sea level records are related to. Satellite altimetry provides absolute sea level measurements related to the global geodetic reference with high precision in the open ocean areas and lower performance in coastal areas, whereas tide gauges provide relative sea level measurements related to the adjacent land, i.e. to the local reference, with high performance in coastal areas. Based on the research objectives and hypothesis proposed, this study resulted in a novel method for reconstructing absolute sea level from satellite altimeter and tide gauge records. The method is documented, analyzed, and applied to enable improved determination of sea level variability and sea level trends for the specific area of the Adriatic Sea during the satellite altimetry era, i.e. from late 1992 onwards. Following the results of the estimates of current sea level trends and the projections of sea level change for the 21st century combined with the computed absolute sea level surfaces and the vertical representation of the land areas, the study was extended on assessing the impact of sea level change on the Adriatic coast, focusing on the eastern Adriatic. The study hence presents the expected area loss of the eastern Adriatic coast throughout the 21st century and detects the possible vulnerable areas that are expected to be significantly influenced by the sea level change.

Vertical land motions on Earth surface by satellite radar altimetry, tide gauge and GPS. A case study : the Gulf of Mexico

Article

Aug 2010

Camille Letetrel

In the context of the current sea level rise, the determination of coastal vertical land motion is crucial for two main reasons. In one hand, tide gauge measurements are affected by vertical displacements and this is a source of uncertainties for the estimation of long-term sea level variations ( those variations are in the order of 1 to 2 mm/yr of sea level rise during the last century according to different authors). On the other hand, coastal subsiding processes could aggravate the effects of sea level rise with rates leading sometimes to 1 cm/yr of relative sea level rise, that is 1 meter over a century, without any acceleration of climatic contribution. This PhD thesis addresses the determination of coastal vertical land movement by GPS and by an original method combining the data from both tide gauge and satellite radar altimetry. The method which is suggested here is based on those of Kuo et al., (2004), repeated and extended by spatio-temporal filtering from EOF analysis of the two kinds of series. The method is applied in the Gulf of Mexico over the period 1950-2009 using the available fifteen tide gauge series of more than 40 years of observations selected from the PSMSL and altimetric series selected from AVISO over the period 1992-2009. The comparison between vertical land movements from the five position time series of the GPS stations collocated at tide gauge locations shows a root mean square error of 0.60 mm/an over the difference , highlighting the high accuracy of the new approach. Beforehand, GPS time series underwent a detailed noise analysis and their associated uncertainties, legitimating the use of GPS series in the correction of tide gauge sea level trends. The uncertainties from GPS vertical velocities are in the order of 0.5 mm/yr which is significantly lower than other analysis of this type. The adjusted method altimetry minus tide gauge presents interesting prospects for the accurate determination of coastal land motion where there are not geodetic measurements.

Monitoring of the ERS-2 Radar Altimeter range measurement stability

Article

Full-text available

Global Mean Sea Level Change Projections up to 2100 Using a Weighted Singular Spectrum Analysis

Article

Full-text available

Nov 2024

This paper forecasts global mean sea level (GMSL) changes from 2024 to 2100 using weighted singular spectrum analysis (SSA) that considers the formal errors of the previous GMSL time series. The simulation experiments are first carried out to evaluate the performance of the weighted and traditional SSA approaches for GMSL change prediction with two evaluation indices, the root mean square error (RMSE) and mean absolute error (MAE). The results show that all the RMSEs and MAEs of the weighted SSA are smaller than those of the traditional SSA, indicating that the weighed SSA can predict GMSL changes more accurately than the traditional SSA. The real GMSL change rate derived from weighted SSA is approximately 1.70 ± 0.02 mm/year for 1880–2023, and the predicted GMSL changes with the first two reconstructed components reaches 796.75 ± 55.92 mm by 2100, larger than the 705.25 ± 53.73 mm predicted with traditional SSA, with respect to the baseline from 1995 to 2014. According to the sixth Assessment Report of Intergovernmental Panel on Climate Change (IPCC AR6), the GMSL change by 2100 is 830.0 ± 152.42 mm/year with the high-emission scenarios is closer to weighted SSA than traditional SSA, though SSA predictions are within the prediction range of IPCC AR6. Therefore, the weighted SSA can provide an alternative future GMSL rise prediction.

Introductory Chapter: Satellite Altimetry – Overview

Chapter

Full-text available

Sep 2023

Tomislav Bašić

Radar satellite altimetry is one of the basic satellite measurement techniques intended primarily for solving global geodetic tasks by means of radar measurements from satellites toward the Earth. Satellite altimetry ensures the collection of high precision global data of uniform accuracy on sea level, which enables monitoring of the geophysical characteristics of the sea and larger water surfaces, that is, marine topography and circulation within liquid water bodies. During the last four decades, satellite altimetry has revolutionized geosciences, especially oceanography, geophysics, and geodesy. This measurement method found its application in modeling the shape of the Earth and the Earth’s field of acceleration of gravity, modeling the relief of the seabed and vertical displacements of the Earth’s crust in coastal areas, and monitoring climate phenomena and long-term climate changes. Satellite altimetry data is distributed in the form of original measurements and products ready for use in geosciences, most often calculated models, or calculation services. This chapter presents the fundamental principles of the radar altimetric measurement method, its historical development, achievements, and expected improvements in technology soon, as well as the scientific and professional results achieved so far in the development and application of technology.

Altimetry for the future: Building on 25 years of progress

Article

Full-text available

Jul 2021
ADV SPACE RES

In 2018 we celebrated 25 years of development of radar altimetry, and the progress achieved by this methodology in the fields of global and coastal oceanography, hydrology, geodesy and cryospheric sciences. Many symbolic major events have celebrated these developments, e.g., in Venice, Italy, the 15th (2006) and 20th (2012) years of progress and more recently, in 2018, in Ponta Delgada, Portugal, 25 Years of Progress in Radar Altimetry. On this latter occasion it was decided to collect contributions of scientists, engineers and managers involved in the worldwide altimetry community to depict the state of altimetry and propose recommendations for the altimetry of the future. This paper summarizes contributions and recommendations that were collected and provides guidance for future mission design, research activities, and sustainable operational radar altimetry data exploitation. Recommendations provided are fundamental for optimizing further scientific and operational advances of oceanographic observations by altimetry, including requirements for spatial and temporal resolution of altimetric measurements, their accuracy and continuity. There are also new challenges and new openings mentioned in the paper that are particularly crucial for observations at higher latitudes, for coastal oceanography, for cryospheric studies and for hydrology. The paper starts with a general introduction followed by a section on Earth System Science including Ocean Dynamics, Sea Level, the Coastal Ocean, Hydrology, the Cryosphere and Polar Oceans and the “Green” Ocean, extending the frontier from biogeochemistry to marine ecology. Applications are described in a subsequent section, which covers Operational Oceanography, Weather, Hurricane Wave and Wind Forecasting, Climate projection. Instruments’ development and satellite missions’ evolutions are described in a fourth section. A fifth section covers the key observations that altimeters provide and their potential complements, from other Earth observation measurements to in situ data. Section 6 identifies the data and methods and provides some accuracy and resolution requirements for the wet tropospheric correction, the orbit and other geodetic requirements, the Mean Sea Surface, Geoid and Mean Dynamic Topography, Calibration and Validation, data accuracy, data access and handling (including the DUACS system). Section 7 brings a transversal view on scales, integration, artificial intelligence, and capacity building (education and training). Section 8 reviews the programmatic issues followed by a conclusion.

Altimetry for the future: Building on 25 years of progress International Altimetry Team à

Article

Full-text available

Mar 2021

In 2018 we celebrated 25 years of development of radar altimetry, and the progress achieved by this methodology in the fields of global and coastal oceanography, hydrology, geodesy and cryospheric sciences. Many symbolic major events have celebrated these developments, e.g., in Venice, Italy, the 15th (2006) and 20th (2012) years of progress and more recently, in 2018, in Ponta Delgada, Portugal, 25 Years of Progress in Radar Altimetry. On this latter occasion it was decided to collect contributions of scientists, engineers and managers involved in the worldwide altimetry community to depict the state of altimetry and propose recommendations for the altimetry of the future. This paper summarizes contributions and recommendations that were collected and provides guidance for future mission design, research activities, and sustainable operational radar altimetry data exploitation. Recommendations provided are fundamental for optimizing further scientific and operational advances of oceanographic observations by altimetry, including requirements for spatial and temporal resolution of altimetric measurements, their accuracy and continuity. There are also new challenges and new openings mentioned in the paper that are particularly crucial for observations at higher latitudes, for coastal oceanography, for cryospheric studies and for hydrology. The paper starts with a general introduction followed by a section on Earth System Science including Ocean Dynamics, Sea Level, the Coastal Ocean, Hydrology, the Cryosphere and Polar Oceans and the ‘‘Green” Ocean, extending the frontier from biogeochemistry to marine ecology. Applications are described in a subsequent section, which covers Operational Oceanography, Weather, Hurricane Wave and Wind Forecasting, Climate projection. Instruments’ development and satellite missions’ evolutions are described in a fourth section. A fifth section covers the key observations that altimeters provide and their potential complements, from other Earth observation measurements to in situ data. Section 6 identifies the data and methods and provides some accuracy and resolution requirements for the wet tropospheric correction, the orbit and other geodetic requirements, the Mean Sea Surface, Geoid and Mean Dynamic Topography, Calibration and Validation, data accuracy, data access and handling (including the DUACS system). Section 7 brings a transversal view on scales, integration, artificial intelligence, and capacity building (education and training). Section 8 reviews the programmatic issues followed by a conclusion.

Altimetry for the future: Building on 25 years of progress

Article

Full-text available

Jul 2021
ADV SPACE RES

Measuring Global Mean Sea Level Changes With Surface Drifting Buoys

Article

Full-text available

Oct 2020
GEOPHYS RES LETT

Shane Elipot

Combining ocean model data and in situ Lagrangian data, I show that an array of surface drifting buoys tracked by a Global Navigation Satellite System (GNSS), such as the Global Drifter Program, could provide estimates of global mean sea level (GMSL) and its changes, including linear decadal trends. For a sustained array of 1,250 globally distributed buoys with a standardized design, I demonstrate that GMSL decadal linear trend estimates with an uncertainty less than 0.3 mm yr⁻¹ could be achieved with GNSS daily random error of 1.6 m or less in the vertical direction. This demonstration assumes that controlled vertical position measurements could be acquired from drifting buoys, which is yet to be demonstrated. Development and implementation of such measurements could ultimately provide an independent and resilient observational system to infer natural and anthropogenic sea level changes, augmenting the ongoing tide gauge and satellites records.

