Intertropical Convergence Zone

"The Calms" redirects here. For other uses, seeCalm (disambiguation).

"Doldrums" redirects here. For other uses, seeDoldrums (disambiguation).

The ITCZ is visible as a band of clouds encircling Earth near the Equator.

TheIntertropical Convergence Zone(ITCZ/ɪtʃ/ITCH,orICZ),known by sailors as thedoldrumsor thecalmsbecause of its monotonous windless weather, is the area where the northeast and the southeasttrade windsconverge. It encircles Earth near thethermal equatorthough its specific position varies seasonally. When it lies near the geographicEquator,it is called thenear-equatorial trough.Where the ITCZ is drawn into and merges with amonsoonalcirculation, it is sometimes referred to as amonsoon trough(a usage that is more common in Australia and parts of Asia).

Meteorology

The ITCZ was originally identified from the 1920s to the 1940s as theIntertropical Front(ITF), but after the recognition in the 1940s and the 1950s of the significance ofwind field convergenceintropicalweather production, the termIntertropical Convergence Zone(ITCZ) was then applied.

The ITCZ appears as a band of clouds, usually thunderstorms, that encircle the globe near the Equator. In theNorthern Hemisphere,thetrade windsmove in a southwestward direction from the northeast, while in theSouthern Hemisphere,they move northwestward from the southeast. When the ITCZ is positioned north or south of the Equator, these directions change according to theCoriolis effectimparted byEarth's rotation.For instance, when the ITCZ is situated north of the Equator, the southeast trade wind changes to a southwest wind as it crosses the Equator. The ITCZ is formed by vertical motion largely appearing asconvectiveactivity of thunderstorms driven by solar heating, which effectively draw air in; these are the trade winds.The ITCZ is effectively a tracer of the ascending branch of theHadley celland is wet. The dry descending branch is thehorse latitudes.

The location of the ITCZ gradually varies with the seasons, roughly corresponding with the location of the thermal equator. As the heat capacity of the oceans is greater than air over land, migration is more prominent over land. Over the oceans, where theconvergence zoneis 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, one of which is usually stronger than the other. When this occurs, a narrow ridge of high pressure forms between the two convergence zones.

ITCZ over oceans vs. land

Seasonal variability of the Intertropical Convergence Zone (ITCZ), Congo air boundary (CAB), tropical rainbelt, and surface winds over Africa (adapted fromDezfuli 2017with modification). This schematic shows that the ITCZ and the region of maximum rainfall can be decoupled over the continents.

The ITCZ is commonly defined as an equatorial zone where the trade winds converge. Rainfall seasonality is traditionally attributed to the north–south migration of the ITCZ, which follows the sun. Although this is largely valid over the equatorial oceans, the ITCZ and the region of maximum rainfall can be decoupled over the continents.The equatorial precipitation over land is not simply a response to just the surface convergence. Rather, it is modulated by a number of regional features such as local atmospheric jets and waves, proximity to the oceans, terrain-induced convective systems, moisture recycling, and spatiotemporal variability of land cover and albedo.

South Pacific convergence zone

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

TheSouth Pacific convergence zone(SPCZ) is a reverse-oriented, or west-northwest to east-southeast aligned, trough extending from the west Pacific warm pool southeastwards towardsFrench Polynesia.It lies just south of the equator during the Southern Hemisphere warm season, but can be more extratropical in nature, especially east of theInternational Date Line.It is considered the largest and most important piece of the ITCZ, and has the least dependence upon heating from a nearbyland massduring the summer than any other portion of themonsoon trough.The southern ITCZ in the southeast Pacific and southern Atlantic, known as the SITCZ, occurs during the Southern Hemisphere fall between3°and10° southof the equator east of the140th meridian westlongitude during cool or neutralEl Niño–Southern Oscillation(ENSO) patterns. When ENSO reaches its warm phase, otherwise known as El Niño, the tongue of loweredsea surface temperaturesdue to upwelling off the South American continent disappears, which causes this convergence zone to vanish as well.

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 severedroughtsor 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 afrontalong the leading edge of the equatorial air.There appears to be a 15 to 25-day cycle in thunderstorm activity along the ITCZ, which is roughly half the wavelength of theMadden–Julian oscillation(MJO).

Within the ITCZ the average winds are slight, unlike the zones north and south of the equator where the trade winds feed. As trans-equator sea voyages became more common, sailors in the eighteenth century named this belt of calmthe doldrumsbecause of the calm, stagnant, or inactive winds.

Role in tropical cyclone formation

HurricanesCeliaandDarbyin the eastern Pacific and the precursor toHurricane Alexin the Intertropical Convergence Zone. (2010)

Tropical cyclogenesisdepends upon low-levelvorticityas one of its six requirements, and the ITCZ fills this role as it is a zone of wind change and speed, otherwise known as horizontalwind shear.As the ITCZ migrates to tropical and subtropical latitudes and even beyond during the respective hemisphere's summer season, increasingCoriolis forcemakes the formation oftropical cycloneswithin this zone more possible. Surges of higher pressure from high latitudes can enhance tropical disturbances along its axis.In the north Atlantic and the northeastern Pacific oceans,tropical wavesmove along the axis of the ITCZ causing an increase in thunderstorm activity, and clusters of thunderstorms can develop under weak vertical wind shear.

