1. School of Mathematics and Statistics, Xi'an Jiaotong University, 710049 Xi'an, China. E-mail: jba@uh.edu
2. Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX 77204, USA
3. Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Borgo Grotta Gigante 42c, Sgonico, Trieste I-34010, Italy
4. Zhou Pei-Yuan Center for Applied Mathematics, Tsinghua University, 100083 Beijing, China