Sea Surface Height Difference Research Articles

Satellite altimeter monitoring the Kuroshio Transport south of Japan

In 1993–1995, we carried out observations of the Kuroshio south of Japan, including direct current measurements and repeated hydrographic surveys along a satellite track of the TOPEX/POSEIDON altimeter. The velocity field of the Kuroshio is determined by geostrophic calculation using the repeated hydrographic survey data, referenced to velocities observed at mid and abyssal depths. The volume transport of the Kuroshio is estimated from this velocity field. The estimated transports of the Kuroshio have a high correlation with sea‐surface height differences across the Kuroshio. Having this relationship and using the altimeter data, we obtained a time series of the Kuroshio transport over seven years at ten‐day intervals. The Kuroshio transport, excluding contributions by local recirculations, is estimated to be 42 × 106 m³/sec on average. The correlation between sea‐surface height difference and transport provides a practical method of long‐term monitoring of the Kuroshio transport using satellite altimetry.

A diagnostic study of the Indonesian Throughflow

The objective of this study is to derive estimates of the mean seasonal variations in the Pacific to Indian Ocean throughflow transport using climatological data of the temperature/salinity fields and surface wind stress. A variable grid global general circulation model (GCM) with 1/6° resolution in Indonesian seas configured with ETOP05 bottom topography is utilized to diagnose the three‐dimensional velocity field. An extensive set of sensitivity experiments is conducted to obtain estimates of the error bars. The computations show that Indonesian Throughflow (ITF) is maximum in boreal summer (13.9±1.3 Sv) and minimum in boreal winter (7.5±1.1 Sv). The annual mean ITF transport amounts to 11.5±1.1 Sv (1 Sv = 106 m3 s−1). These values are within the range of various estimations of ITF transport, and they agree in phase. The largest seasonal variations of the transport occur in the upper 175 m of the water column, exhibiting a 8 Sv amplitude of the seasonal cycle. This causes a substantial variability in the heat transport from the Pacific to the Indian Ocean. Its value increases from 0.4±0.1 PW in January to 1.1±0.1 PW in August (1 PW = 1015 watts). In contrast to previous GCM‐based studies, the annual mean transport is found to be more evenly distributed among the three major outlets from the Indonesian seas (Lombok, Ombai, and Timor Straits). The Luzon Strait inflow gives a considerable contribution to ITF. In winter the inflow of 6.3±1.5 is distributed between the two outflows from South China Sea, which follow the routes through Karimata (4.4±0.5) and Mindoro (1.9±1.5 Sv) Straits. In summer the Karimata pathway is blocked, and the net inflow of 4.7±0.6 Sv exits through the Mindoro Strait west of southern Luzon. The total contribution of the Luzon Strait inflow to the net ITF transport varies from 85% in boreal winter to 35% in summer, with an annual average value of ∼50%. Cross‐correlation analysis of the TOPEX/Poseidon monthly mean sea surface height (SSH) anomalies and the diagnosed transport anomalies demonstrates high correlation of the mean SSH difference between the regions in western Banda Sea and north of Luzon with the heat (0.96) and volume (0.91) transports.

Seasonal Variations of Sea Surface Height in the Gulf Stream Region*

Abstract Based on more than four years of altimetric sea surface height (SSH) data, the Gulf Stream shows distinct seasonal variations in surface transport and latitudinal position, with a seasonal range in the SSH difference across the Gulf Stream of 0.14 m and a seasonal range in position of 0.42° lat. The seasonal variations are most pronounced west (upstream) of about 63°W, near the Gulf Stream’s warm core. The changes in the SSH difference across the Gulf Stream are successfully modeled as a steric response to ECMWF heat fluxes, after removing the large SSH variations due to seasonal position changes of the Gulf Stream. A phase shift between predicted and observed SSH changes in the Gulf Stream suggests that advection may be important in the seasonal heat budget. Consistent with the interpretation of SSH variations as steric, comparisons with hydrographic data suggest that the fall maximum SSH difference is from the upper 250 m of the water column. The maximum volume transport is in the spring. Zonal...

Estimation of the sea state bias in radar altimeter measurements of sea level: Results from a new nonparametric method

The sea state bias (SSB) affecting altimetric measurements of the sea surface height (SSH) is classically estimated using empirical parametric models. The model parameters are determined to minimize the variance of the SSH differences at crossover points or along collinear tracks. It is shown here that so‐calibrated models are not true least squares approximations of the SSB. Simulations indicate that the difference from a true least squares solution is typically a few millimeters to a few centimeters, varying with sea state. This difference proves to be the result of the inevitably imperfect specification of the model's parametric form, which corrupts the calibration process when performed on SSH differences rather than directly on SSH measurements. To avoid specification of a parametric form, we present a new nonparametric version of the SSB estimation problem and propose a general solution based on the statistical technique of kernel smoothing. This solution is then used to obtain the first fully nonparametric estimate of the TOPEX altimeter SSB as a function of both the wind speed and the wave height. The obtained estimate, though based on a limited data set, proves to be better than those obtained with the standard parametric SSB models featured in the TOPEX‐POSEIDON Geophysical Data Records. The improvement is most significant in middle‐ and high‐latitude oceans.

