Hans Burchard

Leibniz Institute for Baltic Sea Research Warnemünde

hans.burchard@io-warnemuende.de

Coastal Ocean Dynamics

Second course: North Sea dynamics

North Seabathymetry

Source: Werner Alpers

North Seacatchment areas and Exclusive Economic Zones

Source: OSPAR Commission

Elbe

Weser

Rhine

Annual cycle of sea surface temperature in North Sea

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

Generation of the semi-diurnal lunar (M2) tide

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

Generation of the semi-diurnal solar (S2) tide

Tidal chart for the M2 tide (phase lines)

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

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.

Tidal chartfor the M2 tide

full lines: amplitude

dashed: phase

(Source: POL)

Tidal chart for the M2 tide (phase lines)

M2 t

idal

cha

rt o

f Sou

ther

n N

orth

Sea

Prandle, 1981

Amphidromic point

Tidal propagation into the Baltic Sea is blocked !

Tides in the Wadden Sea (as seen in 200 m resolution model)

Wadden Sea model:

M4 tidal elevations(phase and amplitude)as validation data.

Gräwe et al., in prep.

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)?

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)

Doggerbank

Doggerbank

Model results by Burchard & Bolding, 2002

Tidal front

Stratification is spatially not homogeneous

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.

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.

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

Tidal Mixing fronts in the Irish Sea

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

Stratification Simpson-Hunter parameter

Environmental effects of seasonal thermal stratification

Surface heat flux (cumulated)during FLEX‘76

Burchard, 2002

Burchard, 2002

Lateral effects are small, such that one-dimensional modelling may be successful

FLEX 1976

Summer stratification along 56°North Sea transect

Summer chlorophyll conc. along 56°North Sea transect

Northern North Sea:Annual cycle ofstratification andprimary production

Burchard et al., 2005Burchard, 2002

Rotating bulk shear in Monterey Bay

Itsweire et al. (1989)

PROVESS-NNS study site(observations: Sep-Nov 1998)

ADCP, CTD, MST

Wind

Bulk property observations in NNS

Wind

Bulk shear squared

Bulk shear directionvs.inertial rotation

Theory I

1D dynamic equations:

Layer averaging:

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.

Application of theory to observations

Impact of bulk shear on diapycnal mixing

Conclusion:Increased interfacialmixing rates correlatewith 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.)