The Intertropical convergence Zone (ITCZ) is a narrow band where the trade winds of the two hemispheres collide. At or near the ITCZ, sea-surface temperature (SST) is at a maximum. As the winds travel over the tropical water, they pick up moisture; where they collide, they are driven upward. The air is also forced upward by convection resulting from heating by the ocean. As the air rises, it cools, the moisture condenses, and heavy rain results. The heat released drives the regional and global atmospheric circulation. It also makes the ITCZ an origin site for cyclones. Another effect of the upward movement of air is relatively calm surface winds. This causes the ITCZ to be an area of downwelling, and, thus, low nutrients and productivity. The ITCZ interacts with other climatological features, affecting weather and climate in the tropics and subtropics.
The ITCZ might be expected to fall directly on the equator, because, on average, this is where the Sun is strongest. In actuality, the ITCZ moves, but tends to fall preferentially in the northern hemisphere, over the eastern Pacific and the Atlantic oceans. The shift can be 10 degrees or more in latitude, or several hundred mi. (km.), and it has significant effects.
The cause appears to be related to the western boundaries of South America and Africa, particularly the northwest to southeast slopes of the boundaries at the equator. This weakens the northeasterly trade winds, thereby warming the water north of the equator and allowing southeasterlies to cross into the north prior to convergence.
The easterly trade winds are the prevalent winds in the tropical Pacific and Atlantic oceans. These winds push the warm surface water west, exposing cooler water, known as the Equatorial Cold Tongue. Stronger trade winds in the south favor upwelling below the equator. Low-level stratus clouds above the cold water and evaporative cooling aided by the winds reinforce the hemispheric temperature difference and move it westward. The Sun is closest to the Earth during northern winter, which may also favor prevalence of a northern ITCZ. The dominance of monsoons over trade winds in the Indian Ocean maintains equatorial symmetry there.
The ITCZ fluctuates over many timescales. Local winds are important seasonally, more distant winds can be significant interannually, and ocean circulation can affect sea-surface temperatures on decadal or greater scales. Normal annual migration is from 10 degrees north latitude in August to 3 degrees north in February. In March and April, a double ITCZ above and below the equator can form. Weak trade winds and subsequent decreased upwelling in El Nino years may move the ITCZ south of the equator in the Pacific.
The asymmetry of the ITCZ aids in establishing an annual, rather than a biannual, weather cycle in the tropical eastern and central Pacific Ocean, even though the sun crosses the equator twice yearly. Seasonal air temperatures there exhibit wide divergence because the seasonal weakening and strengthening of the south-easterlies reduce or enhance upwelling and evaporative cooling. Models indicate that this phenomenon could intensify in response to increased greenhouse gases. Interannual and decadal variation in the ITCZ can result in droughts in some years, and floods in others. Southward displacement is associated with increased the occurrence and intensity of El Nino, while El Nino is suppressed by northward displacement.
The southward shift brings dry conditions to South America and perhaps western Africa, and increased rain in eastern Africa. Warming in one hemisphere or cooling in the other moves the ITCZ in the direction of warming. Past deflection of the ITCZ likely resulted from orbital changes that affected how the sun strikes Earth. Prolonged shifts have been linked to changes in human society, because of their effects on precipitation. The Northern Hemisphere is currently experience greater warming than the Southern Hemisphere because of the greater ocean heat sink in the south. This raises the possibility of change in the El Nino/ Southern Oscillation regime. Changes in the Atlantic Meridional Circulation could also reorganize heat distribution, thereby evoking a response in the ITCZ.