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Global change and the measurement of absolute sea-level
P~.~. Ocem,¢. ~,'~d~ I~. pp. I-:1. I~." Prim,.,d in Great Britain..All night, r,..~cr~.-,I.
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Global Change and the Measurement of Absolute Sea-Level JOHN,~l. DIAMA,~'rE*.THO,~LASE. [YLE*. WILLIAM E. CARTER~" and WOLFOA.~OSC~SREtV " O~ficeof Oceanic & A tmosplwric Rcmrm,ch.~VOA A. RockviUe. M D 20~ :, (,:$.A. *Jo/m OceanographicInstitutions. Inc.. 1753Mussaclu~ats Avenue N3V.$ui~ 800. Washi~o~. DC20036. U.S.A.
~Geodetic Res~rds & DevelopmentLaboratory. Nmiond OceanService. ,~'0.4A. Rockvil~. MD20832. U.S.A. and Lake Lcv'ds Branch. NoAfonalOceanSen'ice..VOAA. Rockville. MD
20~'. U.S.A.
ASSTUCT To quantify properly the long-tern response of sea-level to c11mate change, land motions must be separated from the apparent or relattve sea-level change recorded by conventional tide/sea-level gauges. Here we present a concept for global measurement of the true or "absolute" sea-level change, which combines recent advances tn space-based geodettc techniques with plans for a global sea-level network under the World Climate Research Programme (WCRP). Data from t n i t t a l feastbtl.tty tests show that land motion, due to global (plate tectonic), regtonal (glactal rebound), or lo¢al ( f l u i d wtthdranal) effects, can probably be moasured to ~1¢m (on a single measurement basis) by an innovative combination of Very Long 8aseltne I n t e r f e r ~ t r y (VLSl) and Global Pos|ttontng System (sPa) techniques. By making repeated observations of position at a number of ttde gauges using portable, economIcal GPS receivers tn a differential mode relattve to the fewer, more stable, but more expensive VLSI observatories, i t w i l l be posstble to subtract land motion from the relattve sea-level stgnal. Oecadal to century scale trends at the 1-2m y-1 level w i l l be resolvable tn the sea-level and vertical land motion time series within about a decade. Detection of subsidence or u p l l f t at specific gauges w t l l allow correction for land motion or deletion of bad data when computing regtonal or global, i . e . eustattc, sea-level changes. In addition to their applications in oceanography and climate studies, such data w i l l test models by Peltter and others that relate mantle viscosity and deglactatton history to present rates of crustal subsidence or u p l l f t . I f the predicted crustal motions are confirmed, we can also have more confidence in the use of historical ttde/seaolevel gauge records tn retrospective studies of sea-level change related to c11wte v a r i a b i l i t y on decadal or longer time scales. I t ts concluded that as few as one-third (about 100) of the total number of tide/sea-level gauges (250-300) required for globo1 c11mate research progranms (k"(RTKI and PUGH, 1984) need to be upgraded to "sea-level observatory" status, in terms of sensors and geodetic control, to build a two t i e r sea-level network that w t l l provide the data needed to solve many probloms of both relattve and absolute sea-level change. Such a network wtll !
J. M. DIA~,t,,STE et al.
provide basic information supporting research on a wide range of questions related to "global change".
CONTENTS I. .
3.
4.
Introduction Geodetic radio interferometric surveying in measurement of absolute sea-level 2.1
Application of VLBI
2.2
Application of GPS
Measurement system accuracies 3.1
Geodeticsystems
3.2
Sea-levelmeasurement systems
Design c r i t e r i a for an absolute sea-level network
11
5,
Applications of absolute sea-level data
17
6.
Conclusions
19
7.
Acknowledgements
19
8.
References
20
I . INTRODUCTION
Understanding the circulation of the ocean and i t s role in world climate w i l l require longterm global data bases of ocean observations analogous to the observation sets presently available for the atmosphere.
Of several candidates, the measurement of sea-level stands
out as one of the simplest and most effective.
Recent scientific results indicate that
these measurements have potential for supporting predictions of the onset of El Ni~o (WYRTKI
197g, 1985). They can also provide empirical estimates of heat transport by major ocean currents, and offer independent checks of global warming (or cooling) effects attributable to both natural v a r i a b i l i t y of the climate system and possible man-induced influences on the global climate, such as changes in atmospheric carbon dioxide and other trace gases contributing to a greenhouse warming effect.
Analyses of data from a well-distributed global
network of tide/sea level gauges also contribute to the success of certain s a t e l l i t e a l t i metry mission applications.
Radar altimeter satellites are orders of magnitude more costly
than the proposed sea-level network.
Global change and Ihe mcasuremenI of absolute sca-i¢~cl
3
Sea-level has long been measured by conventional tide gauges at many sites.
Its measurement
is so basic to safe navigation and to the establishment of maritime and coastal zone bo~ndaries that i t has become "routine".
However, the present network is: (a) patchy (i.e. aGgre-
gated in Europe, Japan and the USA);
(b) biased towards the northern hemisphere (only 8
of 155 stations used in a recent "global,' study (BARNETT, 1984) are in the southern hemisphere); and (c) not sited with any regard to ocean circulation patterns. The present network is not a coherent, global network, but an assortment of gauges located according to historic and economic, as opposed to physical, factors. For application to studies of ocean circulation and the ocean's role in climate, sea-level measurements will be needed at selected additional
sites around the world.
For practical
reasons, additional gauges should include updated sensors, digital recording, ancillary data inputs, and satellite telemetry capability. WYRTKI and PUGH (1984) have reviewed the existing global network and proposed potential additional sites (Fig.l). But the addition of more gauges alone is inadequate. For example, when use of a conventional tide/sea-level gauge to measure sea-level over a period of years indicates a trend of rising sea-level, one really cannot be sure whether the sea is rising or the land is sinking. Tide/sea-level gauges actually measure the relative motion between the sea surface and the land. Since the land can change due to a number ~f factors, e.g. effects of glacial rebound, compaction of sediments, or withdrawal of petroleum or ground water, the problem of relative motion must eventually be solved i f sea-level data are to be properly interpreted and used. The development of new geodetic techniques based on the Global Positioning System (GPS) and Very Long Baseline Interferometry (VLBI) has created the opportunity to link the simple, traditional sea-level measurements to a modern, highly accurate global reference frame being established by the international geodesy and radio-astronomy communities. The International Radio Interferometric Surveying (IRIS) programme has adopted a resolution calling for member nations to cooperate with the oceanographic community to establish such a global sea-level reference frame.
T h i s linkage of sea-level measurements to the new, space-based geodetic
networks will ultimately provide the f i r s t measure of absolute sea-level change. The prospect for relating sea-level measurements to a global reference system accurate to the centimetre level introduces the requirement for additional measurements of oceanographic and meteorological variables
at the tide/sea-level gauge sites.
Consequently, there is
a need to measure wind speed and direction, barometric pressure, salinity, air and water temperature (plus the capability for additional sensor inputs, i f needed) at each VLBI/GPSlinked sea-level station site. Such colocated oceanographic and meteorological measurement series, along with the sea-level records and precise geodetic data, will provide unique, invaluable time series and research data sets for satellite programmes, climate studies, meteorological modelling and oceanographic research.
80"
S
60"
40 -
20"
0•
20"
40 +
60"
N
80 °
20"E
FIG.1.
40"
80"
100 °
Existing (e) level network
60"
140"
160 °
180 +
160"
140*
120"
100 °
and p l a n n e d (I) tide/sea-level gauge as p r o p o s e d b y WYRTK[ and PUGH ( 1 9 8 4 ) .
120 °
sites
80"
in
60 °
a
O+ E 20 ~
sea
20"W
global
40 +
Global change ~nd the meusur=mcnt ~)I"Jb,,~)luIa~ca-l¢',~:l
2.
GEOOETICl~a~I)lO INTERFEROMEII~IC SURVEYING IN I~FJLSUREMENT
OF ABSOLUTESEA-LEVEL in order to measure absolute sea-level, both the vertical land motion at the individual sea-level stations and the local sea surface position w i l l need to be monitored relative to an ultra-precise absolute geodetic reference system based on VLBI and GPS technologies. Conceptually, i f not in practice, the approach is straightforward. 2.1
Ap?~(oa~o~ o~ VLBI
The VLBI technique employs two radio telescopes simultaneously observing the same extragalactic radio source.
The wave fronts of the electromagnetic emissions from those enor-
mously distant astronomical sources arrive at the radio telescopes as plane waves. A given wave front w i l l arrive at each observatory at slightly different times, dependent on the overall geometry. Each observatory independently records the arrival time of the signals, using atomic clocks.
The "time-tagged" signals fro~ the pairs of stations are differenced,
and the changes in the differences with time are used to estimate the interstation vectors (length, horizontal and vertical components).
In practice, fixed or mobile antenna systems
can serve as the basis for a given radio observatory. At the present time, the refraction effects produced by atmospheric water vapour are the largest remaining sources of systematic errors limiting the accuracy of VLBI systems for absolute sea-level applications.
However, a n~w generation of water vapour radiometers (WVR)
is currently being tested, which may greatly reduce this source of error (KROGER, DAVIDSON and GARDNER, 1986; WARE, HURSTand ROCKEN, 1986). Both VLBI and GPS are new, rapidly developing geodetic technologies which w i l l have certain limitations in geographic coverage into the 1990s that must be recognised in formulating the observing sequence for a global sea-level monitoring system. There are five types of VLBI systems: (I) permanently installed at dedicated geodetic observatories (G);
(2) perm-
anently installed at shared astronomic-geodetic observatories (S);
(3) movable systems
that require limited dismantling of the telescope to relocate (MV};
(4) mobile units in
which the telescopes are mounted on vehicles that can travel on highways without dismantling (MB); and (5) transportable units that can easily be moved among existing antennas in a time-sharing mode (T), which include all equipment other than the antennas. Table 1 contains a summary of the currently existing and planned f a c i l i t i e s .
The locations of types G and
S observatories are plotted on Figure 2. The concentration of VLBI f a c i l i t i e s in Europe, North America and eastern Asia into 1990 w i l l enable the North Atlantic and Pacific regions to be surveyed most accurately at the earliest dates. The situation is l i k e l y to be very much the samewith GPS i.e. the number and distribution of the base stations i n i t i a l l y w i l l result in s a t e l l i t e orbits being better determined over the northern hemisphere. After 1990, the global distribution of both VLBI stations and GPS satellites should improve substantially.
J.M. DI.~HA,~TECtat.
6
TABLE I: Type of VLBI System
Current and Proposed VLBI Sites Responsible Organisation
Location
Operational Date
Fort Davis, Texas, USA Westford, Massachusetts, USA Mojave, California, USA Richmond, Florida, USA Fairbanks, Alaska, USA Wettzell, FRG Kashima, Japan Shanghai, PRC Kunming, PRC Urumchi, PRC
NOAA NOAA NOAA NOAA NASA IFAG RRL SO SO SO
1980 1981 1983 1984 1984 1984 1984 1987 1989 1990
Bishop, California, USA Hatcreek, California, USA Maryland Point, Maryland, USA Greenbank, West Virginia, USA Kauai, Hawaii, USA Kwajelein Hartebeesthoek, SA SaD Paulo, Brazil Tidbinbillo, Australia Bologna, Italy Sicily, Italy Riyadh, Saudi Arabia Dehra Dun, India Onsala, Sweden
OVRO UCB NRL NRAO NASA USAF CSIR BG[ NASA IDR ISA NOP SI OSO
1980 1983 1983 1980 1984 1984 1986 1986 1986 1986 1986 1988 1989 1980
MV
USA (I unit) Japan (3 units)-
NOAA/NASA NGSJ
1980 1987
MB
USA (2 units) FRG (I unit)
NOAA/NASA IFAG
1984 1988
USA (4 units) USA (I unit) USA (I unit)
NASA NOAA NOAA
1985 1988 1989
G S MV MB T
Permanently installed at dedicated geodetic observatories Permanently installed at shared astronomlc-geodetic observatories ~W}¥able systems that require limited dismantling of the telescope Mobile systems-telescopes on vehicles - no dismantling Transportable systems - all equipment but telescope moves
Key to organisations
NOAA National Oceanic and Atmospheric Administration NASA National Aeronautics and Space Administration IFAG Institut fur Angewandte Geodasie RRL RadioResearch Laboratory NRL Naval Research Laboratory NRAO National Radio Astronomy Observatory USAF United States Air Force SO ShanghaiObservatory NGSJ National Geographical Survey of Japan
Council for Scientific and Industrial Research Brazilian Geophysical Institute BGI Italian Space Agency ISA Institute di Radioastronomia IDR National Observatory Project NOP Survey of India SI OSO Onsala Space Observatory OVRO Owens Valley Radio Observatory University of California at Berkeley UCB
CSIR
FIG.2.
