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.

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

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

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

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?

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.

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.

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

Sea surface temperature Sea surface chlorophyll concentration (AVHRR) (SeaWIFS)

10th July 1999

(Images courtesy of Remote Sensing Group (Plymouth Marine Laboratory))