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50-489: The Mugere River , a major headwater, rises at an elevation of 2,450 metres (8,040 ft) in Burundi. The river has sources at about 2,300 metres (7,500 ft) elevation in the south of Rwanda. Many of the valleys of the higher tributaries are choked by papyrus, which contains seasonal swamp forests. The upstream portion of the river has a catchment area of about 2,650 square kilometres (1,020 sq mi). The lower course of

100-563: A 63 kilometres (39 mi) stretch of the river, with the swamp reaching 6 to 10 kilometres (3.7 to 6.2 mi) up the valleys of tributaries. The upstream part of the river basin has average annual rainfall of about 1,200 millimetres (47 in). Average annual rainfall in the wetlands is about 800 millimetres (31 in). The climate of Rwanda is determined by the Intertropical Convergence Zone , which creates two rainy seasons. One lasts from mid-September to mid-December and

150-559: 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

200-444: A course along which the winds can be expected to blow in the direction of travel. During the Age of Sail , the pattern of prevailing winds made various points of the globe easy or difficult to access, and therefore had a direct effect on European empire-building and thus on modern political geography. For example, Manila galleons could not sail into the wind at all. By the 18th century,

250-445: A grass in the rice genus. Nearer to the shore is a combination of Typha australis , Miscanthidium violaceum , Cladium jamaicense with some papyrus . Cyperus denudatus , Cyperus latifolius and Echinochloa pyramidalis are also found in some parts. Other parts have pure stands of papyrus. The swamp forest near the river is dominated by Bridelia micrantha , Ficus verruculosa , Myrica kandtiana and Phoenix reclinata . Further from

300-440: 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. 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

350-461: A tracer of the ascending branch of the Hadley cell and is wet. The dry descending branch is the horse 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 the convergence zone is better defined,

400-566: A trade wind inversion , which is caused by descending air aloft from within the subtropical ridge . The weaker the trade winds become, the more rainfall can be expected in the neighboring landmasses. The trade winds also transport nitrate- and phosphate-rich Saharan dust to all Latin America , the Caribbean Sea , and to parts of southeastern and southwestern North America. Sahara dust is on occasion present in sunsets across Florida . When dust from

450-705: Is also cited in the 1939 book Wind, Sand and Stars . Trade winds The trade winds or easterlies are permanent east-to-west prevailing winds that flow in the Earth's equatorial region. The trade winds blow mainly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere , strengthening during the winter and when the Arctic oscillation is in its warm phase. Trade winds have been used by captains of sailing ships to cross

500-550: Is known as a temperature inversion. When it occurs within a trade wind regime, it is known as a trade wind inversion. The surface air that flows from these subtropical high-pressure belts toward the Equator is deflected toward the west in both hemispheres by the Coriolis effect . These winds blow predominantly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere . Because winds are named for

550-658: Is no agreement that prevents our population from using the water...of course in a good way." In December 2012 the ministries of Infrastructure and Natural Resources were discussing how best to evaluate the quantity and quality of Akanyaru peat. Two companies, one from India and the other from Turkey, were to take over peat production. A 100MW peat-fired power plant was to be built by the Turkish developer Hakan Mining and Generation Industry and Trade. Intertropical Convergence Zone The Intertropical Convergence Zone ( ITCZ / ɪ tʃ / ITCH , or ICZ ), known by sailors as

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600-512: Is sometimes referred to as a monsoon 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 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 )

650-436: Is the vulnerable papyrus yellow warbler . The sitatunga , an antelope, is found in the swamps. The valley is not protected, and much of the land that is only flooded seasonally is cultivated at other times of the year. The local people also engage in fishing in the river and swamps. The wetlands are increasingly being used for agriculture. In the dry season the local people cut and burn the marsh vegetation, steadily destroying

700-510: Is warm, trade winds are stronger within the tropics. The cold phase of the AO leads to weaker trade winds. When the trade winds are weaker, more extensive areas of rain fall upon landmasses within the tropics, such as Central America . During mid-summer in the Northern Hemisphere (July), the westward-moving trade winds south of the northward-moving subtropical ridge expand northwestward from

750-439: 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. 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

