Sea Level Measurements Research Articles

Recent improvements in GEOSAT altimeter data

Altimetric sea level measurements made by the U.S. Navy's geodetic satellite (Geosat) during its exact repeat mission (1986–1989) have been successfully applied to the study of a variety of oceanic phenomena [Douglas and Cheney, 1990], and comparisons with in situ measurements have shown that monthly mean altimetric sea level is accurate to approximately 5 cm rms [Miller and Cheney, 1990; Taietai, 1989]. Nevertheless, the original release of geophysical data records (GDRs) produced by the National Oceanic and Atmospheric Administration/National Ocean Service [Cheney et al., 1987] showed significant uncertainties in two important fields: the tropospheric water vapor correction and the satellite orbit. Data from other satellites have been used to derive improved water‐vapor fields specifically for Geosat, while satellite ephemerides that are more precise by an order of magnitude have resulted from the application of advanced gravity models and the incorporation of new tracking data. To make these data readily accessible to the scientific community, new versions of the Geosat GDRs have been produced and are being distributed on compact disc/read only memory, or CD‐ROM [Cheney et al., 1991a]. In this report, we document the increased sea level accuracy attainable in the tropical oceans using these new T2 GDRs.

On the effectiveness of Geosat altimeter corrections

Measurements of sea level by a satellite altimeter require a number of corrections for atmospheric and geophysical effects. The paper examines the effectiveness of a large suite of correction terms for Geosat by studying how each correction affects the variances of altimetric sea-surface height differences. Most corrections were found to provide a significant reduction in variance, the most important exception being certain short-period wet-troposphere correlations. The present procedure of using height differences implies nothing about the time-invariant parts of corrections.

Variations of zonal currents in the central tropical Pacific during 1970 to 1987: Sea level and dynamic height measurements

The annual and interannual variations of the major zonal currents of the central tropical Pacific for the period 1970–1987 are described by analyzing two data sets: sea level data from a few selected island stations and ship‐of‐opportunity expendable bathythermograph (XBT) observations. The annual cycles of geostrophic surface current indices (dynamic height or sea level difference between the ridge and the trough defining the current) in the North Equatorial Current (NEC) and North Equatorial Countercurrent (NECC) are found to be in phase (maxima in November), to be strong, and to explain about 50% of the total variance. The annual cycle of volume transport relative to 400 dbar in the NECC is also strong, but NEC volume transport has a weak annual cycle. The discrepancy between the NEC surface current and the transport is due to the variation of the surface current being larger than that of the deep currents. The annual cycle of the transport of the South Equatorial Current (SEC) is 6 months out of phase with that of the NEC and NECC. Phases of the annual cycles of the sea level and dynamic height indices of flow agree well in general, but there are significant differences between the annual amplitudes of the two types of measurements. These differences arise from a variety of sources: dynamic height reference level assumptions, longitudinal spacing of sea level gauges, meridional resolution of sea level array, and different signal to noise ratios for the two sets of indices. The NECC shows marked flow maxima during the peaks of the strong Niños of 1972 and 1982, which were followed by near disappearance of the current the following year; these fluctuations are clearly present in both the surface current indices and the volume transport. In 1976 and 1986–1987 (moderate Niños) the same pattern is not marked. The El Niño signal in NEC transport is weak. There was a strong rise of equatorial sea level during the early phases of the 1972 and 1982 Niños followed by a fall the next year. This resulted in a decrease of the transport of the SEC in the first year followed by an increase to a maximum during the second year; the maximum coincides with a minimum in NECC transport.

