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67-537: 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. The ITCZ appears as a band of clouds, usually thunderstorms, that encircle the globe near the Equator. In

134-403: A Rossby-wave response to the perturbation, transforms into a coherent modon-like structure in the lower layer, which couples with a baroclinic Kelvin wave through a zone of enhanced convection and produces, at initial stages of the process, a self-sustained slowly eastward-propagating zonally- dissymmetrical quadrupolar vorticity pattern. In 2022, Rostami et al advanced their theory. By means of

201-455: A large-scale positive buoyancy anomaly, depressed anomaly, or a combination of them, as soon as this anomaly reaches a critical threshold in the presence of moist-convection at the equator. This MJO-like episode possesses a convectively coupled “hybrid structure” that consists of a “quasi equatorial modon”, with an enhanced vortex pair, and a convectively coupled baroclinic Kelvin wave (BKW), with greater phase speed than that of dipolar structure on

268-452: A new multi-layer pseudo-spectral moist-convective Thermal Rotating Shallow Water (mcTRSW) model in a full sphere, they presented a possible equatorial adjustment beyond Gill's mechanism for the genesis and dynamics of the MJO. According to this theory, an eastward propagating MJO-like structure can be generated in a self-sustained and self-propelled manner due to nonlinear relaxation (adjustment) of

335-730: A period of time, which leads to non-anomalous storm activity in each region of the globe. During the Northern Hemisphere summer season the MJO-related effects on the Indian and West African summer monsoon are well documented. MJO-related effects on the North American summer monsoon also occur, though they are relatively weaker. MJO-related impacts on the North American summer precipitation patterns are strongly linked to meridional (i.e. north–south) adjustments of

402-480: A possible mechanism to explain the genesis and backbone structure of the MJO and to converge some theories that previously seemed divergent. The MJO travels a stretch of 12,000–20,000 km over the tropical oceans, mainly over the Indo-Pacific warm pool , which has ocean temperatures generally warmer than 28 °C. This Indo-Pacific warm pool has been warming rapidly, altering the residence time of MJO over

469-542: A retrograding (i.e. westward moving) circulation pattern in the mid latitudes of the North Pacific. Typical wintertime weather anomalies preceding heavy precipitation events in the Pacific Northwest are as follows: Throughout this evolution, retrogression of the large-scale atmospheric circulation features is observed in the eastern Pacific–North American sector. Many of these events are characterized by

536-501: 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 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

603-467: 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 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

670-642: A vertical movement and to the formation of clouds and precipitation . Large-scale convergence, called synoptic-scale convergence, is associated with weather systems such as baroclinic troughs , low-pressure areas , and cyclones . The large-scale convergence zone formed over the equator, the Intertropical Convergence Zone , has condensed and intensified as a result of the global increase in temperature. Small-scale convergence will give phenomena from isolated cumulus clouds to large areas of thunderstorms . The inverse of convergence

737-653: Is a large-scale coupling between atmospheric circulation and tropical deep atmospheric convection . Unlike a standing pattern like the El Niño–Southern Oscillation (ENSO), the Madden–Julian oscillation is a traveling pattern that propagates eastward, at approximately 4 to 8 m/s (14 to 29 km/h; 9 to 18 mph), through the atmosphere above the warm parts of the Indian and Pacific oceans. This overall circulation pattern manifests itself most clearly as anomalous rainfall. The Madden–Julian oscillation

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804-549: 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 International Date Line . It is considered the largest and most important piece of the ITCZ, and has

871-460: Is case-to-case variability in the amplitude and longitudinal extent of the MJO-related precipitation, so this should be viewed as a general relationship only. In 2019, Rostami and Zeitlin reported a discovery of steady, long-living, slowly eastward-moving large-scale coherent twin cyclones, so-called equatorial modons , by means of a moist-convective rotating shallow water model. Crudest barotropic features of MJO such as eastward propagation along

938-436: Is characterized by an eastward progression of large regions of both enhanced and suppressed tropical rainfall, observed mainly over the Indian and Pacific Ocean. The anomalous rainfall is usually first evident over the western Indian Ocean, and remains evident as it propagates over the very warm ocean waters of the western and central tropical Pacific. This pattern of tropical rainfall generally becomes nondescript as it moves over

1005-509: Is directly related to the position of the Sun or the location of the " energy flux equator," thus the ITCZ shifts corresponding to the seasons. Due to the position of the Sun, the sea surface temperature near the equator (30°S to 30°N), during an equinox , is higher than any other latitudes. During the summer solstice in the Northern Hemisphere (June 21), the ITCZ is shifted north, following

