A result og the heating of part fo the Earth's surface, caused by the concentrated insolation from the overhead sun, which heats air near the surface. THe heated air becomes less dense and rises. This draws in cooler air that flows across the surface to replace the rising air. Air streams are drawn in from both north and south of the equator and meet where air is rising.
A major influence on all tropical climates is the position of the ITCZ and the role of associated subtropical anticyclones. The ITCZ occurs along a line where the NE trade winds, blowing away from the hadley cell in the northern hemisphere, meets the SE trade winds from the southern hemisphere.
Combined with intense heating in the low latitudes, this causes air to rise and this in turn in helped by divergence aloft. The tendency of air to be lifted by convergence is greater on the western sides of the oceans. This increases the frequency of storms in these parts of the tropics.
The ITCZ marks an area of intense low pressure. It is associated with an area of precipitation that moves north and south of the equator throughout the year, following the movement of the sun. It moves further away from the equator in the northern hemisphere than might be expected during the summer months especially over Asia. The same occurs in the southern hemisphere during its summer.
The hot air is rising in the ITCZ becomes cooler with altitude, and condensation leads to dense cumulonimbus cloud formation and heavy rainfall. At high altitude the air cools to the point that the density differences that caused it to rise intially are reduced and it move polewards. The winds at the ITCZ are very light adn the area is known as the Doldrums