Geoscience Reference
In-Depth Information
fresh-water inflow. The complex mechanisms involved
in the deep ocean conveyor belt are still poorly under-
stood.
SUMMARY
The vertical change of pressure with height depends on
the temperature structure. High- (low-) pressure
systems intensify with altitude in a warm (cold) air
column; thus warm lows and cold highs are shallow
features. The upper-level subtropical anticyclones and
polar vortex in both hemispheres illustrate this
'thickness' relationship. The intermediate mid-latitude
westerly winds thus have a large 'thermal wind'
component. They become concentrated into upper
tropospheric jet streams above sharp thermal
gradients, such as fronts.
The upper flow displays a large-scale long-wave
pattern, especially in the northern hemisphere, related
to the influence of mountain barriers and land-sea
differences. The surface pressure field is dominated by
semi-permanent subtropical highs, subpolar lows and,
in winter, shallow cold continental highs in Siberia
and northwest Canada. The equatorial zone is pre-
dominantly low pressure. The associated global wind
belts are the easterly trade winds and the mid-latitude
westerlies. There are more variable polar easterlies,
and over land areas in summer a band of equatorial
westerlies representing the monsoon systems. This
mean zonal (west-east) circulation is intermittently
interrupted by 'blocking' highs; an idealized sequence
is known as the
index cycle
.
The atmospheric general circulation, which trans-
fers heat and momentum poleward, is predominantly
in a vertical meridional plane in low latitudes (the
Hadley cell), but there are also important east-west
circulations (Walker cells) between the major regions
of subsidence and convective activity. Heat and
momentum exchanges in middle and high latitudes
are accomplished by horizontal waves and eddies
(cyclones/anticyclones). Substantial energy is also
carried poleward by ocean current systems. Surface
currents are mostly wind driven, but the slow deep
ocean circulation (global conveyor belt) is due to
thermohaline forcing.
The circulation in the northern hemisphere mid-
latitudes is subject to variations in the strength of the zonal
westerlies lasting three to eight weeks (the index cycle)
and interannual differences in the north-south pressure
gradient in the North Atlantic (the NAO) that lead to a
3 The oceans and atmospheric regulation
The atmosphere and the surface ocean waters are closely
connected both in temperature and in CO
2
concentra-
tions. The atmosphere contains less than 1.7 per cent of
the CO
2
held by the oceans, and the amount absorbed
by the ocean surface rapidly regulates the concentra-
tion in the atmosphere. The absorption of CO
2
by the
oceans is greatest where the water is rich in organic
matter, or where it is cold. Thus the oceans can regulate
atmospheric CO
2
, changing the greenhouse effect and
contributing to climate change. The most important
aspect of the carbon cycle linking atmosphere and ocean
is the difference between the partial pressure of CO
2
in the lower atmosphere and that in the upper ocean
(Figure 2.4). This results in atmospheric CO
2
being
dissolved in the oceans. Some of this CO
2
is sub-
sequently converted into particulate carbon, mainly
through the agency of plankton, and ultimately sinks to
form carbon-rich deposits in the deep ocean as part of a
cycle lasting hundreds of years. Thus two of the major
effects of ocean surface warming would be to increase
its CO
2
equilibrium partial pressure and to decrease the
abundance of plankton. Both of these effects would tend
to decrease the oceanic uptake of CO
2
. This would
increase its atmospheric concentration, thereby pro-
ducing a positive feedback (i.e. enhancing) effect on
global warming. However, as will be seen in Chapter
13, the operation of the atmosphere-ocean system is
complex. Thus, for example, global warming may so
increase oceanic convective mixing that the resulting
imports of cooler water and plankton into the surface
layers might exert a brake (i.e. negative feedback) on
the system warming.
Sea-surface temperature anomalies in the North
Atlantic appear to have marked effects on climate
in Europe, Africa and South America. For example,
warmer sea surfaces off northwest Africa augment West
African summer monsoon rainfall; and dry conditions
in the Sahel have been linked to a cooler North Atlantic.
There are similar links between tropical sea-surface
temperatures and droughts in northeast Brazil. The
North Atlantic Oscillation teleconnection pattern, dis-
cussed above, also shows strong air-sea interactions.
