Convectively Driven Turbulent Mixing in the Upper Ocean

Shay, Thomas J.; Gregg, Michael C.

doi:10.1175/1520-0485(1986)016<1777:cdtmit>2.0.co;2

Cited by 202 publications

(144 citation statements)

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“…The primary source of turbulent energy is the work done by wind stress on the diurnal jet, rsAu, (rather than free convection, which is very small), and so during the day dissipation is unremarkable. Large values are confined to the upper 10-20 m, and are comparable to observations of dissipation at mid-latitudes under conditions of similar heating and wind stress (Lombardo and Gregg, 1989;Shay and Gregg, 1986).…”

Section: 32i Daytime Dissipationsupporting

confidence: 82%

“…During the day, when strong solar heating stabilizes the upper 10-20 m, little turbulent dissipation is observed; what dissipation existed is limited to depths less than about 10 m. During the night, as cooling and wind-mixing erase the daytime warming, dissipation becomes of order 10-7 W kg-1 , indicative of vigorous mixing, down to depths of 80 m, more than twice the depth of the surface mixed layer. In contrast, observations from the upper ocean at mid-latitudes show that the high values of dissipation associated with turbulent mixing penetrate only about 10 m below the mixed layer at night even when the mixed layer is very deep, 0(100 m) (Shay and Gregg, 1986;Lombardo and Gregg, 1989;Price et al, 1986; see Figure 1.2). In the mid-latitude upper ocean, dissipation is also generally much less intense for similar wind and surface heating/cooling conditions.…”

Section: Observations Of the Equatorial Upper Oceanmentioning

confidence: 92%

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Upper ocean dynamics during the LOTUS and TROPIC HEAT experiments

Schudlich¹

1991

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This thesis examines the effect of mean large-scale currents on the vertical structure of the upper ocean during two recent observational programs: the Long Term Upper Ocean Study (LOTUS) and the TROPIC HEAT experiments. The LOTUS experiment took place in the northwest Atlantic Ocean, a mid-latitude region away from strong mean currents, and extended over one entire seasonal cycle. The TROPIC HEAT experiments took place in the central equatorial Pacific Ocean during two 12-day periods in 1984 and 1987, at opposite extremes of the seasonal cycle. We use observations from these field experiments as well as one-dimensional numerical models of the upper ocean to analyze the dynamics of the vertical structure of the upper ocean at the equator and in mid-latitudes. Due to the different nature of the observations, we focus on the long term mean structure of the upper ocean in the LOTUS observations (Chapters 2 and 3), and on the diurnal cycle in the equatorial upper ocean in our analysis of the TROPIC HEAT observations (Chapters 4 and 5).In the LOTUS observations, we find that the observed current is coherent with the wind over low frequencies (greater than an inertial period). Using a wind-relative averaging method we find good agreement with Ekman transport throughout the first summer and winter of the LOTUS experiment, with the exception of a downwind component in the wintertime. The mean current spiral is flat compared to the classic Ekman spiral, in that it rotates less with depth than does the Ekman spiral. The mean current has an e-folding depth scale of 12 m in the summer and 25 m in the winter.Diurnal cycling is the dominant variability in the summer and determines the vertical structure of the spiral. In the winter, diurnal cycling is almost non-existent due to greatly reduced solar insolation. There is a persistent downwind shear in the upper 15 m during the winter which may be partially due to a bias induced by surface wave motion but which is also consistent with a logarithmic boundary layer.The Price et al. (1986) model is reasonably successful in simulating the current structure during the summer, capturing both the mean and the diurnal variation. The model is less successful in the winter, though it does capture the overall depth scale of the current spiral.In our analysis of the TROPIC HEAT observations, we extend the Price et al. (1986) model to the equatorial upper ocean. The model is initialized with the stratification and shear of the Equatorial Undercurrent (EUC), and is driven with heating and wind stress. A surface mixed layer is determined by bulk stability requirements, and a transition layer below the mixed layer is simulated by requiring that the gradient Richardson number be no less than 1/4. A principal result is that the nighttime phase of the diurnal cycle is strongly affected by the EUC, resulting in deep mixing and large dissipation at night consistent with observations of the equatorial upper ocean during TROPIC HEAT. Other features of the equatorial circulation (upwelling and...

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Section: 32i Daytime Dissipationsupporting

confidence: 82%

Section: Observations Of the Equatorial Upper Oceanmentioning

confidence: 92%

Upper ocean dynamics during the LOTUS and TROPIC HEAT experiments

Schudlich¹

1991

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“…Convective mixing and turbulence-Similar to convective mixing at the water surface, which is usually driven by a positive surface buoyancy flux, i.e., by heat loss at the water surface (Shay and Gregg 1986;Anis and Moum 1992;Jonas et al 2003), gravitationally unstable stratification in the BBL can lead to sinking plumes of heavier water and rising plumes of lighter water and hence to the production of TKE and mixing. When buoyancy is generated by a density anomaly Ј, which is associated with a temperature anomaly ␦T, its ability to overcome viscous forces and thermal diffusion and hence give rise to active thermal convection is described by the Rayleigh number: Turner 1973), with g denoting the gravitational acceleration; K T , the molecular diffusivity of heat; , the kinematic viscosity; and h, the height of the unstable layer.…”

