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82-623: In the Indian Ocean, circulation is dominated by the impact of the reversing Asian monsoon winds. As such, the current tends to reverse hemispheres seasonally in that basin. The NECC has a pronounced seasonal cycle in the Atlantic and Pacific, reaching maximum strength in late boreal summer and fall and minimum strength in late boreal winter and spring. Furthermore, the NECC in the Atlantic disappears in late winter and early spring. The NECC

164-614: A concentrated belt that stretches east–west except in East China where it is tilted east-northeast over Korea and Japan. The seasonal rain is known as Meiyu in China, Jangma in Korea, and Bai-u in Japan, with the latter two resembling frontal rain. The onset of the summer monsoon is marked by a period of premonsoonal rain over South China and Taiwan in early May. From May through August,

246-410: A dominant westerly component and a strong tendency to ascend and produce copious amounts of rain (because of the condensation of water vapor in the rising air). The intensity and duration, however, are not uniform from year to year. Winter monsoons, by contrast, have a dominant easterly component and a strong tendency to diverge, subside and cause drought. Similar rainfall is caused when moist ocean air

328-622: A few sub-systems, such as the Indian Subcontinental Monsoon which affects the Indian subcontinent and surrounding regions including Nepal, and the East Asian Monsoon which affects southern China, Taiwan , Korea and parts of Japan. The southwestern summer monsoons occur from June through September. The Thar Desert and adjoining areas of the northern and central Indian subcontinent heat up considerably during

410-498: A geostrophic mass transport per unit latitude, M, as the east-west integral of the meridional derivative of wind stress curl, minus any Ekman transport. The Ekman transport into the current is typically negligible, at least in the Pacific NECC. The total NECC is found by simply integrating M over the relevant latitudes. The Atlantic NECC consists of the eastward zonal transport of water between 3°N and 9°N, with typical widths on

492-547: A low pressure system known as a monsoon trough develops over South-East Asia and Australasia and winds are directed toward Australia. In the Philippines, northeast monsoon is called Amihan . The East Asian monsoon affects large parts of Indochina , the Philippines , China, Taiwan , Korea, Japan, and Siberia . It is characterised by a warm, rainy summer monsoon and a cold, dry winter monsoon. The rain occurs in

574-464: A more even temperature, while land temperatures are more variable. During warmer months sunlight heats the surfaces of both land and oceans, but land temperatures rise more quickly. As the land's surface becomes warmer, the air above it expands and an area of low pressure develops. Meanwhile, the ocean remains at a lower temperature than the land, and the air above it retains a higher pressure. This difference in pressure causes sea breezes to blow from

656-585: A weakening of the Leeuwin Current (LC). The weakening of the LC would have an effect on the sea surface temperature (SST) field in the Indian Ocean, as the Indonesian Throughflow generally warms the Indian Ocean. Thus these five intervals could probably be those of considerable lowering of SST in the Indian Ocean and would have influenced Indian monsoon intensity. During the weak LC, there

738-505: Is Ekman pumping as the tradewinds shift to westerlies causing a pile up of surface water. Some assumptions of the fluid dynamics involved in the process must be made in order to simplify the process to a point where it is solvable. The assumptions made by Ekman were: The simplified equations for the Coriolis force in the x and y directions follow from these assumptions: where τ {\displaystyle \tau \,\!}

820-454: Is a component of wind-driven ocean current. Ekman transport occurs when ocean surface waters are influenced by the friction force acting on them via the wind. As the wind blows it casts a friction force on the ocean surface that drags the upper 10-100m of the water column with it. However, due to the influence of the Coriolis effect , the ocean water moves at a 90° angle from the direction of

902-405: Is an interesting case because while it results from wind-driven circulation, it transports water against the mean westward wind stress in the tropics. This apparent paradox is concisely explained by Sverdrup theory , which shows that the east-west transport is governed by the north-south change in the curl of the wind stress . The Pacific NECC is also known to be stronger during warm episodes of

