Physical forcing of the shelf seas: what drives the motion of ocean? - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 26

buoyancy of water particles, heat exchange modifies the water column stratification.
In addition, differential heating or cooling between different areas will set up hori-
zontal density gradients with attendant buoyancy forces and pressure gradients
which will drive circulations. These motions can be thought of as releasing some of
the potential energy of the density field which is set up by the heating or cooling.
Similarly the input of freshwater, which is lighter than seawater by 26 kg m 3 ,

at the ocean boundaries constitutes an important second source of buoyancy forcing
which injects potential energy and again tends to induce circulations which convert
the potential energy to kinetic energy.

D r

∼

2.2 The seasonal cycle of heating and cooling

...................................................................................
We begin our survey of the forcing mechanisms with energy exchange at the sea
surface, the components of which are shown schematically in Fig. 2.1 . The primary
driver here is Q s , the input of short wavelength (l m) radiation from the Sun,

much of which is absorbed in the ocean. It is not returned to the atmosphere
immediately but heats up the ocean and is ultimately returned back through the
surface either by long wave back radiation (Q b ) or as a combination of heat fluxes
due to evaporation (Q e ) and sensible heat transfer by conduction (Q c ). We shall now
consider the characteristics of each of these fluxes in turn.

∼ 0.5 m

2.2.1 Solar heating, Q s

1.37 kW m 2 . The total
energy intercepted by the Earth's disc, area pr 2 with r the radius of the Earth, is
distributed over a hemisphere of surface area of 4pr 2 , so the average energy input per
square metre at the top of the atmosphere is

Outside the Earth's atmosphere, energy arrives at a rate of ∼

340W m 2 . This incoming radiation
has a spectral energy distribution closely similar to the Planck radiation law for a
black body:

∼

Figure 2.1 The heat fluxes
across the air-sea boundary.
a, b indicate the absorption
and scattering of photons.

Radiation

Q s

Q b

Q e Q c

E

Evaporation Conduction

a a

b

From surface microlayer ~ 1 mm