FEATURE
SATELLITE ALTIMETRY: OBSERVING OCEAN VARIABILITY FROM SPACE
By Lee-Leung Fu, Dudley B. Chelton and Victor Zlotnicki
DIRECT measurement of large-scale ocean determination techniques, the height of the sea sur- currents with current meter arrays is difficult and face relative to the same ellipsoid (the sea level) can costly on basin scales. Since large-scale currents are be obtained by subtracting the altimeter range very nearly in geostrophic balance, their velocity can measurement h from H (Fig. 1). The dynamic sea be calculated from the pressure gradient on an surface elevation hj is then the difference between equigeopotential surface. The surface geostrophic the sea level and the geoid height h,. The magnitude current therefore can be calculated from the devia- of dynamic sea surface elevation ranges from 10cm tion of sea level from the equigeopotential at the (gym-scale currents) to over l m (western boundary ocean surface (the marine geoid). Measuring sea currents and eddies), with variance spread over a level from space by satellite altimetry thus offers a wide range of wavenumbers and frequencies. The unique opportunity for determining the global sur- required measurement accuracy is thus dependent on face geostrophic ocean circulation and its variability. the phenomenon of interest. Generally, an overall Coupled with knowledge of the geoid and the ocean accuracy of better than 10cm is required for observa- density field, satellite altimetry provides the only tions of the dynamic sea surtace elevation to be feasible approach for determining absolute geostro- useful. Present knowledge of the global geoid is phic currents in the global ocean. Repeated altimetric accurate to this level only at scales longer than about observations of sea level at the same locations can 5,000km. At shorter scales, present applications of resolve the variability of surface geostrophic cur- altimetry are therefore limited to the study of tempo- rents, without the requirement for an accurate geoid ral variability of ocean currents obtained from re- model. In this paper, we briefly introduce the tech- peated altimeter measurements at the same locations, nique of satellite altimetry, highlight progress made thus allowing removal of the time-invariant geoid. in the past, assess the challenges that lie ahead, and The accuracy of the sea level measurements then discuss future plans. becomes the primary issue. Uncertainty in the al- The Technique timetric measurement of sea level results from errors A satellite altimeter is a radar that precisely in the satellite range measurement h and the orbit measures the range from the radar antenna to the height H. ocean surface. A schematic summary of altimeter Errors in h can be ascribed to three sources as measurements and the corrections that must be ap- indicated in Fig. 1: instrumental errors, atmospheric plied to obtain dynamic sea surface elevation is refraction, and the interaction between the radar shown in Fig. 1 (p. 6). The altimeter transmits a short pulse and the air-sea interlace. A thorough discus- pulse of microwave radiation with known power sion of the elements of instrumental error is given by toward the sea surface at satellite nadir (the point Chelton (1988). Briefly, instrumental errors consist directly beneath the satellite). This pulse interacts of a random part and a systematic (long-wavelength) with the sea surface, and part of the incident radiation part. The random component is often referred to as reflects back to the satellite, giving the range from the the instrument precision. Since the earliest space- 2-way travel time of the pulse. Near-surface wind borne altimeter flown on Skylab in 1973, the instru- speed and significant wave height also can be deter- ment precision for 1-sec averaged data has improved mined from the power and shape of the returned from about 60cm, which barely resolved the sea signal. surface slope across the Gulf Stream, to better than If the orbit height H of the satellite relative to a 4cm, which is sufficient to detect even small ampli- reference ellipsoid is known from precision orbit tude eddies in the ocean (Table l, p. 10). Unlike random errors, the systematic component of instru- Lee-Leung Fu, Jet Pr~qmlsi(m Laboratoo'. Col(l?,'nia Instttute mental error cannot be reduced by along-track aver- ,!f Te('hmdogy. Pasadena. CA 9ll09: Dudley' B. Chelton, Col- aging of the data. These systematic instrumental lege of Oceano,waphy, Oregon State University: Corvallis, OR errors are still poorly understood and remain a chal- 97331: Victor Zlotnicki, .let Propulsion Laboratory: California lenge to altimeter engineers. Institute of Technolow. Pasadena, CA 91109.
4 OCEANOGRAPHY.NOVEMBER.1988 g r • %
i • u ~.