NASA radar altimeter for the TOPEX/POSEIDON Project

The TOPEX/POSEIDON Project is a joint US and French mission to develop and operate an Earth-orbiting satellite with sensors capable of making accurate measurements of sea level. The NASA radar altimeter (NRA), a fifth-generation US Altimeter, will provide the primary measurement for the TOPEX/POSEIDON Project altimetric mission. The authors present the requirements, altimeter fundamentals, design description, integration and test program, primary elements of ground processing, and assessment for the dual-frequency NASA radar altimeter.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Sea-level changes: Consequences for the Southern Hemisphere

One of the measurable symptoms of man-induced climatic change is a global rise in mean sea-level. A review of the suggested mechanisms for sea-level rise is given, supported by a critical discussion of present predictions and predictive models. The data base on which these predictions are based is geographically inhomogeneous and particularly sparse in the Southern Hemisphere. Some preliminary work which has been done on the possible environmental impact of sea-level rise on coastal areas is described, but very little of this is for Southern Hemisphere countries. It is suggested that since the Southern Hemisphere has particular observational requirements because of a higher ratio of ocean to terrestrial areas, particular attention in international monitoring programmes be given to it. This has special relevance to sea-level measurements in hostile environments such as off Antarctica.

Measurements of electromagnetic bias in radar altimetry

The accuracy of satellite altimetric measurements of sea level is limited in part by the influence of ocean waves on the altimeter signal reflected from the sea surface. The difference between the mean reflecting surface and mean sea level is the electromagnetic bias. The bias is poorly known, yet for such altimetric satellite missions as the Topography Experiment (TOPEX)/Poseidon it is the largest source of error exclusive of those resulting from calculation of the satellite's ephemeris. Previous observations of electromagnetic bias have had a large, apparently random scatter in the range of 1–5% of significant wave height; these observations are inconsistent with theoretical calculations of the bias. To obtain a better understanding of the bias, we have measured it directly using a 14‐GHz scatterometer on the Chesapeake Bay Light Tower. We find that the bias is a quadratic function of significant wave height H1/3. The normalized bias β, defined as the bias divided by the significant wave height, is strongly correlated with wind speed at 10 m, U10, and much less strongly with significant wave height. The mean value for β is −0.034, and the standard deviation of the variability about the mean is ±0.0097. The standard deviation of the variability after removing the influence of wind and waves is ±0.0051 = 0.51%. The results are based on data collected over a 24‐day period during the Synthetic Aperture Radar and X‐Band Ocean Nonlinearities (SAXON) experiment from September 19 to October 12, 1988. During the experiment, hourly averaged values of wind speed ranged from 0.2 to 15.3 m/s, significant wave height ranged from 0.3 to 2.9 m, and air minus sea temperature ranged from −10.2° to 5.4°C. Because U10 can be calculated from the scattering cross section per unit area σ0 of the sea measured by spaceborne altimeters, we investigated the usefulness of σ0 for calculating bias. We find that β is strongly correlated with σ0 and much less strongly with H1/3. The standard deviation of the variability after removing the influence of the radio cross section and waves is ±0.0065 = 0.65%. The results indicate that electromagnetic bias in radar altimetry may be reduced to the level required by the TOPEX/Poseidon mission using only altimetric data. We find, furthermore, that the relationship between σ0 and wind speed agrees with previously published power law relationships within the accuracy of the measurement. The mean value of β, its variability, and the sensitivity of β to wind speed all agree well with previous measurements made using a 10‐GHz radar carried on a low‐flying aircraft. The mean value of β, its variability, and the sensitivity to wind were all significantly larger than previous measurements made using a 39‐GHz radar also carried on a low‐flying aircraft. All experiments included a similar range of wind speeds and wave heights. The SAXON data were, however, much more extensive, and the statistical relationships correspondingly more significant. The mean value of β is very close to the mean value determined from global measurements of sea level made by Geosat.

Large inversions of altimeter and tied tide gauge data for the Mediterranean Sea mean surface

