Estimating transport in Makassar Strait

Monthly averaged current meter data from two moorings in Labani Channel are examined, and a method, based on fitting normal modes, is developed to estimate the transport through Makassar Strait. The data span a depth range from about 210 to 1500 m and a time period from November 1996 to July 1998. They show monthly averaged southward currents in excess of 50 cm s −1 at 250 m , and episodes ranging from 1–6 months of 5– 10 cm s −1 northward flow below 600 m . Estimates of the along-channel flow above and below the data record are made by fitting normal vertical modes, derived from climatological buoyancy frequency profiles, to the data. Tests of the fitting method show that the depth-averaged value is recovered well for profiles truncated between 200 and 250 m , but that the baroclinic structure cannot be recovered if more than the upper ∼50 m of data are missing. However, for some almost full-depth acoustic Doppler profiles taken in Makassar Strait, the reconstructed flow averaged over the upper 250 m is typically found to lie within the bounds provided by the method. The estimated mean depth-integrated transport for 1997 is 6.4 Sv southwards with upper and lower bounds of 16.0 and 4.7 Sv respectively. Over the upper 250 m , the estimated mean transport for 1997 is 2.0 Sv southwards with upper and lower bounds of 9.7 and 0.8 Sv , respectively. The upper (lower) bounds are given by a normal mode reconstruction in which the first (third) baroclinic mode dominates the profile for much of the year; for the best estimate, the second baroclinic mode dominates the profiles through most of the year. The estimated mean net transport range for 1997 encompasses the earlier range published by Gordon et al. (Geophys. Res. Lett. 26 (1999) 3325), where empirical formulae were used to extrapolate the current profiles to the sea surface. The normal-mode reconstruction of the flow, temperature data from T-pods on the western Labani Channel mooring, and temperature and zonal wind data from the TAO moorings in the equatorial Pacific Ocean, provide a consistent description of cool, upwelling (warm, downwelling) baroclinic Rossby waves being scattered into the Indonesian archipelago as the equatorial zonal winds collapse (intensify) at the onset of El Niño (La Niña).