Sea Level Modeling and Realization of Lowest Astronomical Tide (LAT) for Alexandria Harbor, Egypt

Thesis

Full-text available

Nov 2019

Ahmed M. Khedr

Alexandria is the second-largest city in Egypt and the country's largest seaport that serves about 70% of the Egyptian imports and exports, especially the western harbor, which is the main commercial port and contains the Egyptian military naval base. Although Alexandria harbor has significant effect on the Egyptian economical income, there is still a shortage in coastal oceanographic discipline, specifically in the sea-level study that plays an important and effective role in many applications especially in chart production and dredging operations. Sea level is one of the oceanographic parameters that always needed in hydrographic surveying for depth reduction and chart datum realization. An imaginary surface, such as the Lowest Astronomical Tide, is recently used as the vertical tidal datum or chart datum that depths are referred to in all nautical charts. Therefore, sea-level measurements and analysis are always crucial for establishing an accurate datum precisely connected to onshore geodetic vertical datum to achieve the basic needs for mariners and hydrographers. Continuous and precise updating of tidal and terrestrial vertical datums connection and relationship, is of importance to provide essential information to many users including those in; commercial shipping industry, marine construction, water boundaries delimitation, marine safety, coastal areas planning, Engineering, chart datum for nautical charts production, in addition to military operations and many others. The geodetic vertical datum network in Egypt has been set as the mean sea level in Alexandria. This datum was first derived based on sea-level observations for eight years from 1898 to 1906. This imaginary surface was (34 cm) referred to the zero of the graduated staff in Alexandria harbor, and it was called the Egyptian Survey Authority datum. In each annual edition of the Admiralty Tide Tables, it was stated that the chart datum in Alexandria, as a secondary port in Egypt, is (-0.34 m) from mean sea level and all the essential known tidal levels were calculated and referred to Gibraltar as the standard port for Alexandria harbor on Mediterranean Sea. However, limited research has been updating this issue since most of the earlier studies of sea level in Alexandria Harbor dealt with sea level based on statistical computations without referring sea level measurements to a specific geodetic vertical datum. For most of the scientific applications especially in sea-level studies, the International Terrestrial Reference Frame (ITRF) is preferred. ITRF-2014 is the most accurate realization of the international terrestrial reference system. Delft-3D hydrodynamic flow model was used to model hourly sea level time series (19 yrs.). Bathymetric data acquired from chart digitization that was produced from hydrographic surveying operations, together with Era-interim meteorological data (1996-2006) compiled with Ras El-Tin automatic weather station data (2006-2016), all were employed as model's initial conditions beside other physical parameters. Boundary conditions were acquired from Achieving, Validation and Interpretation of the Satellite Oceanographic service (AVISO) for a daily sea level data, compiled with tidal constituents (amplitudes and phase angles) obtained from harmonic analysis of offshore observed sea level data from S4 current meter buoy (depth sensor) and the major tidal constituents parameters in the area from Delft-3D Dashboard tidal data. Model results were validated by results obtained from several observed sea-level data analysis. The analyzed hourly observed sea level time series was accurately referred to the tide gauge zero level which was geo-referenced to the latest geodetic terrestrial reference frame, data was recorded inside the harbor during two time periods (09/11/2008-08/22/2010) and (04/30/2012-10/25/2013). Besides, a short-term sea-level data from S4 buoy offshore outside the harbor during the period (10/26/2008-12/31/2008). From harmonic analysis of both modeled and observed sea level datasets inside and outside the harbor in the two different intervals using Delft-3D tide suit. It was concluded that sea level is mainly derived by tidal power with a power percentage between 53% and 81% to the total sea-level power respectively. These percentages are a result of 13 significant tidal constituents, dominated by the principal diurnal and semi-diurnal lunar tidal constituents (M2-S2-K1-O1). Besides, the solar annual (Sa) along with solar semi-annual (Ssa) tidal constituents which were found to contribute significantly with amplitude percentage ranged between (14% to 23%) for Sa and (2% to 13%) for (Ssa) to the total tidal constituents amplitudes, which reflects the seasonality effect that is related to the annual meteorological variations and thus affects sea-level changes in the area. The amplitude root means square error between both molded and observed datasets equal (0.005 m) and (0.012 m) respectively, that will not affect the accuracy of the major tidal datums determined. The cross-correlation analysis between modeled and observed sea level datasets demonstrated a strong correlation between tidal signals and moderate correlation in residuals with correlation coefficients equal (0.72) and (0.62) respectively, that confirmed the capability of Delft-3D flow model to precisely simulate variabilities and trends of observed oceanic conditions reasonably in Alexandria Harbor. From form factor percentage of both modeled and observed datasets it was signified that the tidal type regime in Alexandria Harbor is semidiurnal with a (0.25) ratio. From analyzing long-term modeled sea-level dataset (19 yrs) during the period (01/01/1996 till 11/30/2015), a positive linear trend was resulted by a rate of 3.4 mm/yr that agrees with the global sea level rise rate. From tidal vertical datums calculations of both modeled and observed datasets, the ellipsoidal heights of lowest and highest astronomical tide datums considering ± 10 cm safety margin were updated and suggested to be (14.29 m) and (15.23 m) referred to the international terrestrial reference frame 2014 respectively, with a range equals (94 cm), while the suggested same datums ellipsoidal height values referred to the world geodetic system 1984 are (14.36 m) and (15.20 m) respectively, with a range equals (84 cm). Finally, the ellipsoidal height values of the most essential used vertical datums referred to both geodetic datums ITRF-2014 and the World Geodetic System 1984 (WGS-84) were re-visited and updated from both modeled and observed sea-level datasets for Alexandria Harbor.

Measuring global mean sea level changes with surface drifting buoys

Preprint

Sep 2019

Shane Elipot

Combining ocean model data and historical in-situ Lagrangian data, I show that an array of surface drifting buoys tracked by a Global Navigation Satellite System (GNSS), such as the GPS-tracked ones of the Global Drifter Program, could provide estimates of global mean sea level (GMSL) and its changes, including linear decadal trends. For a sustained array of 1250 globally distributed buoys with a standardized design, I demonstrate that GMSL decadal linear trend estimates with an uncertainty less than 0.3 mm yr

^{-1}

could be achieved with GNSS daily random error of a couple meters in the vertical direction. This demonstration of the adequacy of the spatial sampling assumes that geodetic-grade GNSS vertical position measurements can be acquired from an array of drifting buoys, which is yet to be demonstrated. Development and implementation of such measurements could ultimately provide an independent and resilient observational system to infer natural and anthropogenic sea level changes, augmenting the on-going tide gauge and satellites records.

Sub-basin-scale sea level budgets from satellite altimetry, Argo floats and satellite gravimetry: A case study in the North Atlantic Ocean

Article

Full-text available

Nov 2016
OCEAN SCI

M. Kleinherenbrink

In this study, for the first time, an attempt is made to close the sea level budget on a sub-basin scale in terms of trend and amplitude of the annual cycle. We also compare the residual time series after removing the trend, the semiannual and the annual signals. To obtain errors for altimetry and Argo, full variance–covariance matrices are computed using correlation functions and their errors are fully propagated. For altimetry, we apply a geographically dependent intermission bias [Ablain et al.(2015)], which leads to differences in trends up to 0.8 mm yr−1. Since Argo float measurements are non-homogeneously spaced, steric sea levels are first objectively interpolated onto a grid before averaging. For the Gravity Recovery And Climate Experiment (GRACE), gravity fields full variance–covariance matrices are used to propagate errors and statistically filter the gravity fields. We use four different filtered gravity field solutions and determine which post-processing strategy is best for budget closure. As a reference, the standard 96 degree Dense Decorrelation Kernel-5 (DDK5)-filtered Center for Space Research (CSR) solution is used to compute the mass component (MC). A comparison is made with two anisotropic Wiener-filtered CSR solutions up to degree and order 60 and 96 and a Wiener-filtered 90 degree ITSG solution. Budgets are computed for 10 polygons in the North Atlantic Ocean, defined in a way that the error on the trend of the MC plus steric sea level remains within 1 mm yr−1. Using the anisotropic Wiener filter on CSR gravity fields expanded up to spherical harmonic degree 96, it is possible to close the sea level budget in 9 of 10 sub-basins in terms of trend. Wiener-filtered Institute of Theoretical geodesy and Satellite Geodesy (ITSG) and the standard DDK5-filtered CSR solutions also close the trend budget if a glacial isostatic adjustment (GIA) correction error of 10–20 % is applied; however, the performance of the DDK5-filtered solution strongly depends on the orientation of the polygon due to residual striping. In 7 of 10 sub-basins, the budget of the annual cycle is closed, using the DDK5-filtered CSR or the Wiener-filtered ITSG solutions. The Wiener-filtered 60 and 96 degree CSR solutions, in combination with Argo, lack amplitude and suffer from what appears to be hydrological leakage in the Amazon and Sahel regions. After reducing the trend, the semiannual and the annual signals, 24–53 % of the residual variance in altimetry-derived sea level time series is explained by the combination of Argo steric sea levels and the Wiener-filtered ITSG MC. Based on this, we believe that the best overall solution for the MC of the sub-basin-scale budgets is the Wiener-filtered ITSG gravity fields. The interannual variability is primarily a steric signal in the North Atlantic Ocean, so for this the choice of filter and gravity field solution is not really significant.

Sub-basin scale sea level budgets from satellite altimetry, Argo floats and satellite gravimetry in the North Atlantic

Article

Full-text available

Jul 2016
OSD

In this study for the first time an attempt is made to close the sea level budget on a sub-basin scale in terms of trend, annual amplitude and residual time series, after removing the trend, the semi-annual and annual signals. To obtain errors for altimetry and Argo full variance-covariance matrices are computed using correlation functions and their errors are fully propagated. For altimetry we apply a geographically dependent intermission bias (Ablain et al., 2015), which leads to differences in trends up to 0.8 mm yr−1. Since Argo float measurements are non-homogeneously spaced, steric sea levels are first objectively interpolated onto a grid before averaging. For the Gravity Recovery And Climate Experiment (GRACE) gravity fields full variance-covariance matrices are used to propagate errors and statistically filter the gravity fields. We use four different filtered gravity field solutions and determine which post-processing strategy is best for budget closure. As a reference the standard 96-degree DDK5-filtered CSR solution is used to compute OBP. A comparison is made with two anistropic Wiener-filtered CSR solutions up to d/o 60 and 96 and a Wiener-filtered 90-degree ITSG solution. Budgets are computed for ten polygons in the North Atlantic, defined in a way that the error on the trend of Ocean Bottom Pressure (OBP) + steric sea level remains within 1 mm yr−1. Using the anisotropic Wiener filter on CSR gravity fields expanded up to spherical harmonic degree 96, it is possible to close the sea level budget in nine-out-of-ten sub-basins in terms of trend. Wiener-filtered ITSG and the standard DDK5-filtered CSR solutions also close the trend budget, if a Glacial Isostatic Adjustment (GIA) correction error of 10–20 % is applied, however the performance of the DDK5-filtered solution strongly depends on the orientation of the polygon due to residual striping. In seven-out-of-ten sub-basins the budget of the annual cycle is closed, using the DDK5-filtered CSR or the Wiener-filtered ITSG solutions.The Wiener-filtered 60- and 96-degree CSR solution in combination with Argo lack amplitude and suffer from what appears to be hydrological leakage in the Amazon and Sahel regions. After reducing the trend, semi-annual and annual signals, 24–53 % of the residual variance in altimetry-derived sea level time series is explained by the combination of Argo steric sea levels and Wiener-filtered ITSG OBP.Based on this, we believe that the best overall solution for the OBP component of the sub-basin scale budgets is the Wiener-filtered ITSG gravity fields. The interannual variability is primarily a steric signal in the North Atlantic, so for this the choice of filter and gravity field solution is not really significant.

