Geoscience Reference
In-Depth Information
25
Sensitivity
to Land Surface
Exchanges
Introduction
As mentioned in Chapter 9, on average across the globe about half the energy
that drives atmospheric circulation enters from the surface of the Earth. Because
two-thirds of the Earth's surface is covered with oceans they provide an important
control on atmospheric circulation. The remaining one-third of the Earth's
surface is continents, and the interaction between land surfaces and the overlying
atmosphere is also a substantial influence in the coupled atmosphere-ocean-land
climate system.
Even at the scale of large-scale atmospheric circulation the differences between
the ocean-atmosphere interactions and land-atmosphere interactions are apparent.
Continents are aerodynamically rougher than the oceans so large-scale circulations
around high pressure at 30°N and 30°S are less strong and persistent over
continents than over the smoother oceans. The seasonal north-south shift in the
pattern of atmospheric pressure and circulation is also influenced by the difference
between ocean-atmosphere interactions and land-atmosphere interactions. As
Fig. 9.5 shows, in the northern hemisphere the oceanic subtropical highs
are farther north and more intense in the northern hemisphere summer than
they are in the southern hemisphere in the southern hemisphere summer. In
the northern hemisphere winter there is also a more marked reversal in the
pressure difference between oceans and the continents. The seasonal differences
in the surface air temperatures over continents relative to those over oceans result
in the Asian-Australian monsoon system, while the presence of the Rocky
Mountains influences the penetration into North America of moist maritime air
in the mid-latitude westerly winds.
In the context of this chapter it is important that the overwhelming majority
of humankind lives on land, and as the human population grows the relative