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984-666: Is directly of "cyclonic" (i.e., monsoon-driven) origin (as opposed to " local convection "). The effects also extend westwards to the Mediterranean, where however the impact of the monsoon is to induce drought via the Rodwell-Hoskins mechanism . Around September, with the sun retreating south, the northern landmass of the Indian subcontinent begins to cool off rapidly, and air pressure begins to build over northern India. The Indian Ocean and its surrounding atmosphere still hold their heat, causing cold wind to sweep down from

1066-414: Is large-scale wind patterns in the open ocean. Open ocean wind circulation can lead to gyre-like structures of piled up sea surface water resulting in horizontal gradients of sea surface height. This pile up of water causes the water to have a downward flow and suction, due to gravity and mass balance. Ekman pumping downward in the central ocean is a consequence of this convergence of water. Ekman suction

1148-533: Is lifted upwards by mountains, surface heating, convergence at the surface, divergence aloft, or from storm-produced outflows at the surface. However the lifting occurs, the air cools due to expansion in lower pressure, and this produces condensation . The monsoon of western Sub-Saharan Africa is the result of the seasonal shifts of the Intertropical Convergence Zone and the great seasonal temperature and humidity differences between

1230-498: Is now used to describe seasonal changes in atmospheric circulation and precipitation associated with annual latitudinal oscillation of the Intertropical Convergence Zone (ITCZ) between its limits to the north and south of the equator. Usually, the term monsoon is used to refer to the rainy phase of a seasonally changing pattern, although technically there is also a dry phase. The term is also sometimes used to describe locally heavy but short-term rains. The major monsoon systems of

1312-519: Is the Trade Winds both north and south of the equator pulling surface waters towards the poles. There is a great deal of upwelling Ekman suction at the equator because water is being pulled northward north of the equator and southward south of the equator. This leads to a divergence in the water, resulting in Ekman suction, and therefore, upwelling. The third wind pattern influencing Ekman transfer

1394-499: Is the wind stress , ρ {\displaystyle \rho \,\!} is the density, u {\displaystyle u\,\!} is the east–west velocity, and v {\displaystyle v\,\!} is the north–south velocity. Integrating each equation over the entire Ekman layer: where Here M x {\displaystyle M_{x}\,\!} and M y {\displaystyle M_{y}\,\!} represent

1476-411: Is the component of Ekman transport that results in areas of upwelling due to the divergence of water. Returning to the concept of mass conservation, any water displaced by Ekman transport must be replenished. As the water diverges it creates space and acts as a suction in order to fill in the space by pulling up, or upwelling, deep sea water to the euphotic zone. Ekman suction has major consequences for

1558-593: Is the possibility of reduced intensity of the Indian winter monsoon and strong summer monsoon, because of change in the Indian Ocean dipole due to reduction in net heat input to the Indian Ocean through the Indonesian Throughflow. Thus a better understanding of the possible links between El Niño , Western Pacific Warm Pool, Indonesian Throughflow, wind pattern off western Australia, and ice volume expansion and contraction can be obtained by studying

1640-481: Is the vertical eddy viscosity coefficient. This gives a set of differential equations of the form In order to solve this system of two differential equations, two boundary conditions can be applied: Things can be further simplified by considering wind blowing in the y -direction only. This means is the results will be relative to a north–south wind (although these solutions could be produced relative to wind in any other direction): where By solving this at z =0,

1722-622: The Eemian interglacial, suggests that they had an average duration of around 64 years, with the minimum duration being around 50 years and the maximum approximately 80 years, similar to today. A study of marine plankton suggested that the South Asian Monsoon (SAM) strengthened around 5 million years ago. Then, during ice periods, the sea level fell and the Indonesian Seaway closed. When this happened, cold waters in

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1804-570: The El Niño-Southern Oscillation (ENSO). Klaus Wyrtki , who first reported this connection, suggested that a stronger than normal NECC could be the cause of an El Niño because of the extra volume of warm water it carried eastwards. There is also a South Equatorial Countercurrent (SECC) that transports water from west to east in the Pacific and Atlantic basins between 2°S and 5°S in the western basin and farther south toward