Reduction of the 59-day error signal in the Mean Sea Level derived from TOPEX/Poseidon, Jason-1 and Jason-2 data with the latest FES and GOT ocean tide models

Article

Full-text available

May 2016
OSD

Mean sea level (MSL) is a prominent indicator of climatic change (Ablain et al., 2015; Cazenave et al., 2014; Leuliette and Willis, 2011), and is therefore of great scientific and societal interest. Since the beginning of the altimeter mission TOPEX/Poseidon, followed by Jason-1 and Jason-2 on similar orbits, and many other missions on different orbits (ERS, EnviSat, etc.), MSL products became essential to the comprehension of Global ocean circulation. Since early in the TOPEX/Poseidon mission (Nerem, 1995) a suspicious signal, having period near 59 days and amplitude of roughly 5 mm, was apparent in the GMSL record. Compared with the 4–5 mm amplitude of the annual signal (Minster et al., 1999), the suspicious 59-day signal has understandably attracted attention. Moreover, the same signal has been subsequently detected in Jason-1 and later Jason-2 MSLs. In 2010, it was the subject of a dedicated session at the Ocean Surface Topography Science Team (OSTST) meeting in Lisbon. The conclusions were this signal is the aliasing of a higher frequency error inherited from the tide model correction: the semi-diurnal wave S2. The source of this error was mainly attributed to TOPEX measurements which are assimilated in ocean tide models. When these models are used in the computation of TOPEX/Poseidon MSL, most of the error cancels. However, this error is communicated to Jason-1 and Jason-2 MSLs. Since 2010, considerable efforts have been undertaken within the ocean tide community in order to correct ocean tide S2-waves from this error, particularly in the Goddard Ocean Tide (GOT) and Finite Element Solution (FES) latest versions. The present paper aims at assessing, quantifying and characterizing the reduction of the 58.77-day error thanks to the latest releases.

Satellite-based wave data and wave energy resource assessment for South China Sea

Article

Dec 2015
RENEW ENERG

Global warming and sea level rise

Article

Dec 2010

W. Llovel

Tide gauge records suggest a rise in sea level rise of ~1.8 mm/yr over the 20st century. More recently, satellite altimetry data reveal a global mean sea level rise of 3.3 mm/yr over 1993-2010. This rise is attributed to Earth's global warming observed since several decades. In this thesis, we analyze observed global mean sea level and its causes over the entire altimetry era (since 1993). Over the recent years (2002-2009), we estimate the effects of ocean thermal expansion and salinity (called steric effects) on sea level, as well as ocean mass change due to land ice and land waters, using Argo and GRACE space gravimetry data. We discuss the regional variability by comparing several datasets for thermal expansion and ocean mass signal. In another study, we investigate terrestrial land water storage variability of the 33 largest river basins worldwide, using GRACE space gravimetry data. We analyze this contribution to the observed global mean sea level inferred by satellite altimetry. In an extension of this study, we analyze the interannual variability of terrestrial land water storage and its impact on sea level variations over the altimetry era and tide gauge era. Finally, we conclude this chapter by studying the sea level budget over the entire altimetry era and the recent years. In a second part, we study the regional patterns in sea level trends. First, we discuss causes of regional variability, mainly non-uniform ocean warming. We then interpret the residual signal (i.e., observed sea level corrected for thermal effects) for the altimetry era. Thereafter, we analyze regional patterns of past sea level over the last decades (1950-2003). The purpose of this study is to provide 2-D regional past sea level reconstruction and obtain some insight on spatial trend patterns and their dominant modes of variability. The ultimate goal is to constrain coupled climate models used by the IPCC (Intergovernment Panel on Climate Change) to predict sea level rise over the 21st century. Moreover, this study highlights a long term signal detected in the reconstructed sea level. This signal is also observed in in situ data and in coupled climate models.

Sea Level Rise - Regional and Global Trends

Conference Paper

Full-text available

Dec 2010

Forootan2013 LSSAofCaspianLevelChanges

Data

Full-text available

Sep 2014

Robust validation of trends and cycles in sea level and tidal amplitude in the Dutch North Sea

Article

Full-text available

Dec 2023

Hessel G. Voortman

In this paper I present a new method for analyzing long series of observed sea levels. My method provides insight in sea level and the changes over time that are required by practicing engineers, assigned the design of new, modified or renovated structures along the coast. After extensively reviewing earlier research of sea level and tide, I present my method. The analysis relies on the application of classic harmonic analysis, which is made operational in a script in the programming language Python. Rigorous statistical testing is introduced to test the significance of trends and cycles in sea level and tide. This application of harmonic analysis and the introduction of formal statistical testing appear both to be new in this field. The method is tested on six locations in the Dutch North Sea, all with continuous sea level records of at least 130 years. The results of the statistical quality tests are shown in this paper. Subsequently, I show my findings related to mean sea level, lunar and solar tide and wind setup. Subsequently I analyze the long-term trends and cycles in Mean Sea Level. Long-period cycles with periods equal to the oceanic perigean and nodal tide are found to be important for a correct interpretation of sea level over time. Statistical tests show that acceleration of the rate of sea level is not significant up to 2021; the last year in the dataset. I compare my results with contemporary projections of sea level rise. The comparison reveals that in the Dutch North Sea the projected rates of rise are a factor two or more higher than the empirical rates established in this paper.

Quality assessment of the nominal attitude model of TOPEX/Poseidon using quaternion data

Article

Mar 2024
ADV SPACE RES

The technical progress and achievements of coastal altimetry waveform retracking over the Asia seas

Chapter

Jan 2023

Références géodésiques pour les futures missions alimétriques: Application à la mission Jason

Thesis

Oct 2001

Laurent Morel

Pour obtenir une meilleure compréhension des interactions entre l’océan et l’atmosphère, les océanographes utilisent des produits de l’altimétrie spatiale tels que l’estimation du niveau des mers. Ils peuvent déterminer les mouvements de cette surface au niveau millimétrique grâce à la connaissance précise de la position du satellite. Toutefois, des effets liés au système de référence peuvent survenir lorsque les repères de référence utilisés pour déterminer l’orbite du satellite changent d’une même mission ou lorsqu’ils sont associés dans les calculs multi-missions. L’objectif de cette thèse consiste à caractériser et à quantifier les conséquences de l’utilisation de repères de référence différents sur l’estimation du niveau des mers. Les résultats obtenus pour la mission TOPEX/Poséidon montrent que l’utilisation de repères de référence avec une réalisation du mètre différente ou avec décalage en X ou Y de l’origine du repère géocentrique ne provoque pas d’erreurs systématiques significatives dans l’estimation du niveau des mers. Par contre, le décalage en Z de l’origine du repère géocentrique entraîne un biais significatif sur le niveau des mers. Nous avons notamment montré que l’incertitude actuelles des repères de référence, cet effet était responsable d’une dérive de 0.4 millimètre par an dans l’estimation du niveau des mers. Nous avons également établi qu’une erreur individuelle sur une coordonnée de station entraînait de faibles effets sur l’estimation du niveau des mers. Les fonctions de transfert établies dans cette étude peuvent apparaître presque négligeables. Cependant, une mission altimétrique peut durer plus de dix ans et utiliser alors des repères de référence établis avec des données de géodésie spatiale antérieures au début de mission. Nous avons montré que les incertitudes qui en découlent provoqueront alors des effets systématiques non négligeables sur l’estimation du niveau des mers.

The Measure of All Things

Article

Full-text available

Sep 2018
HIST STUD NAT SCI

Jérôme Lamy

The TOPEX/POSEIDON satellite mission to observe the oceans triggered the formation of the new specialty of space oceanography from the 1970s to 1990s. Previously, in the 1960s in the United States, traditional oceanographers had shown little interest in the possibilities of space and thus space engineers and physicists worked on the first missions (Seasat in particular). TOPEX/POSEIDON brought together two projects, one American (TOPEX) and the other French (POSEIDON). The gradual crystallization of the disciplinary specialty of space oceanography occurred by making available a platform of instruments able to meet an ensemble of varied needs. Battery failures just before the launch of the joint mission meant that the mission had to focus on the essentials (notably El Niño effects). Subsequently, the discovery of a significant rise in sea levels due to global warming resulted in space oceanography becoming a recognized specialty. The case of TOPEX/POSEIDON shows the original ways in which instruments gained a place in the very large range of oceanographic techniques.

Investigating the 59-Day Error Signal in the Mean Sea Level Derived From TOPEX/Poseidon, Jason-1, and Jason-2 Data With FES and GOT Ocean Tide Models

Article

Feb 2018

Since the beginning of the altimeter mission TOPEX/Poseidon (T/P), followed by Jason-1 and Jason-2 on similar orbits, and many other missions on different orbits (ERS, EnviSat, etc.), mean sea level (MSL) products became essential for the comprehension of global ocean circulation. Since early in the T/P mission, a suspicious signal, having a period of near 59 days and amplitude of roughly 5 mm, was apparent in the Global MSL record. Compared with the 4-5-mm amplitude of the annual signal, the 59-day signal has understandably attracted attention. Moreover, the same signal has been subsequently detected in Jason-1 and later in Jason-2 MSLs. In 2010, the Ocean Surface Topography Science Team (OSTST) concluded this signal as the aliasing of a higher frequency error inherited from the tide model correction: the semi-diurnal wave S2. The source of this error was mainly attributed to T/P measurements, which were assimilated in ocean tide models. When these models are used in the computation of T/P MSL, most of the error cancels. However, this error is communicated to Jason-1 and Jason-2 MSLs. In order to gather and publish the OSTST analyses on this matter, this paper first attempts to list the myriad possibilities for the puzzling 59-day error in MSL. Then, this paper goes deeper into the description of the main contributor to this list: the tide models error. Indeed, since 2010, considerable efforts have been undertaken within the ocean tide community in order to correct ocean tide S2-waves from this error, particularly in the Goddard Ocean Tide (GOT) and finite element solution (FES) latest versions. Comparing several GOT and FES versions and a pure hydrodynamic tide model, this paper assesses, quantifies, and describes a reduction of the MSL 59-day error thanks to the latest releases. These analyses also confirm that a large part of this error has its origins in the T/P mission and has contaminated ocean tide solutions and Jason-1 and Jason-2 MSLs. They also suggest that ocean tide is not the only possible vector. Jason-1 and Jason-2 MSLs contain additional 59-day error--though to a lesser extent--that may either come from the measurements themselves or from another vector.

Trends and interannual variability of mass and steric sea level in the Tropical Asian Seas

Article

Full-text available

Jul 2017

The mass and steric components of sea level changes have been separated in the Tropical Asian Seas (TAS) using a statistically optimal combination of Jason satellite altimetry, GRACE satellite gravimetry and ocean reanalyses. Using observational uncertainties, statistically optimally weighted time series for both components have been obtained in four regions within the TAS over the period January 2005 - December 2012. The mass and steric sea level variability is regressed with the first two principal components (PC1&2) of Pacific equatorial wind stress and the Dipole Mode Index (DMI). Sea level in the the South China Sea is not affected by any of the indices. Steric variability in the TAS is largest in the deep Banda and Celebes seas and is affected by both PCs and the DMI. Mass variability is largest on the continental shelves, which is primarily controlled by PC1. We argue that a water flux from the Western Tropical Pacific Ocean is the cause for mass variability in the TAS. The steric trends are about 2 mm yr−1 larger than the mass trends in the TAS. A signifcant part of the mass trend can be explained by the aforementioned indices and the nodal cycle. Trends obtained from fingerprints of mass redistribution are statistically equal to mass trends after subtracting the nodal cycle and the indices. Ultimately, the effect of omitting the TAS in global sea level budgets is estimated to be 0.3 mm yr−1.

Sea Level Variations Measured by TOPEX/POSEIDON and by Tide Gauges: A Validation Study in the Southern Baltic Sea

Chapter

Jan 1998

To compare sea level heights resulting from satellite altimeter observations and from tide gauge measurements it is necessary to know the tides, the sea surface topography and the geoid in the area under investigation. The paper discusses the characteristics of these components for a test area in the southern Baltic Sea. It is shown that the Baltic Sea is a proper area for such investigations. The method used for the comparison of the sea level heights as well as the results for a possible altimeter drift are presented.

