Geosat Altimetry Research Articles

Comparison of dynamic height variations in the tropical Atlantic during 1987–1989 as viewed in sections Hydrography and Geosat altimetry

During the 1980s the Soviet Union conducted a series of 25 hydrographic cruises in the tropical Atlantic under the Sections (acronym for Energetically Active Zones of the Ocean and Climate Variability) program. As one of its objectives, Sections has produced the most extensive hydrographic data set ever collected in the tropical Atlantic. Here we examine dynamic topography from seven cruises during the years 1987 through 1989 and compare them to corresponding maps of sea level from the Geosat satellite mission. The two independent estimates of sea level variation are well correlated, with the sea level fields explaining approximately half of the variance of dynamic height fields. There is a seasonal variation to the agreement, so that the correlation increases somewhat in summer and fall, as the variability of the height field increases. Wavenumber spectra of both dynamic height and sea level are quite red for wavelengths greater than 400–600 km. Smaller wavelengths cannot be resolved by the hydrographic sampling program. A comparison of the dynamic height and sea level spectra indicates that sea level has somewhat less variability than dynamic height at all wavenumbers.

Modeling interannual variability in the Indian Ocean using momentum fluxes from the operational weather analyses of the United Kingdom Meteorological Office and European Centre for Medium Range Weather Forecasts

A 16‐level general circulation model of the Indian Ocean is used to reproduce interannual variability in ocean circulation from 1987 to 1990 using wind stresses from the United Kingdom Meteorological Office (UKMO) operational forecast model analyses. The results of this integration are compared with those obtained using the Hellerman and Rosenstein [1983] (HR) climatological forcing. The differences in the two wind products are large, as are the resulting surface and subsurface currents. Model results are compared with observations at 55°E and 73°E on the equator. The undercurrent using UKMO stresses is more strongly eastward than that using HR, and in the four years studied, the model always simulates an undercurrent in February‐March at Gan (73°E), contrasting with the inconstancy of its appearance in the years sampled by the observations. Interannual differences in the surface currents can be large, as much as 100 cm s−1 in May on the equator. The years 1987 and 1988 are worthy of special consideration, since 1987 was a drought year over India and 1988 had above average rainfall. Therefore, the period 1987–1988 is repeated using stresses from the European Centre for Medium Range Weather Forecasts (ECMWF). There are large differences between the ECMWF and the UKMO stresses and, consequently, in the model currents, where differences can be as much as 80 cm s−1 over a large area of the equatorial zone and in the Somali current region. At undercurrent depth, the response at Gan (73°E) is very similar in the first half of the year in all three integrations, but differs significantly in the second half with the UKMO and ECMWF stresses inducing a semiannual response, whereas HR leads to an annual cycle. Although interannual differences are large on a month to month basis, they are not long lasting. Interannual variations in model sea surface temperatures (SST) resulting from variations in the surface stress are poorly related to observed SST anomalies. There are no direct measurements of currents in the 1987–1990 period with which to validate the model simulations. There are, however, satellite measurements of the distortion of the sea surface and so comparison is also made between the simulations of dynamic topography from the model and GEOSAT altimetry data. While there is broad‐scale similarity in pattern between the three model simulations and the GEOSAT observations, detailed agreement between the fields is poor. The model results are also compared to the dynamic topography derived from Levitus data.

Dynamic sea surface topography from GEOSAT altimetry

Abstract The so‐called integrated approach in satellite altimetry that attempts to reduce radial orbit and geoid errors due to a mismodeling of the gravity field and to determine a model for the long‐wavelength dynamic sea surface topography simultaneously is described in detail. The method is applied to two years of NOAA GEOSAT altimeter data, using a satellite orbit computed with the NASA GEM‐T2 gravity field model. The results of the analysis show that the adjusted gravity model yields significantly better orbits and an improved geoid model for the data period considered. The RMS of the altimeter sea height residuals is reduced from an initial level of approximately 147 cm to approximately 36 cm. In addition, the RMS of the crossover difference residuals is reduced from approximately 57 cm to approximately 36 cm. Moreover, an improvement of about 1 m in the GEM‐T2 geoid for wavelengths longer than 500 km is realized. The recovered global dynamic sea surface topography model is found to be in good agree...