I
I
I
i
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
LocaLions of major Very Long Baseline lnterfer~netr'y (VLBI) now operational and planned for the period 1987-1990.
I
VLBI Facilities, Present-1990
I
I
I
I
I
installations
i
I
=lr n
@-
0 5"
•~
J. M , DL,~,'.I.~,',,1E et td.
2.2
A?pZ6oc~on~fGPS
The GPS system uses oortable ground-based radio receivers to "observe" the coded radio transmissions from a series of precisely positioned spacecraft in earth orbit. ephemerides provide a geocentric
The s a t e l l i t e s '
reference system, relative to which the local position
of ground-based GPS receivers can be calculated. system employs a single receiver,
The usual application of the GPS
whose position is determined from timing,
range-rate calculations using the coded transmissions.
range and
However, absolute sea-level monitor-
ing operations principally w i l l employ another application of the GPS system.
By simultan-
eously operating two GPS receivers, a r e l a t i v e l y short distance apart (about lOO-2OOkm), the system provides what amounts to a short baseline radio interferometric system, analogous in principle, to the VLBI systems. Here, the s a t e l l i t e radio transmissions take the place of the distant extra-galactic radio sourcesused in VLBI. As with the VLBI systems, water vapour refraction effects again represent the most serious error source limiting the use of GPS for absolute sea-level monitoring.
However, the same
water vapour radiometer technology applicable to VLBI can also support the GPS operations (WARE, HURST and ROCKEN, 1986). The basic concept for GPS operations w i l l be to use a pair of GPS receivers to "step" d i f f e r e n t i a l l y along traverse lines connecting the main VLBI reference stations (fixed) and VLBI station points based on mobile systems with a series of tide gauge/sea-level stations along the open coastlines.
3. 3.1
MEASUREMENTSYSTEM ACCURACIES
Geodetic @seems
Accuracies of continental scale (1500-4000km) and intercontinental scale
(4OO0-70OOkm)
baselines using VLBI observing systems have been discussed by CARTER, ROBERTSON and MACKAY (Ig85), CARTER, ROBERTSON, PYLE and DIAMANTE (1986), (1986).
and KROGER, DAVIDSON and GARDNER,
I t now appears to be well-established that baselines thousands of kilometres in
length can be determined to within a few centimetres, using r e l a t i v e l y short sessions (24 hours or less).
observing
Much of the remaining systematic error, caused by radio ref-
raction effects from atmospheric water vapour, may be removed by the use of new water vapour radiometer systems now becoming available. Consequently,baselines 15OOkm in length, and longer, should be operationally measurable to the ±Icm level of accuracy for a single VLBI measurement session (24 hours or less).
With adequate observing schedules, some VLBI
baselines are now being determined to the ±Icm level even without application of WVR systems, by the use of adjustable scale height models of the atmospheric water vapour distribution in the VLBI data reduction process.
The remaining errors involve sources such as temperature
wind and gravitational deformations of the radio telescope, that w i l l be d i f f i c u l t to model and remove. Therefore, i t appears that the ±Icm level w i l l be the practical accuracy l i m i t
Global chan~c and the measurementof ai~lut~ '~¢;*-!c'.~i
of VLBI measurements for absolute sea-level applications well into the decade of the 1990s. The accuracies of single baselines (up to 50km in length) determined from GPS measurements have been discussed by STRANGE (1985a, lgBSb); WARE, HURST and ROCKEN (1986); PRESCOTT, SVARC and WENDT (1986); and CARTER, ROBERTSON, PYLE and DIAHANTE (1986). Again, a major error source in the vertical component of GPS baselines is atmospheric water vapour effects. As with VLBI, i t may be possible to correct these errors with the latest water vapour radiometer technology. The 100-ZOOkm GPS baselines are conservative values; there are indications that GPS measure-
ments of the baselines may be ultimately good to this ±Icm level over several hundreds of kilometres.
This uncertainty in the operational limits for the GPS baselines also translates
into corresponding uncertainty in the total error budget for a differential stepping process using two GPS units "leap-frogging" between the VLBI anchor points.
I f the GPS performance
allows steps on the order of at least 100km or so, then the 1500km between the VLBI anchor points is expected to allow the total error budget for the single measurement accuracy of VLBI and GPS baselines for absolute sea-level to stay within the ±Icm level. However, i f under the f u l l range of global operations, the GPS limits should turn out to be more pessimistic, then the 1500km distance between VLBI anchor points may turn out to be excessive in some regions of the globe.
In that case, additional VLBI reference points
would need to be established using mobile VLBI systems to reduce the VLBI baseline distances to the 500-750km level. 3.2
Sea-Ce~eZ Measurement Systems
For climate applications, particularly the decadal and longer time scale concerns requiring determination of long-term trends in absolute sea-level, a number of serious problems exists with much of the standard tides/sea-level measurement technology.
These typically employ
float-in-well systems. The float operates in a well with a small orifice (a s t i l l i n g wet1). The s t i l l i n g well acts as a frequency dependent mechanical f i l t e r for the high frequency wave effects.
However, these high frequency effects of the sea surface are not all filtered
out and the s t i l l i n g well is subject to density gradients affecting the float, wave buildup effects, bio-fouling, etc.
The existing designs of s t i l l i n g wells are particularly subject
to hydrodynamic "draw-down" effects wherever there is a significant ambient current.
The
float system also has inertial response problems and experiences gradual buoyancy changes. The accuracy of a single measurement of the sea surface using such systems is not better than about 5cm. Someof this error is due to nearly random effects of the high frequency signals (waves) that can be s t a t i s t i c a l l y reduced or mathematically filtered in multi-year time series, so that long-term trends of a few millimetres per year can be extracted from the data.
However, to the extent that the other errors cited introduce systematic errors
that are not random and cannot be removed a p~(or¢ ,
they may leave small amplitude, long-
period effects that are v i r t u a l l y indistinguishable
(without some independent basis for
J.M. Dt.~.t-~',TE era/.
I()
determining them) from the sea-level trends.
Thesepossible remaining long period systematic
errors can only be dealt with, i f at a l l , by averaging measurements from many gauges, distributed over a wide geographic area.
I t should be noted that effects like hydrodynamic
draw-down, which are always ~0, w i l l never be entirely removed by simple averaging methods, even over a large ensemble of gauges representing a wide geographical area. By replacing the float with a remote sensor not involving the physical contact of a device with the water surface,
a large number of sources
of
systematic error
are either eliminated, or greatly reduced. There is no longer any need for a mechanical f i l t e r in the form of a s t i l l i n g well with a narrow orifice, so that i t is replaced by an open protective well, much less subject to nonlinear f i l t e r i n g effects and bio-fouling, not conducive to density gradient build-ups, and more indicative of the outside ocean environment. The available remote sensors meeting the accuracy requirements and practical power consumption limitations s t i l l require a protective well of some kind, subject to hydrodynamic draw-down. However, the open wells can be protected against major draw-down effects. New gauges of the acoustic type as well as data collection platforms, ancillary meteorological sensors, telemetry systems and newly designed protective wells are being developed and deployed as the New Generation Water Level Measurement System (NGWLMS) by the US National Oceanic and Atmospheric Administration (NOAA) for the US National Water Level Observation Network (SCHERER, 1986a,b). Systems of the NGWLMS type w i l l be required at all of the major sea-level stations, linked by VLBI and GPS, that are intended to provide long-term monitoring of the absolute sea-level.
A schematic representation of the NGWLMS total system concept
is shown in Figure 3.
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Schematic representation of the total system concept for the NO~ New Generation Water Level Measurement System (NGWLMS) from SCHERER (1986b). -
Global chan~e and th~ measurement of absolute ~:a-leveI
11
Based on the design specifications and test results from i n i t i a l prototype systems, a single ~easurement from the NG~LMS should represent nearly an order of magnitude improvement over the existing measurement type, i.e. the single measurement w i l l be good to the :Icm level, with most of the known systematic error effects either removed or greatly reduced. This is consistent with the expectations for the single measurements from the geodetic systems.
4.
DESIGN CONSIDERATIONS FOR AN ABSOLUTE SEA-LEVEL NETWORK
The discussion in the previous sections mainly concerned the determination of absolute sealevel at a single tide/sea-level station or at a pair of stations.
The determination of
the decadal or longer term global sea-level signal, including separation of the absolute component of sea-level from the relative sea-level signal, requires consideration of the design c r i t e r i a for a global sea-level measurement network. The sea-level signals of interest to climate applications have been at two main time scales: (a) an inter-annual scale pertaining to short-term climate fluctuations, and (b) a decadal scale, often assumed to be representative of longer term trends in eustatic sea-level. The long-term trend is generally, but not universally, believed to be indicative of a general secular rise of sea-level, possibly with an increasing rate of rise in recent decades. However, the climate system is also known to exhibit temporal fluctuations on the scale of decades to centuries.
Thesemay be reflected in the sea-level data.
In either case, the
traditional approach in computing mean sea-level trends was based on either a straight or weighted numerical average of corresponding sea-level time series over large regional or global ensembles of stations.
Implicit in this application of averaging procedures is the
assumption that all local oceanographic, meteorological, and geological influences on the water level signal, shorter than the sea-level periods of interest, may be considered to be "noise", i.e. randomly distributed errors relative to the large ensembles of stations. Such higher frequency phenomena are typically of a spatially localised nature and include such effects as currents, barometric pressure, salinity, runoff from rivers, and wave effects. monthiy means,
and
wind
Consequently, the averaging over shorter than seasonal periods, e.g.
is probably consistent with the basic assumption that the higher frequency
contributions to the sea-level signal are random in nature for an appropriate group
of
gauges. In computing the decadal scale trends, it is further implicitly assumed in the traditional approach that the inter-annual scale signals may also be treated statistically as noise relative to the long-term sea-level signal.
However, for the decadal time-scale problem
there now seems to be sufficient evidence to doubt the utility of the conventional averaging approach.
It is now known that the El Ni~o phenomenon, which dominates the inter-annual
signal, is at least of an ocean basin spatial scale, if not of a global scale. must be treated as a systematic error, rather than a random error.
This signal
A systematic error cannot
be elimin~edby a simple filtering process, such as numerical averaging over a large network of stations.
