Coastal Ocean Dynamics Baltic Sea Research Warnemünde Second course: North Sea dynamics Hans Burchard Leibniz Institute for Baltic Sea Research Warnemünde hans.burchard@io-warnemuende.de

North Sea bathymetry Source: Werner Alpers

Exclusive Economic Zones North Sea catchment areas and Exclusive Economic Zones Weser Elbe Rhine Source: OSPAR Commission

Annual cycle of sea surface temperature in North Sea

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

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 loose power during their journey through the North Sea.

Tidal chart for the M2 tide full lines: amplitude dashed: phase (Source: POL)

Tidal chart for the M2 tide (phase lines)

M2 tidal chart of Southern North Sea Amphidromic point M2 tidal chart of Southern North Sea Prandle, 1981

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 How does this affect sediment transport due to tidal asymmetries (M4)? Gräwe et al. (in prep.) Müller et al. (in prep.)

Stratification in the North Sea Besides tides, seasonal stratification is characteristic for the North Sea dynamics. Annual cycle of temprature stratification in the Northern North Sea (as seen from a 1D model) Bolding and Burchard (2002)

Stratification is spatially not homogeneous Tidal front Doggerbank Doggerbank Model results by Burchard & Bolding, 2002

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 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 Sea Stratification Simpson-Hunter parameter Numerical model result, Souza et al., in press.

Environmental effects of seasonal thermal stratification Surface heat flux (cumulated) during FLEX‘76 Burchard, 2002

FLEX 1976 Lateral effects are small, such that one-dimensional modelling may be successful Burchard, 2002

Summer stratification along 56°North Sea transect

Summer chlorophyll conc. along 56°North Sea transect

Northern North Sea: Annual cycle of stratification and primary production Burchard et al., 2005 Burchard, 2002

Rotating bulk shear 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 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 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.)