Rossby Waves in the Pacific Ocean Extracted from Geosat Altimeter Data

Abstract Rossby waves have been detected in the northeast Pacific in time series of in situ hydrographic data, but the limited geographic coverage of such data has restricted the search for Rossby waves to small regions. Two years of continuous Geosat altimetry data were used here to extract the signatures of Rossby waves from the entire Pacific Ocean. This relatively long time series of global data presents an excellent opportunity to examine the surface expression of large-scale ocean phenomena such as Rossby waves. For this analysis the Pacific Ocean was divided into 10-degree square regions, and Rossby waves defined by the Rossby wave dispersion relation were fit to the Geosat data in each region. Rossby waves fit to each 10-degree region were separated by Fourier components in wavenumber space, with frequencies determined by the dispersion relation. Barotropic and first mode baroclinic Rossby waves were fit separately to the Geosat data. Rossby wave amplitudes from the least-squares fit were plotted ...

A comparison of monthly mean sea Level variability determined from Geosat altimetry and a global tide gauge dataset

Abstract Monthly mean sea surface heights derived from precise global orbit recomputations for the first year of the Geosat Exact Repeat Mission have been compared to tide gauge data from 57 island and 71 coastal sites. In general, the monthly means from the different techniques are correlated significantly at island stations, especially in the Tropical Pacific, and the average standard deviation of the monthly mean differences between techniques is 5 cm. This provides a conservative estimate of the current accuracy of deriving monthly mean sea surface topography time series from altimetry. Coastal gauge data in general have a weaker correlation with the nearby deep ocean altimeter data, although groups of stations in the western Pacific do exhibit significant correlation.

A study of the Iceland-Faeroe Front using GEOSAT altimetry and current-following drifters

In this study we investigate the utility of GEOSAT altimetry for monitoring the Iceland-Faeroe frontal zone. Since an expected dynamic topography relief of 10–20 cm over the Iceland-Faeroe Front (IFF) was not much above the 10 cm uncertainty in GEOSAT observations, validation by AVHRR imagery and satellite-tracked drifters constituted an important part of the experiment. Sea Surface Height (SSH) relief of greater than 20 cm occurred a long the western side of the IFF and along the eastern side, north of the Shetland Islands. However, with SSH relief of only 10–15 cm in the central region of the IFF, substantial difficulties were encountered in the ability to unambiguously monitor the location of the front. In contrast, frontal meanders with 20–30 cm SSH relief, current speeds up to 50 cm s −1 and radii of curvature of 25 km, were clearly observed on three occasions during the 2 year study. These meanders first appeared north of the Faeroe Islands, in the region from 6 to 8°W, and propagated southeastward at speeds of about 3.3 km day −1, being lost from view in the Faeroe-Shetland Channel. Their strong signals and lifetimes of 2–3 months would appear to make them important constituents of IFF dynamics.

Transport of the Norwegian Atlantic Current as determined from satellite altimetry

Relatively warm and salty North Atlantic surface waters flow through the Faeroe‐Shetland Channel into the higher latitudes of the Nordic Seas, preserving an ice‐free winter environment for much of the exterior coast of northern Europe. In this study, we have monitored this flow along the Norwegian coast using GEOSAT altimetry on two ascending arcs during the Exact Repeat Mission in 1987–1989. Concurrent undertrack CTD surveys were used to fix a reference surface for the altimeter‐derived sea surface height (SSH) anomalies, in effect creating time series of alongtrack surface dynamic height topographies. Climatologic CTD casts were then used, with empirical orthogonal function (EOF) analysis, to derive relationships between historical surface dynamic heights and vertical temperature and salinity profiles. Applying these EOF relationships to the altimeter signals, we calculated mean transports of volume, heat and salt at approximately 2.9 Sverdrups, 8.1 × 1011 KCal/s and 1.0 × 108 Kg/s, respectively. Although strong variability occurred, a clear seasonal signal could be seen with summer to winter variations of about 50% of the means. Maximum transports occurred in February/March and minimum in July/August.

