The Open Shelf Sea.
1. The primary source of buoyancy is surface heat flux.
Longwave radiation
Qs(1-A) Solar heat
input
Qb
conduction
Qc
evaporation
Qe
h
Qv (advection)
Heat stored =
cp = specific heat capacity of seawater (= 3900 J kg-1 K-1)
T  mean water temperature (in degrees Kelvin)
c p hT
J m-2
Distribution of heat input:
Radiation decays exponentially through the water column, i.e.:
I ( z )  I0e
kz
%I0
0
20
40
60
80 100
k is an attenuation coefficient,
dependent on wavelength of
radiation (e.g. see Kirk, Light &
photosynthesis in aquatic
ecosystems.)
0
-10
depth / m
k=0.1 m-1
-20
In clear water:
55% heat is input into top 1 m
-30
70% is input within 3 m
-40
-50
In typical shelf waters:
>90% input within 5 m
Heat output occurs from the “skin” of the surface.
The tidal currents mix the thermal
structure up from the seabed:
Temperature
0
0
-30
The wind mixes the thermal
structure down from the sea
surface:
-10
depth / m
-20
0
0
Add tidal -40
stress 
-50
-10
-50
Stronger tidal
currents
-20
-30
Add wind-10
stress

depth / m
-40
Temperature
Temperature
-30
depth / m
depth / m
-10
-20
Temperature
-20
-30
-40
-40
-50
-50
Stronger
wind
mixing
The rate of change of the
Potential Energy of a shelf
sea water column, driven
by surface heat flux, can be
derived as:

1 P .E .
g Q

h t heating
2c p
The rate of increase of the
Potential Energy of a shelf
sea water column, driven
by tidal mixing, can be
derived as:
4k b u
1 P .E .

h t tide mixing
3h
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?
3
0
mixed
front
stratified
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.
High u and/or low h will
result in a water column
that remains mixed.
Low u and/or high h will
result in a water column
that stratifies during spring
and summer
warm
cool
cold
Low h/u3
High h/u3
h/u3critical
We can predict this
Enhanced Primary Production
at Tidal Mixing Fronts
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, 677695)
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).
Useful reading: Mann & Lazier, Dynamics of Marine Ecosystems, 2nd ed. (Blackwell Science)
pages 187-196
Sea surface temperature
(AVHRR)
Sea surface chlorophyll concentration
(SeaWIFS)
10th July 1999
(Images courtesy of Remote Sensing Group (Plymouth Marine Laboratory))