Preliminary Analysis Of Spatial And Temporal Variation Characteristics Of Mindanao Undercurrent

The western equatorial Pacific was called "water mass crossroads" due to the confluence and existence there of several currents and water masses. The recirculations of tropical and subtropical gyres are complicated and strengthened there. A lot of flows appear in the surface, such as the North Equatorial Current (NEC), the South Equatorial Current (SEC), the Mindanao Current (MC) and the Kuroshio. Meanwhile, the opposing and corresponding undercurrents exist in the subsurface below the thermocline, including the Equatorial Undercurrent (EUC), the Luzon Undercurrent (LUC), the Mindanao Undercurrent (MUC), the New Guinea Coastal Undercurrent (NGCUC) and the North Equatorial Undercurrent (NEUC). The northward MUC under the MC is a key flow which connects the NGCUC and NEUC, and plays an important role in transporting the subsurface water of the southern hemisphere passing through the equator to the north. At present, there is less knowledge about the characteristics of the MUC. So it is significative to study the MUC to figure out the dynamic structure and change mechanism as well as the mass and heat transportations of the western tropical Pacific circulations.In this paper, the strusture and the varaibility of the MUC have been analyzed using the ARGO data, the WOA01 data, the SODA data and the OFES data. They are also applied of the isopycnic-coordinate transformation, the analysis of hydrological sections, T-S and EOF, and the simulation by HYCOM as well as the sensitive experiment. The result shows as follows.(1) The NGCUC turns to the north after the deep Halmahera Eddy's effect, and then becomes the MUC. The MUC exists on several density surfaces between 26.5σθand 27.5σθ. The main path of MUC acquired from the SODA data is trapped west of 128°E, while the MUC flows northward directly and turns east at 9°N and 12°N, dividing into two branches and joining the two parallel NEUC. However, the results of OFES and HYCOM show that the MUC moves to the north snakelikely influenced by several eddies. By comparison of different outputs of models with different resolution, it is concluded that the path of the MUC is related with the horizontal resolution of the data used. The MUC is consists of some eddies essentially.(2).The MUC is composed of two velocity cores in average state, whose key components are the lower part of the Southern Pacific Tropical Water (SPSW) and the Antarctic Intermediate Water (AAIW), respectively. Relatively the inshore core seems to be more obvious, which locates west of 127°N, approximate 1000m and 27.2σθsurface, carrying the AAIW principally. While the offshore core, known as the weaker one, appears east of 128°E and the surface between 26.6σθto 27.7σθusually, transporting the mixing of the NPIW and the lower part of the SPTW. Sometimes the offshore one connects with the branch of Mindanao Eddy at the surface. However, the two cores may be combined with each other and form a single one in reality.(3).If the potential density of MUC gets to be restricted ranging from 26.5σθto 27.5σθand the salinity greater than 34.4, a more reasonable transportation of MUC could be acquired since it eliminates some upper disturbance and deep ocean circulation and just fits the source property of itself. The transports are 15.65Sv, 17.95Sv and 7.55Sv using the data of OFES, SODA and HYCOM, respectively.(4)Based on the theory of the geostrophic current and the vorticity equation, it is infered that the the generation of the western boundary undercurrents, such as MUC, may be relevant to the vertical movements caused by the local convergence and divergence. The strong western boundary current and the vertical upwelling or downwelling could change the gradient of the thermocline, while the gradient and the surface oppositing flow would affect the formation and strengh of the undercurrent. Take the MUC for example, stronger the MUC, the more downwelling happening, and then the more tilted of the thermocline finally strengthens the MUC. The theroy could be used to explain why the the seasonal variability of the MUC(stronger in spring and weaker in winter) is similar with the MC. Besides, it is a strongly positive correlation of the interannual variability of MUC and MC. Theri transports both have a main period of 7.79 years. The first two modes of EOF analysis shows that the MUC is affected by the gradient of the thermocline as well as the strengh of MC.(5).A high-resolution hybird coordinate ocean model(HYCOM) is used to simulate the climatological northeastern tropical ocean circulation. The output confirms the dual-core structure of the MUC and the eddy-style northward path, substantially, and the MUC shows a similar seasonal variability with prior analysis.(6).The sensitive experiments, which changed the transport of MC, have proven that the MUC's responce to the upper flow and baroclinicity. When the MC gets stronger, the baroclinicity east off the MIndanao Island increases, then the transport of MUC gets bigger if the restrction of potential density broadened, and the main part of MUC strenghening is the deep water below the AAIW, vice versa. The MC's strengh reacts up on the transportation of the lower part of SPTW, which caused by the expansion of the MC's vertical range.