The flux of overflow water through the Charlie‐Gibbs Fracture Zone

Abstract

We report the results of a set of measurements made within the Charlie-Gibbs Fracture Zone in 1988-1989. The principal dam sources are year-long current meter •rds, 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 c•0 > 27.8 kg/m 3. 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 2A + 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 the two wansform 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. Introduction Under the impact of severe winter cooling, the Norwegian-Greenland-Iceland Seas produce dense water which overflows the shallow ridge in the northern North Atlantic lying between Greenland, Iceland and Scotland [Swi•, 1984]. As recent moored current meter measurements show, in two locations the overflow is persistent and steady, namely in the Denmark Strait [Dickson et al., 1990] and in the Faeroe Bank Channel [Saunders, 1990]. In contrast, on the Iceland-Faeroe Ridge it is intermittent [Meincke, 1983]. East of Iceland the overflow water follows a path on the eastern flanks of the Reykjanes Ridge as well as filling the deepest levels of the Iceland Basin [Harvey and Theodorou, 1986]. Near 53øN the overflow encounters the Charlie-Gibbs Fracture Zone (CGFZ) where it passes westward into the Irminger and Newfoundland Basins [Worthington and Volkmann, 1965], see Figure 1 (inset). Recently McCartney [1992] has modified the traditional view discussed above by advancing the concept of a Deep Northern Boundary Current, of which the northern overflow is a constituent, but nevertheless major part. In addition, his deep current contains recirculating Labrador Sea Water and deep water drawn up from midlatitudes. During the past 4 years the author of this paper has been engaged in a field program studying the overflow east of the Mid-Atlantic Ridge. The principal aims of the project are, first, to measure the flux of dense water at source and to determine what fac...

Direct velocity measurements in the Samoan Passage

“…For example, variability of roughly this period has been observed near the equator in satellite sea surface temperature [Legeckis, 1977], sea level [Wyrtki, 1978] It is more interesting to consider how this variability might be caused by the intrinsic dynamics of the northward abyssal flow. Resonances, similar to those occurring in musical wind instruments, arise in one-dimensional open channel flow [Lighthill, 1978] [Saunders, 1994;Schmitz and Hogg, 1978], the Denmark Strait [Dickson and Brown, 1994], the Verna Channel [Hogg et al, 1982], and a gap between the Ceara Rise and the MidAtlantic Ridge [Whitehead and Worthington, 1982]. We speculate that the periodicities of each will be dependent on the bathymetry upstream of the sill.…”

Interaction of the North Atlantic Current with the Deep Charlie Gibbs Fracture Zone Throughflow

“…Earlier observations with moored current meter stations in the northern trench of the CGFZ yielded a mean westward transport of 2.5 Sv with, however, much larger variability than previously reported for the DSOW boundary current off Greenland [Dickson et al, 1990], including intermittent periods of eastward flow [Saunders, 1994]. Moored current observations at the eastern entry of the southern trench yielded no net throughflow [Dickson et al, 1980].…”

Transport of Nordic Seas overflow water into and within the Irminger Sea: An eddy‐resolving simulation and observations

“…A minor contribution also spills over the Wyville-Thomson Ridge. After flowing over the IFR and the FBC sills, and entraining ambient water, the resulting Iceland-Scotland overflow water (ISOW) has been observed to flow southwestward along the northwestern slope of the Iceland Basin [Saunders, 1996], and westward through the Charlie-Gibbs fracture zone (CGFZ) [Saunders, 1994]. It has been suggested that this flow through the CGFZ subsequently turns northward into the Irminger Sea, joining DSOW there [e.g., Dickson and Brown, 1994;Saunders, 2001].…”

“…In contrast, an isopycnic model, in the absence of entrainment parameterization, contains insufficient diapycnal mixing and consequently the overflow properties are retained over unrealistically long distances [Willebrand et al, 2001]. Transport estimates based on the long-term moored instrument arrays of Dickson and Brown [1994], Saunders [1994Saunders [ , 1996, Dickson et al [2008], and Bacon and Saunders [2010] have formed a valuable benchmark for evaluating model performance in simulating overflow transport, which has not been well documented in the past. Some, but not all, of these transport data were used in a recent numerical study by Chang et al [2009].…”

“…There are questions that require further clarification, however. The direct measurements of ISOW transport by Saunders [1994Saunders [ , 1996 lead to unresolved, although not unanticipated, questions concerning a possible branch of overflow water flowing through gaps in the Reykjanes Ridge north of the CGFZ. Questions concerning the transport of ISOW from the Iceland Basin into and within the Irminger Sea have been put forth in reviews by Saunders [2001] and Hansen and Østerhus [2000].…”