**Acknowledgements**

This research was supported by the National Natural Science Foundation of China (Nos. 31270528, 41430966 and 41206082), Natural Science Foundation of Guangdong (No. S2013020012823), Scientific Research Project of Guangzhou (Nos. 201504010005 and 201504010006), the project of Guangdong Provincial Department of Science and Technology (No. 2012A032100004) and the projects of knowledge innovation program of State Key Laboratory of Tropical Oceanography (No. LTOZZ1604).
