Basin-wide exchange between the Pacific and Indian oceans through the Indonesian Archipelago has received attention, both in relation to the oceans' role in the Southern Oscillation1 and in efforts to balance the salt and mass fluxes of the individual basins2-5. Wyrtki6 made the first indirect estimate using hydro-graphic data of a mean annual transport from the Pacific to the Indian Ocean of 2 × 106 m3 s-1 in the upper 200m. Recent estimates have been considerably higher: 14 × 106 m3 s-1 by Piola and Gordon2, 10 × 106 m3 s-1 by Godfrey and Golding5, and 5 × 106 m3 s-1 by Godfrey and Ridgeway3. However, Wunsch et al. 4 concluded that, as a result of inverse calculations on sections across the South Pacific, there was a negligible throughflow transport. A meridional maximum in bomb-produced tritium (half-life 12.4 yr) observed in the South Equatorial Current (SEC) of the Indian Ocean is used here to show direct evidence that in the mean there is a net transport (in the upper 300 m) of 5 × 106 m3 s-1 from the Pacific into the Indian Ocean.
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