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CHAPTER 8 • Insolation Control of Monsoons 139

Africa is a deceptively large landmass compared to of precipitation from the south (Figure 8-3). Rain forest
its appearance on Mercator maps. It stretches from in the year-round wet climate near the equator gives way
37°N to 35°S, with far more of its land area north of northward to a sequence of progressively drier vegeta-
the equator—in fact almost twice the area of the U.S. tion, first the tree-and-grass savannas of the Sahel region
mainland. Because the huge North African land surface and then the desert scrub of the arid Sahara.
is situated at tropical and lower subtropical latitudes, it
is strongly influenced by the overhead Sun. 8-1 Orbital-Scale Control of Summer Monsoons
As a result of strong solar heating during northern
hemisphere summer, a low-pressure region that develops The idea that changing insolation could control the
over west-central North Africa draws moisture-bearing strength of monsoons over orbital time scales was pro-
winds in from the tropical Atlantic (Figure 8-2A). During posed by the meteorologist John Kutzbach in the early
typical summers, this monsoonal rainfall penetrates 1980s, although it had been anticipated to some extent
northward to about 17°N latitude (the southern edge of by Rudolf Spitaler late in the nineteenth century. This
the Sahara Desert) before retreating southward later in concept is called the orbital monsoon hypothesis.
the year. The orbital monsoon hypothesis is a direct logical
During northern hemisphere winter the overhead extension of factors at work in the present monsoon cir-
Sun moves to the southern hemisphere, and solar radia- culations (Figure 8-4). Because seasonal monsoon circu-
tion over North Africa is weaker. Cooling of the North lations are driven by changes in the strength of solar
African land surface by back radiation causes sinking of radiation, orbital-scale changes in summer and winter
air from above, and a high-pressure cell develops at the insolation (Chapter 7) should have produced a similar
surface over the northwestern Sahara Desert (Figure 8- response. If summer insolation was higher in the past
2B). Strong and persistent trade winds associated with than it is today, the summer monsoon circulation should
this high-pressure cell and with similar circulation over have been stronger, with greater heating of the land,
the adjacent North Atlantic blow southwestward from stronger rising motion, more inflow of moist ocean air,
North Africa across the tropical Atlantic. and more rainfall (Figure 8-4B). Conversely, summer
Because the trade winds of the winter monsoon carry insolation levels lower than those today should have dri-
little moisture, winter precipitation is rare in North ven a weaker summer monsoon in the past.
Africa. Only two areas receive much rain in this season: The same kind of reasoning applies to the winter
the northernmost Mediterranean margin, where storms monsoon. Winter insolation minima weaker than the
occasionally form over the nearby ocean, and the tropi- one today should have enhanced the cooling of the land
cal southwest coast (the Ivory Coast), where the moist surface, which should have driven a stronger down-
intertropical convergence zone (ITCZ) remains over and-out flow of dry air from land to sea (Figure 8-4C).
the land. Recall from Chapter 7 that more intense summer
Because most of the rainfall in North Africa occurs in insolation maxima and deeper winter insolation minima
association with the summer monsoon, the distribution always occur together at any one location. As a result,
of major vegetation types reflects the monsoonal delivery stronger in-and-up monsoon flows in summer should

40˚N 40˚N
Trade
winds
30˚ 30˚ H
Dry winter
monsoon
L
20˚ 20˚
ITCZ
Wet summer
10˚ 10˚
monsoon ITCZ
0˚ 0˚

10˚S 10˚S
40˚W 30˚ 20˚ 10˚ 0˚ 10˚ 20˚ 30˚ 40˚E 40˚W 30˚ 20˚ 10˚ 0˚ 10˚ 20˚ 30˚ 40˚E
A Northern hemisphere summer B Northern hemisphere winter
FIGURE 8-2 Monsoon circulations over North Africa Seasonal changes cause (A) a moist
inflow of monsoonal air toward a low-pressure center over North Africa in summer and (B) a dry
monsoonal outflow from a high-pressure center over the land in winter. (Adapted from J. F.
Griffiths, Climates of Africa [Amsterdam: Elsevier, 1972].)