The Outstanding Young Scientist Career Award of the National Natural Science Foundation of China (NSFC) project 40888001 awarded to Professor YUAN Dongliang of the Institute of Oceanology of the Chinese Academy of Science support the study of nonlinear reflection and penetration of equatorial Rossby waves at the Pacific Ocean western boundary. Existing studies have suggested that significant equatorial Rossby waves associated with the interannual events like the El Niño and the South Oscillations (ENSO) propagate to the western equatorial Pacific Ocean and have leaked into the Indonesian seas to influence the so-called Indonesian Throughflow (ITF), and/or are reflected into the equatorial Kelvin waves to influence the eastern equatorial Pacific cold tongue.  The detailed reflection and penetration processes, however, are not clear, except that the circulation in the western Pacific Ocean, particularly in between the Mindanao and the Papua New Guinea, is known to be very complicated and that the reflection/penetration processes involve strong nonlinear interactions between the western boundary currents (WBCs) and the incoming Rossby waves. This research project is funded to investigate the dynamic processes and energy tracks of the reflection and penetration of the equatorial Rossby waves at the porous Pacific western boundary and to estimate the reflection/penetration ratio of the Indonesian archipelago. For these purposes, two key issues are to be resolved by the study of this project. The first is the hydrographic sources and three-dimensional structure of the ocean circulation at the Helmahera Sea and the Malogu Strait, which determines the origin of the ITF; The second is the dynamics of the nonlinear interactions between the WBCs and the incoming Rossby waves, which determine the energy tracks.

　　The first research content must be fulfilled with analyses of in situ hydrographic data. Historical and ARGO data have been analyzed and are found to be far from enough in resolving the circulation structure in the far western Pacific Ocean. We have, therefore, proposed a hydrographic survey of the area to make use of the newly established NSFC ship time funds to fulfill the task. The mapping of the cruise has been designed, which is named the Beginning Indonesian Throughflow Experiment (BITE), and a ship time application has been submitted. A few publications at peer reviewed journals have been finished (see the list at the end).

　　The second research content involves heavily in theoretical studies. A 1.5-layer quasi-geostrophic model and a 1.5-layer shallow-water-equation model are used to study the hysteresis of a WBC flowing by a wide gap. For simplicity, the case of an unidirectional WBC is first considered, which is shown to be able to block the westward propagation of meso-scale eddies from the eastern basin when the WBC is far away from the critical states of the hysteresis. The energy of meso-scale eddies is found to be able to penetrate through the WBC if the WBC is close to critical states of hysteresis from the leaping to the penetrating regimes. Interestingly, when the WBC is close to the critical state from the penetrating to the leaping regimes, an over-reflection is accomplished. In low latitude areas, where the baroclinic deformation radius is large, the Hopf bifurcation of the WBC from penetrating to periodic regimes is found to occur at much lower Reynolds numbers than at middle-to-high latitudes. Effects of winds on the hysteresis evolution have also been investigated. A few publications have been worked out from the study (see the list below). Investigations of the effects of islands, throughflows and periodic oscillating WBCs on the hysteresis are underway.

　　In the next step, we will study the cases that two WBCs meet at the gap of the western boundaries, which represent the meeting of the Mindanao Current with the South Equatorial Current at the Halmaha Seas to generate the ITF and the North Equatorial Countercurrent, and investigate the nonlinear interactions between the WBCs and the incoming Rossby waves.

　　Publications

　　Yuan, D., and R. Li, 2008: Dynamics of the eddy-induced Kuroshio variability in the Luzon Strait. J. Trop. Oceanogr., 27(4), 1-9. (in Chinese with English abstract).

　　Wang Z., D. Yuan, Y. Hou. (2009). Effects of meridional winds on a gap-leaping western boundary current. J. Oceanol. Limn. (in press)

　　Yuan, D., 2009: Long wave dynamics of the Indian Ocean Dipole events. J. Phys. Oceangr., doi:10.1175/2008JPO3900.1.

　　Yuan, D. 2009: Interannual horizontal heat advection in the surface mixed layer over the equatorial Pacific Ocean: Assimilation versus TAO analyses, Theoratical and Applied Climatology (CLIVAR Special issue), 97:3-15, doi:10.1007/s00704-008-0068-7

　　Yuan, D., and Y. Hsueh, 2009: Dynamics of the cross-shelf circulation in the Yellow and East China Seas in winter. (Submitted to Deep Sea Research II)

　　Yuan, D., R. Li, H. Zhou, F. Wang, H. Lei, and D. Hu, 2009: Observation of cross-shelf penetrating fronts off the Changjiang mouth. (Submitted to Deep Sea Res. II)

　　He, L., and D. Yuan, 2009: Variability of Cross-shelf Penetrating Fronts off the Southeast Coasts of China based on multiple-satellite observations. (Submitted to Deep Sea Res. II)

　　Yuan, D., Y. Li, and F. Qiao, 2009: Temperature Inversion in the Yellow Sea Bottom Cold Water in Summer. (Submitted to JO)

　　Zhou hui, Nan feng, Shi maochong, Guo Peifang, Zhou liangming, 2009: Characteristics of water exchange in the Luzon Strait during September of 2006. Chinese Journal of Oceanology and Limnology. (已接收).