A. ..
5:' f : ~"
Figure 3. Upper Panel: Difference bem'een mesoscale sea level variability during the northern hemisphere winter (December. January, February) and the annual average shown on the back cover. Differences are displayed in terms of the square root of the 1 °gridded variance differences, smoothed by a 9 ° square median filter. Positive (negative) values correspond to anomalously high (low) wintertime eddy energy. Lower Panel: Same as the upper panel, except for the southern hemisphere winter (June. July. August). Data for both panels are from the first year of the Geosat exact repeat mission (December 1986-November 1987).
OCEANOGRAPHyoNOVEM BER. 1988 5 Uncertainty in h introduced by atmospheric re- )~ Satelhte fraction result from decreases in the speed of light mainly due to dry gases (primarily oxygen), water vapor, and ionospheric free electrons along the propa- I I H ~nStr urnent Car rectlons gation path of the radar pulse. The range error due to
refraction by dry gases is large (approximately • j''.:r,,~ ]j ~jmr, Atmospheric Refrochon • L ,, ,m,, J,:~, 230cm), but can be accurately corrected with knowl- Correchons ~ , " 'r't'mr') r'l [ 'rt"' edge of sea level atmospheric pressure obtained from • ^l' ' .'l ' I External Geophysical Corr_ct,ons meteorological models. The atmospheric water va- por content, however, cannot be predicted by mete- A~r sea / • ,r ' ,, ." orological models with sufficient accuracy for cor- [nterfoce ~[orrecth}ns 1 recting the water vapor induced range error, which • ,i¢,,,, : , .L ~ hd= H-h-hg has a magnitude of 5-40cm. Accurate concurrent ~.~.~_...... _..~.--~1~, ~-'~'Z~%..~ ~-"-~-.~ .Sea Surf .... measurement of atmospheric water vapor from a
passive microwave radiometer is required for this n :' ....
correction (Tapley et al., 1982b). Similarly, present Topocjrophy ionospheric models are not sufficiently accurate for correcting the ionospheric range error, which has a Fig, 1
magnitude of 0.2-20cm. Direct measurement of the Figure 1. ,4 .schdttldtid ,~llt?ltlltll~V ¢d'ell[llllCtel DIC
6 O~"EA N OGRAPH Y -N ('W EM BER- I ~188 30 I I ' I I I ' I I I I I I ~T~T''Ill '' I detailed wavenumber-frequency characteristics of the transfer function between sea level and atmos- 20 pheric pressure loading are poorly known, and they !o are likely to vary geographically. The accuracy of the inverse barometer correction is thus limited by un- 0 certainties in both the actual atmospheric pressure -10 and in the transfer function between sea level and atmospheric pressure loading. I -20 It is apparent from the above summary that the i -30 I,Ilil:ll III, : !1 I!1.:,111 basic principle of satellite altimetry is deceptively i J FMAM J J ASOND J FMAMJ J ASONDJ FMAM J JASON straightforward. The procedure for attaining highly 1985 1986 1987 Fig 2 accurate measurements of sea level is quite complex, requiring a multidisciplinary approach. None of the 100 I previous nor existing altimeter missions have imple- mented the full suite of corrections discussed above. ,/ Many of the corrections that have been applied are based on relatively crude models and inaccurate observations. Notwithstanding these deficiencies, tremendous progress has been made in application of altimeter data to study the circulation of the ocean. Progress g Applications of satellite altimetry have been re-
I viewed by Fu (1983) and Douglas eta/. (1987). The b- e (,,9 key factor that has allowed progress in the use of z altimetry to study ocean circulation, despite sizable procedure laJ errors in the measurements, is the recognition by for attaining \+ /J oceanographers that there is a separation of scales between signals and errors. For example, the orbit highly accurate error, which is the dominant error in altimetric sea measurements of level measurements+ has a predominant length scale of one orbit revolution (about 40,000kin). For the sea level is quite study of features such as mesoscale eddies+ which 1600 . / . / ' /.