Mesoscale Variability in the Atlantic Ocean from Geosat Altimetry and WOCE High-Resolution Numerical Modeling

Characteristic of the mesoscale variability in the Atlantic Ocean are investigated by analyzing the Geosat altimeter signal between 60°S and 60°N. The rms sea-surface variability for various frequency bands is studied, including the high-frequency eddy-containing band with periods <150 days. Wavenumber spectra and spatial eddy characteristics are analyzed over 10° by 10° boxes covering both hemispheres of the Atlantic Ocean. A comparison, with solutions of a high-resolution numerical experiment, developed as the Community Modeling Effort of the World Ocean Circulation Experiment, aids interpretation of the Geosat results in the tropical and subtropical Atlantic and provides a test of the model fluctuating eddy field. Results from Geosat altimetry show a wavenumber dependence close to k1−5 (k1 being the alongtrack wave-number) over almost the entire Atlantic Ocean except for areas in the tropical and subtropical Atlantic where the rms variability in the eddy-containing band is less than 5 cm, that is, not significantly different from the altimeter noise level. Characteristic eddy length scales inferred from Geosat data are linearly related with the deformation radius of the first baroclinic mode over the whole Atlantic Ocean, except for the equatorial regime (10°S to 10°N). The data-model comparison indicates that the high-resolution model with horizontal grid size of ⅓° and ° in latitude and longitude is quite capable of simulating observed eddy characteristics in the tropics and subtropics. In mid- and high latitudes, however, the model fails to simulate the pronounced poleward decrease in eddy scales. This leads to systematic discrepancies between the model and Geosat observation, with model scales being up to 50% larger than deduced from altimetry.

Seasonal variations in the western tropical Atlantic: Surface circulation from Geosat altimetry and WOCE model results

Sea level variations and geostrophic circulation in the western tropical Atlantic are studied in an intercomparison of Geosat altimetry and the World Ocean Circulation Experiment community model effort high‐resolution model forced with climatological windstress. Overall, the annual cycles of geostrophic current fields of both products compare very well. Special comparison areas are the western North Equatorial Countercurrent (NECC) and the North Brazil Current (NBC) region. Meridional profiles of zonal velocity anomalies show a seasonal meridional migration of the NECC core centered at 5°N and a weaker eastward maximum during fall at 9°N in both products. The Geosat and model seasonal cycles of the NECC core velocity in the region 35°–45°W are highly correlated and agree with respect to the onset of eastward current acceleration and deceleration in May and December, respectively. Geosat time series from November 1986 to June 1989 show year to year differences, in particular an anomalous early NECC acceleration phase in 1987. In the NBC region 54°–58°W, flow anomalies from both Geosat and the model have two westward maxima, in March and June, which appear to be associated with eastward anomalies further offshore.

Circulation in the Gulf of Mexico from Geosat altimetry during 1985–1986

Using altimetry data obtained from the Geosat Geodetic Mission (April 1985 to October 1986), low‐frequency sea surface height (SSH) variations are investigated in the Gulf of Mexico. SSH time series are formed using the method of Fu and Chelton and are used to calculate surface geostrophic current vectors. Spatial patterns of SSH and current vector variations enable the tracking of two major rings shed from the Loop Current. The rings drifted southwestward across the gulf and into the western boundary region at an average speed of about 3.4 cm/s. The buildup of the Loop Current was monitored, as well as the appearance of an eddy of uncertain origin in the southwestern gulf. Verification of the Geosat results are provided with surface drifters, AVHRR imagery, and hydrography.

