Abstract
Satellite altimetry and gravimetry are used to determine the mean seasonal cycle in relative sea level, a quantity relevant to coastal flooding and related applications. The main harmonics (annual, semiannual, terannual) are estimated from 25 years of gridded altimetry, while several conventional altimeter “corrections” (gravitational tide, pole tide, and inverted barometer) are restored. To transform from absolute to relative sea levels, a model of vertical land motion is developed from a high-resolution seasonal mass inversion estimated from satellite gravimetry. An adjustment for annual geocenter motion accounts for use of a center-of-mass reference frame in satellite orbit determination. A set of 544 test tide gauges, from which seasonal harmonics have been estimated from hourly measurements, is used to assess how accurately each adjustment to the altimeter data helps converge the results to true relative sea levels. At these gauges, the median annual and semiannual amplitudes are 7.1 cm and 2.2 cm, respectively. The root-mean-square differences with altimetry are 3.24 and 1.17 cm, respectively, which are reduced to 1.93 and 0.86 cm after restoration of corrections and adjustment for land motion. Example outliers highlight some limitations of present-day coastal altimetry owing to inadequate spatial resolution: upwelling and currents off Oregon and wave setup at Minamitori Island.
Highlights
Satellite altimetry has been used to study the annual and semiannual cycles in sea level since the earliest days of altimetry (Jacobs et al 1992; Knudsen 1994; Nerem et al 1994)
Two kinds of signals considered of little interest have been removed: (1) those forced by the astronomical tidal potential, which at these time scales are close to equilibrium and (2) those forced by atmospheric pressure loading, which are again close to equilibrium at these time scales (Pugh and Woodworth 2014)
The present work is motivated by understanding the contribution of the seasonal cycle to coastal flooding and how that may be incorporated into future high-water projections, including for the many locations where altimetry may be providing the only relevant sea-level measurements
Summary
Satellite altimetry has been used to study the annual and semiannual cycles in sea level since the earliest days of altimetry (Jacobs et al 1992; Knudsen 1994; Nerem et al 1994). The data are in the form of hourly (or faster) measurements from the GESLA-2 database (Woodworth et al 2017), from which we have estimated the seasonal components of relative sea level, the amplitudes and phases of the annual, semiannual, and terannual harmonics. Details about these data are summarized in Appendix B. Appendix A gives further details on phase conventions, including how various inconsistencies have arisen in the published literature for the annual harmonic
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