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

Vertical mixing in the Baltic Sea:A review

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

Elken and Matthäus, 2008

The Baltic conveyor belt determines the Baltic Sea ecosystem.Diapycnal mixing is key for the nutrient supply to the euphotic zone.

Role of vertical mixing for the Baltic Sea ecosystem

Feistel et al., 2006

Salinity in the Central Baltic Sea

Bottom

Surface

Time scales:

Exchange time: 13 aDeep water residence: 21 aSurface water residence: 33 a

Back of the envelope Baltic Sea salt budget

Corresponding uplift: 3 m/a = 10 mm/d

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

sum m er heating

overflow soverflow s

outflow s

seasonal therm ocline

w inter cooling

win

ter

sum

mer

in terna lm ix ing

perm anent ha locline

uplift

in terleaving

in terna lw ave m ixing

bottomcurrententra inm ent

surface w avem ixing

boundarym ixing

convectiveentra inm ent

shear-inducedentra inm ent

d ifferentia ladvection

riverrunoff

w ind stress

coasta lupw elling

sun

Baltic Sea vertical mixing processes

GETM Western Baltic Sea hindcast

Model derived monthly mean vertically integrated physically and numerically induced salinity mixing

Physical mixing Numerical mixing

Mixing in inflows

Contours: densityShading: dissipation rate

Entrainment velocity:≈ 4 · 10-5 m/s(Arneborg et al., 2007)

(Umlauf et al., 2007)

GETM 2DV Slice Model: Transverse gravity current structure

10-6

10-5

10-4

10-3

10-2

10-1

100

-120

-100

-80

-60

-40

-20

0

Kv [m²/s]

z [m

]

DIAMIX99: Mean vertical mixing coefficient

Kv=0.2 N-2

Kv=0.87*10-7m²/s² N-1

Ledwell et al. [1998]Axell [1998]

Matthäus [1990]

Internal mixing in the Baltic Sea: DIAMIX

Dissipation rates Derived diapycnal diffusivities

Lass et al., 2003

Boundary mixing

2.0,2

N

Kv

Bottom boundary mixing

Marginal stability

Upwelling

Enhanced mixing

Observations: QuantAS (IOW & friends)

Boundary mixing may also be due to

breaking internal waves or

grounding meso-scale eddies.

Coastal upwelling

Hard to quantify:

Observations: no T-signal in winter

Models: too low resolution

Lass et al., 1996

sum m er heating

overflow soverflow s

outflow s

seasonal therm ocline

w inter cooling

win

ter

sum

mer

in terna lm ixing

perm anent ha locline

uplift

in terleaving

in terna lw ave m ixing

bottomcurrententra inm ent

surface w avem ixing

boundarym ixing

convectiveentra inm ent

shear-inducedentra inm ent

d ifferentia ladvection

riverrunoff

w ind stress

coasta lupw elling

sun

Summer upwellingtransports phosphatefrom winter water intosurface water.

Contribution of upwelling to diapycnalmixing not yet quantified.

Quantity: Salinity Year: 2005 Station: BY 15 Lat: 57°20' Lon: 20°03' Depth Jan20 Feb25 Apr07 Apr28 May19 Jun16 Jul14 Aug11 Sep01 Sep29 Oct27 Nov16 Dec12 0m - 7.43 7.39 7.31 7.19 7.17 6.93 6.82 6.84 6.97 6.93 7.06 7.13 5m 7.42 7.43 7.39 7.31 7.19 7.17 7.05 6.82 6.85 6.96 6.93 7.06 7.13 10m 7.41 7.43 7.39 7.31 7.18 7.16 7.14 6.83 6.84 6.96 6.93 7.06 7.13 15m 7.41 7.43 7.39 7.32 7.19 7.16 7.15 6.88 6.88 6.95 6.93 7.06 7.13 20m 7.41 7.43 7.39 7.36 7.22 7.18 7.13 6.84 7.08 7.08 6.93 7.06 7.13 30m 7.42 7.45 7.39 7.37 7.36 7.31 7.26 7.07 7.20 7.38 6.94 7.10 7.13 40m 7.41 7.47 7.39 7.43 7.41 7.39 7.36 7.22 7.35 7.48 7.43 7.11 7.14 50m 7.42 7.47 7.41 7.51 7.53 7.49 7.46 7.34 7.44 7.57 7.53 7.13 7.17 60m 7.42 7.46 7.50 7.64 7.73 7.63 7.59 7.68 7.55 7.67 7.63 7.24 7.55 70m 9.54 9.32 9.05 8.77 8.82 8.78 8.44 8.71 8.20 8.08 7.92 7.80 8.24 80m 10.36 10.37 9.73 9.80 9.81 9.85 9.73 9.96 9.68 9.56 8.36 9.49 8.99 90m 10.64 10.86 10.66 10.56 10.40 10.51 10.39 10.47 10.43 10.26 9.72 10.23 10.23

100m 10.90 11.16 11.03 10.98 10.74 11.05 10.96 11.06 10.94 10.67 10.35 10.86 10.58 125m 11.84 11.75 11.68 11.78 11.52 11.91 11.86 11.90 12.01 11.77 11.48 11.85 11.46 150m 12.34 12.23 12.21 12.25 12.19 12.28 12.33 12.31 12.30 12.29 12.23 12.37 12.17 175m 12.47 12.49 12.44 12.45 12.47 12.46 12.53 12.49 12.48 12.45 12.33 12.48 12.45 200m 12.63 12.49 12.59 12.59 12.61 12.60 12.63 12.58 12.61 12.59 12.51 12.59 12.55 225m 12.77 12.61 12.69 12.70 12.73 12.69 12.73 12.66 12.70 12.69 12.61 12.66 12.56 240m 12.80 12.71 12.76 12.74 12.76 12.70 12.74 12.70 12.73 12.73 12.68 12.69 12.68

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

Eddy diffusivityStratification

SalinityTemperature

1D model (GOTM) of Central Baltic Sea upper layers

In terms of (turbulence) observations this complex region is highly undersampled.

The ecosystem impact of these complex structures needs to be assessed.

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.

It is therefore necessary to conduct detailed process studies of small-scalephysical processes.

The Baltic Sea ecosystem depends on the overturning circulationand the diapycnal mixing processes in a highly sensitive way.

Numerical models have to be improved such that these small-scale processes are properly reproduced.

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

sum m er heating

overflow soverflow s

outflow s

seasonal therm ocline

w inter cooling

win

ter

sum

mer

in terna lm ix ing

perm anent ha locline

uplift

in terleaving

in terna lw ave m ixing

bottomcurrententra inm ent

surface w avem ixing

boundarym ixing

convectiveentra inm ent

shear-inducedentra inm ent

d ifferentia ladvection

riverrunoff

w ind stress

coasta lupw elling

sun

There are many things we do not understand here:

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How can we assessthe effects of yet another(engineering) mixingprocess ?