Coastal Ocean Dynamics
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Coastal Ocean Dynamics Second course: North Sea dynamics. Hans Burchard Leibniz Institute for Baltic Sea Research Warnemünde [email protected] North Sea bathymetry. Source: Werner Alpers. North Sea catchment areas and Exclusive Economic Zones. Weser. Elbe.

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Coastal Ocean Dynamics

Secondcourse:

North Sea dynamics

Hans Burchard

Leibniz Institute for

Baltic Sea Research Warnemünde

[email protected]


North Sea

bathymetry

Source: Werner Alpers


North Sea

catchmentareas

and

ExclusiveEconomicZones

Weser

Elbe

Rhine

Source: OSPAR Commission


Annual cycleofseasurfacetemperature in North Sea


Generation ofthe semi-diurnal lunar (M2) tide

http://www2.astro.psu.edu/users/cpalma/astro10/class21.html


Generation ofthe semi-diurnal solar (S2) tide

http://www2.astro.psu.edu/users/cpalma/astro10/class21.html


Tidalchartforthe M2tide (phaselines)

http://en.wikipedia.org/wiki/Amphidromic_point


Why do thetides in the North Sealooklikethis ?

Tidalwavesenterinto North Seathrough

northern boundaryand English Channel.

Phase propagationis c = (g * depth)1/2, g = 9.81 m/s2

(depth = 40 m  c = 20 m/s = 72 km/h)

Due to Earth rotation, tidalwavesare Kelvin waves,

leaning on a coasttotheright.

Energyloss due tobedfriction: tidalwavesloose

power duringtheirjourneythroughthe North Sea.


Tidalchart

forthe M2tide

fulllines: amplitude

dashed: phase

(Source: POL)


Tidalchartforthe M2tide (phaselines)


Amphidromicpoint

M2tidalchartof Southern North Sea

Prandle, 1981


Tidalpropagationintothe Baltic Seaisblocked !


Tides in theWaddenSea (asseen in 200 m resolution model)


WaddenSea model:

M4tidalelevations

(phaseandamplitude)

asvalidationdata.

Gräwe et al., in prep.


Seasonalityoftidalforcing

Howdoesthisaffect

sedimenttransport

due totidalasymmetries (M4)?

Gräwe et al. (in prep.)

Müller et al. (in prep.)


Stratification in the North SeaBesides tides, seasonal stratification is characteristic for the North Sea dynamics.

Annual cycleoftemprature

stratification in the

Northern North Sea

(asseenfrom a 1D model)

Boldingand Burchard (2002)


Stratificationisspatially not homogeneous

Tidal front

Doggerbank

Doggerbank

Model resultsby Burchard & Bolding, 2002


Tidalfronts

Tidalfronts(stratified in deep, mixed in shallowwater) are an environmentallyveryimportantphenomenon.

Thus, in shallowwaterthebottomsedimentis in directcontactwiththesurfacewaters, whereas in deeperwaters, thebottomlayersareclearlyseparatedfromthesurfacewates.


Whatdeterminesthepositionoftidalfronts ?

Stabilising: waterdepthH, surfacebuoyancyfluxQ

(heatflux, netprecipitation).

Destabilising: tidesgivenastidalvelocityamplitudeu.

Importantparameterby dimensional analysis:

(Q * H) / u3

Large:stablystratified; Small:mixed

Note: H / u3isthefamous Simpson-Hunter (1974) parameter.


Tidal mixing fronts in the irish sea
Tidal mixing fronts in the Irish Sea

stratified & deep

mixed & shallow

mixed & very shallow & warm

or

stratified due to river run-off

Satellite images courtesy Alejandro Souza


Tidal mixing fronts in the irish sea1
Tidal Mixing fronts in the Irish Sea

Stratification Simpson-Hunter parameter

Numerical model result, Souza et al., in press.


Environmental effectsofseasonalthermal stratification

Surfaceheatflux (cumulated)

during FLEX‘76

Burchard, 2002


FLEX 1976

Lateral effects

aresmall, such that

one-dimensional

modellingmaybe

successful

Burchard, 2002


Summer stratificationalong 56°North Seatransect


Summer chlorophyllconc. along 56°North Seatransect


Northern North Sea:

Annual cycleof

stratificationand

primaryproduction

Burchard et al., 2005

Burchard, 2002


Rotatingbulkshear in Monterey Bay

Itsweire et al. (1989)


PROVESS-NNS study site

(observations: Sep-Nov 1998)

Wind

ADCP, CTD, MST


Bulk property observations in NNS

Wind

Bulk shear squared

Bulk shear direction

vs.

inertial rotation


Theory I

1D dynamic equations:

Layer averaging:


Dynamic equationforbulkshearsquared:

Conclusion:

Assumingbed stress beingsmall, bulkshearisgenerated

bythealignmentof wind vectorandshearvector.


Application of

theory to

observations


Impact of

bulk shear on

diapycnal mixing

Conclusion:

Increased interfacial

mixing rates correlate

with high shear.

Can we resolve this

in 3D models?


Transect in NNS

Observations (Scanfish data from BSH)

Model results (GETM with adaptive coordinates)

Gräwe et al. (in prep.)


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