Low-frequency fluctuations in the Indonesian throughflow through Lombok Strait

A significant component of the Indonesian throughflow, apparently about 25%, passes through the Lombok Strait. Direct observations in 1985 reported a similar to 2 Sv annual average, with an annual cycle of amplitude similar to 2 Sv. There are also significant fluctuations in this transport in the 0.01-0.1 cpd frequency band. Shallow pressure gauge data (sea level) inside the strait during the current meter observations were of limited use in explaining the large fluctuations in currents. Sea level data at Cilacap, 720 km west (upcoast), however, overlap the current observations for 5 months and correlate exceedingly well (r = 0.87) with the observations inside the strait at these frequencies. These sea level oscillations in the Indian Ocean force fluctuations in the Lombok throughflow that reach 50-70 cm/s, equivalent to 2-3 Sv. Lagged regression analysis indicates Cilacap sea level leads Lombok currents by 1-2 days, suggesting a low-frequency, progressive wave. Simultaneous data in 1989 from four stations extending from the near-equatorial station at Padang at 1 degrees S to Benoa in the Lombok Strait (2000 km downcoast) clearly show the persistent propagation of low-frequency waves of 20- to 40-cm range along this coast. Lagged correlation on station pairs indicates a phase speed consistant with coastally trapped internal Kelvin waves. We speculate that farther eastward progression of these waves to the Timer Passage of Ombai Strait will further modulate the throughflow. The forcing of these waves is not yet identified, but it appears likely that intraseasonal oscillations in the equatorial Indian Ocean winds, as demonstrated by Enfield [1987] for the Pacific, are a probable mechanism. Improved wind data quality in 1991 due to the assimilation of satellite data (special sensor microwave/imager) will allow investigation of remote forcing on more recent data sets.