Nonlinear trend and seasonal signals in Mediterranean Sea level derived by multiresolution wavelet analysis of altimetry
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ORIGINAL PAPER
Nonlinear trend and seasonal signals in Mediterranean Sea level derived by multiresolution wavelet analysis of altimetry data Sofiane Khelifa 1 & Ali Rami 1
Received: 24 September 2014 / Accepted: 24 March 2015 # Saudi Society for Geosciences 2015
Abstract Using multiresolution wavelet analysis, the spectral content of monthly maps of sea level anomaly time series on the Mediterranean Sea derived from satellite altimetry over the period 1993 to 2013 is investigated in order to assess its seasonal changes and its nonlinear trend. The multiresolution decomposition has extracted useful the seasonal signals (annual and semi-annual) and nonlinear trend of the analyzed time series by means of its signals of Bdetails^ and Bapproximations,^ respectively. Details and approximations signals represent, respectively, the high-frequency and the low-frequency contained in the analyzed time series. The amplitude values for the annual signal are less than 10 cm with an average of 6.74 cm, while those for the semi-annual signal are mostly less than 4 cm with an average of 1.79 cm. However, the successive smoothing of the analyzed time series through the signals of approximations has allowed to better identify the rate and time spans of the increase and decrease of the Mediterranean Sea. The filtered trend has a slope about 2.30 mm/year compared to 2.46 mm/ year of the original time series estimated by linear least squares regression.
Keywords Mediterranean Sea . Sea level anomaly . Time series analysis . Multiresolution analysis . Nonlinear trend . Seasonal variations
* Sofiane Khelifa [email protected] 1
Centre of Space Techniques, PO Box 13, 31200 Arzew, Algeria
Introduction The variation of the Mediterranean Sea has been the object of many research studies in order to assess its trend and its seasonal changes, and to understand the mechanisms governing its spatial and temporal variability. Several results for the last years (1993–2013) (Cazenave et al. 2001, 2002; Vigo et al. 2005, 2011; Criado-Aldeanueva et al. 2008; Haddad et al. 2011a, b, 2013; Taibi et al. 2013), based on altimetry data using various tools (least squares method, empirical orthogonal function (EOF), singular spectrum analysis (SSA), etc.), indicate that the mean sea level in the Mediterranean Sea presents a very high interannual variability and does not rise uniformly. The sea level trend estimate is about 7 mm/year indicated by Cazenave et al. (2002) from 6 years (1993– 1998), 2.2 mm/year from 8 years (1992–2000) reported by Fenoglio-Marc (2002), 2.1 mm/year from 13 years (1993– 2005) obtained by Criado-Aldeanueva et al. (2008), 1.7 mm/ year from 17 years (1993–2009), and 2.4 mm/year from 20 years (1993–2012) showed, respectively, by Haddad et al. (2011a, 2011b) and Haddad et al. (2013). This paper is a contribution to these continuous researches on the Mediterranean Sea variability. We apply the wavelet transform method (Daubechies 1992), for time-scale analysis, to the time series of monthly maps of sea level anomaly over the M
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