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

Rui M. PonteAtmospheric and Environmental Research, Inc., Lexington, Massachusetts

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Florent LyardLEGOS, Toulouse, France

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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.

Corresponding author address: Dr. Rui M. Ponte, Atmospheric and Environmental Research, Inc., 131 Hartwell Ave., Lexington, MA 02421-3126. Email: ponte@aer.com

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.

Corresponding author address: Dr. Rui M. Ponte, Atmospheric and Environmental Research, Inc., 131 Hartwell Ave., Lexington, MA 02421-3126. Email: ponte@aer.com

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