Deep-sea research. Part A, Oceanographic research papers最新文献

An Agulhas ring was detected using XBT probes on a cruise between Cape Town and Vema Seamount in the south-east Atlantic Ocean in April 1989. CTD and nutrient data, collected on a second cruise in May 1989, GEOSAT altimeter data for February–April 1989 and cloud-free NOAA-11 satellite imagery from June 1989 were used to characterize the ring. The ring was elliptical (330 km E-W and 165 km N-S, relative to the 16°C isotherm at 200 m depth), evident to at least 1200 m, and centred on 30.5°S, 9.2°E in May, about 700 km west of the Orange River. Its drift velocity was 6.4 ± 1 cm s⁻¹ to the NW. Maximum anticyclonic geostrophic surface currents near its edge were 55 cm s⁻¹ relative to 1150 db. The available potential energy was estimated to be 38.8 × 10¹⁵ J and the kinetic energy 2.3 × 10¹⁵ J using a two layer model of the ring. A cool filament extending 450 km offshore from the Benguela upwelling front was identified in the hydrography and the NOAA-11 imagery. Entrainment velocities (maximum of 75 cm s⁻¹) of mature upwelled water from the Benguela frontal region were inferred from feature tracking.

Distribution of the dissolved atmospheric chlorofluoromethanes (CFMs) F-11 and F-12 in the South Atlantic Ocean are used to study the ventilation and circulation of Antarctic Intermediate Water (AAIW). CFM distributions on an isopycnal surface representative of AAIW are consistent with recently ventilated water entering the subtropical gyre in the southwestern Atlantic and then being advected anticyclonically around this gyre. The westward-flowing northern limb of the gyre apparently divides near the coast of South America with some water flowing southward to recirculate in the gyre, and the balance flowing northward along the coast of Brazil. At the equator this northward current divides again with one branch going eastward along the equator and the other continuing into the Northern Hemisphere. In the eastern tropical Atlantic, the CFM concentrations on this isopycnal surface in the cyclonic gyre are extremely low between the subtropical gyre and the equatorial tongue. Along the prime meridian, the F-11 and F-12 concentrations on the 27.2 θσ isopycnal surface between the mixed layer outcrop and the northern edge of the subtropical gyre are fitted to a one-dimensional advection-diffusion model. This model assumes that the CFMs enter the tubtropical gyre solely by northward diffusion from the mixed layer outcrop to the southern edge of the subtropical gyre, and that their distributions within the gyre are controlled by both advective and diffusive processes. Velocity and eddy diffusion coefficients are calculated from a least-squares fit to the data. These values are then used to calculate a mean oxygen consumption rate which is consistent with rates calculated using models of other time-dependent geochemical tracers.

In this study we investigate the utility of GEOSAT altimetry for monitoring the Iceland-Faeroe frontal zone. Since an expected dynamic topography relief of 10–20 cm over the Iceland-Faeroe Front (IFF) was not much above the 10 cm uncertainty in GEOSAT observations, validation by AVHRR imagery and satellite-tracked drifters constituted an important part of the experiment. Sea Surface Height (SSH) relief of greater than 20 cm occurred a long the western side of the IFF and along the eastern side, north of the Shetland Islands. However, with SSH relief of only 10–15 cm in the central region of the IFF, substantial difficulties were encountered in the ability to unambiguously monitor the location of the front. In contrast, frontal meanders with 20–30 cm SSH relief, current speeds up to 50 cm s⁻¹ and radii of curvature of 25 km, were clearly observed on three occasions during the 2 year study. These meanders first appeared north of the Faeroe Islands, in the region from 6 to 8°W, and propagated southeastward at speeds of about 3.3 km day⁻¹, being lost from view in the Faeroe-Shetland Channel. Their strong signals and lifetimes of 2–3 months would appear to make them important constituents of IFF dynamics.

Acoustic Doppler current profilers (ADCPs) are now commonplace on many ships and have had a considerable impact on modern oceanography. During a recent cruise on R.R.S. Discovery, however, the presence of a large spurious shear in the direction of the ship's motion was noted on a 150 kHz profiler, particularly when steaming into heavy seas. It is thought that this results from the trapping of bubbles near the ship's hull. Parameters representing the state of the wind and sea, and the ship's motions, are combined in a semi-empirical way to produce a function (E) that estimates the severity of this effect and so gives insights into the factors affecting the data quality. The results are compared with a further cruise upon which the transducers were extended beyond the bubble layer. The estimating function allows a meaningful intercomparison for the different weather conditions on the second cruise and shows that the spurious shear was effectively eliminated. Similar problems are now being observed on other vessels, and it is recommended that ADCP transducers should be mounted below the bubble layer whenever possible.

We present the distributions of two chlorofluorocarbons (CFCs) CFC-11 and CFC-12, measured as part of a hydrographic section between Iceland and the equator during July and August 1988. CFC-tagged water has filled the entire subpolar water column and subtropical thermocline in the eastern North Atlantic. Measurable CFC concentrations are observed at the ocean bottom as far south as 35°N, and the CFC penetration depth shoals to ≈750 m in the tropics. Specific features in the CFC distributions include a clear signal of Labrador Sea mid-depth ventilation, a CFC-enriched overflow water boundary current along the Iceland slope, and a mid-depth, equatorial plume of upper North Atlantic Water.

The CFC data are used, in conjuction with the hydrographic data from the cruise, to illustrate the ventilation time-scales and pathways for the water masses in the eastern basin. A subsurface CFC maximum at about 100–200 m depth in the subtropics is shown to be a by-product of the heating and degassing of the seasonal thermocline and of the temperature sensitivity of CFC solubility. The CFC concentrations in the subpolar mode water are undersaturated by 15–18% relative to the atmosphere, reflecting the age of the mode waters and the very deep winter mixed layers in the eastern subpolar gyre. The CFC concentrations in the oxygen minimum off tropical Africa are much lower than the concentrations in the subtropical gyre, supporting previous work that suggests that isolation and enhanced productivity both contribute to the formation of the tropical oxygen minimum. In addition, the CFC inventories at the tropical stations have increased between 1982 and 1983 (TTO/TAS) and the summer of 1988 at a slower rate relative to the subtropical inventories over the same period. Thermocline oxygen utilization rates calculated from the CFC concentration data range from 5 to 10 μmol kg⁻¹y⁻¹ and are in line with previous estimates for the eastern subtropical thermocline. The low CFC concentrations in Mediterranean Water, about one-quarter those in the Labrador Sea Water, are shown to result from entrainment near the Straits of Gibraltar of a large component of low CFC, lower Atlantic thermocline water. Based on the CFC and other transient tracer distributions, the deep eastern basin can be divided into two regions: the Iceland Basin and surrounding area influenced by Iceland-Scotland Overflow Water that is ventilated on a decadal time-scale, and the area south of ≈50°N that has little or no CFC and is ventilated from a southern source on a much longer time-scale. A northward flowing boundary current of low CFC, modified Eastern Basin Bottom Water is also found along the Rockall Plateau and in the Iceland Basin.