The Use of a Precise Reference Frame in Sea Level Change Studies

Chapter

Jan 2000

Long-term sea level variations are an important indicator of global climate change, and their measurement can provide critical information for determining the socio-economic impact of sea level change on coastal land use. Sea level change over the past century is studied almost exclusively using tide gauge records, but they suffer from the unknown effects of land motion and poor spatial distribution. The advent of satellite altimetry, and especially the launch of TOPEX/POSEIDON in 1992, overcomes many of the limitations of tide gauge measurements in that the measurements are global and are tied to the Earth’s center-of-mass in a precise reference frame. However, uncertainties still arise with regard to the long-term performance of the instruments, and the maintenance of the reference frame from one altimeter mission to the next. Here, we discuss the importance of the reference frame for altimeter measurements of sea level. Our results show that care must be taken to maintain the reference frame across multiple geodetic techniques and multiple decades if climate change signals are to be detected. The movement of the tide gauges and the geodetic observatories tracking the altimeter satellites (SLR, DORIS, GPS, etc.) must be precisely monitored through a well-organized international effort.

Gravimetric Methods – Satellite Altimeter Measurements

Chapter

Dec 2015

D.P. Chambers

Satellite altimetry has become an important tool for studying the Earth's oceans. Here, we summarize the basic concepts of satellite radar altimeters, starting from how range is measured, how the precise orbit height is calculated, and how these are combined to determine sea surface height (SSH). Corrections needed to account for path delays of the radar pulse in the atmosphere and biases at the surface are also discussed. These include the ionosphere, wet troposphere, and dry troposphere atmospheric corrections and the sea state bias and inverted barometer surface corrections. The calibration and verification of the SSH measurement are explained, using specific examples from TOPEX/Poseidon, Jason-1, and Jason-2/Ocean Surface Topography Mission. We also comment on some geophysical applications of the altimeter measurement, including measuring the ocean geoid, bathymetry, and global mean sea level. In closing, we briefly discuss other types of satellite altimeters, including laser, delay-Doppler, and wide swath altimeters.

Gravity Field Variability, the Geoid, and Ocean Dynamics

Chapter

Jan 2004

With the advent of accurate satellite altimetry, physical oceanography and geodesy have come to have many overlapping problems. The most fundamental of these problems concerns the detailed determination of the geoid. This gravitational equipotential of the earth is central to a description of the solid earth, and appears as the principle reference surface for computing oceanic currents. From the oceanographer’s point of view, knowledge of the sea surface elevation relative to the geoid determines the absolute circulation of the ocean. Great progress has occurred in recent years in determining the geoid with much improved accuracy, although much remains to be done for the result to be fully useful for oceanographic purposes. The possibility of measurement of the earth’s time variable gravity field and the very high accuracies and precisions which appear possible, raise a myriad of new and interesting challenges for understanding and using the measurements. This present paper summarizes some of the work we have performed (Condi and Wunsch (2003)) in exploring the idea of measuring bottom pressure changes from space, and how these data might be used, but with particular attention paid to basic concepts in ocean dynamics and errors in model approximations.

Correlation Between Multi-Mission Altimeter Time Series and Tide Gauge Registrations in the Caribbean Sea

Chapter

Jan 2002

For the comparison and correlation of altimetric sea level time series with tide gauge registrations a new surface approach is developed and applied to several tide gauges of the Caribbean sea. The approach allows to generate sea level time series from multi mission altimetry, it includes estimates for ocean tide errors that are supposed to increase near the coast and can be used for absolute comparisons if tide gauges reference points have been tied by GPS positioning.

Sea Level Variations in the North Atlantic

Chapter

Jan 2002

Variations in mean sea level are considered as an indicator for global as well as regional environmental change. The spatial distribution of sea level variations can be observed by satellite altimeters with a high precision. The sea level variations in the North Atlantic are studied in this paper by using eight years of sea level data derived from the TOPEX/Poseidon altimeter mission. The mean sea level change for this time period shows regionally remarkable differences and does agree only partly with the mean sea surface temperature change. The annual oscillation has a dominant contribution to the total sea level variability, but within the central Gulf Stream, eddy activity contributes even more strongly to the overall variability. After removing the annual and semi-annual oscillation and an alias period of the altimeter with the M2 tide, the residuals are smoothed by a moving 90 day mean filter and examined by Principal Component Analysis. A low frequency variation with an approximate period of 6-7 years as well as an anomalous sea level change at the end of 1995 have been detected. The analysis of sea surface temperature data also shows a similar long periodic and anomalous behaviour.

Global sea level change characteristics based on multi-altimeter data

Article

May 2012

Based on the AVISO multi-altimeter data obtained by the Centre National d Etudes Spatiales of France during the period from 1993 to 2008, the spatial distributions of long-term change and seasonal change of the global sea level were studied using the stochastic dynamic and EOF methods. The results show the following: (a) From 1993 to 2008, the Pacific sea level rose in the west and dropped in the east; the sea level of the Indian Ocean basically rose; and the Atlantic Ocean had a sea level rise in most areas except the Gulf Stream. (b) The global sea level change was characterized by significant seasonal (annual and semi-annual) variation, which was more significant in the Northern Hemisphere than in the Southern Hemisphere. The most significant seasonal variation occurred in the mid-latitude area. (c) The seasonal sea level variation in the North Indian Ocean was more significant than in the North Pacific and North Atlantic in the same latitude band, (d) The sea level in the regions with strong flow, including the western boundary current and equatorial currents, showed larger variability than that in the surrounding area, (e) Affected by equatorial waves, the sea level change in the low-latitude area was not synchronous with that in the west and east, (f) During the El Nino yecanar, the sea level dropped significantly in the western pacific warm pool and the central equatorial Pacific, and rose signifitly in the equatorial eastern Pacific, while the sea level in the equatorial Indian Ocean changed reversely.

A Note on Sea Level Rise

Article

Aug 2010

A. Parekh

Monitoring global sea level change from 1993 to 2011 using TOPEX and jason altimeter missions

Article

Dec 2012

Multi-satellite altimetry has provided a wealth of data for measuring the long-term sea level change, T/P, Jason-1 and Jason-2 construct a seamless record of global sea level change from 1993 to present. We present the results of our calibration activities, including data comparisons during the tandem period of the missions, during which we resolve the relative bias between the missions, as well as comparisons to independent tide gauge data. When the entire record is assembled, global mean sea level change from 1993-2011 is 3.12±0.4 mm\5a-1, and interannual variation is strongly correlated to El Niño phenomenon.

Gravimetric Methods - Spacecraft Altimeter Measurements

Article

Dec 2007

D.P. Chambers

Satellite altimetry has become an important tool for studying the Earth. Here, we summarize the basic concepts of satellite radar altimeters, including how range is measured, how the precise orbit height is calculated, and how these are combined to determine sea surface height (SSH). Corrections needed to account for path delays of the radar pulse in the atmosphere and biases at the surface are described. These include the ionosphere, wet troposphere, and dry troposphere atmospheric corrections and the sea state bias and inverted barometer surface corrections. The calibration and verification of the SSH measurement is explained, using specific examples from the TOPEX/Poseidon and Jason-1 missions. We also comment on some geophysical applications of the altimeter measurement, including the ocean geoid, bathymetry, tides, and global mean sea level. Finally, we briefly discuss other types of satellite altimeters, including laser, delay-Doppler, and wide-swath altimeters.

A Vision of the Future

Chapter

May 2008

Jeffrey Foss

This one-of-a-kind book provides thoughtful insight into the current relationship between humankind and the environment Beyond Environmentalism is the first book of its kind to present a timely and relevant analysis of environmentalism. The author's decades of experience as a philosopher of science allow him to critically comprehend scientific issues and to develop and explain sound, ethical policies in response to them. The result is a volume that builds a philosophy of nature and helps the reader assess humankind's relationship with and impact on the world around us. This innovative book discusses the inconsistencies, both scientific and philosophical, of popular environmentalism and sheds new perspectives on the issues, causes, and debates that embrace society today. The goal is not to settle environmental issues once and for all, but rather to provide the basis for more reasoned, scientific, and productive debates. The need for a new philosophy of nature is explored through methodological discussion of several topics, including: The rise and fall of scientific proof Nature in religion, romance, and human values Humankind's responsibility to the environment The value of freedom Kinship among species Numerous case studies throughout the book delve into global warming, the "sixth extinction," the precautionary principle, pollution, and other popular issues within environmentalism. Feature boxes guide the reader through complex topics such as eco-sabotage, the Gaia hypothesis, and the urban heat-island effect, while vivid illustrations demonstrate scientific data, theories, and philosophical arguments in a reader-friendly manner. With its balanced approach to provocative issues, Beyond Environmentalism serves as an excellent, thought-provoking supplement for courses on environmental studies at the undergraduate and graduate levels. It is also an interesting and accessible read for anyone with a general interest in environmental issues.

Ocean circulation and climate

Article

Full-text available

Jan 2001

Temporal-spatial variations of sea level over China seas derived from altimeter data of TOPEX/Poseidon, Jason-1 and Jason-2 from 1993 to 2012

Article

Full-text available

Sep 2015
CHINESE J GEOPHYS-CH

Variations of sea level are an important issue associated with global climate change, especially global warming. The sea level rising may lead to serious impact on the social-economic development of a country. Global oceans are continuously observed by altimetry satellite missions for more than 20 years. We use altimeter data of TOPEX/Poseidon (T/P), Jason-1 and Jason-2 from 1993 to 2012 to study the sea level change over China seas. China seas located in the western Pacific Ocean are selected as the study area. T/P MGDR data of 11 to 364 cycles span January 1993 to August 2002. Jason-1 GDR data of 1 to 259 cycles span January 2002 to January 2009. Jason-2 GDR data of 1 to 165 cycles span July 2008 to December 2012. Southern oscillation index (SOI) time series span January 1993 to December 2012. Altimetry data in the tandem stages are used to calibrate biases of sea level anomalies (SLAs) from T/P, Jason-1 and Jason-2 point by point. The spatial distribution of China seas' level change is studied with the continuous tension spline method. Temporal variations of China seas' level are analyzed with the linear fitting method and the wavelet analysis. Relationships between the El Niño-Southern Oscillation (ENSO) and sea level changes of the South China Sea and the East China Sea are studied with the correlation analysis. By unifying altimetry data point by point in tandem stages, the mean differences of sea level heights for Jason-1 vs T/P and Jason-2 vs Jason-1 are 0.21 cm and 0.03 cm, respectively. SLA time series are constructed after corrections of sea level biases from 1993 to 2012. In general, the mean SLA is positive over China seas. SLAs are higher in the south than those in the north, and lower in the west than those in the east. SLAs are negative over the western Bohai Sea, the northern Yellow Sea, the northern Taiwan Strait and the North Bay. SLAs rise with the increasing longitude and decrease with the growing latitude. This spatial distribution of SLAs over China seas is related to the water flux, sea surface wind stress, oceanic dynamics, monsoon, Kuroshio and ENSO. The mean rising rate of sea level for the 20 years is 4.64 mm·a-1 over the whole study area. Sea levels are rising in the Bohai Sea, Yellow Sea, East China Sea and South China Sea with rising rates 4.44 mm·a-1, 2.37 mm·a-1, 3.02 mm·a-1 and 4.25 mm·a-1, respectively. The annual sea level variation is obvious in the study area. The sea level is higher in summer and autumn than in winter and spring. The main cycles of sea level change include one year and 9 years over the whole seas. There are also minor cycles of 0.5 years, 1.5 years, 2 years and 4 years. Cyclical changes of sea level are related to the geographical position, climate, oceanic dynamics and submarine topography. The correlation coefficients between SOIs and SLAs of the South China Sea and the East China Sea are 0.39 and 0.02, respectively. The correlation coefficient is 0.44 after the two-month delay for the South China Sea. The correlation coefficient is 0.17 after the four-month delay for the East China Sea. This indicates that the sea level change over the South China Sea may be largely affected by ENSO. Altimetry data of T/P, Jason-1 and Jason-2 are processed to study the sea level change over China seas from 1993 to 2012. We used the altimetry data in tandem stages to calibrate the sea level biases of these three missions to achieve the seamless SLAs point by point. The spatial distribution of sea level change is given with the continuous tension spline method. The sea level is generally rising at the mean rate of 4.64 mm·a-1 over China seas in these 20 years. The sea level variations change with different seas and seasons. The mean rising rates are 4.44 mm·a-1, 2.34 mm·a-1, 3.02 mm·a-1 and 4.25 mm·a-1 over Bohai Sea, Yellow Sea, East China Sea and South China Sea, respectively. Temporal-spatial change of sea level over the study area is related to oceanic dynamics, submarine topography, monsoon, Kuroshio and ENSO. The main cycles of sea level change are one year and 9 years as derived from the wavelet analysis. El Niño and La Niña events make stronger effects on sea level variations over the South China Sea than the other seas.