--" ~ /./~ 10~ have much shorter along-track scales, the orbit error complex, requiring a can be treated as a secular trend and removed by least multidisciplinary squares procedures without affecting the signals of 272 136 68 34 interest. Moreover, many of the altimetry errors (e.g., approach. PERIOD (days) tides, water vapor range delays, inverse barometer effects, EM bias, etc.) have length scales longer than and lm, respectively (Tapley et al., 1982a). The the oceanic mesoscale and can therefore also be effects of solid earth tides, including the loading of separated from signals associated with mesoscale the ocean, can be modeled with an accuracy of better variability. As a result, mesoscale eddies have been than lcm. The uncertainty of present ocean tide the phenomenon in the ocean most studied from models is at least a factor of ten times larger. How- satellite altimeter data. ever, estimates of ocean tides with centimetric accu- The instrument precision of the Skylab altimeter racies can be obtained from an altimeter flown in a was inadequate to detect unambiguouslyeven the sea carefully chosen orbit that samples the ocean in such level variability associated with the Gulf Stream. a way that the aliased tidal frequencies are well The first oceanographically useful altimeter meas- separated from each other and from the zero and the urements were made by the Geos-3 Mission (April annual frequencies (Parke et al., 1987). 1975-December 1978), which gave continuous cov- The sea level response to atmospheric pressure re- erage only in the western North Atlantic. In spite of sults from the downward force on the sea surface due crude precision and accuracy, the Geos-3 data have to the mass of the overlying atmosphere. At certain been used to produce a map of Gulf Stream mesos- frequencies and wavenumbers, the ocean responds to tale variability that compares well with historical sea level atmospheric pressure in an "inverse ba- ship observations (Douglas et al., 1983). Fu et al. rometer +` sense: the sea level moves up and down by (1987) demonstrated that a seasonal signal in the about l cm per millibar change in atmospheric pres- surface current of the Gulf Stream could be detected sure such that there is no net pressure change at depth from the Geos-3 data. associated with atmospheric pressure changes. These The Seasat altimeter (July-October 1978) was the isostatic changes in sea level are unrelated to surface first designed for oceanographic applications. An geostrophic currents and therefore must be removed on-board microwave radiometer was used to esti- to obtain the dynamic sea surface elevation. The mate the water vapor range correction. Precision
OCEANOGR.~PIlYoNOVEMBER-] 988 7 SEA SURFACE SLOPE VARIABILITY FROM GEOSAT, 11/86 - 9/87
contour interval: .84 arc sec (.2 cm/l
By William F. Haxby ABOVEis shown a map of sea level slope vari- Geosat passes were excluded from the analysis. ability, based on an analysis of colinear Geosat al- Along-track estimates of cy were used to compute timeter profiles of ocean surface topography. The the mean 6 for 0.4 ° geographic grid cells. Un- map was produced through the analysis of a 325 day sampled grid cells were assigned values of 0r based sequence of Geosat exact repeat mission (ERM) al- on bilinear interpolation. Tidal fluctuations pre- timeter data (19 repeat cycles). Altimeter profiles dicted by the Schwiderski tide model were sub- were first adjusted to remove long wavelength orbit tracted from the data prior to analysis, but there is errors, and data points (1 second along-track aver- high variability in some areas that can be attributed ages) with estimated errors greater than 10cm were to tides: the northern margins of Brazil and Austra- eliminated from the data set. The root-mean-square lia, for example. The rms slope variability shown slope variability, ~, was determined from differ- has maximum values of about 30cm per 100km in ences in sea surface slope between individual pro- the regions of brightest colors (pale blues to whites), files and the 325 day mean profile, through a least as shown on the accompanying color scale bar. squares analysis of overlapping 50km track seg- Map and caption contributed by William F. Haxby, Lamont- ments. Tracks that were sampled by less than 10 Doher O' Geological Observatol T. Palisades, NY 10964.