Satellite altimeter measurements and tide gauge data tied (TTG data) to a global geodetic reference frame by space techniques of positioning will provide in a near future consistent and complementary information to map the mean sea surface (MSS) with a subdecimetric absolute accuracy. Information is consistent because, after standard data processing, both kinds of sea level measurements are given relative to the same geocentric reference ellipsoid. Information is also complementary because: first, satellite altimetry draws a dense sampling pattern of the ocean while TTG measurements are local; second, the properties of the data errors are very dissimilar in the two techniques. Both kind of sea level measurements are already available in the Mediterranean basin. This study is the first attempt to compute an accurate MSS model with mixed sets of such data. A total inverse method, previously developed to analyze satellite altimetry, is greatly improved in order to recover a full spectrum of the MSS (down to wavelengths λ ≃ 100 km) and to invert rigorously large data sets including consecutive satellite tracks. The MSS solution derived solely from altimetry in the Mediterranean Sea has a formal error of 12 cm under the repeat satellite tracks, up to 30 cm near the coasts, with a further truncation error of 10 cm. Then an analysis of the discrepancy between this solution and TTG data allows us to point out unsuspected inconsistencies in the two data sets which are clearly identified. Nevertheless the usefulness of the TTG data is demonstrated by an estimate of their impact on the error spectrum of the MSS.

Oceanic heat content variability in the tropical Pacific during the 1982–1983 El Niño

Anomalous heat transport and storage during the 1982–1983 El Niño are investigated using a linear, multimode model forced by observed winds. Heat transport is decomposed into symmetric (about the equator) and antisymmetric components. The former was dominated by anomalous northward Ekman transport which represented an enhancement of the usual seasonal cycle. The latter involved both Ekman and geostrophic transports. Near‐equatorial wind anomalies forced Kelvin and Rossby waves usually associated with El Niño; together these waves set up antisymmetric, geostrophic transport which tended to oppose direct Ekman transport. Because the opposition was imperfect, there was net heat convergence which caused variations in heat content in bands of latitude centered on the equator. Within a fairly narrow band (±5°) heat content was anomalously high preceding El Niño and was depleted following the event. Equatorial heat content anomalies were largely compensated by opposing anomalies in low latitudes of the extraequatorial ocean so that variability over broader bands of latitude about the equator was relatively small. A sampling study employing the model suggests that observational evidence for a heat content variations over the region ±15° is an artifact arising from inadequate spatial resolution offered by the sea level measurement network.

Variability in the Western Equatorial Pacific Ocean during the 1986–87 El Niño/Southern Oscillation Event

Abstract We describe variability in the western Pacific Ocean during the 1986–87 El Nino/Southern Oscillation (ENSO) event, with emphasis on time series measurements of currents, temperature, sea level and winds near the equator at 165°E. Zonal winds were anomalously westerly from mid-1986 to late 1987 and were punctuated by 2–10 m s−1 episodes of westerlies lasting about 10 days to 2 months. Zonal current in the upper 100-m surface layer responded to these wind variations typically within a week, in some cases with speeds exceeding 100 cm s−1 to the east. Zonal current variations in the thermocline below 100 m were generally less coherent with the local wires than currents near the surface. They were also generally less variable, although the Equation Undercurrent disappeared for 3–4 weeks in October-November 1987 at a time when the normal eastward directed zonal pressure gradient force reversed along the equator. Periods of intense and prolonged eastward flow in the surface layer were associated with a ...

Evaluation of Geosat altimeter data with application to tropical Pacific sea level variability

Repeated Geosat altimeter measurements of sea level at ground track intersections (crossovers) and along collinear nests of profiles were used to evaluate Geosat data by (1) construction of a regional, precise surface made up of intersecting sea level profiles, (2) determination of the magnitude of the electromagnetic (EM) bias, and (3) generation of sea level time series and anomaly maps. The first procedure, after elimination of radial ephemeris error and other long‐wavelength signals, established the data's internal precision at a few centimeters. With regard to the EM bias, comparing repeated sea level profiles for different values of significant wave height (SWH) gave a value of about 1% of SWH. The final process yielded a record of sea level whose accuracy was tested by comparison with in situ measurements and wind‐driven model results. On the basis of these analyses we conclude that Geosat data can determine sea level changes with an rms accuracy of approximately 4 cm for time scales of a month or longer. For determining sea level variability, Geosat geophysical data records are a high‐quality, long‐term data set of significant value for the study of ocean dynamics.

Acute mountain sickness and physiological stress during intermittent exposure to high altitude.

