1. State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Hohai University, Nanjing 210098, P. R. China
2. Laboratory of Ocean Dynamics, Third Institute of Oceanography, State Oceanic Administration, Xiamen 361005, P. R. China
This work was supported by State Key Development Program of Basic Research of China (Grant No. 2010CB429001), the National Natural Science Foundation of China (Grant No. 51009062), and the Special Fund of Hydrology-Water Resources and Hydraulic Engineering (Grant No. 2009586812).