Tropical climates - The Global Climate System: Patterns, Processes, and Teleconnections - page 61

Figure 3.2 Average
extreme positions of the
ITCZ. (After Oliver and
Hidore 2002 )
July
January
3.2.1 The intertropical convergence zone
The intertropical convergence zone (ITCZ), an area once designated as the
doldrums, is an east-west oriented low pressure region near the equator. It is
characterized by low surface pressure, and convergence of air masses giving rise
to cloudiness and rainfall. The ITCZ is best developed in the tropical oceans and
is the most prominent climatic feature in the tropics. It plays principal roles in
tropical climate by interacting with the planetary scale circulations of the atmo-
sphere and oceans.
The position of the ITCZ varies seasonally (Figure 3.2 ), occurring in the
hemisphere of the most intense solar radiation. As illustrated in the figure, the
seasonal migration over land is greater than over the oceans. The feature is
obliterated during the onshore monsoon period over the Indian Ocean. The seasonal
migration of the ITCZ is of high significance to the amount of rainfall that occurs
in the semiarid regions that bound the wet tropics. As indicated in the account of
desertification in the Sahel (Section 3.4.1 ), failure of the ITCZ to migrate to its
''normal'' position can lead to drought with resulting social disruptions.
The ITCZ is not a simple band of clouds, and double ITCZs often occur. In
this, an ITCZ is found on each side of the equator. In his review of this
phenomenon, Zhang ( 2001 ) notes that the most noticeable double ITCZs are
usually found over the eastern Pacific during spring while over the western and
central Pacific double ITCZs occur from June to September. It is only during
ENSO events that the eastern Pacific double ITCZ fails to materialize.
For most of the time, the ITCZ south of the equator is weaker than that which
is north. This occurs when the northeast and southeast trade winds converge over
the warm waters and intense convection results in the formation of clouds that
produce heavy precipitation. The south ITCZ is formed when the southern trade
winds blow over the cold upwelling waters near the equator; these decrease vertical
mixing and slowing of surface winds (Liu and Xie 2002 ). The south ITCZ does not
have the cloud creation capability of the north ITCZ, hence its weaker signal.