The effect of intermittent exposure to high altitude (4200 m) hypoxia on symptomatology of acute mountain sickness (AMS) was observed, and the relationship between AMS and stress hormone excretion was examined among shift workers at the United Kingdom Infrared Telescope (UKIRT) facility and a control group of sea level residents. Some of the shift workers experienced AMS during their first day at altitude, but had recovered after 5 days residence at altitude. Upon acute exposure to altitude, none of the shift workers reported severe cases of AMS after 5 days residence at sea level, but some reported severe cases after 45 residence at sea level, thus providing some evidence for a 'carry over' of acclimatization for a 5 day period. Reported symptomatology at sea level was predictive of 24 h. excretion rates of adrenaline and 17-hydroxycorticosteroids at high altitude. No good predictors of symptomatology of AMS at high altitude were found using sea level measures alone.

GEOSAT vs. SEASAT

For the first time, a long‐term global view of sea level change has become available to the ocean science community. The U.S. Navy's GEOSAT altimeter satellite has been measuring ocean surface topography with unprecedented precision since March 1985. GEOSAT represents a dramatic improvement over earlier observations from NASA's SEASAT mission primarily because of higher spatial resolution and longer temporal duration (3+ years vs. 3 months), as depicted on the cover of this issue. The measurement of sea level is an important variable in the determination of ocean circulation. A special session of the 1988 AGU Fall meeting will focus on studies of ocean circulation with the GEOSAT measurements.

Altitude, training and human performance.

The effect of altitude on human performance is complex. Numerous variables are known to change from sea-level measures. Maximum aerobic power is depressed as ascent occurs and this impairs the ability to work maximally. While changes in haematological variables would theoretically counterbalance the loss in aerobic power, they have not been shown to do so. The environmental stress of cold may have positive effects on aerobic capacity at altitude, but this has been little investigated in humans. Pulmonary ventilation increases with altitude and the measure of hypoxic ventilatory response holds some promise of predicting humans who may benefit from altitude conditioning. Cardiac function is well maintained while lung function is not. The preferred fuel for exercise at altitude seems to be fat, while carbohydrate metabolism is dramatically changed. Much is not known of high altitude anorexia and muscle mass loss. Conditioning at altitude is known to benefit performance at altitude. The evidence for a sea-level benefit from altitude training as yet remains elusive. While selected individuals may benefit, the reasons why have not been determined.

Comparisons of sea level and bottom pressure measurements at Drake Passage

Tide gauge and atmospheric sea level pressure measurements from each side of Drake Passage are compared with multiyear time series of bottom pressure at 500 m depth. Synthetic subsurface pressure (SSP), smoothed with a 40‐hour low‐pass filter, derived from tidal and sea level pressure records from the British Antarctic Survey base Faraday is coherent with bottom pressure from southern Drake Passage for periods of 6 to 600 days, but it is about 60° out‐of‐phase at the annual period. The difference between the two series for periods longer than 60 days is mainly annual and is as large as, or larger than, the seasonal variability of bottom pressure itself. Annual variations in open ocean mixed‐layer density near Antarctica are too small to account for the difference. The Faraday SSP series spans 21 years, slightly longer than the 18.6‐year lunar nodal tide, which appears to exist in the record. At the northern side of Drake Passage, SSP from Puerto Williams, Chile, does not compare well with nearby bottom pressure for any band of periods, apparently due to local winds for periods under 100 days and to annual cycles in upper layer density. Barotropic changes in transport of the Antarctic Circumpolar Current cannot be directly estimated using these surface observations, even though over seasonal periods they are an order of magnitude larger than the baroclinic changes due to variations in upper layer density.