Consequently, any network of sea-level stations intended to resolve accurately
~.2
J..~L D l . ~ t ~ ' r E ~t ~1.
the decadal time scale signal must also be designed to resolve the inter-annual scale signal. In a data processing sense, this means that the systematic effects of the inter-annual signal must be modelled from ~ pr(or6 information and solved simultaneously with the longer period signals. In summary, the two critical climate scale sea-level phenomena (inter-annual
and decadal)
seem to be coupled inherently, both with respect to the design of the measurement network, as well as in the data processing sense. Therefore, the density of the stations in the network will be driven by the requirement to distinguish the inter-annual phenomena. In resolving decadal scale sea-level trends and the postulated rise in global sea-level due to greenhouse warming effects from atmospheric carbon dioxide and other atmospheric trace constituents, an additional complication arises. The conventional measurements of sea-level, using shore-based and island sea-level stations, only measure the relative change of the water surface compared to some nearby land-based reference mark. I t is now known that tectonic motion occurs not only on local scales, i.e. subsidence, but over time scales, space scales and amplitudes comparable to the computed trends in the sea-level signal on decadal and longer time scales. These latter effects are due to large-scale plate motions driven by sources deep within the earth, glacial rebound effects and the dynamic loading of the continental
shelves due to increased ocean mass resulting from the retreat of the
last ice age. The glacially-related effects have been shown recently to produce, in some places, a signal comparable in nature to the observed relative sea-level trends (WU and PELTIER, 1983; PELTIER 1984; PETTIER, 1986). The implications are that the experimental design for any global sea-level network must now have the capacity to separate this land (tectonic) signal from the oceanographic (long-term or eustatic) signal by providing appropriate spatial and temporal resolution and also by providing measurements relative to a global absolute geodetic reference system rather than local bench marks alone. Without such a capacity, interpretation of sea-level data is apt to result in spurious conclusions. As noted previously, the implementation of the global VLBI geodetic reference system now underway, offers a unique opportunity to meet absolute sea-level measurement requirements with accuracies to the centimetre level. In addition, all that is required is that periodic geodetic levelling be conducted using conventional methods and/or GPS techniques to tie certain key tidal/sea-level stations to the global VLBI network. The global VLBI framework can be projected to include within the same time period of this proposed global sea-level network (i.e. the Iggos), a total of about 33 stations distributed over the earth. These include: 20 fixed VLBI stations, 4 movable VLBI stations, 3 mobile systems, and 6 transportable systems. The network of approximately 33 VLBI stations provides a reasonably well-distributed global geodetic reference system defined by the established baselines connecting the stations and positioned with respect to the "fixed" astronomical radio sources. Co-located operations
Global change and the measurement of absolute ~a-l¢~,el
|3
with s a t e l l i t e laser ranging systems at selected VLBI stations w i l l provide positions relative to the geocentre.
Hence, the entire VLBI/GPS network w i l l also be referenced to the
geocentre early in i t s implementation schedule. This is important for potential applications to planned s a t e l l i t e radar altimeter missions, e.g. TOPEX.
I t should also be noted that
most of the existing and planned VLBI stations are r e l a t i v e l y close to the coastlines, providing a major advantage for absolute sea-level operations. By using the GPS system in a d i f f e r e n t i a l mode, the basic VLB[ network can be extended to about the same level of accuracy (about Icm in the vertical) to a set of stations at or near coastal and island tide/sea-level gauges. As a practical matter, some geodetic connections between tide gauges and GPS or VLBI reference stations w i l l be made using conventional levelling techniques over short distances.
Where possible, major tidal stations with well-
established historical t i d a l series and well-maintained geodetic control w i l l be selected, but some new stations w i l l also have to be established.
Sea-level stations on open coasts
that are more representative of open ocean conditions w i l l be preferred over stations in protected harbours or estuaries.
The GPS levelling to the VLBI network could be conducted
reasonably several times over the course of a year.
This means that the positions of the
tidal stations and the measured sea surface positions w i l l be determined in the geocentric VLBI/GPS reference system and that a time series of the absolute motion of the tidal bench marks (land reference points) w i l l also be established in that reference system. Analysis of i n i t i a l time series w i l l determine how often the GPS operations w i l l have to be conducted as a practical matter. Individual geodetic measurement accuracies on the centimetre level and measurements repeated at least several times annually w i l l allow any random noise component on the centimetre level to be removed to a level sufficient to resolve long-term trends in absolute motion. Long-term trends in land motion with amplitudes on the order of a few millimetres per year or less, can be resolved using several years of data.
This last conclusion is a consequence
of s t a t i s t i c a l averaging considerations. Now, the new class of sea-level gauges w i l l independently determine the vertical component of the water surface motion r e l a t i v e to the same land bench mark, to a corresponding level of accuracy.
A simple subtraction process allows separation of the long-term land motion
signal from long-term (inter-annual and longer) sea-level signal measured in the geocentric reference frame.
The result is the absolute sea-level signal.
Repeating the process at
other sea-level stations determines the absolute sea-level rise at those locations in the same global reference frame.
Hence, absolute sea-level rise profiles along coastlines can
be determined over large regions, and eventually related globally to island sea-level series.
time
All the time series w i l l be in the common global reference system. Figure 4 depicts
schematically the integrated application of the fixed VLB[ systems, mobile VLB[ systems and GPS units described above. Other operational and instrumental details have been provided by CARTER, ROBERTSON, PYLE and DIAMANTE (1986).
FIG.4.
_ . _
o,e
-
o f / f / / ~ ......-.
,
/ - ~ , ~ ' ~ ~
~
%',,%
"...
.....
~
.°ce,ve, Mobile VLBI
~-~----.__~.-
Schematic depiction of the combined usage of fixed VLBI, mobile VLBI and GPS systems for monitoring the s t a b i l i t y of a tide/sea-level gauge at a station of a g]obal sea-level network (from SCHERER, 1986b).
~
Receiver
Fixed
-4
nl
),
o
Global change and the measurement of absolute sea-level
1.5
Assuming that the ~Icm level of accuracy for both the geodetic measurements and water level measurements is largely limited by remaining errors of a random nature, then the total system error, on the basis of a single measurement of the absolute sea-level, can be approximated by the root mean square (RMS) error for the sum of the geodetic and water levels measurements, or about 1.4om.
Current global analyses suggest that the absolute, long-term sea-level
trend is in the neighbourhood of l-2mm per year (BARNETT, 1984), with comparable values for large-scale vertical land motions due to deglaciation effects (PELTIER, 1984, 1986). A 2mmy - I
trend continuing over a decade produces a 2cm effect, which is well above the
expected noise level of the measurements and should be readily observable. Therefore, in about a decade, i t should be possible to resolve any long-term trends of the land and water levels at an individual absolute sea-level station.
Several decades of measurements from
the entire global network w i l l characterise accurately both the inter-annual sea-level signal and the absolute sea-level signal on decadal and longer time scales. The latter information w i l l assist in distinguishing possible influences of natural long-term v a r i a b i l i t y of the climate system and possible effects of human activities (such as the postulated global carbon dioxide greenhouse warming). I t also should be noted that posslbillties exist for refining the accuracies of the geodetic measurements to about the O.Scm level.
In this case, the total RMS system error would drop
to about lcm for a single observation. Presently, the accuracy of the geodetic measurements is the limiting influence on the length of time series needed to recover s t a t i s t i c a l l y longperiod trends of small amplitude, since water level measurements can be made more frequently than the geodetic measurements. T h i s provides greater statistical reduction of the noise in the signal processing. The accumulation of records from sensors co-located with the tide gauges measuring barometric pressure, air and water temperature, winds, water density, and currents ( i f needed) w i l l allow analysis of the possible meteorological and other oceanographic contributions to water level trends and the implications for any long-term or eustatic sea-level rise.
These con-
siderations establish a network concept meeting the requirements for absolute measurements relative to a fixed geodetic reference system, with the necessary accuracy and resolution in time.
However, the requirements for the spatial resolution s t i l l need to be considered.
In designing the globally distributed network, we already have a number of constraints. One constraint is the need to t i e into the 33 VLBI reference stations and connected GPS stations.
I f the GPS stations are to be maintained to the centimetre level of accuracy
in the vertical, they cannot be an arbitrary distance away from the VLBI stations.
The
problem has already been discussed with respect to the differential stepping process bet~e~ the VLBI anchor points in the v i c i n i t y of a continental coastline.
However, in many
instances, GPS stations at the gauges w i l l have to be tied to a single VLBI reference station through the same GPS stepping process previously described, viz, island cluster situations. The limiting distance in this situation is not well determined at this time, but estimates range from a few hundred kilometres to over a thousand kilometres. For our requirements, we w i l l adopt a value of 500km as sufficiently conservative.
16
J, N|. Ot..~d,-~.TE Ct a!,
What emerges is a concept of 25-30 ocean regions commensurate with the locations of the major VLBI (see f i g . l ) stations and corresponding coastal segments of lO00-30OOkm in length and island clusters each containing about 2-3 absolute sea-level stations.
The total number
of absolute sea-level stations would therefore range between 50-90. Considering the special requirements of s a t e l l i t e altimeter mission calibration and other special applications, a s l i g h t l y higher density of island absolute sea-level stations may be needed. Therefore, an estimate of 100 total stations
appears reasonable.
Global coverage wojld be f a i r l y
good, except in the polar latitudes and certain c r i t i c a l areas such as the indian Ocean. The former areas pose operational and interpretative d i f f i c u l t i e s due to the presence of extensive ice sheets. Therefore, polar stations w i l l be a later phase of the global network development as far as absolute sea-level considerations are concerned. Areas like the Indian Ocean pose problems for s a t e l l i t e orbit determination for missions like TOPEX, so that independent consideration is being given to establishing permanent GPS stations somehwere in these regions.
The extensive measurement series from such stations
would probably a11ow their positions to be determined to the accuracy required here. Considering the results that BARNETT (1984) obtained using 6 regions, the we11-distributed 25 or so regions in the 1000-3000km scale should be able to resolve accurately long period signals.
Such signals are characteristic of the absolute long-term, or eustatic, sea-level
signal, i f
such exists, and the large-scale tectonic and glacially related land motions
along coastlines and islands.
Furthermore, physical considerations suggest that the charac-
t e r i s t i c wave-lengths of the dominant long-period signals should be on the order of a few thousand kilometres or more. Evidencefor this is given by AUBREY and EMERY (i983).
Excep-
tions may occur in tectonically active areas (see EMERY and AUBREY, 1986). However, we must s t i l l resolve the inter-annual oceanic signal and must be able to reduce the shorter wave length (local) oceanic, meteorological and tectonic "noise contributions" in each region as well. alone.
This cannot be done with a global network of about I00 absolute sea-level stations Spatial resolution requirements suggest that at least 9 or 10 stations within or
nearby each region w i l l be required to reduce the local scale noise, i.e. approximately 250-300 stations, in total, over the global ocean. The El NiBo scale ocean signal, as i t is now understood, would indicate a resolution requirement of about lO00km over the equatorial and temperate zones of the global ocean. The surface area of the earth is:
4~ Re2 = 4.5 x 108 km2.
Excluding polar regions and concentrat-
ing on oceanic regions gives an area of approxim@tely 2.5 x I08 km2 that
must be resolved
into a lO00km square grid (106 km2). This would require a well-distributed network of 250 sea-level stations.
Therefore both considerations of spatial and temporal resolution suggest
a global network of about 250-300 tide stations.
Such a global network includes approxim-
ately I00 stations that measure absolute sea-level. I t is important to note that the remaining stations of the global network (i.e. other than the approximately I00 absolute sea-level stations) do not require frequently monitored geodetic ties into the absolute VLBI/GPS reference system. These other stations need only
Global change and the measurement of Jbsolute ~ea-le~cl
IT
serve the purpose of modelling the temporal and spatial characteristics of the local scale and inter-annual effects, in order to f i l t e r such effects from the time series of the absolute sea-level stations.
Thereby, i t will be possible to resolve simultaneously both the
inter-annual and decadal signals in the global sea-level time series. This estimated size of the network is consistent with the proposed global sea-level network of WYRTKI and PUGH (1984), but the special considerations offered here might not lead to all of the same actual locations that they proposed.
5.
