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 I ndian Oceans via the Indonesian Throughflow (ITF) are examined using numeri cal experiments with a 11/2-layer, reduced-gravity model forced with specif ic climatological winds. The model ITF transport, computed as a sum of thro ugh-strait transport, has an annual range of more than 8 Sv tan annual harm onic of amplitude 4.2 Sv and a smaller semiannual harmonic amplitude of 0.5 Sv (Sv = 10(6) m(3) s(-1)), with peak transport from mid-April through Jul y and minimum transport in November and December. Limited long-term observa tions make it difficult to validate these results, but they are consistent with current theory. Experiments with time-varying winds in specific region s show that most of the annual throughflow signal is due to equatorial wind s (from 10 degrees S to 10 degrees N); ITF transport anomalies generated by off-equatorial winds account for less than 1 Sv and are mostly out of phas e with the baseline throughflow signal. Far the particular wind data used i n 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 equatoria l Pacific winds is nearly equivalent to that of the baseline run. In this m odel, the specified wind stress causes annual Rossby waves to be formed in the eastern Pacific by Ekman pumping. These Rossby waves propagate to the w estern boundary of the Pacific, then form coastal Kelvin waves that propaga te through the Indonesian seas. In northern spring, a downwelling wave brin gs elevated sea level to the Pacific side of the Indonesian seas, and the I TF is maximum. In northern fall an upwelling wave reduces the sea level on the Pacific side, and TTF transport is minimum. In the Indian Ocean, monsoo n winds produce equatorial Kelvin waves that propagate eastward and form co astal waves along the southern coasts of Sumatra and Java. A downwelling (u pwelling) wave increases (decreases) sea level on the Indian Ocean side of Indonesia in northern spring (winter), thus acting in opposition to the bas eline ITF variability. The effect of local Ekman pumping is in the opposite sense. Tn northern winter, when remote Indian Ocean winds create an upwell ing coastal wave, the local wind stress provides downward Ekman pumping. In northern spring, locally forced, upward Ekman suction counters the remotel y forced, downwelling Kelvin wave.