Why Does the Indonesian Throughflow Appear to Originate from the North Pacific?

Abstract

Most available wind products show a mean wind stress curl across the Pacific at 2°N that is close to zero, so a Sverdrup model predicts the western boundary current at the Asian coast to be nearly zero at this latitude. Thus, the Australasian circulation, or Indonesian Throughflow-which is estimated by a Sverdrup model to be (16 ± 4) ? 106 m3 s??is predicted to flow westward as a zonal jet from the northern 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 the (relatively fresh) Mindanao Current would all return eastward to feed the North Equatorial Countercurrent. Observed salinities in the Indonesian Throughflow are close to those of the Mindanao Current, suggesting that they are inconsistent with the Sverdrup flow pattern. The outputs of two recent ocean general circulation models are examined; in both models, the Indonesian throughflow is supplied by Mindanao Current water because the South Equatorial Current (SEC) retroflects into the nearby North Equatorial Countercurrent-Equatorial Undercurrent system. In one model, this retroflection is due to the action of a rather large horizontal eddy diffusivity. In a second, fine-resolution model the depth-integrated time-mean flow departs from the Sverdrup prediction within about 3° on either side of the equator; almost complete retroflection of the SEC occurs, associated with a feature similar to the Halmahera Eddy. In both models the retroflected current travels a large distance eastward in the North Equatorial Countercurrent or the undercurrent and then returns west in the North Equatorial Current before entering the Indonesian throughflow. Thus, although the throughflow 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 Pacific. These results suggest that retroflection processes may play an important role in controlling the supply of fresh water from the Pacific to the Indian Ocean.