APPLICATIONSOF ABSOLUTESEA-LEVEL DATA
The potential value of a global sea-level network capable of resolving absolute sea-level changes for the WOCE(US WOCEScience Steering Committee, 1986) and the TOPEXradar altimeter satellite mission was suggested in the last section. The use of tide gauge data for calibration of a satellite based radar altimeter has already been demonstrated for the GEOS3 and SEASAT altimeters. (See TAPLEY, BORN, HAGAR, LORELL, PARKE, DIAMANTE, DOUGLAS,GOAD, KOLENKIEWICZ, MARSH, MARTIN, SMITH, TOWNSEND, WHITEHEAD, BYRNE, FEDOR, HAMMONDand MOGNARD, 197g; DIAMANTE, DOUGLAS, PORTERand MASTERSON, 1982; TAPLEY, BORNEand PARKE, 1982; and KOLI)IKIEWICZ and MARTIN, 198Z.) T h i s application involved use of a single conventional tide gauge on Bermuda providing relative sea-level data for removal of any constant instrument bias in the altimeter height, i.e. an engineering calibration. WUNSCH (1986), using the procedure of WUNSCHand ZLOTNICKI (1984), considered the use of a global network of conventional (relative) sea-level gauges, located mainly on islands, for another type of satellite radar altimeter calibration. WUNSCHconcluded that a properly distributed global network of 30-50 gauges could provide considerable reduction in the error (up to about 80% of the RMS error of the whole surface) effects in satellite altimeter derived sea surfaces.
The error signal analysed by WUNSCHwas caused by only the random
components of the tracking system and satellite orbital
position uncertainties.
WUNSCH
also noted that the potential for the use of a global network of absolute sea-level gauges to reduce systematic error effects had not been considered in his analysis. The satellite radar altimeter calibration problem in the broader sense is concerned with reducing the random and systematic components of error in the derived sea surface topography i.e. the problem of relating the satellite altimeter derived sea surface, with its inevitable spatial and temporal averaging, to the true dynamic sea surface and the dynamic land surface. The problem is compounded by the tendency of radar altimeters to lose electronic signal "lock" at land-water boundaries, making i t d i f f i c u l t to relate accurately such surfaces to coastal relative sea-level measurements and land-based geodetic systems. The orbital position errors tend to have significant systematic components, principally at long wavelengths (~I orbital revolution), and these inevitably contaminate the long wavelength information in the altimetric sea surfaces (TAPLEY and ROSBOROUGH, 1985). Such sea surfaces will tend to have systematic errors in the form of " t i l t s " or "twists" over ocean basin scales.
18
J . M . Ot...,,~t..,,:,.-r~ et al.
The problem is not acute, as long as the interest is only in the significant amplitude features having characteristic wave-lengths shorter than ocean basin scales, and so long as only relative changes in the sea surface over time periods that are short compared with the s a t e l l i t e mission l i f e time are the concern. However, when concerns are with monitoring changes in the large-scale features of the ocean surface over long time periods (decades) for long-term climate research and global ocean dynamics (WOCE) applications, can become serious.
the problem
The systematic error effects which differ slightly over the various
s a t e l l i t e orbital arcs (typically 3-5 days in length) used to determine the sea surface can differ significantly when an orbit manoeuvre is performed, and even more between d i f f e r ent s a t e l l i t e missions. The need to compare relative changes in any significantly large area of the ocean using altimeter-derived sea surfaces from successive s a t e l l i t e missions presents obvious d i f f i c u l ties.
Here, the globally distributed network of absolute sea-level stations achieves its
greatest value for s a t e l l i t e altimetry.
Absolute sea-level stations w i l l be in a common
global, high precision, geodetic reference system based on VLBI, GPS and referenced to the geocentre.
With plans for GPS and laser tracking of radar altimeter satellites, such as
TOPEX, the interconnections should lead to a unified reference system for both satellites and Cn 8¢tu sea-level measurements. This offers possibilities for significant improvement in orbit determination, and removal of substantial systematic errors at long wave-lengths from the altimetric sea-level topographic surfaces. A continuing reference system to relate all sea-level topographic surfaces across different s a t e l l i t e missions would also be provided by such a network.
Furthermore, the altimetric sea-level topographic surfaces, and the
(n s(~u sea-level time series can both be related to the dynamic land surface. In certain respects, the time series of vertical land motions at sea-level station sites may turn out to be even more significant for long-term climate research and global geosciences studies than the absolute sea-level information.
These land motion data w i l l pro-
vide verification and calibration of the geophysical models of PELTIER (1984, 1986) and others that relate mantle viscosity and deglaciation history to present rates of crustal subsidence and u p l i f t .
Similarly, these data w i l l a11ow calibration of relatively simple
empirical models for local ground subsidence due to withdrawal of ground water or o i i . It
is reasonable to expect that the relatively rigid crustal material w i l l exhibit less
v a r i a b i l i t y at decadal-to-century time scales than the coupled atmosphere-ocean fluid system as represented in the time series of the sea surface heights.
Hence, once the geodynamlc
models are verified and calibrated, they should be applicable over wide geographic areas and be able to provide both predictive and retrospective extrapolations over periods of centuries.
Such a confirmation w i l l increase the confidence in the use of historical tide
gauge data in studies of sea-level variations and their correlations to climate v a r i a b i l i t y on decadal and longer time scales.
Global change and the measurement of absolutesea-level
6.
19
CONCLUSIO~IS
We have outlined a concept for development of a two-tiered global sea-level network, t o t a l ing about 250-300 stations,
in which the majority of stations w i l l use conventional or
s l i g h t l y upgraded (e.g. s a t e l l i t e telemetry) technology.
In this global network, there
must be embedded a set of about 100 strategically located "absolute sea-level stations" characterised by improved sea-level sensors, measurement of additional meteorological oceanographic parameters and advanced geodetic control and WVR techniques.
and
using VLBI, interferometric GPS,
Implementation of such a network within the next decade is feasible
because of increased interest in ocean dynamics and global climate problems, the emergence of observational strategies for programmes such as TOGA, WOCEand the World Climate Research Programme (WCRP), and the'transition of a series of new technologies into operational status. The information provided on the long-term trends in the motions of both the sea surface and coastal land surfaces w i l l make these data of great value to "global change" research. I f our expectations are realised, then, as with any other fundamentally new class of measurements, we can anticipate answers to some of our old questions and a whole new set of questions regarding the interactive behaviour of the oceans, cryosphere and solid earth. The accumulating time series of absolute sea-level data w i l l provide a unique set of data for verifying and calibrating complexcoupled models of the ocean and atmosphere (and eventua l l y the cryosphere) for studying the dynamics of the global climate system on decadal and longer time scales.
I t is important to note that sea-level is a measurement that is repres-
entative of the end of the complex causal chain of interacting mechanisms in the climate system.
Hence, i t significantly lags many of the driving mechanisms in time and tends to
integrate the t o t a l i t y of these effects.
Therefore, i t is a v i r t u a l l y unique diagnostic
tool for u t i l i s a t i o n with climate system models, which also purport to integrate
all
effects.
The concept for a global absolute sea-level network is now approaching reality.
In 1987,
NOAA and the US Department of Energy's Carbon Dioxide Research Division, are jointly funding the start of five prototype absolute sea-level stations at Bermuda, Hawaii, Miami {Florida), Duck {North Carolina), and Boston {Massachusetts).
These will all include acoustic pulse-
type water level gauges, installed in parallel with conventional float-in-well gauges, modern satellite data collection platforms, ancillary meteorological and oceanographic instruments and VLBI/GPS operational connections.
These stations will be the first of a new class of
modern sea-level observatories representing a major departure from the familiar tide/sealevel gauge installations now in use.
7.
ACKNOgLEI)GEMENTS
The authors would l i k e to acknowledge K. Wyrtki, Baker f o r
discussions and encouragement.
C. gunsch, O. Pugh, g. Nowlin and D.J.
The authors are also indebted to H. Bezdek who
:1)
J.M.
DI~,XtA.~TEi t
aL
made major contributions to unpublished government documents (DIAMANTEand PYLE, 1983; PYLE, 1985) in which these concepts were originally developed and for fruitfu| discussions and encouragement throughout. We also wish to express our appreciation to W. Hudson for editoria] assistance and to R. Redmond and K. Cromwell for typing and correcting the manuscript.
8.
REFERENCES
AUBREY, D.G. and K.O. EMERY (1983) Eigenanalysis of Recent US Sea Levels. Cont~en~a~ Sh¢~f Research, 2, 21-33. BARNETT, T.P. (1984) The Estimation of "Global" Sea Level Change: A Problem of Uniqueness. Journa~ of Geophys~ca~ R@seareh, 89, 7980-7988. CARTER, W.E., D.S. ROBERTSON,T.E. PYLE and J.M. DIAMANTE (1986) The Application of Geodetic Radio Interferometric Surveying to the Monitoring of Sea L e v e l . Geophy~ica~ Jourma~ of the Roya~ AstronomioaE Sooie~ H, 87, 3-13. CARTER, W.E., D.S. ROBERTSONand J.R. MACKAY(1985) Geodetic Radio Interferometric Surveying: Applications and Results. JournaZ of Geophysioa~ Research, 90, 86, 4577-4587. DIAMANTE, J.M., B.C. DOUGLAS, D.L. PORTERand R.P. MASTERSON, Jr (1982) Tidal and Geodetic Observations for the SEASATAltimeter Calibration Experiment. Journa~ of Geophysioa~ Research, 87, C5, 3199-3206. DIAMANTE, J.M., and T.E. PYLE (1983) ISLANDS: Integm~ted Sea LeueZ a d XauCiea~ Data Seruioes, NOAAIssue Paper. Prepared by Office of the Chief Scientist, National Ocean Service. August 19, 1983, 1-63. EMERY, K.O. and D.G. AUBREY (1986) Relative Sea Level Change from Tide Gauge Records of Western North America. JouBa~ of Geophgaiea~ Research, 91, 814, 13,941-13,953. KOLENKIEWICZ, R. and C.F. MARTIN (1982) SEASATAltimeter Height Calibration. Jou~na~of Geophysioa~ Research, 87, C5, 3189-3198. KROGER, P.M., J.M. DAVIDSON and E.C. GARDNER(1986) Mobile VLBI and GPS Measurement of Vertical Crustal Motion. JournaZ of GeophysieaZ Research, 91, Bg, 9169-9176. PELTIER, W.R. (1984) The Thickness of the Continental Lithosphere. JournaZof GeophysicaZ Reseaeoh, 89, 303-316. PELTIER, W.R. (1986) Deglaciati6n Induced Vertical Motion of the North American Continent and Transient Lower Mantle Rheology. Jour~at of Ceophysiea~ Researoh, 91, B9, 9099-9123. PRESCOTT, W., J. SVARCand K. WENDT(1986) Precision of Global Positioning System Measurements for Crustal Deformation Studies: Initial Results. EOS: Transactions, Amerioan Geophysioa~ Union 67, 911. PYLE, T.E. (1985) G~obaE Sea Levee Progr~ (A HOAA Contribution to TOPEX and b'OCE), Program Deve~opmen~ P ~ a n . Office of Oceanic and Atmospheric Research, National Oceanic and Atmospheric Administration, June 4, 1985, 1-39. SCHERER, W.D. (1986a) NaCionaZOcean Sez~ice's Neu Generation Wa~er ~eve~ Measurement System. FIG, International Congress of Surveyors, June 1-11, 1986, Toronto, Canada, 4, 232-243. SCHERER, W.D. (1986b) Nationa~ Ocean Seruiee's Ne~ GeneraCion Wager LeveZ Measurement SysCem. Applications of Real-Time Oceanographic Circulation Modeling, Symposium Proceedings: The John Hopkins University Applied Physics Laboratory, Laurel, Maryland, May 23-24, 1985, edited by Bruce B Parker, Marine Technology Society, 103-114. STRANGE, W.E. (1985a) The Evaluation of GPS Capabilities for Monitoring Fluid Withdrawal Subsidence. gOS: Transactions, American GeophysioaE Union, 66, 847. STRANGE, W.E. (1985b) Centremeter Level Three Dimensional Crustal Motion Surveys with GPS. EOS: Transao~ions, American Geo~hyaioaE Union, 66, 847. TAPLEY, B.D., G.H. BORN and M.E. PARKE (1982) The SEASAT Altimeter Data and its Accuracy Assessment. Jou~aaZ of GeophysioaZ Research, 87, C5, 3177-3188. TAPLEY, B.D. and G.W. ROSBOROUGH(1985) Geographically Correlated Orbit Error and its Effect on Satellite Altimetry Missions. Jou~naZ of GeopkgsioaZ Research, go, 11,817-11,831 TAPLEY, B.D., G.H. BORN, H.H. HAGER, J. LORELL, M.E. PARKE, J.M. DIAMANTE, B.C. DOUGLAS, C.C. GOAD, R. KOLENKIEWICZ, J.G. MARSH, C.F. MARTIN, S.L. SMITH I l l , W.F. TOWNSEND, J.A. WHITEHEAD, H.M. BYRNE, L.S. FEDOR, D.C. HAMMONDand N.M. MOGNARD(1979) SEASATAltimeter Calibration: Initial Results. Soienoe, 204, 1410-1412.
