Page 162 - Earth's Climate Past and Future

138 PART III • Orbital-Scale Climate Change

Monsoon Circulations creates a region of high pressure over the land in con-
trast to the lower pressure over the still-warm oceans.
In summer, strong solar radiation causes rapid warming The overall atmospheric flow in winter is a down-
of the land but slower and much less intense warming of and-out movement of cold, dry air from the land to the
the ocean. Rapid heating over the continents causes air sea (Figure 8-1B).
to warm, expand, and rise, and the upward movement of Most of the strong summer monsoons occur in the
air creates an area of low pressure at the surface (com- northern hemisphere, where landmasses are large (Asia
panion Web site, pp. 9, 11, 15–18, 21). The flow of air and North Africa) and elevations are high (especially
toward this low-pressure region brings in air from the the Tibetan-Himalayan region of Southeast Asia).
ocean, which also warms and rises (Figure 8-1A). This Monsoons are weaker in the southern hemisphere,
inflow carries water vapor evaporated from nearby where landmasses at tropical and subtropical latitudes
oceans and contributes to monsoonal rainfall. Regions are smaller and high topography is more limited in
of the continents far from the ocean or protected by extent.
intervening mountain ranges may lie beyond the reach Here we focus initially on past variations of the
of the oceanic moisture and bake under the strong sum- North African monsoon for two reasons. First, North
mer radiation. Africa lies far from the high-latitude ice sheets that
During winter, when solar radiation is weaker, air might complicate the direct response of land surfaces to
over the land cools off rapidly, becomes denser than the solar heating. In addition, nearby oceans yield a rich
air over the still-warm ocean, and sinks from higher variety of climate records showing monsoon-related
levels in the atmosphere. This downward movement signals on North Africa.

Strong
solar
radiation

Low
pressure High pressure:
Cooler ocean
Hot land surface
A Summer monsoon

Weak
solar
radiation
FIGURE 8-1 Seasonal monsoon
circulations Seasonal changes in the
strength of solar radiation affect
the surface of the land more than the
ocean. (A) In summer, intense
solar heating of the land causes an
High in-and-up circulation of moist air from
pressure
Low pressure: the ocean. (B) In winter, weak solar
Warmer ocean radiation allows the land to cool off
Cold land surface
and creates a down-and-out
B Winter monsoon circulation of cold dry air.