Surface effects of bottom-generated turbulence in a shallow tidal sea

Abstract

Turbulence in shelf seas strongly affects the spread of pollution (such as oil spills 1 ) as well as the distribution of sediment 2 and phytoplankton blooms 3 . Turbulence is known to be generated intermittently close to the sea bed 4 , but little is known of its evolution through the water column, or to what extent it affects the surface. Here we present observations of the surface effects of bottom-generated turbulence in a tidally in¯uenced and well mixed region of the North Sea, as derived from acoustic and visual images. Although the sea bed in the area is¯at, we ®nd that at any one time, 20±30% of the water surface is affected by boilsÐ circular regions of local upwellingÐof diameter 0.960.2 times the water depth. The signature of individual boils persists for at least 7 minutes and, in accordance with laboratory 5,6 and numerical 7 studies, shows the appearance of eddies. The boils contribute to the replacement of surface waters from depth in unstrati®ed waters, and may therefore enhance the¯uxes of gases between atmosphere and ocean.There are no reported observations of the surface signature of boils in well mixed, open sea. It is, however, clear from dynamical measurements made using current meters that tidal¯ows generate turbulence through the action of shear stress at the sea bed in much the same manner as in channels in laboratory experiments 4,8 . Fluid is intermittently ejected away from the bottom in turbulent bursts that may reach vertical speeds of 25% of the forcing current 4 . Numerical 7 and laboratory 5,6 studies show that the upwardmoving water produces a boil as it impinges on the water surface.Upward-pointing side-scan sonar has been used to observe a wide variety of processes in the upper ocean 9±13 . In experiments designed to study the processes leading to dispersion of an oil plume in the tidally well mixedÐand consequently unstrati®ed 14 Ðsouthern North Sea, a two-beam side-scan sonar system was mounted on a frame set on the sea bed at a depth of 45 m. The sonars operate at 80 kHz and 90 kHz, are set at 908 apart in the horizontal, and produce vertical fan-like beams with axes aligned upwards, 208 from the horizontal 15 . The site is 56 km from the shore and the local sea bed is¯at, with no sand banks or other notable bed forms. The principal acoustic scatterers are bubbles, of typical diameter 20± 200 mm, produced in clouds by wind waves as they break 16 . The clouds are detectable to ranges of ,150 m along the sea surface from the sonar, bubbles accumulating in regions of surface convergence and downwelling 17,18 . Oil reduces wave breaking and acoustic scatter from the sea surface 10 . Acoustic observations were made, however, at least 0.5 km from the oil plume, and in a variety of wind speeds from near calm to 14 m s -1 and in tidal currents reaching 1 m s -1 . letters to nature NATURE | VOL 400 | 15 JULY 1999 | www.nature.com 251 Figure 1 Sonar and video images of boils at the sea surface. The scale is the same in both images (and along both axes), and the tidal current is from ...

Turbulent transport in the outer region of rough-wall open-channel flows: the contribution of large coherent shear stress structures (LC3S)

“…Nimmo Smith et al (1999) show an example of boils (corresponding to large-scale coherent turbulent flow structures impinging on the free surface) in the North Sea reaching the surface from a 30 m depth and argue that the dispersion of material on the surface due to currents exceeds that due to wind-and wave-driven Langmuir circulation when the current speed is greater than about 2 % of the wind speed. The effect of wall-generated turbulence on a free surface is also important in river re-oxygenation (Moog & Jirka 1999), and for the transport of dissolved matter in partially filled large pipes at high discharge levels (Kumar, Gupta & Banerjee 1998).…”

“…Examples of such processes are the exchange of low-solubility gases, such as carbon dioxide and oxygen across the atmosphere-ocean interface under conditions of strong tidal forcing (Nimmo Smith, Thorpe & Graham 1999), and the horizontal dispersion of buoyant material (Thorpe et al 1994;Nimmo Smith et al 1999). Nimmo Smith et al (1999) show an example of boils (corresponding to large-scale coherent turbulent flow structures impinging on the free surface) in the North Sea reaching the surface from a 30 m depth and argue that the dispersion of material on the surface due to currents exceeds that due to wind-and wave-driven Langmuir circulation when the current speed is greater than about 2 % of the wind speed.…”

High-frequency bottom-pressure and acoustic variations in a sea strait: internal wave turbulence

“…To a lesser extent, but dominated at the same short-term periodicity of ∼50 s, vertical motions concentrated in the lower half of the water column accompany the echo and pressure variations. The motions could be turbulent overturns that are characterized by a horizontal length scale of 0.9 H≈20 m (Nimmo Smith et al 1999). They could also manifest internal waves, as near-bottom stratification can be sufficiently large to support them.…”

“…Such vigorous turbulence may fill the entire water column with material whirled up from the bottom. The largest sizes of these turbulent motions are about 0.9 times the water depth (Nimmo Smith et al 1999). This turbulence highly depends on the phase of the tidal current and so do stratification, resuspension of material and internal waves.…”

Surface Imprints of Water-Column Turbulence: A Case Study of Tidal Flow over an Estuarine Sill

“…Boils have been reported for relatively weak stratification in a shallow tidal sea (water depth about 45 m) [3,4]; in a 200-m-deep estuary [5]; over 25-m-deep sand waves [6]; and downstream of 4-m-deep sills [7,8]. Boils have also been reported in association with a headland-induced front [9,10] and with large-amplitude internal waves [11,12]; and boils can be expected to occur in other situations, as well, e.g., in association with upwelling of buoyant outflow water at the edge of an ice shelf.…”