Flow pattern in the Ombai Strait, Indonesia, and its relationship with the Indonesian throughflow

Abstract

Indonesian throughflow has recently been observed in various locations surrounding the Indonesian Seas, including the Ombai Strait, one of the major southern passages. Several oceanographic data sets are available for this strait, including hydrographic (CTD) observations, measurements from a one-year deployment of a current meter mooring, and a two-year sea level measurement from two pressure gauges, one at each end of the strait. North Pacific Intermediate Water and South Pacific Subtropical Water enter the Wetar Strait at its east end, flowing westward through the Ombai Strait toward the Indian Ocean. Some relatively fresh water enters the north Ombai Strait from the Savu Sea to the west. Water from the Flores Sea does not enter the Ombai or Wetar straits through the passage between Alor and Wetar islands. Based on current meter records from December 1995 to early November 1996, the main current direction in the Ombai Strait is westward toward the Indian Ocean, with a mean volume transport estimate of 9.1 Sv over the full depth range 0-1300 m. The along-passage vertical structures of mean current and first EOF modes of time variable currents are highly intensified in the upper 200 m. The low frequency variability of current from direct current measurement in the upper 100 m is significantly correlated with cross-passage sea level difference. In addition, the local forcing of zonal wind stress relates to about 7% of the low frequency current variability in the Ombai Strait. Regression analysis was performed to "calibrate" the cross-passage sea level difference to the contemporaneous direct current measurement. Then, based on geostrophic theory, a "calibrated" two-year cross-passage sea level was used to estimate the Ombai Strait throughflow in the upper 100 m to be 2.6 ± 1.1 Sv in 1996 and 1.9 ± 0.9 in 1997. Tidal currents are dominated by the semidiurnal components M2 and S2. However, the energy of components N2, K2, M4, MS4, Mm and Mf is, in each case, more than 2% of the energy of M2, and that of diurnal components K1 and O1 is approximately 1.6% of the M2 energy.