1072-677: Is divergence, such as the horse latitudes . An example of a convergence zone is the Intertropical Convergence Zone (ITCZ), a low pressure area which girdles the Earth at the Equator . Another example is the South Pacific convergence zone that extends from the western Pacific Ocean toward French Polynesia . The Intertropical Convergence Zone is the result of the northeasterly trade winds and southeasterly trade winds converging in an area of high latent heat and low pressure . As

1139-458: Is quite favorable for tropical storm development. As the MJO progresses eastward, the favored region for tropical cyclone activity also shifts eastward from the western Pacific to the eastern Pacific and finally to the Atlantic basin. An inverse relationship exists between tropical cyclone activity in the western north Pacific basin and the north Atlantic basin, however. When one basin is active,

1206-439: Is suppressed. Each cycle lasts approximately 30–60 days. Because of this pattern, the Madden–Julian oscillation is also known as the 30- to 60-day oscillation , 30- to 60-day wave , or intraseasonal oscillation . Distinct patterns of lower-level and upper-level atmospheric circulation anomalies accompany the MJO-related pattern of enhanced or decreased tropical rainfall across the tropics. These circulation features extend around

1273-649: 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

1340-638: The Congo Basin and East Africa . During the major rainy seasons in East Africa (March to May and October to December), rainfall tends to be lower during when the MJO convective core is over the eastern Pacific, and higher when convection peaks over the Indian Ocean. During 'wet' phases, the normal easterly winds weaken, while during 'dry' phases, the easterly winds strengthen. An increase in frequency of MJO phases with convective activity over

1407-703: The Irish Sea . Flooding in Boscastle , Cornwall, England in August 2004 was the result of thunderstorms developing on a convergence line. Madden%E2%80%93Julian oscillation The Madden–Julian oscillation ( MJO ) is the largest element of the intraseasonal (30- to 90-day) variability in the tropical atmosphere. It was discovered in 1971 by Roland Madden and Paul Julian of the American National Center for Atmospheric Research (NCAR). It

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1474-486: 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 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

1541-551: 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 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

1608-885: The Puget Sound Convergence Zone which occurs in the Puget Sound region in the U.S. state of Washington ; Mohawk–Hudson convergence in the U.S. state of New York ; the Elsinore Convergence Zone in the U.S. state of California ; the Brown Willy effect which can be generated when south-westerly winds blow over Bodmin Moor in Cornwall ; and the Pembrokeshire Dangler which can form when northerly winds blow down

1675-472: The Asian monsoon, normally during the month of July, has been attributed to the Madden–Julian oscillation after its enhanced phase moves off to the east of the region into the open tropical Pacific Ocean. Tropical cyclones occur throughout the boreal warm season (typically May–November) in both the north Pacific and the north Atlantic basins—but any given year has periods of enhanced or suppressed activity within

1742-612: 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 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

1809-410: 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,

1876-461: 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. The South Pacific convergence zone (SPCZ)

1943-439: 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 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

2010-547: 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. 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,

2077-459: The ITCZ may have led to drought in the Sahel in the 1980s. 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

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2144-476: 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

2211-570: The MJO also influences these conditions that facilitate or suppress tropical cyclone formation. The MJO is monitored routinely by both the USA National Hurricane Center and the USA Climate Prediction Center during the Atlantic hurricane ( tropical cyclone ) season to aid in anticipating periods of relative activity or inactivity. The MJO signal is well defined in parts of Africa including in

2278-435: The MJO enhanced activity, winds aloft are westerly. In its wake, or to the west of the enhanced rainfall area, winds aloft are easterly. These wind changes aloft are due to the divergence present over the active thunderstorms during the enhanced phase. Its direct influence can be tracked poleward as far as 30 degrees latitude from the equator in both northern and southern hemispheres, propagating outward from its origin near

2345-432: The Pacific, strong MJO activity is often observed 6 to 12 months prior to the onset of an El Niño episode, but is virtually absent during the maxima of some El Niño episodes, while MJO activity is typically greater during a La Niña episode. Strong events in the Madden–Julian oscillation over a series of months in the western Pacific can speed the development of an El Niño or La Niña but usually do not in themselves lead to

