THE FLUX OF OVERFLOW WATER THROUGH THE CHARLIE-GIBBS FRACTURE-ZONE

We report the results of a set of measurements made within the Charlie-Gibbs Fracture Zone in 1988-1989. The principal data sources are year-long current meter records, 16 in number, and 23 conductivity-temperature-depth stations. The aim was to determine the flux of overflow water passing westward out of the Iceland Basin in the eastern North Atlantic. Within the fracture zone, overflow water is defined either as having salinity greater than 34.94 at the present time or, more generally, as having a potential density sigma0 > 27.8 kg/m3. In 1988-1989 the core salinity had a value equal to that reported in 1981 and 1983 but was fresher by about 0.02 than was found in 1964. The flux was estimated twice per day and was found to fluctuate strongly, with energy spread in a broad band of periods, ranging from 10 to 400 days. No annual signal could be detected. The mean value was 2.4 +/- 0.5 Sv, less than all previous estimates based on hydrography alone. It is suggested that hydrography may systematically overestimate transport because diffusion is neglected. Flux estimates from current meter arrays are rarely used in initializing numerical ocean models; consequently measurements, like those reported here, are an independent and important means for assessing the performance of such models. At the deepest levels, water enters die two transform valleys of the fracture zone from the west. In the northern valley the flow is periodically flushed out by energetic bursts of overflow water. A critical value of an inverse Froude number is shown to define the flushing threshold.