Material transport in a convective surface mixed layer under weak wind forcing

Jean A. Mensa, Tamay M Ozgokmen, Andrew C. Poje, Jörg Imberger

Research output: Contribution to journalArticle

10 Citations (Scopus)

Abstract

Flows in the upper ocean mixed layer are responsible for the transport and dispersion of biogeochemical tracers, phytoplankton and buoyant pollutants, such as hydrocarbons from an oil spill. Material dispersion in mixed layer flows subject to diurnal buoyancy forcing and weak winds (|u10|=5ms-1) are investigated using a non-hydrostatic model. Both purely buoyancy-forced and combined wind- and buoyancy-forced flows are sampled using passive tracers, as well as 2D and 3D particles to explore characteristics of horizontal and vertical dispersion. It is found that the surface tracer patterns are determined by the convergence zones created by convection cells within a time scale of just a few hours. For pure convection, the results displayed the classic signature of Rayleigh-Benard cells. When combined with a wind stress, the convective cells become anisotropic in that the along-wind length scale gets much larger than the cross-wind scale. Horizontal relative dispersion computed by sampling the flow fields using both 2D and 3D passive particles is found to be consistent with the Richardson regime. Relative dispersion is an order of magnitude higher and 2D surface releases transition to Richardson regime faster in the wind-forced case. We also show that the buoyancy-forced case results in significantly lower amplitudes of scale-dependent horizontal relative diffusivity, kD(ℓ), than those reported by Okubo (1970), while the wind- and buoyancy-forced case shows a good agreement with Okubo's diffusivity amplitude, and the scaling is consistent with Richardson's 4/3rd law, kD ~ ℓ4/3. These modeling results provide a framework for measuring material dispersion by mixed layer flows in future observational programs.

Original languageEnglish (US)
Pages (from-to)226-242
Number of pages17
JournalOcean Modelling
Volume96
DOIs
StatePublished - Dec 1 2015

Fingerprint

wind forcing
mixed layer
surface layer
Buoyancy
buoyancy
tracer
diffusivity
convection
buoyancy forcing
Wind stress
Phytoplankton
Oil spills
upper ocean
wind stress
oil spill
flow field
material
Flow fields
Hydrocarbons
phytoplankton

Keywords

  • Relative dispersion
  • Scale-dependent diffusivity
  • Upper ocean turbulence

ASJC Scopus subject areas

  • Atmospheric Science
  • Oceanography
  • Geotechnical Engineering and Engineering Geology
  • Computer Science (miscellaneous)

Cite this

Material transport in a convective surface mixed layer under weak wind forcing. / Mensa, Jean A.; Ozgokmen, Tamay M; Poje, Andrew C.; Imberger, Jörg.

In: Ocean Modelling, Vol. 96, 01.12.2015, p. 226-242.

Research output: Contribution to journalArticle

Mensa, Jean A. ; Ozgokmen, Tamay M ; Poje, Andrew C. ; Imberger, Jörg. / Material transport in a convective surface mixed layer under weak wind forcing. In: Ocean Modelling. 2015 ; Vol. 96. pp. 226-242.
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