Intertropical Convergence Zone
The Intertropical Convergence Zone (ITCZ /ɪtʃ/ ITCH),[1] known by sailors as the doldrums[2] or the calms because of its monotonous windless weather, is the area where the northeast and the southeast trade winds converge. It encircles Earth near the thermal equator though its specific position varies seasonally. When it lies near the geographic Equator, it is called the near-equatorial trough. Where the ITCZ is drawn into and merges with a monsoonal circulation, it is sometimes referred to as a monsoon 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 the Intertropical Front (ITF), but after the recognition in the 1940s and the 1950s of the significance of wind field convergence in tropical weather production, the term Intertropical Convergence Zone (ITCZ) was then applied.[3]
The ITCZ appears as a band of clouds, usually thunderstorms, that encircle the globe near the Equator. In the
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 the convergence zone is better defined, the seasonal cycle is more subtle, as the convection is constrained by the distribution of ocean temperatures.[5] 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
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.[6][7] 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.[6][8][9]
South Pacific convergence zone
The
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.
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.[12] There appears to be a 15 to 25-day cycle in thunderstorm activity along the ITCZ, which is roughly half the wavelength of the Madden–Julian oscillation (MJO).[13]
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 calm the doldrums because of the calm, stagnant, or inactive winds.
Role in tropical cyclone formation
Hazards
In the Age 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.[15] 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 of
Effects of climate change
Based on
Atmospheric convection may 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 reanalyses suggest 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 in outgoing longwave radiation outward of those areas, particularly over land within the mid-latitudes and the subtropics. 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. The IPCC Sixth Assessment Report indicated "medium agreement" from studies regarding the strengthening and tightening of the ITCZ due to anthropogenic climate change.[20]
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, and orbital variations, as well as uncertainties associated with changes in monsoons and the Atlantic meridional overturning circulation. The climate simulations run as part of Coupled 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 in Coupled 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.[20]
In literature
The doldrums are notably described in Samuel Taylor Coleridge's poem The Rime of the Ancient Mariner (1798) and also provide a metaphor for the initial state of boredom and indifference of Milo, the child hero of Norton Juster's classic children's novel The Phantom Tollbooth. It is also cited in the book Wind, Sand and Stars.
See also
- Asymmetry of the Intertropical Convergence Zone
- Chemical equator
- Monsoon trough
- Horse latitudes
- Polar front
- Roaring Forties
References
- ^ "ITCZ". National Weather Service.
- ^ Chisholm, Hugh, ed. (1911). . Encyclopædia Britannica. Vol. 8 (11th ed.). Cambridge University Press. p. 386.
- OCLC 249331900.
Atmosphere, weather, and climate.
- NOAA. 2007-10-24. Retrieved 2009-06-04.
- ^ "Inter Tropical Convergence Zone (ITCZ) - SKYbrary Aviation Safety". www.skybrary.aero. Retrieved 2018-04-12.
- ^ ISBN 9780190228620.
- ISSN 0003-0007.
- S2CID 244551794.
- S2CID 264384015.
- ^ E. Linacre and B. Geerts. Movement of the South Pacific convergence zone Retrieved on 2006-11-26.
- ^ Semyon A. Grodsky; James A. Carton (2003-02-15). "The Intertropical Convergence Zone in the South Atlantic and the Equatorial Cold Tongue" (PDF). University of Maryland, College Park. Retrieved 2009-06-05.
- ISBN 0-13-501149-3.
- ^ Patrick A. Harr. Tropical Cyclone Formation/Structure/Motion Studies. Office of Naval Research Retrieved on 2006-11-26. Archived November 29, 2007, at the Wayback Machine
- ^ C.-P. Chang, J.E. Erickson, and K.M. Lau. Northeasterly Cold Surges and Near-Equatorial Disturbances over the Winter MONEX Area during December 1974. Part I: Synoptic Aspects. Retrieved on 2007-04-26.
- ^ "What are the doldrums?" NOAA. National Ocean Atmospheric Administration's National Weather Service website, 01/07/20.
- ^ "Q & A Turbulences". The Guardian (1 June 2009).
- S2CID 24591761.
- ^ Gulev, Sergey K.; Thorne, Peter W.; et al. (2021). "Changing State of the Climate System". In Masson-Delmotte, Valerie; Zhai, Panmao; et al. (eds.). Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change (PDF). Cambridge, United Kingdom: Cambridge University Press. pp. 287–422. Retrieved 18 January 2023.
- ^ Eyring, Veronika; Gillett, Nathan P.; et al. (2021). "Human Influence on the Climate System". In Masson-Delmotte, Valerie; Zhai, Panmao (eds.). Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change (PDF). Cambridge, United Kingdom: Cambridge University Press. pp. 423–551. Retrieved 18 January 2023.
- ^ a b c Douville, Hervé; Raghavan, Krishnan; Renwick, James; et al. (2021). "Human Influence on the Climate System". In Masson-Delmotte, Valerie; Zhai, Panmao (eds.). Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change (PDF). Cambridge, United Kingdom: Cambridge University Press. pp. 1055–1210. Retrieved 18 January 2023.
External links
- The ITCZ in Africa via the University 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.