Analysis of EnviSat Altimetric Data around Yangtze Estuary by Waveform Retracking

Article

Jun 2015

The waveform data of EnviSat satellite (28°N~32°N, 121°E~125°E) from October 2002 to May 2010 is processed by waveform retracking methodology based on waveform classification. The waveforms in this region are classified into ocean, pre-peak, post-peak, quasi-specular and complex waveforms, with percentages of 89.03%, 2.95%, 0.45%, 3.31% and 4.26%, respectively. For each waveform, correspondent waveform retracking methodology is used. Meanwhile, it is founded that there exist system biases between the sea surface heights(SSH) calculated by different waveform retracking methodologies and calculate optimum threshold level of OCOG, threshold and sub-waveform retracker, with 65%, 45% and 50% respectively. The results of waveform retracking show that the waveform retracking methodology based on waveform classification are prior to any single retracker, which could improve the precision of SSH from 16.62% to 53.86%. In addition, the crossover discrepancies are smaller after using waveform retracking. Specifically, the crossover discrepancies of pass 089 and pass 411 are declined from 1 m to 25 cm or so and the other crossover discrepancies are all around 2~6 cm.

A late Holocene sea-level curve for the east coast of South Africa

Article

Full-text available

Feb 2014

South Africa’s extensive and topographically diverse coastline lends itself to interpreting and understanding sea-level fluctuations through a range of geomorphological and biological proxies. In this paper, we present a high-resolution record of sea-level change for the past ~1200 years derived from foraminiferal analysis of a salt-marsh peat sequence at Kariega Estuary, South Africa. A 0.94-m salt-marsh peat core was extracted using a gouge auger, and chronologically constrained using five radiocarbon age determinations by accelerator mass spectrometry, which places the record within the late Holocene period. Fossil foraminifera were analysed at a high downcore resolution, and a transfer function was applied to produce a relative sea- level reconstruction. The reconstructed sea-level curve depicts a transgression prior to 1100 cal years BP which correlates with existing palaeoenvironmental literature from southern Africa. From ~1100 to ~300 cal years BP, sea levels oscillated (~0.5-m amplitudes) but remained consistently lower than present-day mean sea level. The lowest recorded sea level of −1±0.2 m was reached between 800 and 600 cal years BP. After 300 cal years BP, relative sea level has remained relatively stable. Based on the outcomes of this research, we suggest that intertidal salt-marsh foraminifera demonstrate potential for the high-resolution reconstruction of relative sea-level change along the southern African coastline.

Assessing the Impacts of Sea Level Rise in the Caribbean using Geographic Information Systems

Article

Ratha Sim

Sea Level Changes Detected by Using Satellite Altimeter Data and Comparing with Tide Gauge Records in China Sea

Article

Jan 2003

The Topex/Poseidon (cycles 9-249), Geosat/ERM and ERS-2 (cycles 0-44) altimeter data are used to analyze the sea level changes in China Sea. The sea level changes in studying areas derived respectively from TOPEX/Poseidon and ERS-2 data during the same period are almost coincident with each other. The results are compared with the tide gauge records at different sites of China Sea, and they indicate that the sea level change in China Sea appears to be a rising trend with 2~3 mm per year. Moreover, the trends of the seasonal sea level changes of this region are reverse to that of the global sea. We also investigate some possible implied relationships of local sea level changes to some anomalies of sea surface heights due to such as El Niño/La Niña phenomena in different time and spatial spans.

Global Sea Level Analysis Based on ERS-1 Altimeter Data

Conference Paper

Jan 1997

Global mean sea level observations are necessary to answer the urgent questions about climate changes and their impact on socioeconomy. At GFZ/D-PAF, ERS altimeter data is used to systematically generate geophysical products like sea surface topography, high resolution geoid, short-period and stationary sea surface height models. Based on that experience fully reprocessed ERS-1 altimeter data is used to perform a time series of monthly sea surface height models from April 1992 to April 1995. The reprocessing consists of improved satellite ephemerides, merging of Grenoble tidal model and application of range corrections due to timing errors. The three years time series is taken to estimate the rate of change of global mean sea level. This includes a careful treatment of seasonal effects by a common masking procedure. The obtained rate of change is compared to external results from tide gauges. The relatively short period of three years, however, does not allow definite conclusions with respect to possible secular climate changes.

Effect of the processing methodology on satellite altimetry-based global mean sea level rise over the Jason-1 operating period

Article

Mar 2014

Determining how the global mean sea level (GMSL) evolves with time is of primary importance to understand one of the main consequences of global warming and its potential impact on populations living near coasts or in low-lying islands. Five groups are routinely providing satellite altimetry-based estimates of the GMSL over the altimetry era (since late 1992). Because each group developed its own approach to compute the GMSL time series, this leads to some differences in the GMSL interannual variability and linear trend. While over the whole high-precision altimetry time span (1993-2012), good agreement is noticed for the computed GMSL linear trend (of mm/year), on shorter time spans (e.g., ), trend differences are significantly larger than the 0.4 mm/year uncertainty. Here we investigate the sources of the trend differences, focusing on the averaging methods used to generate the GMSL. For that purpose, we consider outputs from two different groups: the Colorado University (CU) and Archiving, Validation and Interpretation of Satellite Oceanographic Data (AVISO) because associated processing of each group is largely representative of all other groups. For this investigation, we use the high-resolution MERCATOR ocean circulation model with data assimilation (version Glorys2-v1) and compute synthetic sea surface height (SSH) data by interpolating the model grids at the time and location of "true" along-track satellite altimetry measurements, focusing on the Jason-1 operating period (i.e., 2002-2009). These synthetic SSH data are then treated as "real" altimetry measurements, allowing us to test the different averaging methods used by the two processing groups for computing the GMSL: (1) averaging along-track altimetry data (as done by CU) or (2) gridding the along-track data into meshes and then geographical averaging of the gridded data (as done by AVISO). We also investigate the effect of considering or not SSH data at shallow depths as well as the editing procedure. We find that the main difference comes from the averaging method with significant differences depending on latitude. In the tropics, the gridding method used by AVISO overestimates by 11 % the GMSL trend. At high latitudes (above ), both methods underestimate the GMSL trend. Our calculation shows that the CU method (along-track averaging) and AVISO gridding process underestimate the trend in high latitudes of the northern hemisphere by 0.9 and 1.2 mm/year, respectively. While we were able to attribute the AVISO trend overestimation in the tropics to grid cells with too few data, the cause of underestimation at high latitudes remains unclear and needs further investigation.

Long term eustasy and epeirogeny in continents

Article

Full-text available

Jan 1990

Christopher Harrison

Modeling radiation forces acting on TOPEX/Poseidon for precision orbit determination

Article

Full-text available

Jun 1992

Direct anthropogenic contributions to sea level rise in the twentieth century

Article

Full-text available

Jan 1994

GLOBAL compilations of tide records indicate that sea level has been rising throughout the twentieth century1,2, with potentially dangerous consequences for low-lying coastal regions. Thermal expansion of ocean water3 and melting of alpine glaciers4 in response to increased atmospheric temperatures may have been responsible for some of this change, but human activities can also influence sea level directly. For example, the rise in sea level would have been even larger5 if large quantities of water had not been stored in reservoirs, and channelled into aquifers by irrigation projects. Here we show, however, that these and other human activities have together caused a net increase in sea levels over the past century. We estimate that a combination of groundwater withdrawal, surface water diversion and land-use changes has caused at least a third of the observed rise, and suggest that the contributions of climate-related effects must therefore be smaller than has been previously supposed.

Estimating the sea state bias of TOPEX and POSEIDON altimeters from crossover differences

Article

Full-text available

Dec 1994

TOPEX (Ku band) and POSEIDON altimeter measurements at crossover points are used to estimate the sea state bias (SSB) of these two instruments. Different SSB models are tested, ranging from a constant fractions of the significant wave height (SWH) to more elaborate models involving up to four adjustable parameters. For TOPEX, the data show a decrease in the magnitude of the relative bias (SSB/SWH) with SWH. This behavior is well reproduced using a simple empirical modl with two adjustable parameters. The three-parameter SSB model used in the NASA geophysical data records does well in explaining the wind-induced variations of the bias. A model including four adjustable parameters is needed to account for both the wind- and SWH- related variability of the SSB, POSEIDON data analysis reveals a significantly larger SSB than for TOPEX. This bias seems to consist of a skewness plus tracker bias of -2 to -3% of SWH superimposed on a natural electromagnetic (EM) bias whose wind- and SWH-related variations are similar to those of TOPEX.

Could possible changes in global groundwater reservoir cause eustatic sea-level fluctuations

Article

Full-text available

Mar 1990

GPS precise tracking of TOPEX/Poseidon: Results and implications

Article

Full-text available

Jan 1994

A reduced dynamic filtering strategy that exploits the unique geometric strength of the Global Positioning System (GPS) to minimize the effects of force model errors has yielded orbit solutions for TOPEX/POSEIDON which appear accurate to better than 3 cm (1 sigma) in the radial component. Reduction of model error also reduces the geographic correlation of the orbit error. With a traditional dynamic approach, GPS yields radial orbit accuracies of 4-5 cm, comparable to the accuracy delivered by satellite laser ranging and the Doppler orbitography and radio positioning integrated by satellite (DORIS) tracking system. A portion of the dynamic orbit error is in the Joint Gravity Model-2 (JGM-2); GPS data from TOPEX/POSEIDON can readily reveal that error and have been used to improve the gravity model.

