Geoscience Reference
In-Depth Information
Note that seasonal temperature ranges have the following characteristics:
• They are larger than diurnal temperature ranges in high and middle latitudes,
comparable in the subtropics, and smaller in the tropics.
• They are larger on continents than over the oceans at all latitudes, as is the
case for diurnal temperature ranges.
• They are larger in Northern Hemisphere middle latitudes than in Southern
Hemisphere middle latitudes. This is a continentality effect, due to the
greater extent of continents in the Northern Hemisphere.
Seasonality in the tropics and much of the subtropics is more sharply de-
fined by precipitation than by temperature in most regions. Over India, West
Africa, and central South America, precipitation rates are about an order of
magnitude greater in the summer months than in the winter months (
Figure
3.1)
. In other regions, for example, over parts of East Africa, there are two
rainy seasons during the transition seasons (spring and fall) with drier condi-
tions in the solstitial seasons (summer and winter).
The seasonal cycle of precipitation is less pronounced over the oceans than
over continents. This is particularly true across the tropical North Pacific
Ocean where, as seen in
Figure 2.27,
the ITCZ precipitation maximum does
not follow the sun into the Southern Hemisphere during the Northern Hemi-
sphere winter months.
A primary reason for the strong seasonality in precipitation at lower latitudes
is the occurrence of monsoons. The traditional definition of a monsoon climate
is one in which the direction of the low-level wind reverses by 180° between
summer and winter, although this definition is often not strictly applied.
Figure
2.13
shows this reversal over India, as an example. In the summer months, the
flow is directed onto the west coast from the Arabian Sea as the Somali jet car-
ries moisture across the equator to feed powerful rainfall systems. In the winter,
however, the flow is directed off the continent during the dry “winter monsoon.”
The physics of monsoon circulations is discussed in
chapter 7
.
The strongest monsoon system on the planet is the Asian monsoon, which
covers much of southern Asia in boreal summer. Ninety percent of the annual
rainfall in Mumbai, on the west coast of India, is delivered during the monsoon
season and in Chennai, on the east coast, 60% of the annual precipitation is
monsoonal. Other regions with monsoon climates are West Africa, Australia,
South America, and the southwestern United States.
3.2 INTRASEASONAL CLIMATE VARIABILITY
It is easy to understand how diurnal and seasonal variability are generated
in the climate system since insolation provides external forcing on these time
scales. But the climate system also varies on longer time scales. Climate varia-
tions on time scales less than 3 months but greater than about one week are
known as intraseasonal variations. One prominent mode of climate variations
on this time scale is the
Madden- Julian oscillation
(MJO). The MJO is a tropi-
cal oscillation that is apparent in precipitation, cloud, temperature, geopo-
tential height, and circulation observations throughout the full depth of the
troposphere with periods between 40 and 60 days.
