TURBULENT MIXING IN THE MIXED LAYER/THERMOCLINE TRANSITION LAYER

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TURBULENT MIXING IN THE MIXED LAYER/THERMOCLINE
TRANSITION LAYER

• Bryan Rahter and Louis St. Laurent
• Florida State University
• Thanks to
• Support from NSF PO

Photo of Storm over St. George Island by Russel
Grace
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Turbulence in the transition layer
Alford (2003)
QuikSCAT winds

• Wind energy input in the inertial band is
  generally regarded as a direct source of near
  inertial internal waves to the ocean interior.
• This is assumed to support turbulent mixing in
  the thermocline.
• Our study is aimed at quantifying the levels of
  turbulence occurring specifically in the
  transition layer between the mixed-layer and
  thermocline.

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Turbulence in the transition layer
N2(z)
T(z)

• Many studies focus on turbulence occurring in the
  mixed layer
• Examples from microstructure studies
• Oakey (1985), Smyth et al. (1996), Anis Moum
  (1992), Mickett (2008).
• Many other studies focus on the energy transfer
  to internal waves in the thermocline.
• Examples
• DAsaro (1985, 1995), Alford (2001 2003).

mixed layer
thermocline
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Turbulence in the transition layer

• However, shear driven mixing in the transition
  layer inhibits the near-inertial energy transfer
  to waves.
• Plueddemann and Farrar (2006)
• The specific properties of this layer are often
  ignored in models and observations.

N2(z)
T(z)
mixed layer
transition layer
thermocline
Ef
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Data used in our study

• We seek
• time-series turbulence data
• spanning mixed layer to thermocline
• documenting open-ocean conditions.
• FLX91 (FLUX STATS)
• Mid-latitude eastern N. Pacific
• April 1991, 6-day time series
• OSU CHAMELEON (Moum)
• Ref Hebert and Moum (1994)
• NATRE (N. Atlantic Tracer Release)
• Mid-latitude eastern N. Atlantic
• April 1992, 25-day timeseries
• WHOI HRP (Schmitt and Toole)
• Ref. St. Laurent and Schmitt (1999)

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Data used in our study
FLX91
NATRE
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FLX91 time series
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NATRE time series
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NATRE time series
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Analysis procedure

• We examined between 150 (Natre) and 350 (Flx91)
  profiler casts, spanning the length of each
  timeseries.
• Mixed Layer Base
• - Temp. change gt 0.1oC (from surface)
• - Density change gt 0.025 kg/m3
• Transition Layer Base
• - Based on peak in N2 and
• average N2 for thermocline
• Thermocline
• - 100-m thick layer beneath the transition layer
• The dissipation rate ( ) was averaged by layer.
• The diffusivity was also calculated

N2(z)
T(z)
mixed layer
thermocline
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FLX91 dissipation rate (W/kg)
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NATRE dissipation rate (W/kg)
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Analysis results

• Mean diffusivities for the layers
• (cm2/s)
• mixed layer transition layer thermocline
• FLX91 150 0.3 0.5
• NATRE 37 0.08 0.08
• Ratio of average dissipation between layers with
  thermocline
• (equivalent to buoyancy flux ratio)
• mixed layer transition layer
• FLX91 171 8
• NATRE 15 4

Why is FLX91 higher?
Exceptional wind events during FLX91 had twice
the energy of those during NATRE
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Conclusions

• Transition layer dissipation rates are
  consistently elevated above thermocline values
  (by a factor of 4 to 8).
• It appears that the larger dissipation levels of
  FLX91 relative to NATRE were correlated to the
  peak wind events, rather than mean wind levels
  which were comparable.
• Why is this Significant?
• - The enhanced dissipation rates in the
  transition layer represent
• an energy loss term to near inertial waves
  emitting from the
• mixed-layer base.
• - This implies a reduction in energy available
  for turbulent mixing in
• the thermocline.