The Open Shelf Sea. 1. The primary source of buoyancy is surface heat flux. c p = specific heat...
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Transcript of The Open Shelf Sea. 1. The primary source of buoyancy is surface heat flux. c p = specific heat...
![Page 1: 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.](https://reader030.fdocuments.net/reader030/viewer/2022032723/56649d1a5503460f949efcf4/html5/thumbnails/1.jpg)
The Open Shelf Sea.
1. The primary source of buoyancy is surface heat flux.
cp = specific heat capacity of seawater (= 3900 J kg-1 K-1)
mean water temperature (in degrees Kelvin)
Heat stored = J m-2
evaporation
h
Qv (advection)
Qb Qc Qe
Thcp
Longwave radiationconduction
Qs(1-A) Solar heat input
T
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Distribution of heat input:
Radiation decays exponentially through the water column, i.e.:
0 20 40 60 80 100
% I 0
-50
-40
-30
-20
-10
0
dept
h / m
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.)
kzeI)z(I 0
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Tem perature
-50
-40
-30
-20
-10
0
dept
h / m
Stronger tidal currents
Tem perature
-50
-40
-30
-20
-10
0
dept
h / m
T em perature
-50
-40
-30
-20
-10
0
dept
h / m
Add tidal stress
The tidal currents mix the thermal structure up from the seabed:
Tem perature
-50
-40
-30
-20
-10
0
dept
h / m
Add wind
stress
The wind mixes the thermal structure down from the sea surface:
Stronger wind mixing
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pheating cQg
t.E.P
h 21
The rate of change of the Potential Energy of a shelf sea water column, driven by surface heat flux, can be derived as:
h
uk
t.E.P
hb
mixingtide
3
41 30
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)
cp = specific heat capacity of seawater (3900 J kg-1 °C-1)
kb = bottom drag coefficient (~0.003)
= efficiency of tidal mixing (~0.003)
uo = 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.
We can predict this
warm
cold
cool
High h/u3
Low h/u3
h/u3critical
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)
10th July 1999
(Images courtesy of Remote Sensing Group (Plymouth Marine Laboratory))