800-447: The Caribbean Sea into southeastern North America (Florida and Gulf Coast). When dust from the Sahara moving around the southern periphery of the ridge travels over land, rainfall is suppressed and the sky changes from a blue to a white appearance which leads to an increase in red sunsets. Its presence negatively impacts air quality by adding to the count of airborne particulates. Although

850-401: The Coriolis effect imparted by Earth'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 as convective activity of thunderstorms driven by solar heating, which effectively draw air in; these are the trade winds. The ITCZ is effectively

900-419: The Sahara travels over land, rainfall is suppressed and the sky changes from a blue to a white appearance which leads to an increase in red sunsets. Its presence negatively impacts air quality by adding to the count of airborne particulates. The term originally derives from the early fourteenth century sense of trade (in late Middle English ) still often meaning "path" or "track". The Portuguese recognized

950-404: The doldrums 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

1000-588: The Indian Ocean and eastern Africa and a southward displacement over the eastern Pacific and Atlantic oceans. 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 1961 children's novel The Phantom Tollbooth . It

1050-509: The Intertropical Convergence Zone played a role in the loss of Air France Flight 447 , which crashed while flying from Rio de Janeiro–Galeão International Airport to Charles de Gaulle Airport near Paris . 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

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1100-542: The Northern Hemisphere based on results from climate models ; a northward rebound began subsequently following forced 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 to drought in the Sahel in the 1980s. Atmospheric convection may become stronger and more concentrated at

1150-622: The SITCZ, occurs during the Southern Hemisphere fall between 3° and 10° south of the equator east of the 140th meridian west longitude during cool or neutral El Niño–Southern Oscillation (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. Variation in

1200-813: The Southeast US has some of the cleanest air in North America, much of the African dust that reaches the United States affects Florida. Since 1970, dust outbreaks have worsened due to periods of drought in Africa. There is a large variability in the dust transport to the Caribbean and Florida from year to year. Dust events have been linked to a decline in the health of coral reefs across the Caribbean and Florida , primarily since

1250-508: The Southern Hemisphere Hadley cell. The ITCZ shifted north during the mid-Holocene but migrated south following changes in insolation during 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 with cooling induced by aerosols in

1300-633: 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

1350-544: The direction from which the wind is blowing, these winds are called the northeasterly trade winds in the Northern Hemisphere and the southeasterly trade winds in the Southern Hemisphere. The trade winds of both hemispheres meet at the Doldrums . As they blow across tropical regions, air masses heat up over lower latitudes due to more direct sunlight. Those that develop over land (continental) are drier and hotter than those that develop over oceans (maritime), and travel northward on

1400-402: The eighteenth century named this belt of calm the doldrums because of the calm, stagnant, or inactive winds. 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 to tropical and subtropical latitudes and even beyond during

1450-489: The equator during the Southern Hemisphere warm season, but can be more extratropical in nature, especially east of the International Date Line . It is considered the largest and most important piece of the ITCZ, and has the least dependence upon heating from a nearby land mass during the summer than any other portion of the monsoon trough . The southern ITCZ in the southeast Pacific and southern Atlantic, known as

1500-502: The equator while the flow aloft is towards the poles . A low-pressure area of calm, light variable winds near the equator is known as the doldrums , near-equatorial trough, intertropical front, or the Intertropical Convergence Zone . When located within a monsoon region, this zone of low pressure and wind convergence is also known as the monsoon trough . Around 30° in both hemispheres, air begins to descend toward

1550-507: The equator; the ITCZ can then be interpreted as a front along 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 the Madden–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

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1600-510: The flight. Most aircraft flying these routes are able to avoid the larger convective cells without incident. Based on paleoclimate proxies , the position and intensity of the ITCZ varied in prehistoric times along with changes in global climate . During Heinrich 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

1650-778: The habitat. In October 2005 Charles Karangwa, the Butare Director for Economic Affairs, said the people should increase their use of the Akanyaru wetland for crops, particularly maize. In February 2011 the Minister of Lands and Environment told the people of the Gisagara District to make better use of the Akanyaru swamp, with more modern farming methods. According to Minister Kamanzi the Nile Basin Initiative Cooperative Framework did not debar use. He said, "There

