Jan Reissmann, Hans Burchard, Rainer Feistel, Eberhard Hagen, Hans Ulrich Lass, Volker Mohrholz,

1. Vertical mixing in the Baltic Sea: A review Jan Reissmann, Hans Burchard, Rainer Feistel, Eberhard Hagen, Hans Ulrich Lass, Volker Mohrholz, Günther Nausch, Lars Umlauf, and Gunda Wieczorek Leibniz Institute for Baltic Sea Research Warnemünde, Germany hans.burchard@io-warnemuende.de Financially supported by Baltic2020

2. The global conveyor belt dynamics strongly influence the world climate. Diapycnal mixing is a key element of this (Munk 1966).

3. The Baltic conveyor belt determines the Baltic Sea ecosystem. Diapycnal mixing is key for the nutrient supply to the euphotic zone. Elken and Matthäus, 2008

4. Role of vertical mixing for the Baltic Sea ecosystem

5. Salinity in the Central Baltic Sea Surface Time scales: Exchange time: 13 a Deep water residence: 21 a Surface water residence: 33 a Bottom Feistel et al., 2006

6. Back of the envelope Baltic Sea salt budget B.S. excess freshwater: 500 km3/a Mean outflow salinity: 8 g/kg Mean B.S. salt turnover: 4 Gt/a B.S. area at 60 m: 130.000 km2 Vertical salt flux at 60 m: 30 kg/(m2a) Mean salt gradient at 60 m: 0.05 g/kg / m …0.15 g/kg / m Average salt diffusivity: 2.1 · 10-6 m2/s …6.3 · 10-6 m2/s Pycnocline salinity: 10 g/kg Corresponding uplift: 3 m/a = 10 mm/d

7. Baltic Sea vertical mixing processes

8. GETM Western Baltic Sea hindcast

9. Model derived monthly mean vertically integrated physically and numerically induced salinity mixing Physical mixing Numerical mixing

10. Mixing in inflows Entrainment velocity: ≈ 4 · 10-5 m/s (Arneborg et al., 2007) Contours: density Shading: dissipation rate (Umlauf et al., 2007)

11. GETM 2DV Slice Model: Transverse gravity current structure

12. Internal mixing in the Baltic Sea: DIAMIX Derived diapycnal diffusivities Dissipation rates Boundary mixing Lass et al., 2003

13. Bottom boundary mixing Boundary mixing may also be due to breaking internal waves or grounding meso-scale eddies. Upwelling Marginal stability Enhanced mixing Observations: QuantAS (IOW & friends)

14. Coastal upwelling Hard to quantify: Observations: no T-signal in winter Models: too low resolution Contribution of upwelling to diapycnal mixing not yet quantified. Summer upwelling transports phosphate from winter water into surface water. Lass et al., 1996

15. Salinity measurements in the Central Baltic Sea in 2005 (by SMHI)

16. 1D model (GOTM) of Central Baltic Sea upper layers Temperature Salinity In terms of (turbulence) observations this complex region is highly undersampled. Stratification Eddy diffusivity The ecosystem impact of these complex structures needs to be assessed.

17. Conclusions The Baltic Sea vertical overturning may be quantified as bulk. However, the relative importance of contributing processes as well as their spatio- temporal distribution remains highly uncertain and hidden in a black box. This is to a large extent owing to lack of Baltic Sea mixingstudies. The Baltic Sea ecosystem depends on the overturning circulation and the diapycnal mixing processes in a highly sensitive way. It is therefore necessary to conduct detailed process studies of small-scale physical processes. Several initiatives have started: - BATRE – Baltic Sea Tracer Release Experiments – deep water mixing - Intense Microstructure Shear Probe Measurements The Baltic Sea Model Assessment and Teaching Initiative (BaSMATI) Has just been proposed to the BONUS Programme Numerical models have to be improved such that these small-scale processes are properly reproduced.

18. There are many things we do not understand here: @ @ @ @ @ @ @ How can we assess the effects of yet another (engineering) mixing process ?