Atmospheric Circulations - Climate Dynamics - page 131

• Easterly low-level low (Fig. 2.13) occurs in the tropics and subtropics
(the trade wind regime) as Coriolis accelerations act on the low-level
equatorward return flow of the Hadley circulation.
MONSOON CIRCULATIONS
A monsoon climate is technically defined as a 180° change in the direction of
the low-level wind from summer to winter. The winter monsoon features dry
conditions and flow off the land surface, and the summer monsoon is the rainy
period with moist, onshore flow. However, this definition of a monsoon climate
is not always held strictly, and many tropical climates are defined as “monsoon
climates” when they exhibit a strong summer rainy season. Monsoon regions
include Southeast Asia and India, the southwestern United States, West Africa,
East Africa, Australia, and South America.
Both winter and summer monsoon circulations are thermally direct, with
the surface temperature contrast provided by continentality. Thus, the funda-
mental cause of monsoon climates is the different heat capacities of the land
and ocean, which causes the oceans to be warmer than the land in the winter
months and cooler in the summer months ( Fig. 2.7) .
Figure 7.4a shows the low-level (900 hPa) low and geopotential heights
across northern Africa, the Arabian Sea, and India during January, the time
of the winter monsoon. Relatively high geopotential heights cover subtropi-
cal latitudes, with lower geopotential heights to the south. Strong winds are
directed off the Indian subcontinent into the Arabian Sea and the Bay of Ben-
gal—this is the winter monsoon flow. Over West Africa, the flow is from the
northeast, forming the dry and dusty Harmattan of that continent's winter
monsoon.
In July ( Fig. 7.4b) , the low-level wind directions are approximately reversed.
Southwesterly flow is in place across West Africa, with southerly flow cross-
ing the Guinean coast (about 5ºN). A thermal low, indicated by low 900 hPa
geopotential heights, is centered near 20°N. This is the West African summer
monsoon system. Over East Africa and the Arabian Sea, the Somali jet carries
moisture from the Southern Hemisphere across the Horn of Africa and east-
ward onto the Indian subcontinent to fuel, in part, the Asian monsoon. Rather
than being supported by mean flow onto the continent from the Bay of Bengal,
the monsoon rains over Bangladesh arrive in the form of storms known as
monsoon depressions .
The vertical structure of monsoon climates resembles the sketch in Figure
7.1. Near the tropopause in July ( Fig. 7.5a) , high geopotent ial height s form
over the low geopotential heights seen at 900 hPa ( Fig. 7.4b ). Southward out-
flow from the high is diverted to the west by Coriolis forces, and the resulting
easterly flow forms the tropical easterly jet between 10 and 15°N ( Fig. 2.14 ).
Over West Africa, the thermal low (see Fig. 7.4b) occurs within much drier
conditions ( Fig. 2.31 ), so the up-branch of the thermally direct circulation in
this region is not enhanced by the release of latent heat as air parcels rise. As a
consequence, the system is shallower than over India and the upper-level high
occurs at a lower level. This upper-level high is known as the Saharan high ,