Seasonal Variations of Upper Ocean Transport from the Pacific to the Indian Ocean via Indonesian Straits*

Seasonal variations of upper-ocean mass transport between the Pacific and Indian Oceans via the Indonesian Throughflow (ITF) are examined using numerical experiments with a 11⁄2-layer, reduced-gravity model forced with specific climatological winds. The model ITF transport, computed as a sum of through-strait transport, has an annual range of more than 8 Sv (an annual harmonic of amplitude 4.2 Sv and a smaller, semiannual harmonic amplitude of 0.5 Sv (Sv [ 106 m3 s21), with peak transport from mid-April through July and minimum transport in November and December. Limited long-term observations make it difficult to validate these results, but they are consistent with current theory. Experiments with time-varying winds in specific regions show that most of the annual throughflow signal is due to equatorial winds (from 108S to 108N); ITF transport anomalies generated by off-equatorial winds account for less than 1 Sv and are mostly out of phase with the baseline throughflow signal. For the particular wind data used in this study, effects of remote wind forcing in the equatorial Indian Ocean are countered by local winds in the Indonesian seas, and the annual cycle of through-strait transport derived from the model forced only by equatorial Pacific winds is nearly equivalent to that of the baseline run. In this model, the specified wind stress causes annual Rossby waves to be formed in the eastern Pacific by Ekman pumping. These Rossby waves propagate to the western boundary of the Pacific, then form coastal Kelvin waves that propagate through the Indonesian seas. In northern spring, a downwelling wave brings elevated sea level to the Pacific side of the Indonesian seas, and the ITF is maximum. In northern fall an upwelling wave reduces the sea level on the Pacific side, and ITF transport is minimum. In the Indian Ocean, monsoon winds produce equatorial Kelvin waves that propagate eastward and form coastal waves along the southern coasts of Sumatra and Java. A downwelling (upwelling) wave increases (decreases) sea level on the Indian Ocean side of Indonesia in northern spring (winter), thus acting in opposition to the baseline ITF variability. The effect of local Ekman pumping is in the opposite sense. In northern winter, when remote Indian Ocean winds create an upwelling coastal wave, the local wind stress provides downward Ekman pumping. In northern spring, locally forced, upward Ekman suction counters the remotely forced, downwelling Kelvin wave.