WHY DOES THE INDONESIAN THROUGHFLOW APPEAR TO ORIGINATE FROM THE NORTH PACIFIC

Most available wind products show a mean wind stress curl across the P acific at 2-degrees-N that is close to zero, so a Sverdrup model predi cts the western boundary current at the Asian coast to be nearly zero at this latitude. Thus, the Australasian circulation, or Indonesian Th roughflow-which is estimated by a Sverdrup model to be (16 +/- 4) X 10 (6) m3 s-1-is predicted to flow westward as a zonal jet from the north ern tip of Irian Jaya into the Indonesian seas. If this Sverdrup flow pattern were valid, the throughflow would be supplied by (salty) South Equatorial Current water from the northern tip of Irian Jaya, while t he (relatively fresh) Mindanao Current would all return eastward to fe ed the North Equatorial Countercurrent. Observed salinities in the Ind onesian Throughflow are close to those of the Mindanao Current, sugges ting that they are inconsistent with the Sverdrup flow pattern. The ou tputs of two recent ocean general circulation models are examined; in both models, the Indonesian throughflow is supplied by Mindanao Curren t water because the South Equatorial Current (SEC) retroflects into th e nearby North Equatorial Countercurrent-Equatorial Undercurrent syste m. In one model, this retroflection is due to the action of a rather l arge horizontal eddy diffusivity. In a second, fine-resolution model t he depth-integrated time-mean flow departs from the Sverdrup predictio n within about 3-degrees on either side of the equator; almost complet e retroflection of the SEC occurs, associated with a feature similar t o the Halmahera Eddy. In both models the retroflected current travels a large distance eastward in the North Equatorial Countercurrent or th e undercurrent and then returns west in the North Equatorial Current b efore entering the Indonesian throughflow. Thus, although the throughf low does originate in the South Equatorial Current, the high-salinity signal of the SEC is probably obliterated by rainfall along this long path so that the throughflow appears to originate from the North Pacif ic. These results suggest that retroflection processes may play an imp ortant role in controlling the supply of fresh water from the Pacific to the Indian Ocean.