orbit determination has recently resulted in a Seasat the 3.5-month mission, providing the first global orbit accuracy of better than 50cm (Marsh et al., perspective of sea surface topography and its vari- 1988). Seasat altimeter sea level measurements have ability. Using only data from the last month, when been shown to agree with dynamic heights computed Seasat was flown in a 3-day repeat orbit, Cheney et from aircraft expendable bathythermographs to an al. (1983) produced a map showing the global distri- accuracy of 10cm at scales up to order 1000kin bution of mesoscale sea level variability. Mazzega (Bemstein et al., 1982). With the Seasat data, ocean- (1985) and Woodworth and Cartwright (1986) pro- ographers began to develop confidence in the quality duced altimetrically-determined global charts of the and utility of altimeter measurements. M2 tide. Using a mean sea surface computed from An important aspect of the Seasat mission is the the Seasat data, Tai and Wunsch (1984) derived the availability of a nearly continuous global data set for first global absolute dynamic topography, showing
8 OCEANOGRAPHY.NOVEMBER, 1988 The U.S. Navy Geosat Mission has been collecting global altimeter data since March 1985. This multi- year data set has generated a new wave of enthusiasm for satellite altimetry. The primary objective of Geosat is to map the mean sea surface topography with high spatial resolution for Navy applications. This objec- tive was completed during the first 18 months of the mission. The sea level data from this part of the mission have been classified by the Navy, but begin- ning in early November 1986, Geosat was maneu- vered into a 17-day repeat orbit for oceanographic applications. The data from this so-called exact re- peat mission (ERM) are unclassified and have been processed and distributed to the public by the Na- tional Oceanic and Atmospheric Administration. A limitation of the Geosat mission is that the data are somewhat compromised by uncertainty in thewater vapor range correction. Unlike Seasat, there is no on- board microwave radiometer on Geosat. The water vapor range correction is therefore based on Navy model estimates of water vapor. These water vapor fields are at least a factor of two less accurate than microwave radiometer estimates (Tapley etal., 1982b) The reason for the and do not resolve spatial scales of variability shorter than about 2000km. success is the scale Although the sea level data from the first 18 separation between months of the Geosat mission are classified, the differences between altimetric sea level measure- signals and errors. ments at intersections of ascending and descending ground tracks are not. Using a method developed by m) Fu and Chelton (1985), Cheney and Miller (1988) have combined the 18 months of crossover differ- ence data with the ERM data to construct a multi-year gridded array of sea level time series in the tropical Editor's Note: Fluctuating smface geostrophic currents should be closely related to the sea susface slope variability. Pacific. Shown in Fig. 2 (p.7) is the comparison of a which Haxhy's map above sho~w occurs in regions that 2.5 year low-pass filtered altimeter sea level time closely match the re,~ions of high variabilio' q]und~fferenced series with a concurrent monthly-averaged sea level Geosat sea level height data (~f. Fu. Chelton attd ZIotuicki. record from a tide gauge at Majuro in the western this issue, p. 5). The similari(v suggests that sea level height tropical Pacific. The rms difference between these may be a good pro_~y.¢br sea level slope in strong-current re- gions, which is sensible ~f slope changes across a current are two low-pass filtered time series is only 3cm, a mostly due to height changes cot!fined to one flank of the remarkable result considering the errors in orbit current. Both the Haxby attd the Fu et al. maps clearly identi~, height, the large water vapor content, and the strong western boundary" currents, the Antarctic Circumpolar Cur- horizontal gradients of water vapor in the tropics. rent, the Aghulas Retroflection, attd the Brazil-Falkland The reason for the success is the scale separation Cot~luence as areas of high variability. The same areas stand out in modeled fluctuations of thermocline temperature in the between signals and errors, as discussed previously. global ocean (¢f ./)'ont cover and caption by Semtner. this The sea level variability is caused by equatorially issue). trapped motions which have a limited along-track extent, such that long-wavelength orbit height and water vapor errors can be safely removed without sacrificing the signal of interest. The Geosat sea level many qualitatively plausible patterns of the long- observations are presently being used to study the wavelength components of the mean circulation of 1986-87 El Nifio (Cheney and Miller, 1988). the world ocean. Fu and Chelton (1985) used Seasat The Geosat data are also being used to study altimeter data to study the large-scale temporal vari- global mesoscale variability. The ERM data are ability of the Antarctic Circumpolar Current for the collected along closely repeating (within 1kin) ground period July-October 1978. tracks, so the use of the data for studying the temporal The most serious limitation of the Seasat mission variability of ocean currents is fairly straightforward. is its short 3.5-month duration, which has made many Displayed on the back cover is a global map showing scientific results largely qualitative. The Seasat data the geographic distribution of the standard deviation have been extremely useful, however, for developing of sea level measurements made along repeat tracks techniques for analyzing other altimeter data sets. during the first year of the ERM. There are two
