Low‐frequency fluctuations in the Indonesian throughflow through Lombok Strait

Abstract

A significant component of the Indonesian throughflow, apparently about 25%, passes through the Lombok Strait. Direct observations in 1985 reported a ~2 Sv annual average, with an annual cycle of amplitude ~2 Sv. There are also significant fluctuations in this Wansport in the 0.01-0.1 cl• 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 (upcoas0, however, overlap the current observations for 5 months and correlate exceedingly well (r = 0.87) with the observations inside the slrait 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 $v. 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 løS to Benoa in the Lombok Strait (2000 km downcoas0 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 Timor Passage of Ombai Strait will further modulate the throughflow. The forcing of these waves is not yet identified, but it appears likely that intmseasonal oscillations in the equatorial Indian • 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. Recent modeling studies also agree on a significant (10-20 Sv) Pacific-to-Indian transport. The multilayer global model of Semtner and Chervin [1988] indicated a mean 1T of 15-18 Sv; the global Sverdrup model of Godfrey [1989] predicted 16+4 Sv; and the reduced gravity global model of Kindle eta/. [1989] exhibited a mean for 1980-1987 of about 5 Sv, with a seasonal signal of amplitude 4 Sv. A recent basin-scale model study [lnoue and Welsh, 1993] forced by monthly mean climatological winds predicted a total IT ranging from 0.5 Sv toward the Pacific up to 18.1 Sv toward the Indian Ocean in July-October, with a yearly mean of ~10 Sv. This model indicates that the Lombok Strait (Figure 1) carries nearly 40% of the total IT, with the balance passing through the major passages bracketing Timor. While the importance of Lombok Strait is probably overestimated in this model, it is clear that it is a significant conduit for the IT. These modeling studies are generally consistent with results from the observational front. Drifters [Lukas eta/., 1991], hydrographic properties [Ffield and Gordon, 1992], and modeling results [Kindle eta/., 1989; Inoue and Welsh, 1993] identify the Makassar Strait as th...

Effect of the 1997/98 El Niño on Chlorophyll a Variability Along the Southern Coasts of Java and Sumatra

“…In addition to ENSO, other forcings may infl uence chlorophyll a concentrations in the Indonesian seas, such as tides (Ffi eld and Gordon, 1996;Susanto et al, 2000), the Madden-Julian Oscillation (MJO) (Madden and Julian, 1994), Kelvin and Rossby waves (Arief and Murray, 1996;Sprintall et al, 2000), monsoons (Asanuma et al, 2003;Moore et al, 2003;Susanto et al, submitted), and the Indian Ocean Dipole (IOD) (Saji et al, 1999;Webster et al, 1999). Because of its longitudinal extent (90° to 141°E), the Indonesian seas contain a signifi cant part of the tropical ocean productivity.…”

Vertical Structure of Kelvin Waves in the Indonesian Throughflow Exit Passages

“…Syamsudin et al (2004) used altimetric data to estimate that 56% of incoming semiannual Kelvin wave energy enters Lombok Strait, but the amount of intraseasonal Kelvin wave energy entering Lombok Strait has not been well quantified with observations. Few in situ measurements have captured Kelvin waves in the Indonesian archipelago, and most of these have focused on the semiannual Kelvin waves forced during the monsoon transition seasons in May and November (Arief and Murray 1996;Sprintall et al 1999Sprintall et al , 2000Hautala et al 2001;Potemra et al 2002). Little is known about the vertical profile of velocity associated with Kelvin waves (Wijffels and Meyers 2004), particularly in the Indonesian archipelago, which has many sills, islands, and other topographic features that likely affect local Kelvin wave dynamics (S09).…”

On the Indonesian Throughflow in the OCCAM 1/4 degree ocean model

“…In the south there have been measurements in the Lombok Strait from January 1985 to January 1986 (Murray and Arief, 1988;Murray et al, 1990;Arief and Murray, 1996), in the Timor Passage from August 1989 to September 1990 and from March 1992 to April 1993 (Cresswell et al, 1993;Molcard et al, 1994Molcard et al, , 1996, and in the Ombai Strait from November 1995 to November 1996 (Molcard et al, 2001). …”