Estimation of sea surface topography in the Norwegian sea using gravimetry and Geosat altimetry

An analysis of the quasi-stationary sea surface topography (QSST) is carried out in the Norwegian Sea region (54°<o<72°, -25°<λ<20°) using marine gravimetry and one year of Geosat ERM altimetry. As reference models the geopotential model OSU91A and the QSST model OSU89D were used. Two procedures to extract the QSST from mean sea surface heights and gravity anomalies were tested. Spherical FFT techniques were applied in both procedures. The results show that QSST associated with wavelength shorter than 4000 km exists. Relative to the OSU89D model the QSST was found to have a variance of (0.219 m)2 and a correlation length of 1.105°. The circulation pattern recovered in this paper agree with results of oceanographic analysis.

Frontal structure and transport of the Antarctic Circumpolar Current in the south Indian Ocean sector, 40–80°E

Based on recent conductivity-temperature-depth (CTD) data at the Kerguelen-Amsterdam passage plus historical expendable bathy thermograph (XBT) and hydrographie data from the Crozet Basin, the frontal structure and volume transport of the Antarctic Circumpolar Current (ACC) in the south Indian Ocean sector are described. Recent findings on sea-level variability from GEOSAT altimeter data are reviewed as well as barotropic transport variability from bottom pressure measurements at Kerguelen and Amsterdam Islands. The ACC here is mostly concentrated in a narrow frontal zone of 2–3° of latitude, without any noticeable multi-band structure, as a result of the confluence of the Subantarctic and Subtropical Fronts. The Polar Front in this sector is not associated with any prominent current core. This regional particularity of the ACC was further supported by GEOSAT altimetry data. About 80% of the ACC transport (106Sv; 1 Sv=10 6 m 3s −1) passes north of Kerguelen, concentrated along the northern flank of the Kerguelen Plateau; only 30 Sv passes south of Kerguelen. The bottom-pressure-derived barotropic transport variability at Kerguelen-Amsterdam passage was estimated to lie between 10 and 30 Sv, consistent with findings at Drake Passage. It is suggested that the study area constitutes a prominent ACC observatory during the WOCE (World Ocean Circulation Experiment).

Mean flow and variability in the Kuroshio Extension from Geosat altimetry data

Using altimeter data from the Geosat Exact Repeat Mission (ERM), we investigated the mean flow and temporal and spatial variations of the Kuroshio Extension in the region of 140°–180°E and 30°–40°N. Mean surface height profiles were estimated along individual tracks by assuming the velocity profile of the Kuroshio Extension to be Gaussian‐shaped and by successively fitting this synthetic current's height profile to the residual height data. Using the mean profiles from ascending and descending tracks, we derived the mean surface height field by an inverse method and obtained the absolute surface height fields for the first 2.5 years of the Geosat ERM. Both the mean and the instantaneous height fields thus derived compared well with the available hydrographic data and the SST patterns from the NOAA satellites. The mean surface height difference across the Kuroshio Extension attains its maximum around 146°E between the two quasi‐stationary meanders, and its decrease thereafter is mainly due to large‐scale recirculations on the southern side of the Kuroshio Extension. The ratio of the eddy kinetic energy over the mean kinetic energy has a nearly constant value of 1.5–2.0 along the Kuroshio Extension path. Propagation of mesoscale fluctuations in the height fields is generally westward except for the upstream region of the Kuroshio Extension. Effects of deep mean flow and baroclinic shear are found to be important in explaining the observed propagation speeds. In the upstream region of 141°E and 154°E, annual variations in the surface height difference across the Kuroshio Extension(δh) have a September maximum with an average amplitude of 0.2m. For large‐scale interannual fluctuations, anomalies in δh are found to be significantly correlated with those of the current axis positions: a larger surface height difference corresponds to a more northerly position of the Kuroshio Extension. The interannual changes in δh are possibly related to the 86/87 ENSO event in the low‐latitude Pacific Ocean.

