Geoscience Reference
In-Depth Information
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, North Pacific Oscillation and Pacific
North American patterns, discussed above, also
involve strong air-sea interactions.
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 longwave 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 predominantly 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 transfers 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 large-scale 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). Variability in the
Atlantic sector is strongly associated with fluctuations in the north-south pressure gradient (the
North Atlantic Oscillation, or NAO) that lead to a west-east “seesaw' in temperature and other
anomalies. Variability in the Pacific can be associated with patterns such as the North Pacific
Oscillation (NPO) and Pacific Decadal Oscillation (PDO).
The ocean's vertical structure varies latitudinally and regionally. In general, the thermocline is
deepest in mid-latitudes, thus permitting greater turbulent mixing and atmospheric heat exchanges.
The oceans are important regulators of both atmospheric temperatures and CO
2
concentrations.
Ocean dynamics and circulation features are analogous to those in the atmosphere on both the
meso- and macroscale. The wind-driven Ekman layer extends to 100-200m. Ekman transport and
coastal upwelling maintain normally cold sea surfaces off western South America and southwest
Africa in particular.