A Model of Sea Level Rise Caused by Ocean Thermal Expansion

Article

Full-text available

Mar 1991

Warming of the atmosphere as a result of an increased concentration of greenhouse gases is expected to lead to a significant rise is global sea level. We present estimates of the component of this sea level rise caused by thermal expansion of the ocean. These estimates are based on the idea that the upper layers of the main gyres of the ocean are ventilated by the subduction of water at higher latitudes and its subsequent equatorward and downward flow into the main thermocline along surfaces of constant `density'. In this mechanism, heat enters the ocean by an advection process rather than by vertical diffusion, as in previous estimates of the component of sea level rise that is caused by thermal expansion. After the heat initially enters the subtropical gyres by subduction, it is then redistributed to preserve gradients of the depth-integrated pressure field, by an adjustment involving low vertical-mode baroclinic waves. Estimates of historical sea level rise based on this simple ventilation scheme, when combined with estimates of nonpolar glacial melt, are about equal to the observed sea level rise. For a global mean 3.0°C (1.5°C, 4.5°C) temperature rise by 2050 (and with the spatial distribution predicted by three climate models), we estimate the component of sea level rise that is caused by thermal expansion to be about 0.2 to 0.3 m (0.1 m, 0.4 m) by 2050. Low-mode internal Rossby and Kelvin waves appear to be quite efficient at distributing the sea level rise evenly over the earth without major distortions to the thermocline. A delayed warming, as suggested by transient coupled ocean-atmosphere models, can be simulated by using a smaller temperature rise, say 1.5°C rather than 3.0°C, by 2050. Changes in sea level arising from variations in the wind field could be estimated, but so far our calculations are based on the assumption that the wind stress field does not change from its present value. We estimate the maximum rate of sea level rise caused by changes in deep water formation is 0.1 meter per century. Contributions from the cryosphere reported in the literature range from near zero to about 0.35 m. When added to the thermal expansion components, our total sea level rise scenario for 2050 for a temperature rise of 3.0°C (1.5°C to 4.5°C) is about 0.35 m (0.15 and 0.70 m).

Sea Levels, Land Levels, and Tide Gauges

Book

Jan 1991

Changes in sea level caused by global warming can be disastrous to modern civilization. Therefore, it is important to use accurate and reliable methods to monitor any change. During this century, and, in particular, the last three decades, tide-gauge records have been used to show these changes related to the world's oceans. Aubrey and Emery suggest, however, that tidal gauges should not be used unquestioningly as a benchmark for measuring eustatic sea-level changes. Tectonism, subsidence, ocean current variability, and human activity can, and do, affect the accuracy of these records. Understanding the reasons for changes in land and sea levels is essential for the proper development of coastal regions. The results of this study provide guiding data for scientific, engineering, and policy solutions to coastal flooding. Determining the true causes of relative subsidence, and how to use geological and oceanological controls, will allow us to exist within our natural environment, rather than force nature to conform to our legal and temporary 'remedies.'

Some extensions to the response method applied to TOPEX/Poseidon (abstract)

Article

Jan 1994

The JGM-3 gravity model

Article

Jan 1994
ANN GEOPHYS-GERMANY

Byron D. Tapley

New sigma0 calibration for the TOPEX altimeter

Article

Jan 1994

Monitoring of Changes in Global Mean Sea Level Using Geosat Altimeter

Chapter

Jan 1992

Gravity model development for TOPEX/POSEIDON: joint gravity models 1 and 2

Article

Jan 1994

R.S. Nerem

The TOPEX/POSEIDON (T/P) prelaunch Joint Gravity Model-1 (JGM-1) and the postlaunch JMG-2 Earth gravitational models have been developed to support precision orbit determination of T/P. Each of these models is complete to degree 70 in spherical harmonics and was computed from a combination of satellite tracking data, satellite altimetry, and surface gravimetry. While improved orbit determination accuracies for T/P have driven the improvements in the models, the models are general in application and also provide an improved geoid for oceanographic computations. The postlaunch model, JGM-2, which includes T/P satellite laser ranging (SLR) and Doppler orbitography and radiopositioning integrated by satellite (DORIS) tracking data, introduces radial orbit errors for T/P that are only 2 cm RMS with the commission errors of the marine geoid for terms to degree 70 being ± 25 cm. Geoid errors still prevent the absolute determination of the oceanic dynamic topography for wavelengths shorter than about 2500km. -from Authors

The temporal and spatial characteristics of TOPEX/POSEIDON radial orbit error

Article

Jan 1995

J.A. Marshall

This study examines the spatial and temporal characteristics of the T/P radial orbit error, as assessed through the analysis of laser tracking residuals and orbit comparisons with independently generated trajectories. Spectral analyses of the orbit differences between the orbits indicate that the predominant power is at the once-per-orbital revolution frequency with 2-3 cm peaks. When the orbit differences are colinearly aligned to a fixed geographic grid and spectral analysis is performed at each geographical grid point, a nearly 60-day period is found with maximum amplitudes in the 2-4 cm range. We demonstrate that the ~60 day error period seen at fixed geographic locations arises from weaknesses in the dynamic ocean tidal models used in the orbit calculations. New tidal models have been developed which significantly reduce this error. -from Authors

TOPEX altimeter range stability estimated from calibration mode data

Article

Jan 1994

Contribution of Small Glaciers to Global Sea Level

Article

Jan 1984

M.F. Meier

Observed long-term changes in glacier volume and hydrometeorological mass balance models yield data on the transfer of water from glaciers, excluding those in Greenland and Antarctica, to the oceans, The average observed volume change for the period 1900 to 1961 is scaled to a global average by use of the seasonal amplitude of the mass balance. These data are used to calibrate the models to estimate the changing contribution of glaciers to sea level for the period 1884 to 1975. Although the error band is large, these glaciers appear to accountfor a third to half of observed rise in sea level, approximately that fraction not explained by thermal expansion of the ocean.

Optimal Averaging of Seasonal Sea Surface Temperatures and Associated Confidence Intervals (1860–1989)

Article

Jun 1994

Optimal averaging (OA) is used to compute the area-average seasonal sea surface temperature (SST) for a variety of areas from 1860 to 1989. The OA gives statistically improved averages and the objective assignment of confidence intervals to these averages. The ability to assign confidence intervals is the main advantage of this method. Confidence intervals reflect how densely and uniformly an area is sampled during the averaging season. For the global average, the early part of the record (1860–1890) and the times of the two world wars have largest uncertainties. Analysis of OA-based uncertainty estimates shows that before 1930 sampling in the Southern Hemisphere was as good as it was in the Northern Hemisphere. From about 1930 to 1950, uncertainties decreased in both hemispheres, but the magnitude of the Northern Hemisphere uncertainties reduced more and remained smaller. After the early 1950s uncertainties were relatively constant in both hemispheres, indicating that sampling was relatively con...

Climate and Sea Level Change: Observations, Projections and Implications

Article

Feb 1994

Spectroscopy of the word ocean tides from a finit element hydrodynamic model

Article

Dec 1994

An atlas of the main components of the tides has been produced on the basis of a finite element hydrodynamic model, with the aim of offering the scientific community, using satellite altimetric data, a prediction of the tidal contribution to sea surface height variations under the ground tracks of the satellites that is totally independent of altimetric measurements. Eight constituents, M2, S2, N2, K2, 2N2, K1, O1 and Q1 have been simulated. Five secondary constituents Mu2, Nu2, L2, T2, and P1, required to insure a priori correct predictions, have been deduced by admittance. The accuracy and precision of these solutions have been estimated by reference to the harmonic constituents data set available from analysis of the entire collection of pelagic, plateau and coastal observations made to date, and archived by International Association for Physical Sciences of the Oceans and International Hydrographic Bureau. The performances of the tidal predictions made on the basis of this new set of solutions was tested by looking at the residual RMS differences at the crossover points of sea level measurements supplied by the TOPEX/POSEIDON mission, when using these predictions. -from Authors

Global sea level acceleration

Article

Aug 1992
J GEOPHYS RES

Bruce C. Douglas

Greenhouse warming scenarios commonly forecast an acceleration of sea level rise in the next 5 or 6+ decades in the range 0.1–0.2 mm/yr2. Long tide gauge records (75 years minimum) have been examined for past apparent sea level acceleration (i.e., deviation from a purely linear rise) and for indication of how long it might take to detect or verify a predicted future acceleration. For the 80-year period 1905–1985, 23 essentially complete tide gauge records in 10 geographic groups are available for analysis. These yielded the apparent global acceleration −0.011 (±0.012) mm/yr2. A larger, less uniform set of 37 records in the same 10 groups with 92 years average length covering the 141 years from 1850 to 1991 gave for acceleration 0.001 (±0.008) mm/yr2. Thus there is no evidence for an apparent acceleration in the past 100+ years that is significant either statistically, or in comparison to values associated with global warming. Estimating how well a global acceleration parameter could be determined in a relatively short time was accomplished by dividing the 1905–1985 data set into four equal time spans. The formal 1σ uncertainty (about 0.2 mm/yr2) of global acceleration from these 20-year periods is more than an order of magnitude larger than for the 80- and 141-year cases owing to the existence of large interdecadal and longer variations of sea level. This means that tide gauges alone cannot serve as a leading indicator of climate change in less than at least several decades. Confirming the prediction of a particular model at the 95% confidence level or differentiating between model predictions will take much longer. The time required can be significantly reduced if the interdecadal fluctuations of sea level can be understood in terms of their forcing mechanisms and then removed from the tide gauge records.

Modeling radiation forces acting on TOPEX/Poseidon for precision orbit determination

Article

Jan 1994

Geodetic satellites, such as GEOSAT, SPOT, ERS-1, and TOPEX/Poseidon require accurate orbital computations to support the scientific data they collect. The TOPEX/Poseidon mission requirements dictate that the mismodeling of the nonconservative forces of solar radiation, Earth albedo and infrared reradiation, and spacecraft thermal imbalances produce in combination more than a 6-cm radial rms orbit error over a 10-day period. Therefore, a box-wing satellite form was investigated to model the satellite as the combination of flat plates arranged in the shape of a box and a connected solar array.

Airborne measurement of absolute sea-surface heights

Article

May 1993

Topographic profiles may be measured from an aircraft by differencing precise radar or laser altitudes with ellipsoidal heights obtained by means of kinematic interferometric-mode GPS. Over the ocean, the difference between the two altitudes is the geoid height plus dynamic oceanographic effects such as tides, currents, eddies, etc. Aircraft might provide, on a local scale, altimetry measurements similar to those from GEOSAT and ERS-1. Such airborne measurements could be applied to the determination of a local high-resolution geoid or for oceanographic studies.Several long over-water altimetry profiles were obtained during the 1991 field campaign of the Greenland Aerogeophysics Project (GAP91). Kinematic interferometric GPS between multiple short or zero-baseline airborne receivers and several sets of stationary GPS receivers on the ground provided precise three-dimensional positioning for airborne gravity, magnetic and topographic profiling sensor systems mounted aboard a P-3 Orion aircraft. The logistics of surveying a large region (long GPS baselines and poor satellite coverage) limit the accuracy of the GPS positioning, but the geoid signal is clearly visible in the over-water profiles. In spite of GPS baseline lengths of up to 1000 km, differences between modeled geoid heights and measured sea-surface heights ranged from about 1-2 meters r.m.s. Although the differences are primarily due to errors in the GPS positions, some portion is due to errors in the model geoid, uncompensated tides, sea-ice contamination of the radar data and other errors in the radar. We believe that this represents the first airborne measurement of absolute sea-surface heights.

Greenland Ice Sheet: Is It Growing or Shrinking?