1700-614: The importance of the trade winds (then the volta do mar , meaning in Portuguese "turn of the sea" but also "return from the sea") in navigation in both the north and south Atlantic Ocean as early as the 15th century. From West Africa, the Portuguese had to sail away from continental Africa, that is, to west and northwest. They could then turn northeast, to the area around the Azores islands, and finally east to mainland Europe. They also learned that to reach South Africa, they needed to go far out in

1750-469: The importance of the trade winds to England's merchant fleet for crossing the Atlantic Ocean had led both the general public and etymologists to identify the name with a later meaning of "trade": "(foreign) commerce". Between 1847 and 1849, Matthew Fontaine Maury collected enough information to create wind and current charts for the world's oceans. As part of the Hadley cell , surface air flows toward

1800-418: 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

1850-465: 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. Less certain are

1900-509: The ocean, head for Brazil, and around 30°S go east again. (This is because following the African coast southbound means sailing upwind in the Southern hemisphere.) In the Pacific Ocean, the full wind circulation, which included both the trade wind easterlies and higher-latitude westerlies , was unknown to Europeans until Andres de Urdaneta 's voyage in 1565. The captain of a sailing ship seeks

1950-461: The other from March to May. Climate change may be increasing the severity of both floods and droughts. In the 1997-98 El Nino episode a large number of agricultural plantations in the shallows and swamps of the Nyabarongo and Akanyaru river basins were destroyed. Beside the open channels the main plants are Pistia stratiotes (water cabbage), Leersia hexandra (southern cutgrass) and Oryza barthii ,

2000-519: 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, 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

2050-477: The respective hemisphere's summer season, increasing Coriolis force makes the formation of tropical cyclones within 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 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. In

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2100-409: The river common species include Acacia polyacantha and Albizia gummifera . The marshland is an area of great biodiversity, particularly of birdlife. There are records of at least 54 bird species with wetland habitats in the swamp region. These include migrant Malagasy pond heron , pallid harrier and great snipe . Lesser kestrel have been seen. The near-threatened papyrus gonolek is present as

2150-513: The river is a belt of permanent wetlands about 7 kilometres (4.3 mi) wide that ends where it reaches the Nyabarongo River. In this 80 kilometres (50 mi) stretch the river drops from an elevation of 1,465 metres (4,806 ft) to 1,400 metres (4,600 ft). The swamp belt is fed from the right by lakes Cyohoha North and Cyohoha South . The Burundi side of the swamp belt has about 14,600 hectares (36,000 acres) of permanent swamp along

2200-399: 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. The ITCZ is commonly defined as an equatorial zone where

2250-433: The surface in subtropical high-pressure belts known as subtropical ridges . The subsident (sinking) air is relatively dry because as it descends, the temperature increases, but the moisture content remains constant, which lowers the relative humidity of the air mass. This warm, dry air is known as a superior air mass and normally resides above a maritime tropical (warm and moist) air mass. An increase of temperature with height

2300-584: The trade wind inversion. Trade winds originate more from the direction of the poles (northeast in the Northern Hemisphere, southeast in the Southern Hemisphere) during the cold season, and are stronger in the winter than the summer. As an example, the windy season in the Guianas , which lie at low latitudes in South America , occurs between January and April. When the phase of the Arctic oscillation (AO)

2350-406: 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

2400-416: The western periphery of the subtropical ridge. Maritime tropical air masses are sometimes referred to as trade air masses. All tropical oceans except the northern Indian Ocean have extensive areas of trade winds. Clouds which form above regions within trade wind regimes are typically composed of cumulus which extend no more than 4 kilometres (13,000 ft) in height, and are capped from being taller by

2450-729: The world's oceans for centuries. They enabled European colonization of the Americas , and trade routes to become established across the Atlantic Ocean and the Pacific Ocean . In meteorology , they act as the steering flow for tropical storms that form over the Atlantic, Pacific, and southern Indian oceans and cause rainfall in North America , Southeast Asia , and Madagascar and East Africa . Shallow cumulus clouds are seen within trade wind regimes and are capped from becoming taller by

2500-401: Was then applied. The ITCZ appears as a band of clouds, usually thunderstorms, that encircle the globe near the Equator. In the Northern Hemisphere , the trade winds move in a southwestward direction from the northeast, while in the Southern Hemisphere , they move northwestward from the southeast. When the ITCZ is positioned north or south of the Equator, these directions change according to

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