Character And Mechanism Of Seasonal And Interannual Variabilities Of The Circulations Near The Low Latitude Western Boundary Of The North Pacific

The only relatively complete horologic observations of compared winter cruises(2010 La Ni?a winter and 2012 normal winter) in the low-latitude western boundary of the North Pacific Ocean, and the observation during 2011 summer, combined with the mooring ADCPs measurements, the altimeter data, the Argo data and the other data, are analyzed to explore the seasonal variation and ENSO-influenced interannual variability of two western boundary currents(WBCs), the Kuroshio and the Mindanao Current(MC), the mechanisms of which are discussed. Based on the cruise observations in recent years, for the first time, the thesis discusses the distribution character and migration pathway of the water masses and their interannual variations at the entrance of the Indonesia Throughflow(ITF). The volume transports of the east Sulawesi Sea and Maluku Strait during different ENSO phase are evaluated.The geostrophic currents across 18oN, 8oN and 129/130oE indicate that in the upper layer, the MC and the NEC is stronger in summer than in winter, while the variation of the Kuroshio is not really obvious, suggesting the relationship between the seasonal variations of the NEC and the MC. Both the geostrophic transports and the moored ADCPs observations have shown that the Kuroshio decreases significantly whereas the Mindanao Current increases significantly during the 2010 La Ni?a event than in the 2012 normal year, the transport anomalies of which are found much larger than the total meridional transport anomalies integrated across the interior North Pacific. The comparison suggests that the WBCs may play an important role in warm water accumulation in the equatorial Pacific during La Ni?a event. The altimeter data shows that the sea level anomaly around Philippine coast is correlated significantly with that in the equatorial western Pacific, suggesting during a La Nina event, downwelling Rossby waves arrives at the western boundary, which then excite downwelling coastal Kelvin waves to propagate into the South China Sea. We suggest that these Kelvin waves propagate clockwise and raise the sea level along Philippine coast, and then generating southward anomalies of the WBCs.The surface drifters recovered a cyclonic eddy with diameter of 400 km in the eastern Sulawesi Sea during 2012 winter. The concurrently Argo profiles indicate that the eddy is shed by the MC, suggesting significant nonlinear interactions between the WBCs and the Indonesian seas. The observations show a strengthened North Equatorial Countercurrent at 130oE, a westward intensified transport in the east Sulawesi Sea, and a southward anomaly of the surface warm water in the Maluku Strait during La Ni?a winter. The interannual anomaly of the upper layer circulation in the east Sulawesi Sea and the Maluku Strait during La Ni?a event is observed for the first time, suggesting both channels may have great influence in the interannual variations of the ITF.The low-salinity water in the east Indonesian seas flows into the western Pacific, mixing with the pycnocline water, which may contribute the formation of the relatively fresh tropical subsurface water between the North Pacific Tropical Water and the South Pacific Tropical Water.