Analysis of chlorophyll-a concentration around the South China Sea from ocean color images

In this study, we will discuss the spatial and temporal variation of the chlorophyll-a concentration (Chl-a) around the SCS. All Chl-a data are derived from Aqua satellite MODIS (Moderate Resolution Imaging Spectroradiometer) visible ocean color data during the period from 2002 to 2006. The Empirical Orthogonal Function (EOF) analysis is applied to see the dominant patterns of the Chl-a variation in the study area. From EOF mode 1 results of Chl-a, we find that the higher Chl-a is mainly along the coastal area of the SCS and rapidly decreasing offshore. The Chl-a EOF mode 2 indicates a higher Chl-a jet along the Vietnam coast in summer, and a high Chl-a meandering off the northwestern Luzon in winter. From Chl-a EOF mode 2 results we can see a clear Chl-a vibration between the eastern and western coast of the SCS. The Chl-a along the western coast enhances in summer but decays in winter, the reversed situation happens in the eastern coast. The covariance Chl-a EOF1 results emphasize again a clear high Chl-a plume along the Vietnam coast in summer. Furthermore, we also can retrieve sea surface temperature (SST) data from Aqua satellite MODIS thermal infrared bands. The gradient SST EOF mode 1 results show the colder water is mainly along the northwestern SCS. Meanwhile, the warm Kuroshio water intrusion to the Luzon Strait happens mainly in winter. The gradient SST EOF2 results indicate two circulation gyres in the SCS, one nearly covers the whole northern SCS and the other smaller one is in the southern SCS. These two gyres are all warmer in summer and cooler in winter. The spatial-mean removed joint EOF results of Chl-a and SST indicate the SST pattern is much clear than the Chl-a pattern in the SCS, it is also found the Kuroshio intrusion to the Luzon Strait in winter and two circulation gyres in the northern and southern SCS.