Global change ~nd the measurement ~f ~-olu¢c ~c~-Ic~cl
.'I
WAR~, R., R.J. HURSTand C. ROCKEN(1986) GPS Baselines: Comparing Tropospheric Corrections Based on WVR Data to those Based on Surface Meteorological Data. cos= Tr~r.~aet~ons
A~er~eun Geoph~ea~ Un~on, 67, 44, 911.
~OC~ SCIENCE STEERING COMMITTEE, 1986) ~beZdOcean Cieo~c:~on ~peei~en¢, ScaCus ~eport on CS :VOCEP~anning. US WOCE Planning Report No.3, Contributions to th~ ~lanning of WOCE, January 1986. WU, P. and W.R. PELTIER (1983) Glacial Isostatic Adjustment and the Free Air Gravity Anomaly as a Constraint on Deep Mantle Viscosity. Geo?~)s~ ~o~n.a~ of the Ro?=~ As~eo-
no~oa~ Soe~y,
74, 377-450.
WU~SCH, C. (1986) CaP,beating an A~t¢.~eCer: Ho~ m~n~ T~de Gc:~g~s ~s enowgh? Unpublished Technical Paper in the form of a private communication to K. Wyrtki, 3. Pugh, D. Cartwright, T. Barnett, M. Lefebvre, J-F. Minster, W. Nowlin, T. Pyle and G. Needler. Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA 02139, January 27, 1986. WU~SCH, C. and V. ZLOTNICK[ (1984) The Accuracy of Altimeter Surfaces. Geoph~s~cc; Jo,~=~ of ~he Ro~aZ AsCeonom~ou~ Soe(et~ 78, 795-808. WYRTKI, K. (1979) The Response of Sea Surface Topography to the 1976 El NiBo. :o,~a~ of
Ph~s~ea~ Oeeanogeaph~, 9, 1223-1231
WYRTKI, K. (1985) Water Displacements in the Pacific and the Genesis of El Ni~o Cycles.
Journa~ of Geoph~s~e~ ReseaPoh, 90, C4, 7129-7132.
WYRTKI, K. and D. PUGH (1984) P~n joe ~ G~ob~Z Sea ~e~eZ ~ecwo~k. INF-563.
IOC/CCCOPublication
,l,"4.s,'.i ~*. S, lat - ~! C o p ~ n ~ t .~) l ~ " Pct'Fm~on J~mm.lh Ltd,
Global Change and the Measurement of Absolute Sea-Level JOHN,~l. DIAMA,~'rE*.THO,~LASE. [YLE*. WILLIAM E. CARTER~" and WOLFOA.~OSC~SREtV " O~ficeof Oceanic & A tmosplwric Rcmrm,ch.~VOA A. RockviUe. M D 20~ :, (,:$.A. *Jo/m OceanographicInstitutions. Inc.. 1753Mussaclu~ats Avenue N3V.$ui~ 800. Washi~o~. DC20036. U.S.A.
~Geodetic Res~rds & DevelopmentLaboratory. Nmiond OceanService. ,~'0.4A. Rockvil~. MD20832. U.S.A. and Lake Lcv'ds Branch. NoAfonalOceanSen'ice..VOAA. Rockville. MD
20~'. U.S.A.
ASSTUCT To quantify properly the long-tern response of sea-level to c11mate change, land motions must be separated from the apparent or relattve sea-level change recorded by conventional tide/sea-level gauges. Here we present a concept for global measurement of the true or "absolute" sea-level change, which combines recent advances tn space-based geodettc techniques with plans for a global sea-level network under the World Climate Research Programme (WCRP). Data from t n i t t a l feastbtl.tty tests show that land motion, due to global (plate tectonic), regtonal (glactal rebound), or lo¢al ( f l u i d wtthdranal) effects, can probably be moasured to ~1¢m (on a single measurement basis) by an innovative combination of Very Long 8aseltne I n t e r f e r ~ t r y (VLSl) and Global Pos|ttontng System (sPa) techniques. By making repeated observations of position at a number of ttde gauges using portable, economIcal GPS receivers tn a differential mode relattve to the fewer, more stable, but more expensive VLSI observatories, i t w i l l be posstble to subtract land motion from the relattve sea-level stgnal. Oecadal to century scale trends at the 1-2m y-1 level w i l l be resolvable tn the sea-level and vertical land motion time series within about a decade. Detection of subsidence or u p l l f t at specific gauges w t l l allow correction for land motion or deletion of bad data when computing regtonal or global, i . e . eustattc, sea-level changes. In addition to their applications in oceanography and climate studies, such data w i l l test models by Peltter and others that relate mantle viscosity and deglactatton history to present rates of crustal subsidence or u p l l f t . I f the predicted crustal motions are confirmed, we can also have more confidence in the use of historical ttde/seaolevel gauge records tn retrospective studies of sea-level change related to c11wte v a r i a b i l i t y on decadal or longer time scales. I t ts concluded that as few as one-third (about 100) of the total number of tide/sea-level gauges (250-300) required for globo1 c11mate research progranms (k"(RTKI and PUGH, 1984) need to be upgraded to "sea-level observatory" status, in terms of sensors and geodetic control, to build a two t i e r sea-level network that w t l l provide the data needed to solve many probloms of both relattve and absolute sea-level change. Such a network wtll !
J. M. DIA~,t,,STE et al.
provide basic information supporting research on a wide range of questions related to "global change".
CONTENTS I. .
3.
4.
Introduction Geodetic radio interferometric surveying in measurement of absolute sea-level 2.1
Application of VLBI
2.2
Application of GPS
Measurement system accuracies 3.1
Geodeticsystems
3.2
Sea-levelmeasurement systems
Design c r i t e r i a for an absolute sea-level network
11
5,
Applications of absolute sea-level data
17
6.
Conclusions
19
7.
Acknowledgements
19
8.
References
20
I . INTRODUCTION
Understanding the circulation of the ocean and i t s role in world climate w i l l require longterm global data bases of ocean observations analogous to the observation sets presently available for the atmosphere.
Of several candidates, the measurement of sea-level stands
out as one of the simplest and most effective.
Recent scientific results indicate that
these measurements have potential for supporting predictions of the onset of El Ni~o (WYRTKI
197g, 1985). They can also provide empirical estimates of heat transport by major ocean currents, and offer independent checks of global warming (or cooling) effects attributable to both natural v a r i a b i l i t y of the climate system and possible man-induced influences on the global climate, such as changes in atmospheric carbon dioxide and other trace gases contributing to a greenhouse warming effect.
Analyses of data from a well-distributed global
network of tide/sea level gauges also contribute to the success of certain s a t e l l i t e a l t i metry mission applications.
Radar altimeter satellites are orders of magnitude more costly
than the proposed sea-level network.
Global change and Ihe mcasuremenI of absolute sca-i¢~cl
3
Sea-level has long been measured by conventional tide gauges at many sites.
Its measurement
is so basic to safe navigation and to the establishment of maritime and coastal zone bo~ndaries that i t has become "routine".
However, the present network is: (a) patchy (i.e. aGgre-
gated in Europe, Japan and the USA);
(b) biased towards the northern hemisphere (only 8
of 155 stations used in a recent "global,' study (BARNETT, 1984) are in the southern hemisphere); and (c) not sited with any regard to ocean circulation patterns. The present network is not a coherent, global network, but an assortment of gauges located according to historic and economic, as opposed to physical, factors. For application to studies of ocean circulation and the ocean's role in climate, sea-level measurements will be needed at selected additional
sites around the world.
For practical
reasons, additional gauges should include updated sensors, digital recording, ancillary data inputs, and satellite telemetry capability. WYRTKI and PUGH (1984) have reviewed the existing global network and proposed potential additional sites (Fig.l). But the addition of more gauges alone is inadequate. For example, when use of a conventional tide/sea-level gauge to measure sea-level over a period of years indicates a trend of rising sea-level, one really cannot be sure whether the sea is rising or the land is sinking. Tide/sea-level gauges actually measure the relative motion between the sea surface and the land. Since the land can change due to a number ~f factors, e.g. effects of glacial rebound, compaction of sediments, or withdrawal of petroleum or ground water, the problem of relative motion must eventually be solved i f sea-level data are to be properly interpreted and used. The development of new geodetic techniques based on the Global Positioning System (GPS) and Very Long Baseline Interferometry (VLBI) has created the opportunity to link the simple, traditional sea-level measurements to a modern, highly accurate global reference frame being established by the international geodesy and radio-astronomy communities. The International Radio Interferometric Surveying (IRIS) programme has adopted a resolution calling for member nations to cooperate with the oceanographic community to establish such a global sea-level reference frame.
T h i s linkage of sea-level measurements to the new, space-based geodetic
networks will ultimately provide the f i r s t measure of absolute sea-level change. The prospect for relating sea-level measurements to a global reference system accurate to the centimetre level introduces the requirement for additional measurements of oceanographic and meteorological variables
at the tide/sea-level gauge sites.
Consequently, there is
a need to measure wind speed and direction, barometric pressure, salinity, air and water temperature (plus the capability for additional sensor inputs, i f needed) at each VLBI/GPSlinked sea-level station site. Such colocated oceanographic and meteorological measurement series, along with the sea-level records and precise geodetic data, will provide unique, invaluable time series and research data sets for satellite programmes, climate studies, meteorological modelling and oceanographic research.
80"
S
60"
40 -
20"
0•
20"
40 +
60"
N
80 °
20"E
FIG.1.
40"
80"
100 °
Existing (e) level network
60"
140"
160 °
180 +
160"
140*
120"
100 °
and p l a n n e d (I) tide/sea-level gauge as p r o p o s e d b y WYRTK[ and PUGH ( 1 9 8 4 ) .
120 °
sites
80"
in
60 °
a
O+ E 20 ~
sea
20"W
global
40 +
Global change ~nd the meusur=mcnt ~)I"Jb,,~)luIa~ca-l¢',~:l
2.
GEOOETICl~a~I)lO INTERFEROMEII~IC SURVEYING IN I~FJLSUREMENT
OF ABSOLUTESEA-LEVEL in order to measure absolute sea-level, both the vertical land motion at the individual sea-level stations and the local sea surface position w i l l need to be monitored relative to an ultra-precise absolute geodetic reference system based on VLBI and GPS technologies. Conceptually, i f not in practice, the approach is straightforward. 2.1
Ap?~(oa~o~ o~ VLBI
The VLBI technique employs two radio telescopes simultaneously observing the same extragalactic radio source.