Article

Apr 1990

The estimation of “global” sea level change: A problem of uniqueness

Article

Sep 1984

T. P. Barnett

An objective method of estimating regional averages of coherent sea level (SL) change is developed. The technique is applied to a large set of SL data representative of most of the world's continental margins. The results show highly coherent SL changes over many of the regions studied. The method is then applied to the regional averages themselves to develop an overall estimate of the coherent pattern of SL variations existing in the historical SL data set. The pattern is characterized by a coherent rise of SL in all regions except Alaska, Scandinavia (both areas of notorious crustal uplift), and Southeast Asia, where SL appears to be falling. The analysis suggests little or no change in SL prior to the early 1900's. The period since that time has seen an increase in SL that is optimally fit by a linear trend of 23 cm/century. The study results suggest that it is not possible to uniquely determine either a global rate of change of SL or even the average rate of change associated with the existing (inadequate) data set. Indeed, different analysis methods, by themselves, can cause 50% variations in the estimates of SL trend in the existing data set. A signal/noise analysis suggests it should be easy to detect small, future changes in the SL trends estimated for the period 1930-1980. However, detection of theoretically predicted low-frequency signals (e.g., caused by CO2 warming) will be difficult in view of the huge, low-frequency, natural variability associated with glacial/tectonic processes.

An Improved Real-Time Global Sea Surface Temperature Analysis

Article

Jan 1993

The monthly global sea surface temperature (SST) analysis of Reynolds using real-time in situ and satellite SST data has now been improved by using sea ice data to simulate SSTs in ice-covered regions. The simulated SSTs now become the external boundary condition for the analysis solution. This technique eliminates any high-latitude satellite biases and extends the analysis to the ice edge. The analysis with the ice data has been computed for the period January 1982 to present. 7 refs., 6 figs.

Global sea level rise

Article

Apr 1991

Bruce C. Douglas

Published values for the long-term, global mean sea level rise determined from tide gauge records exhibit considerable scatter, from about 1 mm to 3 mm/yr. This disparity is not attributable to instrument error; long-term trends computed at adjacent sites often agree to within a few tenths of a millimeter per year. Instead, the differing estimates of global sea level rise appear to be in large part due to authors' using data from gauges located at convergent tectonic plate boundaries, where changes of land elevation give fictitious sea level trends. In addition, virtually all gauges undergo subsidence or uplift due to postglacial rebound (PGR) from the last deglaciation at a rate comparable to or greater than the secular rise of sea level. Modeling PGR by the ICE-3G model of Tushingham and Peltier (1991) and avoiding tide gauge records in areas of converging tectonic plates produces a highly consistent set of long sea level records. The value for mean sea level rise obtained from a global set of 21 such stations in nine oceanic regions with an average record length of 76 years during the period 1880-1980 is 1.8 mm/yr {plus minus} 0.1. This result provides confidence that carefully selected long tide gauge records measure the same underlying trend of sea level and that many old tide gauge records are of very high quality.

Ice3G: A New Global Model Of Late Pleistocene Deglaciation Based Upon Geophysical Predictions Of PostGlacial Relative Sea Level Change

Article

Mar 1991

a new high resolution global model of late pleistocene deglaciation is inferred on the geophysical predictions oof postglacial relative sea level variations in which the ice-ocean-solid earth interaction is treated in a gravitationally self-consistent fashion. The radial viscoelastic structure of the planet is assumed known on the basis of previously published sensitivity tests on solutions of the forward problem. Only radiocarbon controlled relative sea level histories from sites that were actually ice covered (with one of two additions) are employed to constrain the model, leaving relative sea level (RSL) data from sites that were not ice covered to be employed to confirm its consistency. Results for these confirmatory analyses are reported elsewhere. The deglaciation model, ICE-3G, is compared to previous models derived by several independent means and tested against a number of additional observations other than sea level histories, including geologically controlled retreat isochrones, oxygen-isotope data from deep-sea sedimentary cores, and coral terrace elevations. The latter two observations strongly constrain the net sea level rise that has occurred since the onset of deglaciation and therefore the mass of ice that melted during the last glacial-interglacial transition.

Global sea level change: Determination and interpretation (95RG00355)

Article

Jan 1995

Bruce C. Douglas

Sea level rise at Key West, Florida, 1846-1992: America's longest instrument record?

Article

Sep 1993
GEOPHYS RES LETT

The continuous series of sea level at Key West, Florida commenced in 1913, but we have discovered sporadic measurements that date back to 1846. From records at the U. S. Army Corps of Engineers and the U. S. Coast and Geodetic Survey, the sea level series has been connected to a Summary (common) Datum. Thus, a gappy record of monthly and annual mean heights (H[t]), perhaps the United States' longest series over San Francisco (ca. 1854) or New York (ca. 1956), can be tested to ascertain if the rise in relative sea level at this site is stationary. Applying first and second order least squares and two-phase regression analyses, we find that dH/dt is 0.19+/-0.01 cm/yr, and that d2H/dt2=[9.6+/-8.6].10-3 cm/yr2 the two-phase regression shows H[t] rising 0.15+/-0.03 cm/yr before ca. 1925 and 0.23+/-0.01 cm/yr afterwards. Neither the second-order regression coefficient nor d2H/dt2 nor the two-phase calculation are significant above the 75% confidence level, but all three are weakly consistent with accelerated rise. For the epoch 1951-1987, Key West sea level, corrected for post-glacial rebound, is best explained by concurrent measurements of 0-1,000 db dynamic height anomaly change.

Reply [to “Comments on ‘Observation of a monthly variation in global surface temperature data’”]

Article

Nov 1994

Clive Best

An analysis of global surface temperature data is described. The data originates from the European Centre for Medium Range Weather Forecasting, and covers the period 1986–1991 with a spatial resolution of 1.125 degrees. A global average temperature, Tav, defined as the area weighted average of local temperatures, has been calculated for each day in this time period. In addition to clear seasonal variations, a small monthly oscillation is visible. The data for each year has been fitted to the form Tav=A+Bx+Ccos(Dx+E), where x is the day within the year. Subtracting the fitted seasonal variation from the data isolates the remaining monthly oscillations. The magnitude of this oscillation is about 0.2°K. A Fourier analysis shows the time period to be 30±3 days, and an anti-correlation between southern and northern hemispheres is observed. The oscillation is found to be stronger towards the polar regions and absent in the tropics. Possible physical and model effects are considered.

The GPS flight experiment on TOPEX/Poseidon

Article

Sep 1994

The precision orbit determination (POD) experiment on TOPEX/POSEIDON using the Global Positioning System (GPS) is yielding concrete results. Orbit consistency and accuracy tests indicate that GPS is routinely providing satellite altitude with an accuracy of better than 3 cm. Here we review the GPS experiment, its basic concepts, POD techniques and key results, and discuss the possible cost and performance benefits that may flow to future missions.

Impact of Mount Pinatubo Aerosols on Satellite-derived Sea Surface Temperatures

Article

Apr 1993

Richard W. Reynolds

The June 1991 eruptions of Mount Pinatubo produced new stratospheric aerosols that were greater than the aerosols from the 1982 eruptions of El Chichon. These new aerosols strongly affected the advanced very high resolution radiometer (AVHRR) retrievals of sea surface temperature in the tropics where occurred with magnitudes greater than 1[degrees]C. The time dependence of these biases are shown. In addition, a method to correct these biases is discussed and integrated into the National Meteorological Center's optimum interpolation sea surface temperature analysis.

The temporal and spatial characteristics of TOPEX/POSEIDON radial orbit error

Article

Jan 1995

Satellite orbit error has long been the bane of oceanographers who analyze altimetry data. However, radial orbit error on TOPEX/POSEIDON (T/P) has been reduced to the 3 to 4-cm root-mean-square (rms) level over a 10-day repeat cycle, which represents an order of magnitude improvement over earlier altimetry missions such as Geosat. Consequently, oceanographers are now able to directly evaluate the absolute ocean topography to unprecedented accuracy levels. While significantly reduced, the T/P orbit error still requires quantification. This study examines the spatial and temporal characteristics of the T/P radial orbit error, as assessed through the analysis of laser tracking residuals and orbit comparisons with independently generated trajectories. Spectral analyses of the orbit differences between the orbits determined from satellite laser ranging and Doppler Orbitography and Radiopositioning Integrated by Satellite data and the independently determined reduced dynamic Global Positioning System (GPS) ephemerides indicate that the predominant power is at the once-per-orbital revolution frequency with 2- to 3-cm peaks. When the orbit differences are colinearly aligned to a fixed geographic grid and spectral analysis is performed at each geographic grid point, a nearly 60-day period is found with maximum amplitudes in the 2- to 4-cm range. The contribution of both conservative and nonconservative force and measurement mismodeling to this error signal are assessed. We demonstrate that the ~60-day error period seen at fixed geographic locations arises from weaknesses in the dynamic ocean tidal models used in the orbit calculations. New tidal models have been developed which significantly reduce this error. Second-generation orbits incorporating many model improvements have been computed and demonstrate a significant reduction in the radial orbit error signals. Some orbit error still exists, and methods for further model improvements and the possibility of achieving 1-cm radial rms orbit accuracy in T/P are discussed.

TOPEX/Poseidon: The 2-cm solution

Article

Dec 1994

The unprecedented accuracy of TOPEX/POSEIDON (T/P) altimeter data warrants a new evaluation of the methods typically used to form time series of sea level change. Whereas explicit removal of orbit error has always been required as a first step in altimeter data processing, the T/P analysis presented here is based simply on unadjusted, monthly averages. This approach has the advantage of retaining the large-scale ocean signal, which would be distorted by orbit adjustment. Using 16 months of data, we have evaluated the T/P monthly means on spatial scales ranging from mesoscale to global. In the tropical Pacific, comparisons with 17 island tide gauge records and dynamic height derived from 36 thermistor moorings show that the altimeter data have an accuracy of approximately 2 cm when averaged over spatial scales a few hundred kilometers. On basin scales in the northern hemisphere, similar agreement is found between the T/P data and the dynamic height climatology of Levitus (1982). These new altimeter observations are thus providing the first reliable view of global sea level changes on seasonal-to-interannual timescales.

Calibration of the TOPEX/POSEIDON altimeters at Lampedusa: additional results at Harvest

Article

Dec 1994

The Italian Lampedusa island in the Mediterranean Sea was chosen by the Centre National d'Etudes Spatiales (CNES) to support the verification of altimeters on board the TOPEX/POSEIDON satellite. The orbit was phased in such a way that a descending pass overflies a small islet called Lampione, 20 km west of Lampedusa. During the TOPEX/POSEIDON verification phase, until mid-December 1992, the satellite passed nine times over Lampedusa. The Lampedusa site was equipped with all the required instrumentation to provide independent and accurate knowledge of the altimeter system components: sea level, orbit, and geophysical corrections. In addition, a reprocessing of the altimeter data was performed to better adjust the waveform parameters estimation especially when quasi-specular echoes due to flat sea conditions have disturbed the on-board estimation. Three different local orbits computed by expert groups (Centre d'Etudes et de Recherches Geodynamiques et Astronomiques, Delft University of Technology, and Jet Propulsion Laboratory), plus CNES and NASA global orbits were considered. The bias of the CNES altimeter (SSALT) was then calculated, based on six overflights, giving a mean value of 0.7 cm. A SSALT bias also was calculated at Harvest, the NASA calibration site, using six overflights. The deduced bias is 1.3 cm. Integrating both site results gives a mean SSALT bias of 1 cm with an uncertainty of 2.4 cm. The NASA altimeter (ALT) bias computed at Lampedusa is -18.5 cm with an uncertainty of 3.4 cm, but this result must be carefully considered because it was obtained with only two overflights. Considering the Lampedusa SSALT calibration results and those derived from statistical repeat track analysis giving an estimation of the relative bias between SSALT and ALT (e.g., Le Traon et al., this issue) a bias of -14.8 cm with an uncertainty of 2.6 cm was found for the ALT altimeter.

Precise Centre National d'Etudes Spatiales orbits for TOPEX/POSEIDON: is reaching 2 cm still a challenge?

