Intertropical Convergence Zone

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

Infra-red image from GOES 14 showing the intertropical convergence zone.

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

The Intertropical Convergence Zone (ITCZ) is the area encircling the earth near the equator where winds originating in the northern and southern hemispheres come together.

The ITCZ was originally identified from the 1920s to the 1940s as the "Intertropical Front" (ITF), but after the recognition in the 1940s and 1950s of the significance of wind field convergence in tropical weather production, the term "ITCZ" was then applied.^[1]

Where the ITCZ is drawn into and merges with a monsoonal circulation, it is sometimes referred to as a monsoon trough, a usage more common in Australia and parts of Asia. In the seamen's speech the zone is referred as The Doldrums because of its erratic weather patterns with stagnant calms and violent thunderstorms.

The ITCZ appears as a band of clouds, usually thunderstorms, that circle the globe near the equator. In the Northern Hemisphere, the trade winds move in a southwesterly direction, while in the Southern Hemisphere, they move northwesterly. The point at which the trade winds converge forces the air up into the atmosphere, forming the ITCZ.^[2]

The tendency for thunderstorms in the tropics is to be short in their duration, but can produce intense rainfall. It is estimated that 40 percent of all tropical rainfall rates exceed 25 mm per hour (one inch per hour). Greatest rainfall typically occurs when the midday Sun is overhead. On the equator this occurs twice a year in March and September, and consequently there are two wet and two dry seasons. Further away from the equator, the two rainy seasons merge into one, with one wet season and one dry season. In the Northern Hemisphere, the wet season occurs from May to July, in the Southern Hemisphere from November to February.^[2]

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.

South Pacific Convergence Zone

The South Pacific Convergence Zone (SPCZ) is a reverse-oriented, or west-northwest to east-southeast aligned, trough extending from the west Pacific warm pool southeastwards towards French Polynesia. It lies just south of the Equator during the Southern hemisphere warm season, but can be more extratropical in nature, especially east of the Dateline. It is considered the largest and most important piece of the ITCZ, and has the least dependence upon heating from a nearby landmass during the summer than any other portion of the monsoon trough.^[3] The southern ITCZ in the southeast Pacific and southern Atlantic, known as the SITCZ, occurs during the Southern hemisphere fall between three and ten degrees south of the equator east of the 140th meridian west longitude during cool or neutral ENSO patterns. When ENSO reaches its warm phase, otherwise known as El Niño, the tongue of lowered sea surface temperatures due to upwelling off the South American continent disappears, which causes this convergence zone to vanish as well.^[4]

Effects on weather

The ITCZ moves farther away from the equator during the Northern summer than the Southern one due to the North-heavy arrangement of the continents.

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.

In some cases, the ITCZ may become narrow, especially when it moves away from the equator; the ITCZ can then be interpreted as a front along the leading edge of the equatorial air.^[5] There appears to be a 15-25 day cycle in thunderstorm activity along the ITCZ, which is roughly half the wavelength of the Madden-Julian Oscillation, or MJO.^[6]

Within the ITCZ the average winds are slight, unlike the zones north and south of the equator where the trade winds feed. Early sailors named this belt of calm the doldrums because of the inactivity and stagnation they found themselves in after days of no wind. To find oneself becalmed in this region in a hot and muggy climate could mean death in an era when wind was the only effective way to propel ships across the ocean. Even today the leisure and competitive sailors attempt to cross the zone as quickly as possible as the erratic weather and wind patterns may cause unexpected delays.

Role in tropical cyclone formation

Tropical cyclogenesis depends upon low-level vorticity as one of its six requirements, and the ITCZ fills 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 makes 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.

Hazards

Some aviation experts are proposing that the Intertropical Convergence Zone might have played a role in the loss of Air France Flight 447, which left Rio de Janeiro's Galeão International Airport on Sunday, May 31, 2009, at 7 p.m. (4:00 p.m. EDT) and had been expected to land at Paris's Roissy Charles de Gaulle Airport on Monday June 1, 2009, at 11:15 a.m.^[7] However, this area rarely causes any problems to civil aviation and the loss of the aeroplane is unexplained at this time.

See also

• Horse latitudes
• Doldrums

Notes

References

• Short NASA article with high resolution photo

External links

• The ITCZ in Africa