Empirical Mode Decomposition and Fourier analysis of Caspian Sea level's time series

Abstract

The present study described Caspian Sea's water level changes by using the recorded data in three ports namely Anzali, Nowshahr, and Amirabad. The data are the mean values of daily records within a three-year period. Primary determinations revealed that the water level drops to the lowest amount in the winter but then starts to increase towards the spring. Afterwards, the water level peaks in the summer and again it decreases in the autumn. It was also detected that water level decreased 22 cm annually in the given period. Fourier Transform and Empirical Mode Decomposition (EMD) were adopted for the decomposition of time series. Fourier analysis indicated that the dominant sinusoidal component possessed a period and amplitude of 294 days and 0.148 m, respectively. EMD yielded six Intrinsic Mode Functions (IMFs) and one residual mode. The average period of each IMF was calculated through zero-down crossing method. Afterwards, the spectrum of each IMF and their summit characteristics were extracted. A maximum value of 0.127 m was observed at the frequency of 0.0034 cycle/day in the IMF6 spectrum, which perfectly corresponds to the dominant sinusoidal component in time series.