1886-756: The Hadley circulation during boreal winter. It is associated with the development of the Siberian High and the movement of the heating maxima from the Northern Hemisphere to the Southern Hemisphere. North-easterly winds flow down Southeast Asia, are turned north-westerly/westerly by Borneo topography towards Australia. This forms a cyclonic circulation vortex over Borneo, which together with descending cold surges of winter air from higher latitudes, cause significant weather phenomena in

1968-623: The Himalayas and Indo-Gangetic Plain towards the vast spans of the Indian Ocean south of the Deccan peninsula. This is known as the Northeast Monsoon or Retreating Monsoon. While travelling towards the Indian Ocean, the cold dry wind picks up some moisture from the Bay of Bengal and pours it over peninsular India and parts of Sri Lanka . Cities like Chennai , which get less rain from

2050-543: The Intertropical Convergence Zone (ITCZ). In the western basin, the NECC may merge with the Equatorial Undercurrent (EUC) below the surface. Generally, the current weakens to the east in the basin, with estimated flows of 21 Sv, 14.2 Sv, and 12 Sv in the western, central, and eastern Pacific, respectively. Like the Atlantic NECC, the Pacific NECC undergoes an annual cycle. This is a result of

2132-785: The Sahara and the equatorial Atlantic Ocean. The ITCZ migrates northward from the equatorial Atlantic in February, reaches western Africa on or near June 22, then moves back to the south by October. The dry, northeasterly trade winds , and their more extreme form, the harmattan , are interrupted by the northern shift in the ITCZ and resultant southerly, rain-bearing winds during the summer. The semiarid Sahel and Sudan depend upon this pattern for most of their precipitation. The North American monsoon ( NAM ) occurs from late June or early July into September, originating over Mexico and spreading into

2214-683: The South China Sea led to a timing of the monsoon beginning 15–20 million years ago and linked to early Tibetan uplift. Testing of this hypothesis awaits deep ocean sampling by the Integrated Ocean Drilling Program . The monsoon has varied significantly in strength since this time, largely linked to global climate change , especially the cycle of the Pleistocene ice ages. A study of Asian monsoonal climate cycles from 123,200 to 121,210 years BP, during

2296-562: The return of the westerlies ) is the result of a resurgence of westerly winds from the Atlantic, where they become loaded with wind and rain. These westerly winds are a common phenomenon during the European winter, but they ease as spring approaches in late March and through April and May. The winds pick up again in June, which is why this phenomenon is also referred to as "the return of the westerlies". The rain usually arrives in two waves, at

2378-470: The 1890s. Ekman transport has significant impacts on the biogeochemical properties of the world's oceans. This is because it leads to upwelling (Ekman suction) and downwelling (Ekman pumping) in order to obey mass conservation laws. Mass conservation, in reference to Ekman transfer, requires that any water displaced within an area must be replenished. This can be done by either Ekman suction or Ekman pumping depending on wind patterns. Ekman theory explains

2460-756: The Asian monsoon has been linked to the uplift of the Tibetan Plateau after the collision of the Indian subcontinent and Asia around 50 million years ago. Because of studies of records from the Arabian Sea and that of the wind-blown dust in the Loess Plateau of China, many geologists believe the monsoon first became strong around 8 million years ago. More recently, studies of plant fossils in China and new long-duration sediment records from

2542-704: The EASM grew in strength, but it has been suggested to have decreased in strength during Heinrich events . The EASM expanded its influence deeper into the interior of Asia as sea levels rose following the LGM; it also underwent a period of intensification during the Middle Holocene, around 6,000 years ago, due to orbital forcing made more intense by the fact that the Sahara at the time was much more vegetated and emitted less dust. This Middle Holocene interval of maximum EASM

