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Published byTamsyn Shepherd Modified over 9 years ago
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The Open Shelf Sea. 1. The primary source of buoyancy is surface heat flux. c p = specific heat capacity of seawater (= 3900 J kg -1 K -1 ) mean water temperature (in degrees Kelvin) Heat stored = J m -2 evaporation h Q v (advection) QbQb QcQc QeQe Longwave radiation conduction Q s (1-A) Solar heat input
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Distribution of heat input: Radiation decays exponentially through the water column, i.e.: k=0.1 m -1 In clear water: 55% heat is input into top 1 m 70% is input within 3 m In typical shelf waters: >90% input within 5 m Heat output occurs from the “skin” of the surface. k is an attenuation coefficient, dependent on wavelength of radiation (e.g. see Kirk, Light & photosynthesis in aquatic ecosystems.)
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Stronger tidal currents Add tidal stress The tidal currents mix the thermal structure up from the seabed: Add wind stress The wind mixes the thermal structure down from the sea surface: Stronger wind mixing
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The rate of change of the Potential Energy of a shelf sea water column, driven by surface heat flux, can be derived as: The rate of increase of the Potential Energy of a shelf sea water column, driven by tidal mixing, can be derived as: Heating > tide-mixing water column stratifies in summer Heating < tide-mixing water column remains vertically mixed = 1.6 x 10 -4 °C -1 volume expansion coefficient of seawater Q = rate of heat flux through surface (W m -2 ) c p = specific heat capacity of seawater (3900 J kg -1 °C -1 ) k b = bottom drag coefficient (~0.003) = efficiency of tidal mixing (~0.003) u o = tidal current amplitude (m s -1 ) h = depth (m) What happens if the two rates are equal?
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mixed front stratified
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Shelf Sea (or Tidal Mixing) Fronts. These are the transition regions between the permanently mixed and seasonally stratified shelf waters. By running the phys1d program with a range of values for h and/or u you can investigate the effects of tidal mixing on a shelf sea water column.phys1d We can predict this warm cold cool High h/u 3 Low h/u 3 h/u 3 critical Low u and/or high h will result in a water column that stratifies during spring and summer High u and/or low h will result in a water column that remains mixed.
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As the existence of shelf sea fronts became recognised, parallel observations of the biology and chemistry of the fronts showed: 1. Fronts separate the low nutrient, stratified surface water from higher nutrient mixed water (Morin et al., 1985. J. Mar. Biol. Assoc., 65, 677- 695) 2. Fronts are often observed to be regions of high chlorophyll biomass (Pingree et al., 1975. Nature, 258, 672-677) 3. Fronts are regions of enhanced primary production (Horne et al., 1989. Scientia Marina, 53, 145-158). Enhanced Primary Production at Tidal Mixing Fronts Useful reading: Mann & Lazier, Dynamics of Marine Ecosystems, 2nd ed. (Blackwell Science) pages 187-196
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Sea surface temperature Sea surface chlorophyll concentration (AVHRR)(SeaWIFS) 10 th July 1999 (Images courtesy of Remote Sensing Group (Plymouth Marine Laboratory))
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