TURBULENT MIXING IN THE MIXED LAYER/THERMOCLINE TRANSITION LAYER

1. 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

2. 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.

3. Ef Turbulence in the transition layer N2(z) T(z) mixed 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). thermocline

4. uz Turbulence in the transition layer N2(z) T(z) mixed layer 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. thermocline Ef

5. 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)

6. Data used in our study FLX91 NATRE

7. FLX91 time series

8. NATRE time series

9. NATRE time series

10. Analysis procedure N2(z) T(z) mixed layer 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: thermocline

11. FLX91 dissipation rate (W/kg)

12. NATRE dissipation rate (W/kg)

13. 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

14. 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.