Energetics of global ocean tides from Geosat altimetry

The authors' global ocean tide maps derived from Geosat altimetry are here improved in quality and coverage; they compare favorably with ground truth data at about the same rms level as Schwiderski's models, slightly better in lunar than in solar tides. Diurnal admittances clearly show Kelvin wave structure in the southern ocean and confirm Platzman's resonant mode at 28.5±0.1 hours with an apparent Q of about 4. Driving energy enters dominantly in the North Pacific for the daily tides and is strongly peaked in the tropical oceans for the half‐daily tides. Global rates of working on all major tide constituents except S2 agree well with independent results from analyses of gravity through satellite tracking. Comparison at S2 is improved by allowing for the air tide in gravitational results but suggests deficiencies in all solar tide models. Estimates of Q factors based on potential energy (approximately 8 for daily tides and 13 for half‐daily tides) agree reasonably well with hydrodynamic models.

Examining several Southern Ocean data sets

The Southern Hemisphere ocean covers approximately 45% of the Earth's surface and poses a major scientific challenge to initiatives such as the World Ocean Circulation Experiment (WOCE) and the Joint Global Ocean Flux Study (JGOFS) programs, whose goals include elucidation of its deep circulation and its contributions to global bio‐geochemical budgets. The Southern Ocean is generally regarded as the region south of about 30°S and, in many respects, is quite different from its northern counterpart. Unlike the oceanic and atmospheric flows in the Northern Hemisphere, the Circumpolar Current “Nowlin and Klinck, 1986” and surface winds have relatively unobstructed circulations around Antarctica that allow the exchange of water between basins and result in the most intense wind forcing in the global ocean.An analysis of the annual cycle of sea level using Geosat altimetry has shown that seasonal changes in the surface ocean circulation are much weaker in the Southern Hemisphere than in the Northern Hemisphere. Also, comparison of the pigment fields obtained from the Nimbus‐7/Coastal Zone Color Scanner (CZCS) indicates that the biology of the northern and southern oceans is quite different in that the northern oceans exhibit a pronounced spring bloom, while the Southern Ocean is relatively constant the year round “Esaias et al., 1986; Feldman et al. 1989”. The cause of this dissimilarity is a topic of some debate, with various arguments focusing on differences in trace nutrient supply, physical forcing, and illumination.

The tectonic evolution of the South Atlantic from Late Jurassic to present

An improved tectonic database for the South Atlantic has been compiled by combining magnetic anomaly, Geosat altimetry and onshore geologic data. We used this database to obtain a revised plate-kinematic model. Starting with a new fit-reconstruction for the continents around the South Atlantic, we present a high-resolution isochron map from Chron M4 to present. Fit reconstructions of South America and Africa that require rigid continental plates result in substantial misfits either in the southern South Atlantic or in the equatorial Atlantic. To achieve a fit without gaps, we assume a combination of complex rift and strike-slip movements: 1. (1) along the South American Parana-Chacos Basin deformation zone, 2. (2) within marginal basins in South America (Salado, Colorado Basin) and 3. (3) along the Benue Trough/Niger Rift system in Africa. These faults are presumed to have been active before or during the breakup of the continents. Our model describes a successive “unzipping” of rift zones starting in the southern South Atlantic. Between 150 Ma (Tithonian) and approximately 130 Ma (Hauterivian), rifting propagated to 38 °S, causing tectonic movements within the Colorado and Salado basins. Subsequently, between 130 Ma and Chron M4 (126.5 Ma), the tip of the South Atlantic rift moved to 28 °S, resulting in intracontinental deformation along the Parana-Chacos Basin deformation zone. Between Chron M4 and Chron MO (118.7 Ma) rifting propagated into the Benue Trough and Niger Rift, inducing rift and strike-slip motion. After Chron MO, the equatorial Atlantic began to open, while rifting and strike-slip motion still occurred in the Benue Trough and Niger Rift. Since Chron 34 (84 Ma), the opening of the South Atlantic is characterized by simple divergence of two rigid continental plates.

