Effects of Unresolved High-Frequency Signals in Altimeter Records Inferred from Tide Gauge Data

Abstract

Rapid sea level signals are poorly resolved, or ?aliased,? by the TOPEX/Poseidon altimeter, due to its ?10-day repeat cycle (Nyquist period TN ? 20 days). To assess the potential for temporal aliasing in the altimeter data, the authors analyze hourly sea level records from a set of 110 island and coastal tide gauges. Time series are detided to focus on nontidal effects. Large variance is found at periods <TN (more than 50% of the total variance at high latitudes), with important contributions from the shortest periods (<2 days). The inverted barometer correction reduces high-frequency variance at mid- and high latitudes but not at low latitudes. As inferred from the tide gauge analysis, in the worst-case scenario (one data point per altimeter repeat cycle), the ratio of aliased to measured variance can range from ?0.1 at the annual period to ?0.8 near TN. Moderate data averaging (three points per altimeter repeat cycle, given a ?3.3-day near-repeat orbit) can substantially reduce the aliasing, however. The impact of aliasing is, in general, worse for coastal than for island stations.