Section: Observations and Analysesmentioning

confidence: 99%

“…The buoyancy flux on the continental shelf was much higher because of the thermal expansivity of seawater, which is also an order of magnitude larger than that for cold freshwater. Following the method of Shay and Gregg (1986), the upper bound of J and the thickness h can be used to charac- Evaluating t* for h ϭ 2.5 m and J ഠ 7 ϫ 10 Ϫ9 W kg ഠ 2.6 mm s Ϫ1 . If convective turbulence was an additional source of TKE, then dissipation rates should be enhanced during the cycle of unstable stratification.…”

Section: B Bmentioning

confidence: 99%

Shear-induced convective mixing in bottom boundary layers on slopes

2005

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Field data from a stratified lake demonstrate that buoyancy-driven convective mixing can be an important mechanism for the formation of well-mixed bottom boundary layers (BBLs) on slopes. Convective turbulence is caused by differential transport of stratified water masses along the slope of a basin. Because the current velocity usually decreases toward the sediment, water at some distance from the bottom is transported faster than is water below. During upslope flow, this process leads to transport of heavier water on top of lighter water and hence to unstable stratification within the BBLs. Analogously, strong BBL stratification occurs during downslope flow. High-frequency acoustic Doppler current profiler (ADCP) and temperature measurements in the BBLs of a lake revealed the cyclic occurrence of convective turbulence driven by periodic across-slope currents of internal seiching. The estimated maximum buoyancy flux within the BBLs was in good agreement with the highest observed dissipation rates of turbulent kinetic energy. This suggests that, in our specific case, classical bottom shear production and the mentioned buoyancy-driven (convective) production contributed by similar amounts to the turbulent kinetic energy.It is a frequently observed feature of many stratified water bodies that a well-mixed bottom boundary layer (BBL) exists directly above the sediment surface. The height of such well-mixed layers varies between a few meters in lakes and reservoirs (Gloor et al. 2000;Hondzo and Haider 2004;Lemckert et al. 2004) to several tens of meters in oceans (Caldwell 1978;Lentz and Trowbridge 1991). The turbulent kinetic energy (TKE) required to generate and maintain such mixed layers is usually assumed to be produced by the bottom friction of basin-or large-scale currents (Fricker and Nepf 2000; Wüest et al. 2000), shoaling and critical reflection of high-frequency internal waves (Thorpe 1997; Imberger 1998), or the interaction of large-scale currents with rough topography (Rudnick et al. 2003).Enhanced mixing along the boundaries has been demonstrated in numerous studies (Macintyre et al. 1999; Ledwell et al. 2000;Garrett 2003). Moreover, tracer measurements in lakes (Goudsmit et al. 1997) and ocean basins (Ledwell and Bratkovich 1995; Ledwell and Hickey 1995) revealed the potential importance of boundary mixing for basin-scale 1 To whom correpsondence should be addressed. Present address: Limnological Institute, University of Konstanz, Mainaustrasse 252, D-78464 Konstanz, Germany (andreas.lorke@uni-konstanz.de). AcknowledgmentsWe thank M. Schurter and D. Finger for their great help in the field. The constructive input from two reviewers significantly improved the clarity of the manuscript; D. Richter, M. Schmid, and D. McGinnis provided helpful comments; and M. Stubbs kindly corrected the English.

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“…Maximum onshore winds occurred during the November survey when the most plumes were observed; the fewest plumes were observed during the August survey, when winds were surface mixed layer in June (Figure 9). The deep winter mixed layer is driven by convection [Shay and Gregg, 1986]. These density profiles were used to estimate the depth range in which bank water would appear with the assumption of no mixing for each survey (Hc) ( Table 1).…”

Section: Evidence For Saline Plumes In Exuma Soundmentioning

confidence: 99%

Dense saline plumes in Exuma Sound, Bahamas

Hickey¹,

MacCready²,

Elliott³

et al. 2000

J. Geophys. Res.

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Abstract. Cold dense plumes have been associated with coral killoff on tropical shelves as well as with sediment movement from banks to the deep basins adjacent to shallow banks. This paper presents evidence that plumes of dense, salty water generated over shallow banks entrain ambient water rather than descending intact to a density compensation level (as has sometimes been assumed) and that plumes can spread laterally distances of tens of kilometers from their source. In Exuma Sound, dense, salty water appears to be formed year-round by cooling or by evaporation over shallow banks. The water is forced by tidal currents through the channels between islands onto the narrow shelves of Exuma Sound where it likely moves as gravity currents. The gravity currents entrain basin water as they cross the shelf and cascade over the steep shelf edge into Exuma Sound, after which they are advected by ambient currents around the sound. A numerical streamtube model was used to explore early details of plume evolution. The model predicts that the plumes entrain 2.

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Convectively Driven Turbulent Mixing in the Upper Ocean

Cited by 202 publications

References 0 publications

Upper ocean dynamics during the LOTUS and TROPIC HEAT experiments

Upper ocean dynamics during the LOTUS and TROPIC HEAT experiments

Shear-induced convective mixing in bottom boundary layers on slopes

Dense saline plumes in Exuma Sound, Bahamas

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