Hazards

In theAge of Sail,to find oneself becalmed in this region in a hot and muggy climate could mean death when wind was the only effective way to propel ships across the ocean. Calm periods within the doldrums could strand ships for days or weeks.Even today, leisure and competitive sailors attempt to cross the zone as quickly as possible as the erratic weather and wind patterns may cause unexpected delays.

In 2009, thunderstorms along the Intertropical Convergence Zone played a role in the loss ofAir France Flight 447,which crashed while flying fromRio de Janeiro–Galeão International AirporttoCharles de Gaulle AirportnearParis.The aircraft crashed with no survivors while flying through a series of large ITCZ thunderstorms, and ice forming rapidly on airspeed sensors was the precipitating cause for a cascade of human errors which ultimately doomed the flight. Most aircraft flying these routes are able to avoid the largerconvectivecells without incident.

Effects of climate change

See also:Effects of climate change on the water cycle

Titaniumconcentrations in sediment within theCariaco Basinhave been used as a paleoclimate proxy to infer shifts in the ITCZ.

Based onpaleoclimate proxies,the position and intensity of the ITCZ varied in prehistoric times along withchanges in global climate.DuringHeinrich eventswithin the last 100 ka, a southward shift of the ITCZ coincided with the intensification of the Northern Hemisphere Hadley cell coincident with weakening of the Southern Hemisphere Hadley cell. The ITCZ shifted north during themid-Holocenebut migrated south following changes ininsolationduring the late-Holocene towards its current position. The ITCZ has also undergone periods of contraction and expansion within the last millennium.A southward shift of the ITCZ commencing after the 1950s and continuing into the 1980s may have been associated withcooling inducedbyaerosolsin the Northern Hemisphere based on results fromclimate models;a northward rebound began subsequently followingforced changesin the gradient in temperature between the Northern and Southern hemispheres. These fluctuations in ITCZ positioning had robust effects on climate; for instance, displacement of the ITCZ may have led todrought in the Sahelin the 1980s.

Atmospheric convectionmay become stronger and more concentrated at the center of the ITCZ in response to a globally warming climate, resulting in sharpened contrasts in precipitation between the ITCZ core (where precipitation would be amplified) and its edges (where precipitation would be suppressed).Atmospheric reanalysessuggest that the ITCZ over the Pacific has narrowed and intensified since at least 1979, in agreement with data collected by satellites and in-situ precipitation measurements. The drier ITCZ fringes are also associated with an increase inoutgoing longwave radiationoutward of those areas, particularly over land within the mid-latitudes and thesubtropics.This change in the ITCZ is also reflected by increasing salinity within the Atlantic and Pacific underlying the ITCZ fringes and decreasing salinity underlying central belt of the ITCZ. TheIPCC Sixth Assessment Reportindicated "medium agreement" from studies regarding the strengthening and tightening of the ITCZ due to anthropogenic climate change.

Less certain are the regional and global shifts in ITCZ position as a result of climate change, with paleoclimate data and model simulations highlighting contrasts stemming from asymmetries in forcing from aerosols, voclanic activity, andorbital variations,as well as uncertainties associated with changes inmonsoonsand theAtlantic meridional overturning circulation.The climate simulations run as part ofCoupled Model Intercomparison Project Phase 5(CMIP5) did not show a consistent global displacement of the ITCZ under anthropogenic climate change. In contrast, most of the same simulations show narrowing and intensification under the same prescribed conditions. However, simulations inCoupled Model Intercomparison Project Phase 6(CMIP6) have shown greater agreement over some regional shifts of the ITCZ in response to anthropogenic climate change, including a northward displacement over the Indian Ocean and eastern Africa and a southward displacement over the eastern Pacific and Atlantic oceans.

In literature

The doldrums are notably described inSamuel Taylor Coleridge's poemThe Rime of the Ancient Mariner(1798) and also provide a metaphor for the initial state of boredom and indifference of Milo, the child hero ofNorton Juster's classic children's novelThe Phantom Tollbooth.It is also cited in the bookWind, Sand and Stars.

See also

• Asymmetry of the Intertropical Convergence Zone
• Chemical equator
• Monsoon trough
• Horse latitudes
• Polar front
• Roaring Forties

External links

• The ITCZ in Africavia theUniversity of South Carolina
• "A Shifting Band of Rain",Scientific American(March 2011)
• Duane E. Waliser and Catherine Gautier, November 1993:"A Satellite-derived Climatology of the ITCZ".J. Climate,6,2162–2174.