The wave fronts of the electromagnetic emissions from those enor-
mously distant astronomical sources arrive at the radio telescopes as plane waves. A given wave front w i l l arrive at each observatory at slightly different times, dependent on the overall geometry. Each observatory independently records the arrival time of the signals, using atomic clocks.
The "time-tagged" signals fro~ the pairs of stations are differenced,
and the changes in the differences with time are used to estimate the interstation vectors (length, horizontal and vertical components).
In practice, fixed or mobile antenna systems
can serve as the basis for a given radio observatory. At the present time, the refraction effects produced by atmospheric water vapour are the largest remaining sources of systematic errors limiting the accuracy of VLBI systems for absolute sea-level applications.
However, a n~w generation of water vapour radiometers (WVR)
is currently being tested, which may greatly reduce this source of error (KROGER, DAVIDSON and GARDNER, 1986; WARE, HURSTand ROCKEN, 1986). Both VLBI and GPS are new, rapidly developing geodetic technologies which w i l l have certain limitations in geographic coverage into the 1990s that must be recognised in formulating the observing sequence for a global sea-level monitoring system. There are five types of VLBI systems: (I) permanently installed at dedicated geodetic observatories (G);
(2) perm-
anently installed at shared astronomic-geodetic observatories (S);
(3) movable systems
that require limited dismantling of the telescope to relocate (MV};
(4) mobile units in
which the telescopes are mounted on vehicles that can travel on highways without dismantling (MB); and (5) transportable units that can easily be moved among existing antennas in a time-sharing mode (T), which include all equipment other than the antennas. Table 1 contains a summary of the currently existing and planned f a c i l i t i e s .
The locations of types G and
S observatories are plotted on Figure 2. The concentration of VLBI f a c i l i t i e s in Europe, North America and eastern Asia into 1990 w i l l enable the North Atlantic and Pacific regions to be surveyed most accurately at the earliest dates. The situation is l i k e l y to be very much the samewith GPS i.e. the number and distribution of the base stations i n i t i a l l y w i l l result in s a t e l l i t e orbits being better determined over the northern hemisphere. After 1990, the global distribution of both VLBI stations and GPS satellites should improve substantially.
J.M. DI.~HA,~TECtat.
6
TABLE I: Type of VLBI System
Current and Proposed VLBI Sites Responsible Organisation
Location
Operational Date
Fort Davis, Texas, USA Westford, Massachusetts, USA Mojave, California, USA Richmond, Florida, USA Fairbanks, Alaska, USA Wettzell, FRG Kashima, Japan Shanghai, PRC Kunming, PRC Urumchi, PRC
NOAA NOAA NOAA NOAA NASA IFAG RRL SO SO SO
1980 1981 1983 1984 1984 1984 1984 1987 1989 1990
Bishop, California, USA Hatcreek, California, USA Maryland Point, Maryland, USA Greenbank, West Virginia, USA Kauai, Hawaii, USA Kwajelein Hartebeesthoek, SA SaD Paulo, Brazil Tidbinbillo, Australia Bologna, Italy Sicily, Italy Riyadh, Saudi Arabia Dehra Dun, India Onsala, Sweden
OVRO UCB NRL NRAO NASA USAF CSIR BG[ NASA IDR ISA NOP SI OSO
1980 1983 1983 1980 1984 1984 1986 1986 1986 1986 1986 1988 1989 1980
MV
USA (I unit) Japan (3 units)-
NOAA/NASA NGSJ
1980 1987
MB
USA (2 units) FRG (I unit)
NOAA/NASA IFAG
1984 1988
USA (4 units) USA (I unit) USA (I unit)
NASA NOAA NOAA
1985 1988 1989
G S MV MB T
Permanently installed at dedicated geodetic observatories Permanently installed at shared astronomlc-geodetic observatories ~W}¥able systems that require limited dismantling of the telescope Mobile systems-telescopes on vehicles - no dismantling Transportable systems - all equipment but telescope moves
Key to organisations
NOAA National Oceanic and Atmospheric Administration NASA National Aeronautics and Space Administration IFAG Institut fur Angewandte Geodasie RRL RadioResearch Laboratory NRL Naval Research Laboratory NRAO National Radio Astronomy Observatory USAF United States Air Force SO ShanghaiObservatory NGSJ National Geographical Survey of Japan
Council for Scientific and Industrial Research Brazilian Geophysical Institute BGI Italian Space Agency ISA Institute di Radioastronomia IDR National Observatory Project NOP Survey of India SI OSO Onsala Space Observatory OVRO Owens Valley Radio Observatory University of California at Berkeley UCB
CSIR
FIG.2.
I
I
I
i
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
LocaLions of major Very Long Baseline lnterfer~netr'y (VLBI) now operational and planned for the period 1987-1990.
I
VLBI Facilities, Present-1990
I
I
I
I
I
installations
i
I
=lr n
@-
0 5"
•~
J. M , DL,~,'.I.~,',,1E et td.
2.2
A?pZ6oc~on~fGPS
The GPS system uses oortable ground-based radio receivers to "observe" the coded radio transmissions from a series of precisely positioned spacecraft in earth orbit. ephemerides provide a geocentric
The s a t e l l i t e s '
reference system, relative to which the local position
of ground-based GPS receivers can be calculated. system employs a single receiver,
The usual application of the GPS
whose position is determined from timing,
range-rate calculations using the coded transmissions.
range and
However, absolute sea-level monitor-
ing operations principally w i l l employ another application of the GPS system.
By simultan-
eously operating two GPS receivers, a r e l a t i v e l y short distance apart (about lOO-2OOkm), the system provides what amounts to a short baseline radio interferometric system, analogous in principle, to the VLBI systems. Here, the s a t e l l i t e radio transmissions take the place of the distant extra-galactic radio sourcesused in VLBI. As with the VLBI systems, water vapour refraction effects again represent the most serious error source limiting the use of GPS for absolute sea-level monitoring.
However, the same
water vapour radiometer technology applicable to VLBI can also support the GPS operations (WARE, HURST and ROCKEN, 1986). The basic concept for GPS operations w i l l be to use a pair of GPS receivers to "step" d i f f e r e n t i a l l y along traverse lines connecting the main VLBI reference stations (fixed) and VLBI station points based on mobile systems with a series of tide gauge/sea-level stations along the open coastlines.
3. 3.1
MEASUREMENTSYSTEM ACCURACIES
Geodetic @seems
Accuracies of continental scale (1500-4000km) and intercontinental scale
(4OO0-70OOkm)
baselines using VLBI observing systems have been discussed by CARTER, ROBERTSON and MACKAY (Ig85), CARTER, ROBERTSON, PYLE and DIAMANTE (1986), (1986).
and KROGER, DAVIDSON and GARDNER,
I t now appears to be well-established that baselines thousands of kilometres in
length can be determined to within a few centimetres, using r e l a t i v e l y short sessions (24 hours or less).
observing
Much of the remaining systematic error, caused by radio ref-
raction effects from atmospheric water vapour, may be removed by the use of new water vapour radiometer systems now becoming available. Consequently,baselines 15OOkm in length, and longer, should be operationally measurable to the ±Icm level of accuracy for a single VLBI measurement session (24 hours or less).
With adequate observing schedules, some VLBI
baselines are now being determined to the ±Icm level even without application of WVR systems, by the use of adjustable scale height models of the atmospheric water vapour distribution in the VLBI data reduction process.
The remaining errors involve sources such as temperature
wind and gravitational deformations of the radio telescope, that w i l l be d i f f i c u l t to model and remove. Therefore, i t appears that the ±Icm level w i l l be the practical accuracy l i m i t
Global chan~c and the measurementof ai~lut~ '~¢;*-!c'.~i
of VLBI measurements for absolute sea-level applications well into the decade of the 1990s. The accuracies of single baselines (up to 50km in length) determined from GPS measurements have been discussed by STRANGE (1985a, lgBSb); WARE, HURST and ROCKEN (1986); PRESCOTT, SVARC and WENDT (1986); and CARTER, ROBERTSON, PYLE and DIAHANTE (1986). Again, a major error source in the vertical component of GPS baselines is atmospheric water vapour effects. As with VLBI, i t may be possible to correct these errors with the latest water vapour radiometer technology. The 100-ZOOkm GPS baselines are conservative values; there are indications that GPS measure-
ments of the baselines may be ultimately good to this ±Icm level over several hundreds of kilometres.
This uncertainty in the operational limits for the GPS baselines also translates
into corresponding uncertainty in the total error budget for a differential stepping process using two GPS units "leap-frogging" between the VLBI anchor points.
I f the GPS performance
allows steps on the order of at least 100km or so, then the 1500km between the VLBI anchor points is expected to allow the total error budget for the single measurement accuracy of VLBI and GPS baselines for absolute sea-level to stay within the ±Icm level. However, i f under the f u l l range of global operations, the GPS limits should turn out to be more pessimistic, then the 1500km distance between VLBI anchor points may turn out to be excessive in some regions of the globe.
In that case, additional VLBI reference points
would need to be established using mobile VLBI systems to reduce the VLBI baseline distances to the 500-750km level. 3.2
Sea-Ce~eZ Measurement Systems
For climate applications, particularly the decadal and longer time scale concerns requiring determination of long-term trends in absolute sea-level, a number of serious problems exists with much of the standard tides/sea-level measurement technology.
These typically employ
float-in-well systems. The float operates in a well with a small orifice (a s t i l l i n g wet1). The s t i l l i n g well acts as a frequency dependent mechanical f i l t e r for the high frequency wave effects.
However, these high frequency effects of the sea surface are not all filtered
out and the s t i l l i n g well is subject to density gradients affecting the float, wave buildup effects, bio-fouling, etc.
The existing designs of s t i l l i n g wells are particularly subject
to hydrodynamic "draw-down" effects wherever there is a significant ambient current.
The
float system also has inertial response problems and experiences gradual buoyancy changes. The accuracy of a single measurement of the sea surface using such systems is not better than about 5cm. Someof this error is due to nearly random effects of the high frequency signals (waves) that can be s t a t i s t i c a l l y reduced or mathematically filtered in multi-year time series, so that long-term trends of a few millimetres per year can be extracted from the data.
However, to the extent that the other errors cited introduce systematic errors
that are not random and cannot be removed a p~(or¢ ,
they may leave small amplitude, long-
period effects that are v i r t u a l l y indistinguishable
(without some independent basis for
J.M. Dt.~.t-~',TE era/.
I()
determining them) from the sea-level trends.
Thesepossible remaining long period systematic
errors can only be dealt with, i f at a l l , by averaging measurements from many gauges, distributed over a wide geographic area.
I t should be noted that effects like hydrodynamic
draw-down, which are always ~0, w i l l never be entirely removed by simple averaging methods, even over a large ensemble of gauges representing a wide geographical area. By replacing the float with a remote sensor not involving the physical contact of a device with the water surface,
a large number of sources
of
systematic error
are either eliminated, or greatly reduced. There is no longer any need for a mechanical f i l t e r in the form of a s t i l l i n g well with a narrow orifice, so that i t is replaced by an open protective well, much less subject to nonlinear f i l t e r i n g effects and bio-fouling, not conducive to density gradient build-ups, and more indicative of the outside ocean environment. The available remote sensors meeting the accuracy requirements and practical power consumption limitations s t i l l require a protective well of some kind, subject to hydrodynamic draw-down. However, the open wells can be protected against major draw-down effects. New gauges of the acoustic type as well as data collection platforms, ancillary meteorological sensors, telemetry systems and newly designed protective wells are being developed and deployed as the New Generation Water Level Measurement System (NGWLMS) by the US National Oceanic and Atmospheric Administration (NOAA) for the US National Water Level Observation Network (SCHERER, 1986a,b). Systems of the NGWLMS type w i l l be required at all of the major sea-level stations, linked by VLBI and GPS, that are intended to provide long-term monitoring of the absolute sea-level.
A schematic representation of the NGWLMS total system concept
is shown in Figure 3.
