Sea level anomaly measurements from satellite coastal altimetry and tide gauges at the entrance of the Gulf of California

Abstract

The Sea Level Anomalies (SLA) obtained from tide gauges and coastal altimetry are compared in four locations around the entrance area of the Gulf of California, Mexico to validate the coastal altimetry data in our region. We perform two analyses using SLA data from X-TRACK coastal altimetry product (1 Hz along track) for the missions Topex/Poseidon (TP), Jason 1–2-3 (Ja), Geosat Follow-On (GFO) and Envisat (EN). The first compares SLA from X-TRACK with the SLA obtained from tide gauge subtracting 31 tidal components. In the second comparison, we start from the total water level envelope for altimetry data, which results from adding the Sea Level Anomalies, tides and atmospheric corrections. A harmonic analysis is performed to subtract only the most relevant tidal constituents in our region. We obtain 18 tidal constituents based on the harmonic analysis of the four tide gauges (16 with amplitudes > 1 cm and including the LDA2 and MM constituent). However, the number of tidal constituents that can be removed from the altimetric data varies due to their tidal alias periods which, in turn depend on their repetition periods and orbits. Therefore, we extract only 18/16/11 constituents for TP-Ja/GFO/EN respectively. The same number of constituents are removed from the tide gauges for comparison. The results show greater similarity between the SLA time series of altimeters and tide gauges for the second analysis, obtaining higher correlations (R > 0.6) and smaller root mean square differences (Rmse < 11 cm) for the TP-Ja and GFO missions and less than 18 cm for the EN mission. The tidal constituents inside the gulf undergo significant changes in amplitude and phase over short distances. Because of this, large differences can be found between the SLA time series of tide gauges when comparing with altimetry points located at distances 40–80 km away. However, when comparing points located less than 30 km away the comparisons are acceptable with differences attributable to the coastal dynamics, and therefore we consider them to be properly validated.