Intensive measurements of turbulence and shear in the equatorial undercurrent

Gregg, Michael C.; Peters, Hartmut; Wesson, J. C.; Oakey, Neil S.; Shay, Thomas J.

doi:10.1038/318140a0

Cited by 160 publications

(110 citation statements)

References 16 publications

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“…3a) at maximum. The magnitude and vertical structure of the observed turbulent parameters agrees rather well with those inferred from microstructure measurement programs carried out in the central equatorial Pacific (Gregg et al 1985;Peters et al 1988;Moum et al 1989;Lien et al 1995). However, in comparison to the central equatorial Pacific, Hummels et al (2013) report for the equatorial ACT region a reduction in the night time enhancement of turbulence, which is referred to as deep cycle turbulence (Moum and Caldwell 1985).…”

Section: Turbulent Mixing Within the Actsupporting

confidence: 70%

“…In fact, they were able to balance the seasonal ML heat budget at this location for the period from May to November when incorporating estimates of the diapycnal heat flux. In the ACT as well as in the PCT region, turbulent mixing could be associated with shear instabilities (Gregg et al 1985;Hummels et al 2013). Within the entire equatorial ACT vertical shear of horizontal velocity is significantly elevated due to opposing currents, namely the westward-directed northern branch of the South Equatorial Current (nSEC) at the surface and the Equatorial Undercurrent (EUC) flowing eastward along the thermocline.…”

Section: Introductionmentioning

confidence: 99%

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Diapycnal heat flux and mixed layer heat budget within the Atlantic Cold Tongue

et al. 2014

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Section: Turbulent Mixing Within the Actsupporting

confidence: 70%

Section: Introductionmentioning

confidence: 99%

Diapycnal heat flux and mixed layer heat budget within the Atlantic Cold Tongue

et al. 2014

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“…For example at a high oceanic value of y = 0.1 S-' (e.g. Gregg et al 1985), shear has less effect on viral contact rate than movement of 1 body-length S-', which is a minimal swimming speed (Fig. 3).…”

mentioning

confidence: 99%

Viral dynamics: a model of the effects of size shape, motion and abundance of single-celled olanktonic organisms and other particles

Murray¹,

Jackson²

1992

Mar. Ecol. Prog. Ser.

313

339

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The transport of aquatic viruses to particles can be described in terms of diffusive transport from solution. Such transport is influenced by motion of the water relative to the particle. Because transport rate is determined purely by physical factors it is independent of whether the particle is a host or non-host organism. The low viral diffusivity relative to that for dissolved nutrients makes transport enhancement from organism swimming more important for viruses. The virus contact rate with bacteria is relatively unaffected by such motions because of small bacterial sizes. Transport rates for phytoplankton and protozoa can increase over an order of magnitude when swimming motions are considered. Although larger organisms have much higher transport rates per individual, their far lower concentrations In sea water should make small organisms the preferred targets for viruses. Rates of host-virus interactions in culture are closely related to predictions from transport theory. There is a fairly close relationship between bacterial populations and virus disappearance rates in the marine environment, suggesting that non-host organisms are a major cause of viral mortality at the higher ionic strengths typical of sea water. Other factors, such as UV radiation, must also be included when estimating viral mortality in seacvater.

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“…Recent upper ocean measurements made during the TROPIC HEAT program (Niiler, 1987) revealed a large diurnal cycle in turbulent dissipation at the equator (Gregg et al, 1985;Mourn and Caldwell, 1985;Peters et al, 1988;Mourn et al, 1989), indicating a very strong daily variation in mixing at the equator that was previously unknown. 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.…”

Section: Observations Of the Equatorial Upper Oceanmentioning

confidence: 99%

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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Intensive measurements of turbulence and shear in the equatorial undercurrent

Cited by 160 publications

References 16 publications

Diapycnal heat flux and mixed layer heat budget within the Atlantic Cold Tongue

Diapycnal heat flux and mixed layer heat budget within the Atlantic Cold Tongue

Viral dynamics: a model of the effects of size shape, motion and abundance of single-celled olanktonic organisms and other particles

Upper ocean dynamics during the LOTUS and TROPIC HEAT experiments

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