~ L
"'rift,.
"'",,.,t,o
I ~
-'."~
l ,,,,,,'
R~te s,,s°~ ~II~rLIK n n~ .,.'---ill"
FIG.3.
,~,.
~,~,~°'~
"//N
GO[$ .:
~,rJ"
N~ o:~,:, " /
Schematic representation of the total system concept for the NO~ New Generation Water Level Measurement System (NGWLMS) from SCHERER (1986b). -
Global chan~e and th~ measurement of absolute ~:a-leveI
11
Based on the design specifications and test results from i n i t i a l prototype systems, a single ~easurement from the NG~LMS should represent nearly an order of magnitude improvement over the existing measurement type, i.e. the single measurement w i l l be good to the :Icm level, with most of the known systematic error effects either removed or greatly reduced. This is consistent with the expectations for the single measurements from the geodetic systems.
4.
DESIGN CONSIDERATIONS FOR AN ABSOLUTE SEA-LEVEL NETWORK
The discussion in the previous sections mainly concerned the determination of absolute sealevel at a single tide/sea-level station or at a pair of stations.
The determination of
the decadal or longer term global sea-level signal, including separation of the absolute component of sea-level from the relative sea-level signal, requires consideration of the design c r i t e r i a for a global sea-level measurement network. The sea-level signals of interest to climate applications have been at two main time scales: (a) an inter-annual scale pertaining to short-term climate fluctuations, and (b) a decadal scale, often assumed to be representative of longer term trends in eustatic sea-level. The long-term trend is generally, but not universally, believed to be indicative of a general secular rise of sea-level, possibly with an increasing rate of rise in recent decades. However, the climate system is also known to exhibit temporal fluctuations on the scale of decades to centuries.
Thesemay be reflected in the sea-level data.
In either case, the
traditional approach in computing mean sea-level trends was based on either a straight or weighted numerical average of corresponding sea-level time series over large regional or global ensembles of stations.
Implicit in this application of averaging procedures is the
assumption that all local oceanographic, meteorological, and geological influences on the water level signal, shorter than the sea-level periods of interest, may be considered to be "noise", i.e. randomly distributed errors relative to the large ensembles of stations. Such higher frequency phenomena are typically of a spatially localised nature and include such effects as currents, barometric pressure, salinity, runoff from rivers, and wave effects. monthiy means,
and
wind
Consequently, the averaging over shorter than seasonal periods, e.g.
is probably consistent with the basic assumption that the higher frequency
contributions to the sea-level signal are random in nature for an appropriate group
of
gauges. In computing the decadal scale trends, it is further implicitly assumed in the traditional approach that the inter-annual scale signals may also be treated statistically as noise relative to the long-term sea-level signal.
However, for the decadal time-scale problem
there now seems to be sufficient evidence to doubt the utility of the conventional averaging approach.
It is now known that the El Ni~o phenomenon, which dominates the inter-annual
signal, is at least of an ocean basin spatial scale, if not of a global scale. must be treated as a systematic error, rather than a random error.
This signal
A systematic error cannot
be elimin~edby a simple filtering process, such as numerical averaging over a large network of stations.
Consequently, any network of sea-level stations intended to resolve accurately
~.2
J..~L D l . ~ t ~ ' r E ~t ~1.
the decadal time scale signal must also be designed to resolve the inter-annual scale signal. In a data processing sense, this means that the systematic effects of the inter-annual signal must be modelled from ~ pr(or6 information and solved simultaneously with the longer period signals. In summary, the two critical climate scale sea-level phenomena (inter-annual
and decadal)
seem to be coupled inherently, both with respect to the design of the measurement network, as well as in the data processing sense. Therefore, the density of the stations in the network will be driven by the requirement to distinguish the inter-annual phenomena. In resolving decadal scale sea-level trends and the postulated rise in global sea-level due to greenhouse warming effects from atmospheric carbon dioxide and other atmospheric trace constituents, an additional complication arises. The conventional measurements of sea-level, using shore-based and island sea-level stations, only measure the relative change of the water surface compared to some nearby land-based reference mark. I t is now known that tectonic motion occurs not only on local scales, i.e. subsidence, but over time scales, space scales and amplitudes comparable to the computed trends in the sea-level signal on decadal and longer time scales. These latter effects are due to large-scale plate motions driven by sources deep within the earth, glacial rebound effects and the dynamic loading of the continental
shelves due to increased ocean mass resulting from the retreat of the
last ice age. The glacially-related effects have been shown recently to produce, in some places, a signal comparable in nature to the observed relative sea-level trends (WU and PELTIER, 1983; PELTIER 1984; PETTIER, 1986). The implications are that the experimental design for any global sea-level network must now have the capacity to separate this land (tectonic) signal from the oceanographic (long-term or eustatic) signal by providing appropriate spatial and temporal resolution and also by providing measurements relative to a global absolute geodetic reference system rather than local bench marks alone. Without such a capacity, interpretation of sea-level data is apt to result in spurious conclusions. As noted previously, the implementation of the global VLBI geodetic reference system now underway, offers a unique opportunity to meet absolute sea-level measurement requirements with accuracies to the centimetre level. In addition, all that is required is that periodic geodetic levelling be conducted using conventional methods and/or GPS techniques to tie certain key tidal/sea-level stations to the global VLBI network. The global VLBI framework can be projected to include within the same time period of this proposed global sea-level network (i.e. the Iggos), a total of about 33 stations distributed over the earth. These include: 20 fixed VLBI stations, 4 movable VLBI stations, 3 mobile systems, and 6 transportable systems. The network of approximately 33 VLBI stations provides a reasonably well-distributed global geodetic reference system defined by the established baselines connecting the stations and positioned with respect to the "fixed" astronomical radio sources. Co-located operations
Global change and the measurement of absolute ~a-l¢~,el
|3
with s a t e l l i t e laser ranging systems at selected VLBI stations w i l l provide positions relative to the geocentre.
Hence, the entire VLBI/GPS network w i l l also be referenced to the
geocentre early in i t s implementation schedule. This is important for potential applications to planned s a t e l l i t e radar altimeter missions, e.g. TOPEX.
I t should also be noted that
most of the existing and planned VLBI stations are r e l a t i v e l y close to the coastlines, providing a major advantage for absolute sea-level operations. By using the GPS system in a d i f f e r e n t i a l mode, the basic VLB[ network can be extended to about the same level of accuracy (about Icm in the vertical) to a set of stations at or near coastal and island tide/sea-level gauges. As a practical matter, some geodetic connections between tide gauges and GPS or VLBI reference stations w i l l be made using conventional levelling techniques over short distances.
Where possible, major tidal stations with well-
established historical t i d a l series and well-maintained geodetic control w i l l be selected, but some new stations w i l l also have to be established.
Sea-level stations on open coasts
that are more representative of open ocean conditions w i l l be preferred over stations in protected harbours or estuaries.
The GPS levelling to the VLBI network could be conducted
reasonably several times over the course of a year.
This means that the positions of the
tidal stations and the measured sea surface positions w i l l be determined in the geocentric VLBI/GPS reference system and that a time series of the absolute motion of the tidal bench marks (land reference points) w i l l also be established in that reference system. Analysis of i n i t i a l time series w i l l determine how often the GPS operations w i l l have to be conducted as a practical matter. Individual geodetic measurement accuracies on the centimetre level and measurements repeated at least several times annually w i l l allow any random noise component on the centimetre level to be removed to a level sufficient to resolve long-term trends in absolute motion. Long-term trends in land motion with amplitudes on the order of a few millimetres per year or less, can be resolved using several years of data.
This last conclusion is a consequence
of s t a t i s t i c a l averaging considerations. Now, the new class of sea-level gauges w i l l independently determine the vertical component of the water surface motion r e l a t i v e to the same land bench mark, to a corresponding level of accuracy.
A simple subtraction process allows separation of the long-term land motion
signal from long-term (inter-annual and longer) sea-level signal measured in the geocentric reference frame.
The result is the absolute sea-level signal.
Repeating the process at
other sea-level stations determines the absolute sea-level rise at those locations in the same global reference frame.
Hence, absolute sea-level rise profiles along coastlines can
be determined over large regions, and eventually related globally to island sea-level series.
time
All the time series w i l l be in the common global reference system. Figure 4 depicts
schematically the integrated application of the fixed VLB[ systems, mobile VLB[ systems and GPS units described above. Other operational and instrumental details have been provided by CARTER, ROBERTSON, PYLE and DIAMANTE (1986).
FIG.4.
_ . _
o,e
-
o f / f / / ~ ......-.
,
/ - ~ , ~ ' ~ ~
~
%',,%
"...
.....
~
.°ce,ve, Mobile VLBI
~-~----.__~.-
Schematic depiction of the combined usage of fixed VLBI, mobile VLBI and GPS systems for monitoring the s t a b i l i t y of a tide/sea-level gauge at a station of a g]obal sea-level network (from SCHERER, 1986b).
~
Receiver
Fixed
-4
nl
),
o
Global change and the measurement of absolute sea-level
1.5
Assuming that the ~Icm level of accuracy for both the geodetic measurements and water level measurements is largely limited by remaining errors of a random nature, then the total system error, on the basis of a single measurement of the absolute sea-level, can be approximated by the root mean square (RMS) error for the sum of the geodetic and water levels measurements, or about 1.4om.
Current global analyses suggest that the absolute, long-term sea-level
trend is in the neighbourhood of l-2mm per year (BARNETT, 1984), with comparable values for large-scale vertical land motions due to deglaciation effects (PELTIER, 1984, 1986). A 2mmy - I
trend continuing over a decade produces a 2cm effect, which is well above the
expected noise level of the measurements and should be readily observable. Therefore, in about a decade, i t should be possible to resolve any long-term trends of the land and water levels at an individual absolute sea-level station.
Several decades of measurements from
the entire global network w i l l characterise accurately both the inter-annual sea-level signal and the absolute sea-level signal on decadal and longer time scales. The latter information w i l l assist in distinguishing possible influences of natural long-term v a r i a b i l i t y of the climate system and possible effects of human activities (such as the postulated global carbon dioxide greenhouse warming). I t also should be noted that posslbillties exist for refining the accuracies of the geodetic measurements to about the O.Scm level.
In this case, the total RMS system error would drop
to about lcm for a single observation. Presently, the accuracy of the geodetic measurements is the limiting influence on the length of time series needed to recover s t a t i s t i c a l l y longperiod trends of small amplitude, since water level measurements can be made more frequently than the geodetic measurements. T h i s provides greater statistical reduction of the noise in the signal processing. The accumulation of records from sensors co-located with the tide gauges measuring barometric pressure, air and water temperature, winds, water density, and currents ( i f needed) w i l l allow analysis of the possible meteorological and other oceanographic contributions to water level trends and the implications for any long-term or eustatic sea-level rise.
These con-
siderations establish a network concept meeting the requirements for absolute measurements relative to a fixed geodetic reference system, with the necessary accuracy and resolution in time.
However, the requirements for the spatial resolution s t i l l need to be considered.
In designing the globally distributed network, we already have a number of constraints. One constraint is the need to t i e into the 33 VLBI reference stations and connected GPS stations.
I f the GPS stations are to be maintained to the centimetre level of accuracy
in the vertical, they cannot be an arbitrary distance away from the VLBI stations.
The
problem has already been discussed with respect to the differential stepping process bet~e~ the VLBI anchor points in the v i c i n i t y of a continental coastline.
However, in many
instances, GPS stations at the gauges w i l l have to be tied to a single VLBI reference station through the same GPS stepping process previously described, viz, island cluster situations. The limiting distance in this situation is not well determined at this time, but estimates range from a few hundred kilometres to over a thousand kilometres. For our requirements, we w i l l adopt a value of 500km as sufficiently conservative.
