TURBULENT MIXING IN THE MIXED LAYER/THERMOCLINE TURBULENT MIXING IN THE MIXED LAYER/THERMOCLINE TRANSITION LAYERTRANSITION LAYER
Bryan Rahter and Louis St. LaurentBryan Rahter and Louis St. Laurent
Florida State UniversityFlorida State University
Thanks to:Thanks to:
Support from NSF POSupport from NSF PO
Photo of Storm over St. George Islandby Russel Grace
Turbulence in the transition layer
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.
Alford (2003)
QuikSCAT winds
Turbulence in the transition layer
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:D’Asaro (1985, 1995), Alford (2001; 2003).
mixed layer
T(z) N2(z)
thermocline Ef
ε = 15 / 2( )ν ∂u / ∂z( )2.
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.
transitionlayer
mixed layer
T(z) N2(z)
uz
thermocline Ef
dE
dt=1fddt
τ ⋅u( )−ρ ε dz−δ
0
∫ −E f ,
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)
FLX91
NATRE
FLX91 time series
NATRE time series
NATRE time series
Analysis procedure
We examined between 150 (Natre) and 350 (Flx91) profiler casts, spanning the length of each timeseries.
Mixed Layer Base:
- Temp. change > 0.1oC (from surface)
- Density change > 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:
mixed layer
T(z)N2(z)
thermocline
.2Nkv εΓ=
FLX91 dissipation rate (W/kg)
NATRE dissipation rate (W/kg)
Analysis resultsMean 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
kv
( )( ) 0
20
200
22
200
2
0 /
/
ε
ε
ε
ε=
Γ
Γ==
NN
NN
Nk
Nk
J
J vb
€
ε0( )
Exceptional wind events during FLX91 had twice the energy of those during NATRE
Why is FLX91 higher?
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.