Local crossover analysis of exactly repeating satellite altimeter data

The crossover adjustment plays a central role in the processing of satellite altimeter measurements. The usual procedure is to form sea surface height differences at crossover points, solve for the radial orbit error (with due attention to the singular nature of the estimation problem) and then to construct altimetric sea-level maps using the mean sea surface heights at the crossover points. Our approach is very different, to make direct use of measurements at crossover points without differencing and to estimate simultaneously orbit parameters, mean sea surface height and sea surface height variability in a single, unified adjustment. The technique is suited for repeat data over an area small enough that adjoining passes may be considered to be parallel and to permit the solution of a set of linear equations of dimension equal to the number of crossover points. The size of the numerical problem is almost independent of the number of repeat cycles of the altimeter mission. Explicit recognition is given to the rank defect of the least-squares estimation problem; we show that, for an orbit model with r parameters, the rank defect of the local crossover problem is exactly r2. The defect may be overcome by choosing an appropriate set of constraints – either giving a best fit of mean sea surface heights to a reference surface, or minimising orbit parameters, or a minimum norm solution in which both mean sea surface heights and orbit parameters are minimised. There is no need to choose a reference pass, all passes are treated equally and data gaps are easily accommodated. Numerical results are presented for the south-western Indian Ocean, based on the first 2 years of altimeter data from the Geosat Exact Repeat Mission.

On the temporal variability of the transport between Amsterdam and Kerguelen islands

Data from over 3 years of bottom pressure recording together with Geosat altimetry and the Fine Resolution Antarctic Model numerical ocean model have been used to investigate the temporal variability of the Antarctic Circumpolar Current between Amsterdam and Kerguelen islands in the southern Indian Ocean. An upper limit to the standard deviation of the variability in transport derived from pressure differences has been calculated to be 10–15 Sv, subject to an assumption of barotropic flow dominating the variability and after the application of a 31‐day low‐pass filter. This is slightly larger than the variability obtained in a similar experiment at the Drake Passage a decade ago. Although the seasonal cycles in pressure difference and in sea surface height difference measured by altimetry are different each year, their means are consistent with the presence of a semiannual component with an amplitude in terms of (assumed barotropic) transport of the order of 5 Sv, which is comparable to the Drake Passage findings. In addition, the pressure difference data contain a large, quasistationary annual component of amplitude of approximately 5 mbar, a possible source of which is different seasonal steric fluctuations at the two sites. Insofar as the bottom recorder data can be considered ground truth for the altimetry, results from the different techniques show some features in common, although significant differences between data sets remain, for which possible explanations are offered. It is concluded that when they are used for monitoring transport fluctuations, bottom pressure recorders should be accompanied by inverted echo sounders and comprehensive hydrographic measurements.

Determining the mean Gulf Stream and its recirculations through combining hydrographic and altimetric data

The altimetric data from the first 2.5‐year Geosat Exact Repeat Mission were used to estimate the mean sea surface height (SSH) field in the region of the Gulf Stream and its recirculation gyres. Assuming the instantaneous surface velocity field is composed of an eastward flowing jet and two westward recirculating flows, we used the time‐varying surface data from the altimeter to determine the shape of the along‐track mean SSH profiles and the historical hydrographic data to constrain the net SSH difference across the Gulf Stream system. The two‐dimensional mean SSH field was determined by objectively mapping the mean height profiles along the ascending and descending tracks. The SSH jump across the mean Gulf Stream has a maximum of 1.15m around 65°W and drops to an almost constant 0.9m downstream of the New England Seamount Chain (NESC). While the SSH jump associated with the mean northern recirculating flow is mostly uniform, we found that the Gulf Stream's southern recirculation has two local gyres that are separated by the NESC. An attempt was then made to estimate the mean deep circulation in this region by comparing the mean SSH field derived from the altimetry data and the surface dynamic height field based on the historical hydrographic data. Despite the large uncertainties, the mean deep flow pattern thus estimated agrees favorably with the overall circulation pattern from the long‐term current meter observations. Like the well‐defined northern recirculation gyre, we found that a continuous southward flow exists along 57.5°W, which follows closely along the deep layer potential vorticity contours. To the south of the Gulf Stream, the deep circulation consists of two separated recirculation gyres; the recirculation gyre to the east of the NESC appears to be trapped around the Corner Rise.

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.