Currents off south-eastern Australia: results from the Australian coastal experiment

The Australian Coastal Experiment was conducted off the east coast of New South Wales between September 1983 and March 1984. The experiment was conducted with arrays of current meters spanning the continental margin at three latitudes (37.5�, 34.5�, and 33.0�S.), additional shelf moorings at 29� and 42�S. coastal wind and sea-level measurements, monthly conductivity-temperature-depth probe/expendable bathythermograph (CTD/XBT) surveys, and two satellite-tracked buoys. Over the continental shelf and slope, the alongshore component of the current generally exceeded the onshore component, and the subtidal (&lt;0.6 cpd, cycles per day) current variability greatly exceeded the mean flow. Part of the current variability was associated with two separate warm-core eddies that approached the coast, causing strong (&gt;50 cm sec-1), persistent (&gt;8 days), southward currents over the continental slope and outer shelf. Temperature and geostrophic velocity sections through the eddies, maps of ship's drift vectors and temperature contours at 250 m, and the satellite-tracked drifter trajectories showed that these eddies were similar in structure to those observed previously in the East Australian Current region. Both eddies migrated generally southward. Eddy currents over the shelf and slope were rare at Cape Howe (37.5�S.), more common near Sydney (34.5�S.), and frequent at Newcastle (33.0�S.), where strong northward currents were also observed. Near Sydney, the eddy currents over the slope turned clockwise with depth between 280 and 740 m, suggesting net downwelling there. Repeated CTD sections also indicated onshore transport and downwelling at shallower levels; presumably, upwelling occurred farther south where the eddy currents turned offshore. Periodic rotary currents over the continental slope near Sydney and Newcastle indicated the presence of small cyclonic eddies on the flank of a much larger anticyclonic eddy. Between early October and late January, no strong southward currents were observed over the continental margin near Sydney. Data from this 'eddy-free' period were analysed further to examine the structure and variability of the coastal currents. Much of this variability was correlated with fluctuations in coastal sea-level (at zero lag) and with the wind stress (at various lags). The coherence and phase relationships among current, wind-stress, and sea-level records at different latitudes (determined from spectral analysis and frequency-domain empirical orthogonal functions) were consistent with the equatorward propagation of coastal-trapped waves generated by winds in phase with those near Cape Howe. Time-domain empirical orthogonal functions show that the current fluctuations decayed with distance from shore and with depth, as expected of coastal-trapped waves.

Overview of Pacific sea level variability

Abstract Sea level measurements have proven to be extremely valuable for investigations of the dynamics of the world's oceans. Although a comprehensive review of studies using sea level is beyond the scope of this article, it can give an idea of the impact that these data have had on the development of our understanding of ocean and ocean‐atmosphere dynamics. The discussion is restricted geographically and temporally to the variability of the Pacific Ocean on time scales longer than several days and is organized into two broad categories. The main section is concerned with the description of a few selected phenomena that are conveniently divided according to their time scale. For example, research studies concerned with the dynamics of the seasonal cycle of the Pacific Ocean are covered. This category also contains a description of the important work on interannual sea level fluctuations and their relationship to global climate disturbances, followed by a short discussion of the use of sea level variabili...

Calculation of sea level time series from noncollinear GEOSAT altimeter data

Abstract Oceanic temporal variability is calculated from satellite altimeter data by differencing sea level measurements repeated at the same location. These differences yield variations independent of any permanent undulations of sea level. If satellite groundtracks are collinear (nested), altimeter measurements are repeated in a regular fashion continuously along a repeating groundtrack, and calculation of sea level variation is straightforward. For an orbit that repeats only at intersections (crossovers) of the groundtrack, the problem is more complicated, but results equal in accuracy to those obtainable from a collinear analysis can be achieved. For both collinear and crossover analyses, long wavelength error in the altitude of the satellite must be eliminated by using a model for the orbit error and adjusting each pass of data into a reference pass in its nest for the collinear case or into a reference grid of intersecting passes for nonnested tracks.

Indonesian through flow and the associated pressure gradient

The flow of water from the western Pacific to the eastern Indian Ocean through the Indonesian archipelago is governed by a strong pressure gradient. Dynamic height computations determine the average sea level difference as 16 cm and show that most of the pressure gradient is contained in the upper 200 m. Sea level data from Davao in the Philippines and from Darwin in Australia are used to determine the annual signal and the interannual variations of the pressure gradient for the years 1966 to 1985. The annual signal has a maximum during the southeast monsoon in July and August and a minimum in January and February. Interannual variations are not related to the Southern Oscillation because sea level is low at both stations during El Niño events, and thus there is little influence on the sea level difference. The mechanism of the through flow is discussed, but a determination of its numerical value will have to await direct measurements. A comparison of the sea level difference with results from a numerical model by Kindle shows satisfactory agreement. It is concluded that the variability of the through flow can be monitored by sea level measurements.

