An improved Hilbert-Huang transform (HHT) based on ensemble empirical mode decomposition (EEMD) is applied to analyze monthly averaged time series of remotely-sensed surface chlorophyll concentration from September 1997 to July 2009 and the relevant hydrological and meteorological factors northwest of the Luzon Island in the South China Sea. On the basis of modes separated by EEMD, we develop a correlation index using the cosine values of the phase differences between a pair of intrinsic mode functions (IMFs), of which one is from chlorophyll and the other is from another variable. Our exploration of the relation between chlorophyll and physical environment shows that a seasonal pattern and an interannual oscillation are common characteristics in the time series. Quasi-annual periodicity accounts for 81% of the total variance of chlorophyll, and there are two other modes (the quasi-biennial oscillation and 4-year period, respectively). Further more, it is revealed that a significant positive correlation between seasonal modes of Ekman pumping velocity and chlorophyll, and a weak positive relationship between quasi-biennial modes of Ni?o3 SST and chlorophyll. Besides, all the other variables show out-of-phase correlation with chlorophyll at different time-scales. These findings demonstrate the usefulness of HHT in climate time series analysis.
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