OCEANOGRAPHyoNOVEMBERo 1988 9 significant improvements in this map over the map of ]'~ll)le 1. %ill:Jill:c> ,,1 l}i, !<.,,' ]I!,';HI ~,I~I~H, i,i,C/~l<,ii i i Cheney et al. (1983), obtained from one month of -,',',,>1 ;,',,'~;,.z',',l,/;~t;~ ;m,l,,:l,]t m,u~,.-> f<,~ ,;,~l,.> the much closer ground track spacing (equatorial q];j, ];,I, , 197:~1 t;(; IJH}O crossings separated by 164kin for Geosat vs. 935km t ;<,,. ?, I ltlT;,~ 2t) 2(Jtl ] .q(l() for the repeat orbit period of the Seasat mission). q,,~-~tt , ]t)?"l L 51) ](H) It is well-known that the wind stress over the (;,'<,~,,t , 19~.',i 4 7,1) IIHI ocean is stronger in winter than in summer, so the part EI~q I , ]b!*lJl 5 L() IH() of the mesoscale variability that is directly driven by I',,i>,, P,>-,~,[,,~ , [']!11~ l 1() 2t) wind would be expected to have a similar seasonal cycle. Shown in Fig. 3 (p. 5) are global maps of the difference in sea level variability between the annual Table 1 mean (shown on the back cover) and winter and sufficient for the study of basin-scale variability and summer seasonal means. Significant seasonal vari- the time-mean circulation of the ocean. These are ations are found in many' regions, among which the among the least known characteristics of the ocean, northeastern Pacific, the southeastern Pacific, and and yet they bear profound influence on the climate the northeastern Atlantic are indeed regions where system of the earth. Satellite altimetry has the prom- Over the winds are a significant source of eddy energy. The ise /'or making significant advancements in these past fifteen years, mechanisms for the seasonal variability evident in areas. The challenge facing the oceanographic coin- other regions le.g., the Gulf Stream, the Kuroshio, munity is to push present capabilities one step further the satellite altimeter and the Antarctic Circumpolar Current) are yet to be to realize this promise. explored. It is also possible that part of the apparent Challenges has matured from an seasonal differences result from other sources of To be useful for studying the basin-scale variabil- experimental sensor variability unrelated to the winds, e.g. long-period ity and the time mean circulation of the ocean, the instabilities. (Also see the article by Gordon in this altimetric sea level measuremenl must have an over- into an important issue, the,fi'onl cover model results by Semtner. and all accuracy on the order of 10cm or better. Given instrument for the sidebar by Ha.vhv accompanyi#tg this article present and projected near-future capabilities in (p, 8-9) - Eel.) precision orbit determination, the overall accuracy oceanographic The sampling scheme of the Geosat ERM offers will be dominated by orbit error of this same order. It research. an opportunity to map the synoptic field ofmesoscale is therefore essential that the accuracy of each of the eddies for the study of their dynamics. To illustrate other corrections applied to the altimeter range the sampling capability of Geosat, shown in Fig. 4 measurement be kept to a level much better than (p. 7) is a wavenumber-frequency spectrum of sea 10cm. level variability estimated from ERM data from a To evaluate the accuracies of the important al- region south of Africa where the eddy energy is timeter geophysical algorithms, a workshop was among the strongest in the world. The bulk of the held at the Oregon State University in Corvallis, eddy energy is contained in a spectral domain with Oregon, in August 1987 (Chelton, 1988). A major wavelengths longer than 200km, indicating that the conclusion from this workshop was the identification Geosat l l0km track spacing (at 40 ° latitude) is of two areas where the physical basis for correction adequate for resolving the eddy spatial scales. At algorithms is not adequately understood. The first is wavelengths from 300 to 600kin, the frequency dis- the instrument and air-sea interface correction: the tribution of energy shows a rapid drop in energy at second is the inverse barometer correction. Most of periods slightly longer than 34 days, the shortest the existing algorithms in the first category are based period resolved by the data. In this wavenumber on empirical studies, so the resulting corrections are band, the Geosat 17-day repeat data are thus adequate prone to bias errors related to sea state and altimeter for resolving the eddy