Long-term sea level change has ordinarily been estimated from tide gauge data. However, two fundamental problems are encountered using tide gauge measurements for this purpose. First, tide gauges only measure sea level change relative to a crustal reference point, which may be moving vertically at rates comparable to the true sea level signals (Douglas, 1995). Second, tide gauges have limited spatial distribution and suboptimal coastal locations (Barnett, 1984; Groger and Plag, 1993) and thus provide poor spatial sampling of the open ocean. Douglas (1991, 1992) has argued that by selecting tide gauge records of at least 50 years in length and away from tectonically active areas, even a limited set of poorly distributed tide gauges can give a useful estimate of global sea level rise. However, averaging over such a long time period makes the investigation of shorter term changes problematic at best.
Over the past decade researchers interested in sea level variations have had an exciting new tool added to their arsenal. During this time satellite altimetry—the measurement of sea surface height from space—has come of age as an oceanographic observing technology. Beginning in 1992, with the launch of a joint U.S. and French project called TOPEX/POSEIDON (hereinafter, T/P), researchers have had routine access to high-quality data proven useful for many applications. For readers interested in the broad scope of the applications of the T/P data, a good starting point for further reading is the special issue of the Journal of Geophysical Research (1995, Vol. 100, No. C12), which was dedicated to science results and investigations using T/P data. In this chapter we will focus on one particular application, using T/P data for measurement of long-term sea level change. We will primarily focus on low-frequency changes in sea level associated with changes in total ocean volume, as discussed by Douglas in Chapter 3, rather than on interannual to decadal changes associated with dynamical changes in the large-scale ocean circulation like those discussed by Sturges and Hong in Chapter 7. We will, however,
Sea Level Rise
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briefly mention some interesting interannual changes we have observed in total ocean volume associated with the 1997-1998 El Niño/Southern Oscillation (ENSO) event.
Why is satellite altimetry such an interesting new tool for the study of long-term sea level change? To answer this question let us consider some issues related to how altimeters, as opposed to tide gauges, sample the global sea surface height field.
Begin by imagining a densely spaced, global set of perfect tide gauges, that is, tide gauges all geodetically leveled together and perfectly measuring the sea surface height variations typical of the open ocean. Then aside from lacking perfection, the actual tide gauge network is simply a subset of this hypothetical dense global set. Now imagine that the total volume of the real ocean did not change at all in time, but that the ocean mass was allowed to redistribute. For example, there could be a tilt from east to west across an ocean basin that increased slowly in time. In this case the average sea level computed from the dense set of gauges would properly return a value that did not vary in time. The real tide gauge network, on the other hand, could not do so. Depending on how the actual gauges were distributed with respect to the east-west tilt, there would be a trend in the average sea level that could be mistakenly interpreted as ocean volume change. The main purpose of this very simple exercise is to point out that because satellite altimeters survey the entire global ocean, or very close to all of it, it is plausible that altimeter measurements can separate ocean volume changes from low-frequency redistributions of ocean mass with significantly shorter time series. Thus, even though less than a decade of precise altimetric data from T/P exists, these data might be as useful as much longer tide gauge series for certain problems.
We can extend this example further by imagining an ocean whose volume was indeed changing with time, but in which there were no dynamic redistributions of mass. Although this example is as artificial as the previous one, it will serve to illustrate a slightly different point. In this case, in principle just one perfect tide gauge would be adequate for determining the volume change rate. But this assumes, of course, that the volume change rate is the same everywhere on the planet. Unfortunately, models of the ocean volume change rate associated with an enhanced greenhouse effect (e.g. Russell et al., 1999) suggest that this may not be the case. If these model simulations are correct, then even in our simple example where there are no mass redistributions to deal with, the relatively poor spatial sampling of the real tide gauge network can again lead to misleading conclusions. Obviously, the nearly complete global sampling afforded by satellite altimetry is a unique advantage for studies of very low frequency sea level change. No conceivable tide gauge network can provide the capabilities of a T/P-class altimeter satellite.
The foregoing examples were unrealistic in that the measurements, from both the tide gauges and the altimeter, were assumed to be free of error. Of course, each is subject to various errors (for example, Douglas in Chapter 3
discusses tide gauge errors in some detail; see also Pugh, 1987). A careful assessment of the errors in both is required before it will be clear how these observations should be combined to arrive at reliable estimates of very-low-frequency sea level change. This chapter focuses on satellite altimetry and considers the errors in those measurements, especially errors that take the form of a long-term drift. Obviously such errors are potentially fatal to the use of altimetric heights to measure real long-term trends in sea level. But the examples above do serve to show that there is great promise in using altimetric heights to make determinations of very-low-frequency sea level change.
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