Article

Dec 1994

To achieve maximum benefit from the altimetric data collected by the French-aAmerican TOPEX/POSEIDON spacecraft, radial orbit accuracy of 10 cm or better is required. This unprecedented requirement led the French Space Agency Centre National d'Etudes Spatiales (CNES) to develop a new high-accuracy tracking system, Doppler orbitography and radiopositioning integrated by satellite (DORIS), and a new precision orbit production facility, the Service d'Orbitographie DORIS. A global effort produced new models and a new orbit determination strategies. The result of these efforts has been assessed after 1 year of operation. The original goal has clearly been met, and the TOPEX/POSEIDON orbits produced by NASA and CNES agree beter than the 5 cm root mean square (RMS) level in the radial direction. At this level of accuracy, traditional techniques cannot correctly describe the actual orbit error, and some new procedures are proposed.

Global Sea Level Change From Satellite Altimetry

Article

Jan 1992

We examine the feasibility of using satellite altimeter data to measure the long-term change of global sea level (estimated from tide gauge data to be a rise of approximately 0.2 cm yr-1). Two and one-half years of collinear Geosat altimeter data (1986-1989) are used together with a 17-day set of Seasat altimetry (July-August 1978) having nearly the same ground track. A consistent set of precise orbits was used throughout, and residual orbit error was removed as a sinusoidal fit to approximately 3-day arcs of sea level collinear differences. The globally averaged Geosat data show sea level falling at 1.2 ± 0.3 cm yr-1 over the first 2 years, even after removal of tide errors and instrument biases not accounted for in the Geosat geophysical data records. This unrealistic result is found to be due largely to long-term error in the ionospheric model for the single-frequency Geosat altimeter. The Geosat-Seasat comparison, based on data 10 years apart, shows an apparent sea level rise of 1.0 cm yr-1. Assuming this result is also unrealistic, a possible explanation is a biased scale to the Doppler-determined Geosat orbit which, unlike Seasat, did not have the benefit of laser tracking. It is also possible that the Geosat altimeter (without external in-orbit calibration) had a bias of the order of 10 cm. We conclude that for satellite altimetry to make a fundamental contribution to monitoring global mean sea level change, both the altimeter (including its media corrections) and the orbit model which provides a geocentric reference for the ocean surface will need continuing and careful calibration with absolute standards.

Application of satellite altimetry to ocean circulation studies: 1987-1994

Article

Jan 1995

The ocean plays a key role in determining the global climate and its time evolution. To understand this role and subsequently develop techniques for predicting future climate, one must understand the dynamics of the global ocean circulation—the movement of water that transports mass, heat, salt, and other biogeochemical properties of the ocean that are closely linked to the processes of climate change. The only viable approach to observing the global ocean circulation with sufficient resolution and consistent sampling is the use of a satellite radar altimeter to measure the height of the sea surface—the sea level (Wunsch and Gaposchkin, 1980; Stewart, 1983; Wunsch, 1992). After removing the effects of the tides and atmospheric pressure from the observation, the deviation of the sea surface from the geoid, called the ocean dynamic topography, is readily related to the velocity of the surface geostrophic flow—a component of the surface flow on which the surface pressure force is balanced by the Coriolis force due to the Earth's rotation. Moreover, the ocean dynamic topography provides a strong constraint for determining the ocean circulation through the entire water column via the dynamic equations governing the fluid motion. Precise measurement of the shape of the global sea surface thus provides a powerful tool for studying the dynamics of the ocean circulation.

The Supplementary Report to the IPCC Scientific Assessment

Article

Jan 1992

Large-scale response of the ocean to atmospheric forcing at low frequencies

Article

Aug 1991

Carl Wunsch

Estimates of the time-dependent sea surface topography on large (500-2500 km) and time (30 days to 2 years) scales from altimetry and tide gauge data are compared with estimates of the surface meteorological fields (winds and pressure) on the same scales. The comparisons are made in the North Pacific and North Atlantic. Despite the obvious remaining high noise level in the sea surface topography, there is a significant correlation between the atmosphere and the time-varying ocean circulation. These relations are quantified using simple multichannel regression models. With no time lags, typically 50% of the sea level variance can be described by the wind curl and divergence and the pressure field. Some areas produce classical static inverted barometer effects, but other suggest an amplified response. The motions are believed to characterize most of the water column.

Improved global sea surface temperature analyses

Article

Climate change: the IPCC Scientific Assessment

Book

Jan 1990

Book review of the intergovernmental panel on climate change report on global warming and the greenhouse effect. Covers the scientific basis for knowledge of the future climate. Presents chemistry of greenhouse gases and mathematical modelling of the climate system. The book is primarily for government policy makers.

A preliminary tidal analysis of TOPEX/POSEIDON altimetry

Article

Dec 1994

Approximately 12 months of data from the TOPEX/POSEIDON satellite altimeter mission are analyzed for the major short-period oceanic tides. A harmonic analysis is performed on data captured within bins defined on a deep-ocean grid, which, owing to tidal aliasing considerations, must have a relatively coarse spatial resolution. Our analysis is in terms of corrections to the Schwiderski and Cartwright-Ray models, and it confirms many of the Schwiderski differences previously reported by Cartwright and Ray. Our differences with respect to the Geosat-based Cartwright and Ray model form a sectorial pattern in M2 with high/low differences separated roughly 180° in longitude. We suggest that these sectorial errors were most likely induced by Geosat's relatively large orbit error. Comparisons to independent data validate the improved TOPEX/POSEIDON solutions; in situ ” ground truth” shows M2 RMS differences of 4.10cm (Schwiderski), 3.86cm (Cartwright and Ray), 2.63cm (this paper). Global rates of energy dissipation confirm earlier estimates for M2, and show improved agreement with satellite tracking studies for K1 and S2. These preliminary exercises confirm that TOPEX/POSEIDON should result in a new generation of improved global tidal models.

Climate change 1992: the supplementary report to the IPCC scientific assessment

Book

Jan 1992

Global sea level change: Determination and interpretation

Article

Jul 1995

Bruce C. Douglas

The notion of sea level rise brings to the popular mind the specter of deep inundation of coastal regions. One pictures skyscrapers emerging from the waters like so many sleeping flamingos standing in the shallows of a lake. Of course if all of the world's ice sheets suddenly melted or collapsed, this vision would apply to New York City and its coastal counterparts. But there is a general consensus that such a calamity is not an immediate threat [Houghton et al, 1990]. The actual situation for the recent historical past and near future appears to be more benign, but with nonetheless extremely significant, even devastating impacts due to erosion and flooding of coastal areas.

Man, water, and global sea level

Article

Nov 1991
Eos Trans. AGU

B. Fong Chao

I'd like to call attention to an anthropogenic influence on global sea level change. Although far less interesting for geophysicists who study natural processes, the phenomenon in question has made a significant difference, and hence ought to be taken into account in the sea level budget.Observational evidence shows that the global sea level has risen at the rate of 1.6-2 mm/yr for the last several decades [e.g., Trupin and Wahr, 1990; Douglas, 1991]. How much of [the rise] results from natural fluctuation, how much is anthropogenic, and what are the sources and mechanisms of the rise are among the key questions asked in this era of concerns about enhanced greenhouse effect and global warming. Presumably, as the global temperature rises, the sea level will rise for two reasons: thermal expansion (the steric change), and addition of water (the eustatic change). A primary candidate for the source of the latter is melting land ice in the form of polar ice sheets and mountain glaciers. Practically nothing useful is known about the present-day mass balance of the polar ice [Zwally, 1989; Douglas et al., 1990]. The best estimate for the rate of mountain glacier mass wastage, based on scanty data, amounts to a contribution of 0.46 (±.26) mm/yr of higher sea level between 19007ndash;1961 [Meier, 1984].

Glacial Rebound Sea-Level Change and Mantle Viscosity

Article

Feb 1990

Kurt Lambeck

Satellite orbit perturbations due to the geopotential

Article

Jan 1986

George Walton Rosborough

Satellite-borne radar altimeters can measure the distance from the satellite platform to the ocean surface with an accuracy of five centimeters or less. To fully exploit the geophysical information contained in this data, it is necessary to have independent knowledge of the geocentric satellite distance to a comparable level of accuracy. Current altimeter satellite ephemerides fail this requirement by at least an order of magnitude. The radial errors contained in the estimated orbits are primarily a result of inadequacies in the modeling of the Earth's gravity field. To better understand the limitations this orbit error imposes on the analysis of the altimeter data, this study details the temporal and spatial characteristics of the radial orbit perturbations produced by the geopotential. The temporal description fully describes the frequency spectrum of the perturbations, and the spatial description provides a direct method for quantifying the perturbations on a geographic basis. These results are used to analyze the expected radial orbit perturbations of the altimeter satellite to be flown by the proposed TOPEX mission. To facilitate other possible applications, the geopotential perturbations in the along-track and cross-track orbit directions are presented.

Search for acceleration in records of European mean sea-level

Article

Mar 1990
INT J CLIMATOL

P. L. Woodworth

Long records from European tide gauges have been inspected for evidence of modifications in the rate of change, or ‘accelerations’, of mean sea level (MSL). In general, no evidence was found for MSL accelerations significantly different from zero over the period 1870 to the present, although non-zero accelerations were observed at individual stations. Changes in surface air pressure may have been responsible for an overall slightly negative acceleration observed in northern Europe. In order to extend the study to time-scales longer than a century, data from the oldest European MSL records at Brest, Sheerness, Amsterdam and Stockholm starting in 1807, 1834, 1700 and 1774, respectively, were also investigated with the result that a positive acceleration of order 0·4 (mm year−1) per century appears to be typical of European Atlantic coast and Baltic MSL over the last few centuries. Although of interest as an indicator of past climate change, this small low-frequency MSL acceleration is an order of magnitude less than that anticipated over the next few decades as a result of greenhouse warming. A conceptual study of possible future Newlyn (UK) tide gauge data has shown that the MSL acceleration anticipated from the global warming should be apparent in the records by the early part of the next century.

Variability in a homogeneous global ocean forced by barometric pressure

Article

Aug 1993
DYNAM ATMOS OCEANS

Rui M. Ponte

The nature of the oceanic response to pressure loading is explored using a constant-density, shallow-water numerical model driven by atmospheric pressure fields from the European Centre for Medium Range Weather Forecasts. The model has realistic bottom topography and coastlines and is run for 1 year (1986) on a global domain. Meridional gradients in mean sea-level are generally large (10–20 cm over 20–30°), particularly in high southern latitudes. Sea-level variability is strong in mid- and high latitudes (typical standard deviations of 10–15 cm), but weakens towards the equator. Results indicate a significant contribution of pressure-driven fluctuations to the observed large-scale sea-level variability in mid- and high latitudes, away from western boundary regions. Pressure-induced velocity signals are, in contrast, generally small compared with other types of variability.The validity of the inverted barometer approximation is found to be strongly dependent on frequency and geographical location. Globally, the approximation is not reliable for periods shorter than approximately 2 days, but failure at longer periods occurs over extensive regions (e.g. the tropical Atlantic and Pacific, and the Southern Ocean). Nonisostatic contributions to the sea-level variability are substantial in many areas, including the tropics, the high-latitude North Atlantic, the Gulf of Mexico, and several other boundary regions. The dynamical signals are partly associated with the excitation of several high-frequency normal modes. Some of these features have a spatial structure and period very similar to normal modes calculated by Platzman and collaborators. Their presence in the model indicates that atmospheric pressure forcing is a possible mechanism for normal mode excitation.

Measuring global mean sea level variations using TOPEX/POSEIDON altimeter data

Abstract

No full-text available

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