Estimating the Surface Transport of Meandering Oceanic Jet Streams from Satellite Altimetry: Surface Transport Estimates for the Gulf Stream and Kuroshio Extension

Abstract A method has been developed to detect the location, width maximum velocity, and suffice transport of a meandering oceanic jet stream from satellite altimetry. Ale method is based on a nonlinear least squares scheme, in which Gaussian-shaped velocity profiles are fitted to explain the observed current variability in the meandering region. The method has been applied to 25 Geosat tracks in the Kuroshio Extension and Gulf Stream for the first you of the Geosat Exact Repeat Mission. A reasonably smooth circulation pattern has emerged for the surface transport for both currents (at opposed to the widely varying direct transport estimates). After leaving the coast, both currents have surface strength of the order of 90 × 103 m2 a−1(equivalent to a 90 cm height jump, i.e., the sea surface height difference across the jet). They continue to gain strength downstream until a maximum of the order of 130 × 10 3 m2> a−1 is reached (near 150°E for the Kuroshio and 63°W for the Gulf Stream). The Gulf Stream the...

Precision orbit determination for the Geosat Exact Repeat Mission

Precise ephemerides have been determined for the Navy's Geodesy Satellite (Geosat) Exact Repeat Mission (ERM) using an improved gravity field model, PTGF‐4A (Shum et al., 1989). The Geosat orbits were computed in a terrestrial reference system which is tied to the reference system defined by satellite laser ranging (SLR) to LAGEOS through a survey between the TRANET Doppier receiver and the SLR system located at Wettzell, Federal Republic of Germany. The remaining Doppler tracking station coordinates were estimated simultaneously with the geopotential in the PTGF‐4A solution. In this analysis, three continuous 17‐day Geosat orbits, which were computed using the 46‐station TRANET data and global altimeter crossover data, have a crossover residual root‐mean‐square (rms) of 20 cm, indicating that the Geosat radial orbit error is of the order of 20 cm. The orbits computed based on data collected by a 7‐station OPNET tracking network and crossover data have the same level of accuracy. Collinear sea surface height analysis using the improved orbits for two Geosat 17‐day repeat cycles yields an rms height difference of 16 cm. Spectral analysis of the sea surface height difference indicates that the long‐wavelength orbit error has been reduced significantly. Finally, the Geosat altimeter time tag bias, based on 54 days of the Geosat ERM altimeter crossover data, is 5±3 ms.

Global nondynamic orbit improvement for altimetric satellites

The largest source of error in satellite altimetry is in the radial position of the satellite. Radial orbit errors of more than a few decimeters prohibit basin‐scale studies of sea surface height variability. We explore nondynamic techniques for reducing this error. Sea surface height differences at intersections of satellite altimeter profiles (crossover data) provide a strong constraint on radial orbit error but do not uniquely define it. The portion of orbit error that is a function of latitude and longitude only produces no crossover differences and therefore cannot be recovered with crossover data. Using mathematics (inclination functions) originally developed for satellite dynamics, we determine the entire class of orbit error functions not recoverable with crossover data. These functions are mappings of surface spherical harmonics into the orbit plane. For example, the l = 1, m = 0 surface harmonic maps into sinusoidal orbit error with a frequency of once per orbit. Nonzonal harmonics map into linear combinations of three or more frequencies that are linked by the inclination functions. Between frequencies of 0 and 2.2 cycles per orbit there are nine orbit error components that cannot be recovered using crossover data. These components are uniquely defined, however, by nine globally distributed radial tracking points. Fewer tracking points are sufficient if a smoothness criteria is applied to the orbit correction curve. Our findings suggest that radial orbit error can be significantly reduced by including a few globally distributed radar reflectors (or transponders) in the tracking network.

Seasat altimeter observations of dynamic topography in the Gulf Stream Region

Individual Seasat altimeter profiles in the western North Atlantic have been differenced with the best available geoid model to remove the gravitational component. The resulting sea surface height profiles compare remarkably well with independent oceanographic observations. The Gulf Stream is clearly apparent in each profile as a 1‐ to 2‐m step, and known positions of cyclonic and anticyclonic rings correspond with depressions and elevations, respectively, with amplitudes as large as 95 cm. Some of the most important altimeter data for analyzing the dynamic ocean signal were gathered during the last month of the mission when the same ground track was repeated every 3 days. These data allow detailed examination of time‐varying ocean phenomena, since the gravitational component is time‐invariant. One set of collinear passes clearly shows a cyclonic ring as it moves out from underneath the spacecraft's track, an event which was simultaneously observed with the aid of a satellite‐tracked surface buoy. Another striking feature seen in the altimeter data set is the apparent variability exhibited by the Gulf Stream. On time scales of a few days, surface transport indicated by the sea surface height difference across the stream varied by nearly 30%, and over the entire 3‐month period much larger fluctuations were observed, suggesting significant changes in total mass transport. Altimetry may provide an effective means of determining the time and space scales associated with these variations.