time scales as well. Ocean antenna mispointing. The potential for ground-based modelers are beginning to incorporate synoptic Geosat analysis of altimeter waveform data to reduce instru- observations into numerical ocean models to im- mental and air-sea interface errors has been identi- prove simulations and even predictions of ocean fied as a high priority item for investigation. The dynamic processes (Walstadt, personal communica- inverse barometer correction is actually a research tion). problem for physical oceanographers. An improved Over the past fifteen years, the satellite altimeter understanding of the wavenumber-frequency re- has matured from an experimental sensor into an im- sponse of the ocean to atmospheric pressure forcing portant instrument for oceanographic research. While is required. measurement accuracy has undergone an improve- Since the largest source of error in altimetric sea ment of more than an order of magnitude, it is still not level measurements is uncertainty in orbit height, 1 0 OCEANOGRAPHY.NOVEM BIJR. 1988 any significant improvement in the accuracy of al- Administration (NASA) and the French space agency timeter estimates of sea level will require improve- Centre National d'Etudes Spatiales (CNESI. This ments in the knowledge of the earth's gravity field. A mission has been optimally designed for studying the major accomplishment has recently been made by world ocean circulation. The satellite will orbit at an Marsh et al. (1988) in developing a much improved altitude of 1335km with an inclination of 65.1 °. The gravity model thal is able to achieve an rms orbit relatively high orbit was chosen to minimize both the accuracy of 25cm. This model is derived solely from atmospheric drag on the satellite and the influence of satellite tracking data and is better than the best gravity errors on orbit accuracy. The orbit inclination previous "'satellite-only'"gravity model by a factor of was chosen to satisfy rather stringent requirements two. By incorporating Seasat altimeter data into the imposed by tidal aliasing and precision orbit deter- model development effort, an orbit accuracy of about ruination. The primary Topex/Poseidon altimeter, 18cm can be attained (Marsh et al., 1988). After designed and built by NASA, will make simultane- "'tuning" the model using the tracking data for a ous measurements at two frequencies (5.3 GHz and particular altimeter satellite, the orbit accuracy for 13.5 GHz) so that the ionospheric range correction that satellite can be even further improved. can be directly estimated from the combined meas- As noted previously, an accurate geoid model is urements. The other Topex/Poseidon altimeter is an required lk)r obtaining information about the time- experimental solid-state altimeter designed and built mean ocean circulation. The required accuracy of a by CNES for future low-power and low-cost altime- few cm over a length scale of 100km cannot be ter missions. Other sensors include a three-frequency achieved by the Marsh et al. (1988) gravity model microwave radiometer for determination of atmos- based on existing satellite tracking data. The only pheric water vapor for the water vapor range correc- feasible approach is to fly a gravity mission utilizing tion, a Doppler radio tracking system for precision satellite-to-satellite tracking or gradiometry tech- orbit tracking, and an experimental Global Position- The combined Topex/ niques (Topex Science Working Group, 1981). ing System t GPS) tracking system that has the poten- Poseidon and ERS-1 Temporal variations in the geoid can generally be tial of providing continuous orbit tracking with an ignored for the study of ocean circulation. The geoid accuracy of better than 10cm (Yunck et al., 1985). altimeter data sets will therefore need not be measured concurrently with the The specification for overall measurement accuracy provide information altimetry measurements. The time-mean circulation for a single pass is 14cm, which is dominated by an can be derived retrospectively when a sufficiently rms orbit error of 13cm. However, orbit improve- about ocean variability accurate geoid eventually becomes available. ment will be a continuing research effort that is with unprecedented Besides the technical challenges of improving the expected to eventually reduce the orbit error to less overall measurement accuracy of altimeter observa- than 10cm. The mission is scheduled to be launched accuracy and temporal tions, new techniques for utilizing the altimeter data by the French Ariane launch vehicle in December must be developed. Major research efforts are under- 1991. and spatial resolution. way to develop techniques for combining altimeter The ERS-1 mission is planned by the European data, in situ observations, and numerical and statisti- Space Agency and is scheduled to be launched in cal models to extract the most information possible September 1990. Sensors will include a single-fre- from these different approaches for studying ocean quency altimeter and a microwave radiometer. The circulation. The