On the choice of orbits for an altimetric satellite to study ocean circulation and tides

The choice of an orbit for satellite altimetric studies of the ocean's circulation and tides requires an understanding of the orbital characteristics that influence the accuracy of the satellite's measurements of sea level and the temporal and spatial distribution of the measurements. Three orbital parameters determine the temporal and spatial sampling characteristics of the satellite: the orbital altitude, the inclination of the orbit, and the repetition period. The eccentricity of the chosen orbit should be less than 0.001 to minimize altimeter errors caused by uncertainty in determining the time of each altimeter measurement and to maintain the rate of height variation over the oceans within the limits of the altimeter tracker. The choice of the satellite's orbital altitude is constrained by the influences of atmospheric drag and errors in models of the gravity field, which decrease with height, and the required complexity of the altimeter (e.g., the power and/or antenna size required) and the effect on satellite systems of radiation from Earth's radiation belts, which increase with height. The choice of inclination is constrained by a desire to measure sea level over all the ocean, by the requirement that the acute angle between intersections of the subsatellite track be as large as possible, and by the time phasing of the repeat of the satellite's ground track as a function of inclination, which determines the aliased frequencies of the measurements of the oceanic tides. The choice of repetition period is constrained by tradeoffs between temporal and spatial coverage and by the aliasing of tidal constituents. Combining the above constraints leads to a set of orbits in a narrow range of altitude and inclination, called here the “Topex/Poseidon window.” This window extends from 1100 to 1500 km altitude and 62° to 66° inclination for a 10‐day repeat period and consists of three subwindows following lines of equal orbit plane precession. There are four choices of orbits within these subwindows that overfly the planned calibration sites at Bermuda and Dakar. These are at 64.80° and 1335 km, 62.01° and 1252 km, 65.84° and 1255 km, and 62.69° and 1173 km. These choices have a minimum separation between the aliased frequencies of the eight largest diurnal and semidiurnal tides of one cycle per 4.71 years, one cycle per 4.86 years, one cycle per 4.87 years, and one cycle per 4.27 years, respectively. Thus there will be less than one cycle separation over the nominal (3 years) mission. Of these four choices, the first at 64.8° and 1335 km is preferable, because it is less sensitive to error in estimating the upcoming solar cycle (which will be a maximum during the mission), and because it is more nearly centered in terms of inclination than the other choices. For other calibration sites, other choices would have to be made. Additional work, beyond the scope of this paper, will also be necessary to determine if the Topex/Poseidon mission requires a frozen orbit (i.e., an orbit for which there are no long‐term variations in the eccentricity and argument of perigee of the orbit). If a frozen orbit is required, then the first and third choices listed above have frozen orbits with an eccentricity that is acceptable for Topex/Poseidon.

Estimates of Heat Content Variations from Sea Level Measurements in the Central and Western Tropical Pacific from 1979 to 1985

Abstract A method is developed to estimate oceanic heat content (0–300 m) variations from sea level measurements in the tropical Pacific. To this end, statistical relationships between heat content and steric level, used as a surrogate variable for the sea level, are derived from climatological data. These relationships are then applied on independent datasets and the predictive ability of the method is determined regionally by comparing heat content estimated from XBT and sea level measurements recorded in three tropical Pacific islands (Christmas Fanning and Truk) during the 1979–85 period. Good qualitative agreements are found between the two heat content estimates with correlations R = 0.78 to 0.94 and rms differences of average temperature of 0.25° to 0.50°C over an observed range of 6°C. Quantitative disagreements are observed in the central Pacific during the fall 1982 (El Ninc) period. These deficiencies in the method are found to be primarily due to intense and unusual salinity fluctuations at th...