Physical forcing of the shelf seas: what drives the motion of ocean? - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 30

technique to compensate for atmospheric absorption and achieve an accuracy of
∼

0.3 C in the estimation of surface temperatures. Infra-red radiation is rapidly
absorbed in seawater so that the outgoing radiation from the surface comes from a
thin micro-layer of thickness 1mm whose temperature may differ from that of the

surface mixed layer by up to 1 C (Schluessel et al., 1990 ). As we shall see in Chapter 8 ,
this use of satellite infra-red sensors in the mapping of SST has proved an invaluable
tool for shelf sea oceanography.

∼

2.2.3 Heat exchange by evaporation and conduction, Q e and Q c

In addition to the input and export of energy through short and long wave radiation,
there are two other mechanisms of heat transfer across the ocean-atmosphere
boundary. The most important of these, which frequently accounts for the major
part of heat transfer back to the atmosphere, is the loss of heat through evaporation
from the ocean surface. In this process, high velocity water molecules escape from the
surface into the atmosphere, taking with them above average kinetic energy which
constitutes a loss of latent heat from the ocean. This latent heat transfer amounts
to L H ∼

10 6 J for each kilogram evaporated, so that for an evaporation rate E v
(kg m 2 s 1 ), the evaporative heat loss Q e ¼

2.5

E v L H .
In contrast to the radiation terms Q s and Q b , both of which can be measured
relatively easily with radiometers, evaporation is generally not easily measured even
on land. Over the ocean, it often has to be estimated by a semi-empirical method
which relates E v to bulk parameters:

10 3

Q e ¼ E v L H ¼ 1 : 5 a W ð q s q a Þ L H ð 2 : 4 Þ

where q a and q s are the specific humidity and its saturated value at SST respectively,
W is the wind speed at anemometer height (usually 10 metres) and the constant
1.5 10 3 is the Dalton number.

The second non-radiative heat exchange mechanism is the direct transfer of heat by
conduction Q c in response to air-sea temperature differences, i.e. simply the transfer
of heat between a relatively warm fluid to a relatively cool fluid. This heat transfer is
referred to as 'sensible heat', which means that its effects are solely observed as a
change in temperature (in contrast to latent heat where the heat flux is associated with a
change in state). Heat loss via conduction is usually a much smaller term than the
evaporative heat flux, but like the evaporative term it is difficult to determine directly
and is usually estimated from bulk parameters according to the semi-empirical relation:

10 3 c a a W Q c ¼ 1 : 45 ð T s T a Þ ð 2 : 5 Þ

where T s and T a are the sea surface and air temperatures respectively, c a 1000 J

kg 1 K 1 is the specific heat capacity of air, and r a 1.3 kg m 3 is the density of air.

10 3 is termed the Stanton number.

The coefficient 1.45