Geoscience Reference
In-Depth Information
weaker maxima
2
C in the southern hemisphere. This difference would seem to
reflect the fact that the northern hemisphere has a higher proportion of land which
possesses a much lower thermal capacity than the ocean. Land surface temperatures,
therefore, exhibit a considerably larger range of variation which influences SST
through heat transport by winds from land to ocean.
Although it serves well as a general indicator of seasonal heat exchange, the simple
picture of SST variation in
Fig. 2.8
(see colour plates) does not have sufficient
horizontal resolution to separate out differences between the shelf seas and the open
ocean and within the shelf seas. We should also remember that the picture in
Fig. 2.8
represents only the surface temperature change and does not fully reflect changes in
water column heat content, which also depends on water column depth and the
strength of vertical mixing.
∼
2.2.6
Thermal expansion and buoyancy changes
The transfer of heat in and out of the ocean does not directly drive currents in the
ocean, but it has the important effect of changing the density of seawater and hence
its buoyancy. As we saw in 1.5.1, the full variation of density (r) with temperature
(T), salinity (S) and pressure (p) is expressed in the equation of state by a series of
complicated polynomial functions (UNESCO,
1981
). For our purposes, it is often
sufficient to use a linearised equation of state:
ð
T
;
S
;
p
Þ¼
0
ð
1
a
ð
T
T
0
Þþ
b
ð
S
S
0
Þþ
g
p
Þ
ð
2
:
10
Þ
which represents the changes of density over a restricted range of temperature salinity
and pressure from a reference density r
0
(T
0
, S
0
, 0). The parameter a
0
@
1
¼
@T
is the
0
@
0
@
1
1
thermal expansion coefficient; b
@p
are the equivalent parameters for
salinity and pressure changes. A positive heat input of
¼
@S
and g
¼
Q (J m
3
) increases the
D
temperature by:
¼
Q
c
p
:
T
ð
2
:
11
Þ
The increase in temperature causes a reduction in density
D
r which imposes a
positive buoyancy force b (N m
3
) given by:
g
a
Q
b
¼
g
¼
g
a
0
T
¼
:
ð
2
:
12
Þ
C
p
Since this heating is concentrated near the surface, the upper water column develops
vertical gradients of density with lower density water on top. If we wanted to
re-distribute, or mix, the low density surface water throughout the water column,
we would need to supply some energy. You can think of the mixing as acting to
raise the centre of mass of the water column, which involves doing work; this is an
idea that we will utilise further in
Chapter 6
. So the water column is made more stable
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