Abyssal plain hills and internal wave turbulence

Abstract

Abstract. A 400 m long array with 201 high-resolution NIOZ temperature sensors was deployed above a north-east equatorial Pacific hilly abyssal plain for 2.5 months. The sensors sampled at a rate of 1 Hz. The lowest sensor was at 7 m above the bottom (m a.b.). The aim was to study internal waves and turbulent overturning away from large-scale ocean topography. Topography consisted of moderately elevated hills (a few hundred metres), providing a mean bottom slope of one-third of that found at the Mid-Atlantic Ridge (on 2 km horizontal scales). In contrast with observations over large-scale topography like guyots, ridges and continental slopes, the present data showed a well-defined nearhomogeneous "bottom boundary layer". However, its thickness varied strongly with time between < 7 and 100 m a.b. with a mean around 65 m a.b. The average thickness exceeded tidal current bottom-frictional heights so that internal wave breaking dominated over bottom friction. Near-bottom fronts also varied in time (and thus space). Occasional coupling was observed between the interior internal wave breaking and the near-bottom overturning, with varying up-and down-phase propagation. In contrast with currents that were dominated by the semidiurnal tide, 200 m shear was dominant at (sub-)inertial frequencies. The shear was so large that it provided a background of marginal stability for the straining high-frequency internal wave field in the interior. Daily averaged turbulence dissipation rate estimates were between 10 −10 and 10 −9 m 2 s −3 , increasing with depth, while eddy diffusivities were of the order of 10 −4 m 2 s −1 . This most intense "near-bottom" internal-wave-induced turbulence will affect the resuspension of sediments.

Variance of Bottom Water Temperature at the Continental Margin of the Northern South China Sea

“…While varied around 2.6 × 10 −4 m 2 s −1 in the range from 10 −5 to 10 −3 m 2 s −1 . These values are comparable to those observed in the abyssal plain of the Pacific Ocean (van Haren, 2018b) and globally in the deep ocean (Kunze, 2017). Figure 9 shows the spatial distribution of , 〈 χ θ 〉 m and interpolated by an inverse distance weighted method, where the searching neighborhood was specified as a circle with radius of 30 km, and at least 2 sites were involved.…”

“…Here, θ typically varied around 2.062°C in the observational period, except for a temperature jump to 2.067°C between days 155 and 164, which was detected by both Loggers 1 and 2. We speculated that the temperature shifts at days 155 and 164 might be attributed to the changes in flow direction across the slope, as it was reported that such variation would lead to a sharp change of temperature (Winters, 2015; van Haren, 2018b). At site 15, temperature fluctuation was small within a range of 0.003°C and the tidal signal was not so obvious, except for the period after day 172.…”

“…Comparable turbulence mixing has been accessed in recent abyssal ocean studies. In a site with an ocean depth of 4,114 m in the north‐east equatorial Pacific abyssal plain, the average ɛ and K ρ were 1.2 × 10 −10 m 2 s −3 and 7 × 10 −5 m 2 s −1 , respectively, within the depth range of 7–407 m above the ocean bottom (van Haren, 2018b). In another study on the dissipation driven by internal wave mixing in the global oceans deeper than 2000 m, the average dissipation rate and diffusivity were estimated at 10 −9 m 2 s −3 and 10 −4 m 2 s −1 , respectively, in the bottom 500 m above the seafloor (Kunze, 2017).…”

“…This corresponds to weaker mixing of χ θ ≈ 10 −14 °C 2 s −1 and ɛ θ ≈ 10 −10 m 2 s −3 . van Haren (2018b) found that the tides associated with internal waves of different frequencies occasionally penetrated close to the ocean bottom with a temperature chain measurement in the hilly Pacific abyssal plain (depth 4,114 m). Our results indicate that the influence of internal waves (tides) can reach much closer to the ocean floor (≈0.5 m above bottom).…”

“…Over the depth range of different sites from 2,700 to 4,000 m, the fitted temperature gradient d θ /d z gradually decreased from 6.9 × 10 −5 to 1.2 × 10 −5 °Cm −1 , and the fitted buoyancy frequency N from 4.7 × 10 −4 to 1.9 × 10 −4 rads −1 (6.3–2.4 cpd). Thus, the overall background was under weak stratification according to the classification of N < 10 f by van Haren (2018b). In comparison, we calculated the vertical temperature gradient d θ /d z and buoyancy frequency N of individual profiles in the depth ranges of 10, 50, 100, and 200 m above the ocean bottom by using a first‐order difference scheme.…”