altimeter measures the surface geo- mission orbit is sun-synchronous with an altitude of strophic circulation globally. In contrast, m situ 800kin and an inclination of 98 °. The overall meas- measurements from current meter moorim, s and urement accuracy of the ERS- 1 altimeter is expected hydrography measure the vertical structure of the to be comparable to that of the Seasat altimeter. The ocean at specific locations. By combining these two Topex/Poseidon and ERS-1 altimeter data sets will observational data sets with models using data as- be complementary, and to the extent that they over- similation techniques, optimized 4-dimensional pic- lap, the Topex/Poseidon orbit will help calibrate the tures of ocean circulation can be obtained. The chal- ERS-1 orbit. The combined Topex/Poseidon and lenges include development of techniques for capi- ERS-1 altimeter data sets will provide information talizing on the strengths of altimetric data. In particu- about ocean variability with unprecedented accuracy lar, methods must be developed for utilizing the long- and temporal and spatial resolution. With the in situ wavelength altimetric sea level (for which the geoid observations, numerical modeling, and data assimi- is known accurately) or time-variable sea level from lation efforts anticipated as part of the World Ocean repeated altimetric measurements at the same loca- Circulation Experiment, the decade of the 1990s tions. promises major advancements in our understanding Future Missions of the circulation of the world ocean. There are two satellite altimeter systems sched- Acknowledgements uled to be launched in the early' 199(ts. One consists The research described in this paper was carried out at the Jet Propulsion Laboratory, California h>titute of Technology, under of a pair of altimeters on the Topex/Poseidon dedi- contract with the National Aeronautics and Space Administration cated altimetric satellite. The other is one of several (NASA). and at Oregon State University under Contract 958127 instruments on the European Remote Sensing satel- funded under the NASA Topex/Poseidon Announcement of lite ERS-1. The Topex/Poseidon Mission is jointly Opportunity. planned by the U.S. National Aeronautics and Space [ PLEASE TURN TO PAGE 58 ] OCI]ANO(IR-\PIIY.N()VI:M BI{R. 198g 1 l great success during the forty-five years rectly to the education and training of marine The importance of effective communica- since the school's founding. The resources science students. tion is fully recognized among students at available to students in the form of facili- The OTMSS sponsors a number of field the RSMAS, and the planned workshops ties, curriculum and fitculty are extensive. trips to various environments in South will combine expert instruction with prac- The RSMAS library is accessible to stu- Florida and the Caribbean, stressing the tical experience. Individual workshops will dents 24 hours a day, 365 days a year. Each importance of an interdisciplinary approach cover written and verbal skills, and will be student receives nearly unlimited time on in the marine sciences. Through these or- lead by selected faculty members noted for the school's computing system, which in- ganized trips, students gain direct exposure their presentation skills. cludes links to a variety of academic and to, and an increased appreciation of, the The above are just a few of the ways that research networks. And, of course, individ- sub-disciplines of their fellow students. graduate students at the University of ual divisions provide facilities equipped to Field trip leaders are faculty members or Miami's Rosenstiel School of Marine and satisfy research needs within their respec- students with specialties in the areas to be Atmospheric Science are enriching the tive sub-disciplines. The small student-to- visited. Beginning this fall, the OTMSS educational experience for themselves and faculty ratio (-2) enables students to work will sponsor a series of workshops directed their fellow students. We encourage other closely with their advisors and to play an at improving students' presentation skills. students to share their successes. active role in research. These factors and more come together at the RSMAS to cre- inertial boundary current from an irregular ate an exceptional educational environment. THE SOUTH ATLANTIC coastline. ,I. Phvv. Occam~gr., 16,281)-289. [CONTINUED FROM PAGE 17] Despite the excellent provisions of the Rintoul, S., 1988: Mass, heal and nument fluxes in the RSMAS, students have gone a step further Revealed by hwerled Echo sounders..l.Geophys. Atlantic Ocean determined by inverse methods. and established two organizations to ad- Res.. 92. rC'2j, 1914-1922. Ph,D. thesis, Massachusetts Institute of Tech- dress student needs and concerns. These Gordon, A. L.. 1986: hater-Ocean Exchange of Th- nologyfWoods Hole Oceanographic Institute Joint Program, 287 pp. are the Marine Science Graduate Student ermocline Water..I Geophys. Rcs.. 91(C4~: Roden. G., 1986: Thermohaline fronts and baroclinic Organization (MSGSO) and the Organiza- 5037- 5046. Gordon, A. L. J. R. E. Lutjeharms and M. L. flov, m the Argentine Basin during the austral tion of Tropical Marine Science Students Gri.indlingh 1987: Stratification and Circulation spring of 1984..I Gcophys. Res., Ol. 5075- (OTMSS). Soon after the founding of the at the Agulhas Retroflcction. D¢'¢7~-Sea Res 5093. RSMAS, the MSGSO was established to ,']4(4). 