STUDY OF SEASONAL TRANSPORT VARIATIONS IN THE INDONESIAN SEAS

Seasonal transport variations between the Pacific and Indian Oceans vi a the Indonesian seas were studied by the Euler-Lagrangian method. The velocity field was calculated with a fairly high resolution robust di agnostic model, The model well reproduces the features of seasonal var iations in the Indonesian seas. The total volume transport of the Indo nesian throughflow is 20 +/- 3 Sv (1 Sv = 10(6) m(3) s(-1)), the maxim um being from boreal spring to boreal summer and the minimum in boreal winter. The values are similar to those of previous general circulati on models with a wide Indonesian passage despite resolution of the pre sence of the many small islands in the Indonesian seas. Although a lar ge portion of the net transport is contained in the upper layer, deep transport below 1000-m depth is about 5 Sv. This value corresponds to approximately 25% of the total transport, which means that disregard o f the deep transport leads to underestimation of the volume transport of the throughflow. Tracking of numerous labeled particles in the calc ulated velocity field clarified the sources and pathways of the Indone sian throughflow. The major route is a western one through both the Ma kassar and Lombok Straits. Most of the North Pacific water supplied fr om the Mindanao Current passes along this route, entering the Indian O cean within several months with almost no loss of its properties (inte nse vertical mixing around the Lombok sill reported by observations co uld not be reproduced in our model). In contrast, South Pacific water takes the eastern route into the eastern Indonesian seas and subsequen tly mixes with waters from the North Pacific and Indian Oceans in the Banda Sea, which means that it has a long travel time (at least a few years). Water taking the eastern route therefore loses its original pr operties before arriving in the Indian Ocean. The transport processes also are significantly affected by seasonal variations in equatorial c irculation in the western Pacific. In the surface layer, North Pacific water is vigorously supplied to the western route only from boreal sp ring to summer in association with the linkage between the current flo wing through the Makassar Strait and the Mindanao Current. In other se asons, because the Mindanao Current is strongly linked with the North Equatorial Countercurrent and the New Guinea Coastal Current primarily by northeasterly monsoonal winds, its upper water flows back to the P acific Ocean. In the subsurface layer, a pronounced inflow of Mindanao Current water into the western route occurs from boreal winter to spr ing, when the subsurface link between that current and the Equatorial Undercurrent tends to weaken. In the deep, the quasi-steady transport of Pacific water into the Indian Ocean via the eastern route is fed by the westward deep current in the equatorial Pacific.