2412-409: The amount of outgoing long wave radiation, the stronger the thunderstorm complexes, or convection, is within that region. Enhanced surface (upper level) westerly winds occur near the west (east) side of the active convection. Ocean currents, up to 100 metres (330 ft) in depth from the ocean surface, follow in phase with the east-wind component of the surface winds. In advance, or to the east, of

2479-512: The bulk of its annual precipitation . Storms in this region can last for several days or more and are often accompanied by persistent atmospheric circulation features. Of particular concern are extreme precipitation events linked to flooding . Strong evidence suggests a link between weather and climate in this region from studies that have related the El Niño Southern Oscillation to regional precipitation variability. In

2546-412: The child hero of Norton Juster 's classic 1961 children's novel The Phantom Tollbooth . It is also cited in the 1939 book Wind, Sand and Stars . Convergence zone A convergence zone in meteorology is a region in the atmosphere where two prevailing flows meet and interact, usually resulting in distinctive weather conditions . This causes a mass accumulation that eventually leads to

2613-515: The diabatic moist-convective environment on the equator. In 2020, a study showed that the process of relaxation (adjustment) of localized large-scale pressure anomalies in the lower equatorial troposphere, generates structures strongly resembling the Madden Julian Oscillation (MJO) events, as seen in vorticity, pressure, and moisture fields. Indeed, it is demonstrated that baroclinicity and moist convection substantially change

2680-601: 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 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

2747-482: The eastern Pacific might have contributed to the drying trend seen in the Congo Basin in the last few decades. There is strong year-to-year (interannual) variability in Madden–Julian oscillation activity, with long periods of strong activity followed by periods in which the oscillation is weak or absent. This interannual variability of the MJO is partly linked to the El Niño–Southern Oscillation (ENSO) cycle. In

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2814-419: The equator at around 1 degree latitude, or 111 kilometres (69 mi), per day. The MJO's movement around the globe can occasionally slow or stall during the Northern Hemisphere summer and early autumn, leading to consistently enhanced rainfall for one side of the globe and consistently depressed rainfall for the other side. This can also happen early in the year. The MJO can also go quiet for

2881-454: The equator, slow phase speed, hydro-dynamical coherent structure, the convergent zone of moist-convection, are captured by Rostami and Zeitlin's modon. Having an exact solution of streamlines for internal and external regions of equatorial asymptotic modon is another feature of this structure. It is shown that such eastward-moving coherent dipolar structures can be produced during geostrophic adjustment of localized large-scale pressure anomalies in

2948-446: The globe and are not confined to only the eastern hemisphere. The Madden–Julian oscillation moves eastward at between 4 m/s (14 km/h, 9 mph) and 8 m/s (29 km/h, 18 mph) across the tropics, crossing the Earth 's tropics in 30 to 60 days—with the active phase of the MJO tracked by the degree of outgoing long wave radiation, which is measured by infrared -sensing geostationary weather satellites . The lower

3015-403: 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. There appears to be a 15 to 25-day cycle in thunderstorm activity along the ITCZ, which is roughly half the wavelength of

3082-526: The intraseasonal time scale. Interaction of the BKW, after circumnavigating all around the equator, with a new large-scale buoyancy anomaly may contribute to excitation of a recurrent generation of the next cycle of MJO-like structure. Overall, the generated "hybrid structure” captures a few of the crudest features of the MJO, including its quadrupolar structure, convective activity, condensation patterns, vorticity field, phase speed, and westerly and easterly inflows in

3149-580: 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 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

3216-607: 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 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

3283-528: 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 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,

3350-480: The location of extreme west coast precipitation events. Extreme events in the Pacific Northwest are accompanied by enhanced precipitation over the western tropical Pacific and the region of Southeast Asia called by meteorologists the Maritime Continent , with suppressed precipitation over the Indian Ocean and the central Pacific. As the region of interest shifts from the Pacific Northwest to California ,

3417-449: The lower and upper troposphere. Although the moisture-fed convection is a necessary condition for the ``hybrid structure” to be excited and maintained in the proposed theory in this theory, it is fundamentally different from the moisture-mode ones. Because the barotropic equatorial modon and BKW also exist in “dry” environments, while there are no similar “dry” dynamical basic structures in the moisture-mode theories. The proposed theory can be

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3484-471: 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 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

3551-483: The onset of a warm or cold ENSO event. However, observations suggest that the 1982-1983 El Niño developed rapidly during July 1982 in direct response to a Kelvin wave triggered by an MJO event during late May. Further, changes in the structure of the MJO with the seasonal cycle and ENSO might facilitate more substantial impacts of the MJO on ENSO. For example, the surface westerly winds associated with active MJO convection are stronger during advancement toward El Niño and

