Physical forcing of the shelf seas: what drives the motion of ocean? - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 28

%

Figure 2.3 The fraction of energy absorbed
by clear ocean water (dashed line) and shelf
seawater (dash-dot line). Based on (Ivanoff,
1977 ) with permission from Pergamon Press.

50 60 70 80 90 100

0

5

10

15

20

where K d (l)(m 1 ) is the diffuse attenuation coefficient which varies with wavelength
l. When K d does not vary with depth, the energy flux decays exponentially with
increasing depth, i.e.

E 0 e K d z E d ð z Þ¼ ð 2 : 3 Þ

where E 0 (l) is the energy flux at wavelength l incident at the surface. Note that we
define z 0 at the sea surface, with z negative below the sea surface.

Most of the energy entering the ocean is absorbed or backscattered in the first few
metres below the surface. The infra-red (IR) and ultra-violet (UV) components are
attenuated particularly rapidly on length scales of a few mm or less. Only in the blue
and green regions of the spectrum, which are included in the PAR wavelengths, does
energy penetrate more than a few metres down the water column. Peak penetration in
the clearest ocean water is at a wavelength of l

¼

¼ 0.45 m m at which the diffuse

0.03 m 1 ; this implies that attenuation coefficient K d ∼ 5% of downward energy

flux at this wavelength reaches a depth of 100 metres. In the shelf seas, attenuation
values are usually considerably larger (0.1-0.4 m 1 ) so that less than 5% of radiation
in the blue-green penetrates beyond a depth of

∼

20 metres. We shall return to the
biologically important question of the vertical distribution of PAR in Chapter 5 , but
for the moment we focus on the downward flux of the total radiant energy which is
the primary source of heat for the ocean.
Measurements of total energy absorption (Ivanoff, 1977 ) in oceanic and shelf
seawater types are shown in Fig. 2.3 . For clear, open ocean waters, 55% of the
incoming solar energy is absorbed in the first metre of the water column while more
than 90% is absorbed in the first 15 metres. The higher turbidity levels typical of shelf
seawaters reduce the 90% absorption level to

∼

5 metres or less. Since this is usually
only a small fraction of the water column depth, we may often regard the heat input
as being concentrated at the surface. A widely used and rather better approximation,
of which we shall make use later, is to treat the heat input from a downward radiation
flux of I Wm 2 as a component of 0.55I input at the very surface of the water
column, while the remaining 0.45I diminishes with depth as e K av z , where K av depends
on the water clarity and can be thought of as an average attenuation coefficient for
the remaining components of the spectrum. In shelf seas, K av is typically

∼

0.1-0.4m 1 ,

∼