During the 2007-08 Spanish Antarctic campaign, two moorings of bottom pressure sensors were carried out over a ten week period. This paper presents the results of the tidal analysis from sea level records obtained at Deception and Livingston islands (South Shetland Islands, Antarctica). The main objective of this paper is to present a detailed study of the tidal characteristics at these two islands, for which statistical and harmonic analysis techniques are applied to the tidal records. A geodetic network was used to reference the pressure sensors. Geometric levelling, with an accuracy of 1 mm, allowed us to link the tidal marks with geodetic vertices located on Livingston and Deception islands. The amplitudes and phase lags obtained by harmonic analysis are compared to the harmonic constants of several coastal stations and co-tidal and co-range charts. Results show an evident influence of tides in the sea level signal, with a clear mixed semi-diurnal behaviour and a daily inequality between high and low waters. Measurements of salinity and temperature were made using electronic sensors. Results from this study showed that salinity and temperature were strongly influenced by tides. Seawater temperature varied in a manner that was consistent with the time series of residual bottom pressure.
IntroductionAccording to Dragani et al. (2004), the first observations of the sea levels at Deception Island were made in 1991. Tidal data were obtained over four days in summer by a visual tide staff located near the Decepción Station (Argentina) geodetic benchmark (BARG). This geodetic benchmark, belonging to the Deception Island geodetic network (REGID), was used as the vertical and horizontal datum. Based on these tidal data, an orthometric height value of 2.547 m for the BARG was calculated. This orthometric level was translated by geometric levelling to the point LN00, the fundamental point of the levelling network of Deception Island (RENID); the mean sea level obtained relative to LN00 was 5.
Instrumentation and dataData were obtained using two moorings, each with a SAIV TD304 bottom pressure sensor at points near the coast, obtaining average local values of depth of the tide gauge of 3.147 m at Deception Island and 7.475 m at Livingston Island. The tidal observations at DECMAR and LIVMAR stations were made between February 2011 and April 2013.Pressure (P), temperature (T) and conductivity (C) data were recorded at the stations with a CTD SAIV SD 204 sensor that has an accuracy of ± 0.02 ppt for salinity, S, (derived from P, T and C) and ± 0.01ºC for T. Additionally, AQUAlogger 520 PT were used. These sensors have an accuracy of ± 0.05°C for T and 0.005% of full scale (10 m) for P. The geometric levelling was made using a Leica model NA2 optical level with an accuracy of ± 0.7 mm for one kilometre double run levelling. The GPS data for positioning of benchmarks and stations were taken from Vidal et al.
MethodologyTo convert hydrostatic pressure into a sea level equivalent height P = P a + ρgh was used, where P is the pressure registered by the sensor of the tide gauge, P a is the atmospheric pressure, g is the acceleration due to gravity (average value calculated for Deception Island is 9.822083 m s -2 ) and ρ the density of the water in the study area, calculated using the records of T and S.Vidal et al. (2012) used a constant reference value of 990.8 mb (hPa) as P a which corresponds to the average value of P a in the region during the period 1978-89 (Rakusa-Suszczewski et al. 1992. The sea level for any particular time can be corrected by adding or subtracting the changes caused by P a changes on the reference values: at low enough frequency the correction is c. -1 cm of sea surface height for every +1 mb (hPa) of P a (Chelton & Enfield 1986). For this study, records of P a were taken concurrently with the tidal measurements, obtaining average values of 988.39 mb at Deception Island and 987.27 mb at Livingston Island.
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