D4 Sea Level Measurement

Topic

Sea-level measurement combines two complementary data sources: coastal tide gauges, which provide a very long-term local record, and satellite altimetry, which offers global coverage. Tide gauges are instruments with a history spanning over 150 years; they measure either the total weight of the water column pressing on the sensor (as pressure) or the height of the sea surface directly. A global reference network comprises nearly three hundred such stations distributed along the world's coastlines. Harmonic analysis of the sea-level data produced by these gauges allows for the separation of the tidal signal—the sum of several perfectly predictable astronomical constituents—from the non-tidal residual, which captures both meteorological variability and longer-term trends. It is precisely this separation that enables a single tide gauge record to serve both for predicting tomorrow's tides and for measuring climate change over the coming decades.

Satellite altimetry, meanwhile, measures the distance between the satellite and the ocean surface with an accuracy of one to two centimeters, using a radar altimeter that transmits pulses in the Ku-band at a frequency of approximately 13.6 gigahertz. The sea-level height at any given point is calculated by determining the difference between the satellite's orbit—known with high precision relative to a geocentric reference ellipsoid—and the measured distance to the surface. However, arriving at this final result requires correcting for several effects: the delay introduced by free electrons in the ionosphere, the delay caused by water vapor and dry air in the troposphere, and the impact of the sea state itself on the reflected signal. By combining successive satellites—with periods of overlap between missions—a uniform, continuous sea-level record is constructed, spanning from 1993 to the present day. The separation between the tidal signal and the residual—mentioned earlier—is, in fact, the core of this entire measurement process: harmonic analysis fits a set of constituents with fixed amplitudes and phases to the observed series, with each constituent linked to a distinct astronomical cycle. What remains after subtracting these tidal constituents is the residual; this residual allows for the distinction of meteorological variability (driven by low pressure and wind), seasonal variability, interannual variability (associated with the El Niño-Southern Oscillation and the Atlantic Multidecadal Oscillation), and long-term sea-level trends.

Along most of the world's coastlines, two constituents typically dominate all others: the principal lunar semidiurnal component and the principal solar semidiurnal component. Their amplitudes and phases can be precisely determined after just one or two years of continuous recording, making it possible—in practice—to predict tides at any location with minimal prior observation. Thus, from the tide gauge that has monitored the coast for over a century to the satellite measuring height from space—and including the harmonic analysis that isolates the predictable tide from the rest—sea-level measurement ultimately reconstructs, through widely varying methods, a single sea-level height that rises and falls due to a combination of astronomical, meteorological, and climatic factors.

ID:1665