16
J, N|. Ot..~d,-~.TE Ct a!,
What emerges is a concept of 25-30 ocean regions commensurate with the locations of the major VLBI (see f i g . l ) stations and corresponding coastal segments of lO00-30OOkm in length and island clusters each containing about 2-3 absolute sea-level stations.
The total number
of absolute sea-level stations would therefore range between 50-90. Considering the special requirements of s a t e l l i t e altimeter mission calibration and other special applications, a s l i g h t l y higher density of island absolute sea-level stations may be needed. Therefore, an estimate of 100 total stations
appears reasonable.
Global coverage wojld be f a i r l y
good, except in the polar latitudes and certain c r i t i c a l areas such as the indian Ocean. The former areas pose operational and interpretative d i f f i c u l t i e s due to the presence of extensive ice sheets. Therefore, polar stations w i l l be a later phase of the global network development as far as absolute sea-level considerations are concerned. Areas like the Indian Ocean pose problems for s a t e l l i t e orbit determination for missions like TOPEX, so that independent consideration is being given to establishing permanent GPS stations somehwere in these regions.
The extensive measurement series from such stations
would probably a11ow their positions to be determined to the accuracy required here. Considering the results that BARNETT (1984) obtained using 6 regions, the we11-distributed 25 or so regions in the 1000-3000km scale should be able to resolve accurately long period signals.
Such signals are characteristic of the absolute long-term, or eustatic, sea-level
signal, i f
such exists, and the large-scale tectonic and glacially related land motions
along coastlines and islands.
Furthermore, physical considerations suggest that the charac-
t e r i s t i c wave-lengths of the dominant long-period signals should be on the order of a few thousand kilometres or more. Evidencefor this is given by AUBREY and EMERY (i983).
Excep-
tions may occur in tectonically active areas (see EMERY and AUBREY, 1986). However, we must s t i l l resolve the inter-annual oceanic signal and must be able to reduce the shorter wave length (local) oceanic, meteorological and tectonic "noise contributions" in each region as well. alone.
This cannot be done with a global network of about I00 absolute sea-level stations Spatial resolution requirements suggest that at least 9 or 10 stations within or
nearby each region w i l l be required to reduce the local scale noise, i.e. approximately 250-300 stations, in total, over the global ocean. The El NiBo scale ocean signal, as i t is now understood, would indicate a resolution requirement of about lO00km over the equatorial and temperate zones of the global ocean. The surface area of the earth is:
4~ Re2 = 4.5 x 108 km2.
Excluding polar regions and concentrat-
ing on oceanic regions gives an area of approxim@tely 2.5 x I08 km2 that
must be resolved
into a lO00km square grid (106 km2). This would require a well-distributed network of 250 sea-level stations.
Therefore both considerations of spatial and temporal resolution suggest
a global network of about 250-300 tide stations.
Such a global network includes approxim-
ately I00 stations that measure absolute sea-level. I t is important to note that the remaining stations of the global network (i.e. other than the approximately I00 absolute sea-level stations) do not require frequently monitored geodetic ties into the absolute VLBI/GPS reference system. These other stations need only
Global change and the measurement of Jbsolute ~ea-le~cl
IT
serve the purpose of modelling the temporal and spatial characteristics of the local scale and inter-annual effects, in order to f i l t e r such effects from the time series of the absolute sea-level stations.
Thereby, i t will be possible to resolve simultaneously both the
inter-annual and decadal signals in the global sea-level time series. This estimated size of the network is consistent with the proposed global sea-level network of WYRTKI and PUGH (1984), but the special considerations offered here might not lead to all of the same actual locations that they proposed.
5.
APPLICATIONSOF ABSOLUTESEA-LEVEL DATA
The potential value of a global sea-level network capable of resolving absolute sea-level changes for the WOCE(US WOCEScience Steering Committee, 1986) and the TOPEXradar altimeter satellite mission was suggested in the last section. The use of tide gauge data for calibration of a satellite based radar altimeter has already been demonstrated for the GEOS3 and SEASAT altimeters. (See TAPLEY, BORN, HAGAR, LORELL, PARKE, DIAMANTE, DOUGLAS,GOAD, KOLENKIEWICZ, MARSH, MARTIN, SMITH, TOWNSEND, WHITEHEAD, BYRNE, FEDOR, HAMMONDand MOGNARD, 197g; DIAMANTE, DOUGLAS, PORTERand MASTERSON, 1982; TAPLEY, BORNEand PARKE, 1982; and KOLI)IKIEWICZ and MARTIN, 198Z.) T h i s application involved use of a single conventional tide gauge on Bermuda providing relative sea-level data for removal of any constant instrument bias in the altimeter height, i.e. an engineering calibration. WUNSCH (1986), using the procedure of WUNSCHand ZLOTNICKI (1984), considered the use of a global network of conventional (relative) sea-level gauges, located mainly on islands, for another type of satellite radar altimeter calibration. WUNSCHconcluded that a properly distributed global network of 30-50 gauges could provide considerable reduction in the error (up to about 80% of the RMS error of the whole surface) effects in satellite altimeter derived sea surfaces.
The error signal analysed by WUNSCHwas caused by only the random
components of the tracking system and satellite orbital
position uncertainties.
WUNSCH
also noted that the potential for the use of a global network of absolute sea-level gauges to reduce systematic error effects had not been considered in his analysis. The satellite radar altimeter calibration problem in the broader sense is concerned with reducing the random and systematic components of error in the derived sea surface topography i.e. the problem of relating the satellite altimeter derived sea surface, with its inevitable spatial and temporal averaging, to the true dynamic sea surface and the dynamic land surface. The problem is compounded by the tendency of radar altimeters to lose electronic signal "lock" at land-water boundaries, making i t d i f f i c u l t to relate accurately such surfaces to coastal relative sea-level measurements and land-based geodetic systems. The orbital position errors tend to have significant systematic components, principally at long wavelengths (~I orbital revolution), and these inevitably contaminate the long wavelength information in the altimetric sea surfaces (TAPLEY and ROSBOROUGH, 1985). Such sea surfaces will tend to have systematic errors in the form of " t i l t s " or "twists" over ocean basin scales.
18
J . M . Ot...,,~t..,,:,.-r~ et al.
The problem is not acute, as long as the interest is only in the significant amplitude features having characteristic wave-lengths shorter than ocean basin scales, and so long as only relative changes in the sea surface over time periods that are short compared with the s a t e l l i t e mission l i f e time are the concern. However, when concerns are with monitoring changes in the large-scale features of the ocean surface over long time periods (decades) for long-term climate research and global ocean dynamics (WOCE) applications, can become serious.
the problem
The systematic error effects which differ slightly over the various
s a t e l l i t e orbital arcs (typically 3-5 days in length) used to determine the sea surface can differ significantly when an orbit manoeuvre is performed, and even more between d i f f e r ent s a t e l l i t e missions. The need to compare relative changes in any significantly large area of the ocean using altimeter-derived sea surfaces from successive s a t e l l i t e missions presents obvious d i f f i c u l ties.
Here, the globally distributed network of absolute sea-level stations achieves its
greatest value for s a t e l l i t e altimetry.
Absolute sea-level stations w i l l be in a common
global, high precision, geodetic reference system based on VLBI, GPS and referenced to the geocentre.
With plans for GPS and laser tracking of radar altimeter satellites, such as
TOPEX, the interconnections should lead to a unified reference system for both satellites and Cn 8¢tu sea-level measurements. This offers possibilities for significant improvement in orbit determination, and removal of substantial systematic errors at long wave-lengths from the altimetric sea-level topographic surfaces. A continuing reference system to relate all sea-level topographic surfaces across different s a t e l l i t e missions would also be provided by such a network.
Furthermore, the altimetric sea-level topographic surfaces, and the
(n s(~u sea-level time series can both be related to the dynamic land surface. In certain respects, the time series of vertical land motions at sea-level station sites may turn out to be even more significant for long-term climate research and global geosciences studies than the absolute sea-level information.
These land motion data w i l l pro-
vide verification and calibration of the geophysical models of PELTIER (1984, 1986) and others that relate mantle viscosity and deglaciation history to present rates of crustal subsidence and u p l i f t .
Similarly, these data w i l l a11ow calibration of relatively simple
empirical models for local ground subsidence due to withdrawal of ground water or o i i . It
is reasonable to expect that the relatively rigid crustal material w i l l exhibit less
v a r i a b i l i t y at decadal-to-century time scales than the coupled atmosphere-ocean fluid system as represented in the time series of the sea surface heights.
Hence, once the geodynamlc
models are verified and calibrated, they should be applicable over wide geographic areas and be able to provide both predictive and retrospective extrapolations over periods of centuries.
Such a confirmation w i l l increase the confidence in the use of historical tide
gauge data in studies of sea-level variations and their correlations to climate v a r i a b i l i t y on decadal and longer time scales.
Global change and the measurement of absolutesea-level
6.
19
CONCLUSIO~IS
We have outlined a concept for development of a two-tiered global sea-level network, t o t a l ing about 250-300 stations,
in which the majority of stations w i l l use conventional or
s l i g h t l y upgraded (e.g. s a t e l l i t e telemetry) technology.
In this global network, there
must be embedded a set of about 100 strategically located "absolute sea-level stations" characterised by improved sea-level sensors, measurement of additional meteorological oceanographic parameters and advanced geodetic control and WVR techniques.
and
using VLBI, interferometric GPS,
Implementation of such a network within the next decade is feasible
because of increased interest in ocean dynamics and global climate problems, the emergence of observational strategies for programmes such as TOGA, WOCEand the World Climate Research Programme (WCRP), and the'transition of a series of new technologies into operational status. The information provided on the long-term trends in the motions of both the sea surface and coastal land surfaces w i l l make these data of great value to "global change" research. I f our expectations are realised, then, as with any other fundamentally new class of measurements, we can anticipate answers to some of our old questions and a whole new set of questions regarding the interactive behaviour of the oceans, cryosphere and solid earth. The accumulating time series of absolute sea-level data w i l l provide a unique set of data for verifying and calibrating complexcoupled models of the ocean and atmosphere (and eventua l l y the cryosphere) for studying the dynamics of the global climate system on decadal and longer time scales.
I t is important to note that sea-level is a measurement that is repres-
entative of the end of the complex causal chain of interacting mechanisms in the climate system.
Hence, i t significantly lags many of the driving mechanisms in time and tends to
integrate the t o t a l i t y of these effects.
Therefore, i t is a v i r t u a l l y unique diagnostic
tool for u t i l i s a t i o n with climate system models, which also purport to integrate
all
effects.
The concept for a global absolute sea-level network is now approaching reality.
In 1987,
NOAA and the US Department of Energy's Carbon Dioxide Research Division, are jointly funding the start of five prototype absolute sea-level stations at Bermuda, Hawaii, Miami {Florida), Duck {North Carolina), and Boston {Massachusetts).
These will all include acoustic pulse-
type water level gauges, installed in parallel with conventional float-in-well gauges, modern satellite data collection platforms, ancillary meteorological and oceanographic instruments and VLBI/GPS operational connections.
These stations will be the first of a new class of
modern sea-level observatories representing a major departure from the familiar tide/sealevel gauge installations now in use.
7.
ACKNOgLEI)GEMENTS
The authors would l i k e to acknowledge K. Wyrtki, Baker f o r
discussions and encouragement.
C. gunsch, O. Pugh, g. Nowlin and D.J.
The authors are also indebted to H. Bezdek who
:1)
J.M.
DI~,XtA.~TEi t
aL
made major contributions to unpublished government documents (DIAMANTEand PYLE, 1983; PYLE, 1985) in which these concepts were originally developed and for fruitfu| discussions and encouragement throughout. We also wish to express our appreciation to W. Hudson for editoria] assistance and to R. Redmond and K. Cromwell for typing and correcting the manuscript.
8.
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IOC/CCCOPublication