TheIntertropical Convergence Zone (ITCZ/ɪtʃ/ITCH, orICZ),[2] known by sailors as thedoldrums[3] or thecalms because of its monotonous windless weather, is the area where the northeast and the southeasttrade winds converge. It encircles Earth near thethermal equator, though 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 amonsoonal circulation, it is sometimes referred to as amonsoon trough (a usage that is more common in Australia and parts of Asia).
The ITCZ was originally identified from the 1920s to the 1940s as theIntertropical Front (ITF); however, after the recognition of the significance ofwind field convergence intropical weather production in the 1940s and 1950s, the termIntertropical Convergence Zone (ITCZ) was then applied.[4]
The ITCZ appears as a band of clouds, typically thunderstorms, that encircle the globe near the Equator. In theNorthern Hemisphere, thetrade winds move 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 effect imparted 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 asconvective activity of thunderstorms driven by solar heating, which effectively draw air in; these are the trade winds.[5] The ITCZ is effectively a tracer of the ascending branch of theHadley cell and 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 thethermal 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 zone is better defined, the seasonal cycle is more subtle, as the convection is constrained by the distribution of ocean temperatures.[6] 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.
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.[1][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.[1][8][9]
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 mass during the summer than any other portion of themonsoon trough.[10] The southern ITCZ in the eastern tropical Pacific and southern tropical Atlantic, known as the SITCZ, occurs during the Southern Hemisphere fall between3° and10° south of the equator east of the140th meridian west longitude 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 temperatures due to upwelling off the South American continent disappears, which causes this convergence zone to vanish as well.[11]
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 severedroughts 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 afront 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 theMadden–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 calmthe doldrums because of the calm, stagnant, or inactive winds.
Tropical cyclogenesis depends upon low-levelvorticity 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 horizontalwind shear. As the ITCZ migrates to tropical and subtropical latitudes and even beyond during the respective hemisphere's summer season, increasingCoriolis force makes the formation oftropical cyclones within this zone more possible. Surges of higher pressure from high latitudes can enhance tropical disturbances along its axis.[14] In the tropical north Atlantic and the eastern portion of the tropical north Pacific oceans,tropical waves move along the axis of the ITCZ causing an increase in thunderstorm activity, and clusters of thunderstorms can develop under weak vertical wind shear.[citation needed]
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.[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 ofAir France Flight 447, which crashed while flying fromRio de Janeiro–Galeão International Airport toCharles de Gaulle Airport nearParis.[16] 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 largerconvective cells without incident.
Based onpaleoclimate proxies, the position and intensity of the ITCZ varied in prehistoric times along withchanges in global climate. DuringHeinrich events within 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-Holocene but migrated south following changes ininsolation during the late-Holocene towards its current position. The ITCZ has also undergone periods of contraction and expansion within the last millennium.[18] A southward shift of the ITCZ commencing after the 1950s and continuing into the 1980s may have been associated withcooling induced byaerosols in the Northern Hemisphere based on results fromclimate models; a northward rebound began subsequently followingforced changes in 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 Sahel in the 1980s.[19][20]
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 inoutgoing longwave radiation outward 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 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, volcanic activity, andorbital variations, as well as uncertainties associated with changes inmonsoons and 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.[20]
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 1961 children's novelThe Phantom Tollbooth. It is also cited in the 1939 bookWind, Sand and Stars.
Atmosphere, weather, and climate.
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