3618-485: The other is normally quiet, and vice versa. The main reason for this appears to be the phase of the MJO, which is normally in opposite modes between the two basins at any given time. While this relationship appears robust, the MJO is one of many factors that contribute to the development of tropical cyclones. For example, sea surface temperatures must be sufficiently warm and vertical wind shear must be sufficiently weak for tropical disturbances to form and persist. However,

3685-539: The position of the Sun. The ITCZ is shifted farther south during the winter solstice (in the Northern Hemisphere), when the solar radiation is focused at 23.5°S. Convergence zones also occur at a smaller scale. Convergence lines form rows of showers or thunderstorms over a more local area. Sea breezes colliding can trigger development of a convergence line. The heavy rain caused in a short period of time can cause severe flooding. Some examples are

3752-492: The precipitation pattern in the eastern tropical Pacific. A strong relationship between the leading mode of intraseasonal variability of the North American Monsoon System, the MJO and the points of origin of tropical cyclones is also present. A period of warming sea surface temperatures is found five to ten days prior to a strengthening of MJO-related precipitation across southern Asia. A break in

3819-465: The primarily cooler ocean waters of the eastern Pacific, but reappears when passing over the warmer waters over the Pacific Coast of Central America . The pattern may also occasionally reappear at low amplitude over the tropical Atlantic and higher amplitude over the Indian Ocean. The wet phase of enhanced convection and precipitation is followed by a dry phase where thunderstorm activity

3886-454: The progression of the heaviest precipitation from south to north along the Pacific Northwest coast over a period of several days to more than one week. However, it is important to differentiate the individual synoptic -scale storms, which generally move west to east, from the overall large-scale pattern, which exhibits retrogression. A coherent simultaneous relationship exists between the longitudinal position of maximum MJO-related rainfall and

3953-478: The region of enhanced tropical precipitation shifts further to the east. For example, extreme rainfall events in southern California are typically accompanied by enhanced precipitation near 170°E. However, it is important to note that the overall link between the MJO and extreme west coast precipitation events weakens as the region of interest shifts southward along the west coast of the United States. There

4020-518: The scenario of the quasi-barotropic "dry" adjustment, which was established in the framework of one-layer shallow water model and consists, in the long-wave sector, in the emission of equatorial Rossby waves, with dipolar meridional structure, to the West, and of equatorial Kelvin waves, to the East. If moist convection is strong enough, a dipolar cyclonic structure, which appears in the process of adjustment as

4087-414: The season. Evidence suggests that the Madden–Julian oscillation modulates this activity (particularly for the strongest storms) by providing a large-scale environment that is favorable (or unfavorable) for development. MJO-related descending motion is not favorable for tropical storm development. However, MJO-related ascending motion is a favorable pattern for thunderstorm formation within the tropics, which

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4154-433: 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. Sometimes, a double ITCZ forms, with one located north and another south of

4221-424: The surface easterly winds associated with the suppressed convective phase are stronger during advancement toward La Niña. Globally, the interannual variability of the MJO is most determined by atmospheric internal dynamics, rather than surface conditions. The strongest impacts of intraseasonal variability on the United States occur during the winter months over the western U.S. During the winter this region receives

4288-486: 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 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

4355-406: The tropical Pacific compared to stronger warm and cold episodes. In these winters, there is a stronger link between the MJO events and extreme west coast precipitation events. The typical scenario linking the pattern of tropical rainfall associated with the MJO to extreme precipitation events in the Pacific Northwest features a progressive (i.e. eastward moving) circulation pattern in the tropics and

4422-556: The tropical Pacific, winters with weak-to-moderate cold, or La Niña, episodes or ENSO-neutral conditions are often characterized by enhanced 30- to 60-day Madden–Julian oscillation activity. A recent example is the winter of 1996–1997, which featured heavy flooding in California and in the Pacific Northwest (estimated damage costs of $ 2.0–3.0 billion at the time of the event) and a very active MJO. Such winters are also characterized by relatively small sea surface temperature anomalies in

4489-467: The two trade winds converge, the cool, dry air collects moisture from the warm ocean and rises, contributing to cloud formation and precipitation. The low pressure area that is created by the movement of the trade winds acts as a vacuum , drawing in the cooler, dry air from high pressure areas (divergence zones), creating a convection cell commonly known as the Hadley Cell . Sea surface temperature

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