565-599. Whitworth, T. and W. Nowlin, 1987: Water masses and currents of the Southern Ocean at the insure student representation within the Olson. D. B. and R. H. Evans, 1986: Rings of the Greenwich meridian.,l. Ge,q#zys Res.. 92,6462- University. It has remained an active voice Agulhas Current. De(T Sea Rcs, 33, 27-42. Ou, H.W. and W. de Ruuter, 1986: Separation of an 6476. I throughout the school's history. A repre- sentative of the MSGSO occupies a voting position on the school's Academic Com- timetry. J. Ge(q~hys. Rex. 92, 74%754. mittee and participates in the formulation SATELLITE ALTIMETRY Fu, L.-L.. and D.B. Chelton, 1985: Observing large- ]CONTINUED FROM PAGE I1] scale temporal variability of ocean currents by of policy regarding students. The MSGSO satellite altimetry: with application to the Antarc- also provides a number of services such as References tic Circumpolar Current..I. Ge(q~hy,s'. Res., 90, short-term, no-interest loans and new stu- Bernstem. R.L., G.H. Born and R.H. Whrimer, 1982: 4721-4739. dent orientation intended to help with the Seasat altimeter detemfinanon of ocean curi'ent Marsh. J.G., (and 19 other authors~, 1988: A new problems of "'student life.'" As a means of variability. ,I Geophys. Re,~.. ,~7, 3261-3268. gravitational model for the earth from satellite Born, G.H., M.A, Richards and G.W. Rosborough. tracking data: GEM-T1. ,I. Geoph)'s Rcs.. 93, promoting student-faculty exchange as well lqS2: An empirical determination of the eflects 6169-6215. as student-student interaction, the MSGSO of sea-state bias on the SEASAT altimeter. J. Mazzega, P.. 1985:M2 model of the global ocean tide operates a commons complete with music Ge,g#U.~. Re,Y, 87, 3221-3226. derived from SEASAT altimetry. Mar. Geod., 9, and a full bar. Finally, the MSGSO has Chehon, D.B., 1988: WOCE/NASA Altimeter Algo- 335-363. created a travel fund program to provide rithm Workshop. U.S, WOCE Tech. Rep. No. 2. Parke, M.E.. R.H. Stewart. DL. Farless and D.E. U.S. Plalming Office for WOCE, College Sta- Carlwright, 1987: On the choice of orbits for an financial assistance to students presenting tion, TX., 70 pp. altimetric satelhte to ~,tudy ocean circulation and the results of their research at scientific Chene>, R.E., J.G. Marsh and B.D. Beckley, 1983: tides. ,1. Gc(q)h3".~. Res.. 92, 11,693-11,707. meetings. Global mesoscale variability from col linear tracks Tat, C,-K., and C. Wunsch, 1984: An estimate of Monies allotted to this fund annually are of Seasat altimeter data. ,I. Gccq~h3'.s Res, ."¢& global absolute dynamic topography. ,I. Phvs 4343-4354. Oceam~er,. 14. 457-463. matched by the Office of the Dean. Exclud- Cheney. R.E., and L. Miller. 1988: Mapping the Tapley. B.D., G.H. Born, and M.E. Parke, 1982a: The ing the Dean's matching funds, the MSGSO 1986-I987 El Nifio with Geosat alnmeter data. SEASAT altimeter data and its accuracy receives no monetary support from the Eos Trans. Amer. Geop/Us. Ulfion. h9 754-755. assessment. ,I. Ge~q#Us. Res.. h'7, 317%3188. University, and all of its annual budget is Douglas. B.C., R.E. Cheney and R.W. Agreen. 1983: Tapley, B.D., J.B. Lundberg and G.H. Born, 1982b: met through fund-raising activities. Chief Eddy energy of the Nowthwest Atlantic and Gulf The SEASAT altimeter ,set tropospheric range of Mexico determined from Geos-3 altimetry..I con-cotton. ,I. Geophys. Res., 87. 3213-3220. among these is the Annual MSGSO Stu- Ge~q#n's Res , &~.'. t~595-9603. Topex Science Working Group, 1981: Satellite al- dent Auction, an event in which businesses Douglas, B.C., D.C. iMcAdoo and R.E. Cheney, tinletllC measurements of the ocean. Doc. 400- and individuals donate their services and 1087: Oceanographic and geophysical applica- 1 I I, Jet Propul. Lab., Pasadena, Calif. merchandise. The donations are auctioned tions of satellite alnmetry. Rcv. Gcophys.. 25. Woodworth, P.L., and D.E. Caltwrlght. 1986: Ex- with earnings going to the MSGSO. Last ~75-880. traction of the M2 ocean tide from SEASAT Fu, L.-L., 1983: Recent progress in the apphcation of ahimeter data. G(,ophys J Roy. ,,1silo. Sot., N4, year the event raised nearly $8,000. satellite altunelrv to observing the mesoscule 227-255. The Organization of Tropical Marine variability and general circulation of the oceans. Yunck, T.P,, S.C. Wu and S.M. Lichtcn, 1985: A Science Students was established in 1986 Roy. Geol~hys. Spa('e Phys., 2 l, 1657-1666. GPS measurement system for prect',e satellite to promote activities which contribute di- Fu, L-5., J. Vasquez and M.E. Parke, 1987: Seasonal tracking and geodesy. ,I. Astromlul. Sot.. 33, variability of the Gulf Stream from satellite al- 367-380. I 58 OCEANOGR \PItY.NOVEMBER. Igx8