The intertropical convergence zone (ITCZ) is the ascending arm of the northern and southern hemisphere Hadley cells, where the trade winds from each hemisphere meet. The rising warm and humid air at the ITCZ is why the region is characterized by clouds and thunderstorms. Because the Coriolis force is weak near the ITCZ, winds here tend to be very low; hence, the region is also known by sailors as the doldrums.
The mean position of the ITCZ is over the equator; however, its position changes seasonally to follow the thermal equator. It is northerly during the northern hemisphere summer and southerly during the southern hemisphere summer. The ITCZ moves further poleward over land than over the ocean because the land surface heats faster than the sea. Furthermore, because the northern hemisphere has more land area than the southern hemisphere, the average position of the ITCZ in the northern hemisphere tends to be at higher latitude compared to the southern hemisphere.
Thunderstorm activity in the ITCZ tends to by cyclical, with enhanced thunderstorms and rainfall occurring every three to four weeks. This cycle is caused by a phenomenon known as the Madden Julien Oscillation (MJO), which is an eastward propagating wave of enhanced or suppressed rainfall that travels around the equator. The wavelength of the MJO is so long that it divides the planet into two regions, one with drier than average conditions and the other with wetter than average conditions. The intensity of these variations depends on the strength of the MJO. Weather models incorporate the MJO into their forecasts; however, specific information on the MJO and its impact on tropical weather is available from the Global Tropics Hazards and Benefits Outlook on the NOAA website.
In the southern hemisphere the ITCZ branches southwards over the Western Pacific Ocean to form the South Pacific convergence zone (SPCZ). Like the ITCZ, the SPCZ is characterized by clouds and deep convection, leading to thunderstorms and unsettled weather. The SPCZ is one of the most important weather features in the South Pacific and is responsible for the majority of the dry season squalls, thunderstorms, and rainfall in the region. The SPCZ starts in SE Asia and stretches southeast to French Polynesia. The strength of the SPCZ is influenced by a number of climate drivers, including monsoon activity over India, the El Nino southern oscillation, and the Interdecadal Pacific oscillation.
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