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for the lecture on Wednesday October 16
Global Pattern of Temperature and Rainfall
If we look at the climate of Planet Earth without going into very much detail, we can see a number of very basic features that seem to depend only upon latitude, as sketched in Fig. 1:
Fig. 2 Annual mean rainfall based on station data over land and infrared satellite
imagery over the sea. For monthly mean maps and animations see
Fig. 5 is an image from todays global composite satellite loop. You can clearly see the ITCZ, the southern hemisphere summer monsoons, the cloud free desert belts and both the Northern and Southern Hemisphere storm tracks.
The inter-tropical convergence zone is
largely restricted to the Atlantic and Pacific Oceans. Over the tropical
continents the rainfall is dominated by the monsoons. The word 'monsoon'
is derived from Arabic 'mausim' a season, meaning in this case, a seasonal
wind. It was originally used to describe the winds over the northwest part
vacations in that part of the country).
In contrast to the ITCZ, which stays quite
close to 7°N year round, the monsoons shift back and forth between
Northern and Southern Hemispheres, following the sun, so to speak.
Many equatorial stations experience equinoctial (March and September) rainy
seasons when the monsoons pass overhead, whereas stations at, say, 10°N
or 10°S experienced well marked summer rainy seasons and winter dry
seasons. Monthly rainfall maps clearly reveal the 'annual march of
The Subtropical Anticyclones
As noted above, the deserts are located
in the transition zone between the trade winds and the westerlies.
During summer the winds in these transition zones develop a strong 'anticyclonic'
spin. Meteorologists and oceanographers use the term 'cyclonic' to denote
'in the same sense as the earth's rotation' and 'anticyclonic' to denote
'in the opposite sense as the earth's rotation'. In the Northern
Hemisphere, the sense of the earth's rotation (looking down from above)
is counterclockwise-- we know that because the sun rises in the east and
sets in the west. Hence, in the Northern Hemisphere, the word 'cyclonic'
is synonymous with counterclockwise and 'anticyclonic' with 'counterclockwise.
With a bit of mental gymnastics, you can easily verify that in the Southern
Hemisphere the terms have the opposite meaning. The words cyclone
and anticyclone are from the same root. They denote cyclonic circulation
and anticyclonic circulation, respectively. Hence, anticyclone' denotes
Note the subtropical anticyclones over the Atlantic, Pacific and Southern Indian Oceans in Fig. 5. They are much more prominent in the summer seasons in their respective hemispheres. Hence, the Northern Hemisphere subtropical anticyclones are easier to identify in the lower panel of the figure and vice versa. Note how it always works out that the winds during summer are equatorward on the west coasts of the continents..(e.g., on the coasts California, Chile, Portugal, Namibia) and poleward on the east coasts of the continents (e.g., eastern United States and China, Japan, Argentina and Angola).
The subtropical anticyclones are also commonly
referred to as 'subtropical highs' because they are regions of high barometric
Note that cyclonic circulations are also
apparent in Fig. 5 The place
to look for them is in high latitudes of the winter hemisphere: i.e., over
the Aleutians and south of Greenland in the top panel of the figure.
These features are referred to as the Aleutian and Icelandic lows.
They're semipermanent features of the wintertime circulation. They're
marked by stormy weather and intermittent periods of high winds.