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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
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 ?