SEASONAL CIRCULATION IN THE NORTHWESTERN TROPICAL PACIFIC OCEAN DIAGNOSED WITH THE ASSIMILATION DATA FROM 1989 THROUGH 1997

An assimilation data set based on the Geophysical Fluid Dynamics Laboratory (GFDL) Modular Ocean Model version 3 (MOM3) and the NODC XBT data set is used to examine the circulation and its variabilities in the western tropical Pacific, with special emphasis on the seasonal variations. It is shown that the assimilated and observed mean velocities and transports of the major flows in the western tropical Pacific agree well. The flows in the north Pacific, including the North Equatorial Current (NEC), Kuroshio, Mindanao Current (MC) and North Equatorial Countercurrent (NECC) west of 140°E display the seasonal cycles almost in the same phase, with the biggest transport in spring and the smallest in autumn. The phase of the NECC seasonal cycle east of 140°E is opposite to that in the west. Besides of the annual cycle, there seems to be a semi annual fluctuation of the NECC transport possibly resulting from the phase lag between seasonal cycles of the NEC and NGCC. Strong in summer during the southeast monsoon, the seasonal cycle of the Indonesian Throughflow (ITF) is closely linked with those of both the MC and the New Guinea Coastal Current/Undercurrent (NGCC/NGCUC), but not as strong as that in observations probably caused by the superimposed seasonal and interannual variations. Variations on the interannual time scale are also discussed, but only indistinct interannual variations of the flows related to the ENSO are revealed during 1989～1997. Transport of the NEC, Kuroshio and NECC are slightly larger in the E1 Nino years when that of the ITF is weaker, while the MC has little ENSO related variation. There were also quasi biennial signals superimposing the ENSO like oscillations in the flows, but their relationships with the ENSO are still unclear.