Hans Burchard Leibniz Institute for Baltic Sea Research Warnemünde
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Transcript of Hans Burchard Leibniz Institute for Baltic Sea Research Warnemünde
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Hans Burchard
Leibniz Institute for Baltic Sea Research Warnemünde
Coastal Ocean Dynamics
Second course: North Sea dynamics
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North Seabathymetry
Source: Werner Alpers
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North Seacatchment areas and Exclusive Economic Zones
Source: OSPAR Commission
Elbe
Weser
Rhine
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Annual cycle of sea surface temperature in North Sea
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http://www2.astro.psu.edu/users/cpalma/astro10/class21.html
Generation of the semi-diurnal lunar (M2) tide
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http://www2.astro.psu.edu/users/cpalma/astro10/class21.html
Generation of the semi-diurnal solar (S2) tide
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Tidal chart for the M2 tide (phase lines)
http://en.wikipedia.org/wiki/Amphidromic_point
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Why do the tides in the North Sea look like this ?
Tidal waves enter into North Sea through northern boundary and English Channel.
Phase propagation is c = (g * depth)1/2, g = 9.81 m/s2
(depth = 40 m c = 20 m/s = 72 km/h)
Due to Earth rotation, tidal waves are Kelvin waves,leaning on a coast to the right.
Energy loss due to bed friction: tidal waves loosepower during their journey through the North Sea.
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Tidal chartfor the M2 tide
full lines: amplitude
dashed: phase
(Source: POL)
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Tidal chart for the M2 tide (phase lines)
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M2 t
idal
cha
rt o
f Sou
ther
n N
orth
Sea
Prandle, 1981
Amphidromic point
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Tidal propagation into the Baltic Sea is blocked !
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Tides in the Wadden Sea (as seen in 200 m resolution model)
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Wadden Sea model:
M4 tidal elevations(phase and amplitude)as validation data.
Gräwe et al., in prep.
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Seasonality of tidal forcing
Müller et al. (in prep.)
Gräwe et al. (in prep.)
How does this affect sediment transport due to tidal asymmetries (M4)?
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Stratification in the North Sea
Besides tides, seasonal stratification is characteristic for the North Sea dynamics.
Bolding and Burchard (2002)
Annual cycle of temprature stratification in the Northern North Sea (as seen from a 1D model)
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Doggerbank
Doggerbank
Model results by Burchard & Bolding, 2002
Tidal front
Stratification is spatially not homogeneous
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Tidal fronts
Tidal fronts (stratified in deep, mixed in shallow water) are an environmentally very important phenomenon.
Thus, in shallow water the bottom sediment is in direct contact with the surface waters, whereas in deeper waters, the bottom layers are clearly separated from the surface wates.
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What determines the position of tidal fronts ?
Stabilising: water depth H, surface buoyancy flux Q (heat flux, net precipitation).
Destabilising: tides given as tidal velocity amplitude u.
Important parameter by dimensional analysis:
(Q * H) / u3
Large: stably stratified; Small: mixed
Note: H / u3 is the famous Simpson-Hunter (1974) parameter.
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Tidal mixing fronts in the Irish Sea
Satellite images courtesy Alejandro Souza
stratified & deepmixed & shallowmixed & very shallow & warm
or
stratified due to river run-off
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Tidal Mixing fronts in the Irish Sea
Numerical model result, Souza et al., in press.
Stratification Simpson-Hunter parameter
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Environmental effects of seasonal thermal stratification
Surface heat flux (cumulated)during FLEX‘76
Burchard, 2002
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Burchard, 2002
Lateral effects are small, such that one-dimensional modelling may be successful
FLEX 1976
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Summer stratification along 56°North Sea transect
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Summer chlorophyll conc. along 56°North Sea transect
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Northern North Sea:Annual cycle ofstratification andprimary production
Burchard et al., 2005Burchard, 2002
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Rotating bulk shear in Monterey Bay
Itsweire et al. (1989)
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PROVESS-NNS study site(observations: Sep-Nov 1998)
ADCP, CTD, MST
Wind
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Bulk property observations in NNS
Wind
Bulk shear squared
Bulk shear directionvs.inertial rotation
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Theory I
1D dynamic equations:
Layer averaging:
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Dynamic equation for bulk shear squared:
Conclusion:
Assuming bed stress being small, bulk shear is generated by the alignment of wind vector and shear vector.
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Application of theory to observations
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Impact of bulk shear on diapycnal mixing
Conclusion:Increased interfacialmixing rates correlatewith high shear.
Can we resolve this in 3D models?
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Transect in NNS
Observations (Scanfish data from BSH)
Model results (GETM with adaptive coordinates)
Gräwe et al. (in prep.)