Geophysical Applications of Satellite Altimetry

Publications related to geophysical applications of Seasat and Geosat altimetry are reviewed for the period 1987-1990. Problems discussed include geoid and gravity errors, regional geoid heights and gravity anomalies, local gravity field/flexure, plate tectonics, and gridded geoid heights/gravity anomalies.

Loop current eddy shedding estimated using Geosat altimeter data

A method for extracting periodic signals from altimeter data is applied in the Gulf of Mexico using data from the first two years of the Geosat Exact Repeat Mission. The least squares fitting of selected frequencies of sine and cosine functions to the time series of sea surface height at each point of the Geosat ground track is used to estimate the dominant frequency in the gulf for this time period. Optimal interpolation of the sine and cosine coefficients at the dominant frequency to a uniformly spaced grid allows reconstruction of the time series in the gulf and shows eddies shedding from the Loop Current with a period of approximately 10.5 months, consistent with shedding periods determined from numerical models and analysis of satellite infrared data. This technique is a novel method for investigation of periodic signals in Geosat altimetry and is not severely affected by the data outages present in the time series.

Eddy Variability in the Kuroshio Extension as Revealed by Geosat Altimetry: Energy Propagation Away from the Jet, Reynolds Stress, and Seasonal Cycle

Altimetric sea level data around the Kuroshio Extension, collected during the first year of the Geosat Exact Repeat Mission (ERM) (starting in November 1986), have been analyzed for energy propagation away from the jet, the Reynolds stress pattern, and the seasonal cycle of the eddy field. Space-time diagrams and wavenumber-frequency spectra are employed to discern the dominant direction of phase propagation, revealing westward and inward phase propagation towards the jet (i.e., energy propagation away from the jet), which is consistent with the notion that the widespread eddy variability from the jet is the result of transient Rossby wave response to forcing provided by growing, pulsating, and decaying meanders of the jet. It can be shown that any theory based on the linear instability cannot normally account for the energy propagation, because any linear instability that is wavelike away from the jet is by necessity not the fastest growing linear instability. Rather, a nonlinear instability mechanism is needed, whose nonstationary finite amplitude behaviors can excite Rossby waves away from the jet. The computed Reynolds stress pattern is consistent with previous observation indicating that, on the whole, the jet is reinforced, rather than weakened, thus favoring the baroclinic instability as the dominant source for meandering growth. A seasonal cycle in the intensity of both the eddy field and the surface transport of the Kuroshio Extension is found, which is weakest during winter and progressively strengthens from spring to fall.

Sea level variabilities in the Gulf Stream between Cape Hatteras and 50°W: A Geosat study

Sea level residuals relative to a 2‐year mean sea level in the Gulf Stream downstream of Cape Hatteras (between 80°W and 50°W longitude) are studied using Geosat altimetry between November 1986 and December 1988. Spectral slopes computed at four nominal locations along the Gulf Stream (72°W, 65°W, 60°W, and 55°W) range between −1.5 and −0.9, significantly less steep than the −2.5 spectral slope found at Bermuda. Because of increased short‐wavelength energy, the spectra tend to become more “white” going eastward, but this trend is not statistically significant. Energy at the annual cycle decreases eastward from around 40% of the total variance at 72°W to 20% of the total variance at 55°W. Maximum residual geostrophic sea surface velocities associated with the annual cycle were of the order of 70 cm/s. Comparison between the Gulf Stream path from sea level residual maps and the north wall edge observed from NOAA infrared (IR) imagery shows an rms difference of 49 km between 75°W and 60°W longitude, but not all sea level maps show a clearly defined Gulf Stream. Using the IR estimates for Gulf Stream position, sea level residuals were modeled as the meandering of a sea level front shaped as 0.5 m · tanh {(X–X0)/[50 km · sin (α)]}, with α the angle between the Geosat track and the Gulf Stream edge and X0 is based on the observed IR north wall positions. This showed that the annual signal, in terms of the location and phase of its maxima, is dominated by the meandering of the Gulf Stream.