AY Honors/Weather - Advanced/Answer Key

The thunderstorms of the Intertropical Convergence Zone form a line across the eastern Pacific Ocean.

The Intertropical Convergence Zone (ITCZ), also known as the Intertropical Front, Monsoon trough or the Equatorial Convergence Zone, is a belt of low pressure girdling Earth at the equator. It is formed by the vertical ascent of warm, moist air from the latitudes above and below the equator.

The air is drawn into the intertropical convergence zone by the action of the Hadley cell, a macroscale atmospheric feature which is part of the Earth's heat and moisture distribution system. It is transported aloft by the convective activity of thunderstorms; regions in the intertropical convergence zone receive precipitation more than 200 days in a year.

Position

The location of the intertropical convergence zone varies over time. Over land, it moves back and forth across the equator following the sun's zenith point. Over the oceans, where the convergence zone is better defined, the seasonal cycle is more subtle, as the convection is constrained by the distribution of ocean temperatures.

Sometimes, a double ITCZ forms, with one located north and another south of the equator. When this occurs, a narrow ridge of high pressure forms between the two convergence zones, one of which is usually stronger than the other.

In this way it moves between the tropic of Capricorn and of Cancer, being at the tropic of Cancer on the 21st of June and at the Tropic of Capricorn on the 21st of December causing the longest and shortest days and the equinox's on the 21 March and September.

Vertical velocity at 500 hPa, July average. Ascent (negative values) is a tracer for the ITCZ and is concentrated close to the solar equator; descent (positive values) is more diffuse.

Effects on weather

Variation in the location of the intertropical convergence zone drastically affects rainfall in many equatorial nations, resulting in the wet and dry seasons of the tropics rather than the cold and warm seasons of higher latitudes. Longer term changes in the intertropical convergence zone can result in severe droughts or flooding in nearby areas.

Within the ITCZ the average winds are slight, unlike the zones north and south of the equator where the trade winds feed in. Early sailors named this belt of calm the doldrums because of the low spirits they found themselves in after days of no wind.[1] To find oneself becalmed in this region in a hot and muggy climate could mean death in an era when wind was the only major motive force.

Role in tropical cyclone formation

Tropical cyclogenesis depends upon low-level vorticity as one of its six requirements, and the ITCZ/monsoon trough fill this role as it is a zone of wind change and speed, otherwise known as horizontal wind shear. As the ITCZ migrates more than 500 km from the equator during the respective hemisphere's summer season, increasing coriolis force allows the formation of tropical cyclones within this zone more possible. In the north Atlantic and the northeastern Pacific oceans, tropical waves move along the axis of the ITCZ causing an increase in thunderstorm activity, and under weak vertical wind shear, these clusters of thunderstorms can become tropical cyclones.

See also

• Horse latitudes
• Monsoon trough
• Tropical cyclogenesis

References

• Short NASA article with high resolution photo

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