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2624-601: The East Asian Winter Monsoon (EAWM) became stronger as the subarctic front shifted southwards. An abrupt intensification of the EAWM occurred 5.5 million years ago. The EAWM was still significantly weaker relative to today between 4.3 and 3.8 million years ago but abruptly became more intense around 3.8 million years ago as crustal stretching widened the Tsushima Strait and enabled greater inflow of

2706-506: The GDP and employs 70% of the population) is heavily dependent on the rains, for growing crops especially like cotton , rice , oilseeds and coarse grains. A delay of a few days in the arrival of the monsoon can badly affect the economy, as evidenced in the numerous droughts in India in the 1990s. The monsoon is widely welcomed and appreciated by city-dwellers as well, for it provides relief from

2788-845: The Late Holocene, significant glacial accumulation in the Himalayas still occurred due to cold temperatures brought by westerlies from the west. During the Middle Miocene , the July ITCZ, the zone of rainfall maximum, migrated northwards, increasing precipitation over southern China during the East Asian Summer Monsoon (EASM) while making Indochina drier. During the Late Miocene Global Cooling (LMCG), from 7.9 to 5.8 million years ago,

2870-458: The NECC owes its existence to the fact that the ITCZ is not located at the equator, rather several degrees of latitude to the north. The rapid relative change in the coriolis parameter (a function of latitude) near the equator combined with the ITCZ being located north of the equator leads to similar rapid changes in the surface Ekman transport of the ocean and areas of convergence and divergence in

2952-451: The Pacific were impeded from flowing into the Indian Ocean. It is believed that the resulting increase in sea surface temperatures in the Indian Ocean increased the intensity of monsoons. In 2018, a study of the SAM's variability over the past million years found that precipitation resulting from the monsoon was significantly reduced during glacial periods compared to interglacial periods like

3034-615: The Southwest Monsoon first hits the Western Ghats of the coastal state of Kerala , India, thus making this area the first state in India to receive rain from the Southwest Monsoon. This branch of the monsoon moves northwards along the Western Ghats ( Konkan and Goa ) with precipitation on coastal areas, west of the Western Ghats. The eastern areas of the Western Ghats do not receive much rain from this monsoon as

3116-535: The Southwest Monsoon, receive rain from this Monsoon. About 50% to 60% of the rain received by the state of Tamil Nadu is from the Northeast Monsoon. In Southern Asia, the northeastern monsoons take place from October to December when the surface high-pressure system is strongest. The jet stream in this region splits into the southern subtropical jet and the polar jet. The subtropical flow directs northeasterly winds to blow across southern Asia, creating dry air streams which produce clear skies over India. Meanwhile,

3198-458: The air temperature remains relatively stable for two reasons: water has a relatively high heat capacity (3.9 to 4.2 J g K ), and because both conduction and convection will equilibrate a hot or cold surface with deeper water (up to 50 metres). In contrast, dirt, sand, and rocks have lower heat capacities (0.19 to 0.35 J g K ), and they can only transmit heat into the earth by conduction and not by convection. Therefore, bodies of water stay at

3280-408: The annual Rossby wave. Early each year increased winds in the eastern Pacific generate a region of lower sea level. Over the following months this propagates westward as an oceanic Rossby wave . Its fastest component, near 6°N, reaches the western Pacific around mid-summer. At higher latitudes the wave travels more slowly. As a result in the western Pacific the NECC tends to be weaker than normal in

3362-583: The arrival at the Eastern Himalayas, the winds turns towards the west, travelling over the Indo-Gangetic Plain at a rate of roughly 1–2 weeks per state, pouring rain all along its way. June 1 is regarded as the date of onset of the monsoon in India, as indicated by the arrival of the monsoon in the southernmost state of Kerala. The monsoon accounts for nearly 80% of the rainfall in India. Indian agriculture (which accounts for 25% of

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3444-507: The beginning of June, and again in mid- to late June. The European monsoon is not a monsoon in the traditional sense in that it doesn't meet all the requirements to be classified as such. Instead, the return of the westerlies is more regarded as a conveyor belt that delivers a series of low-pressure centres to Western Europe where they create unsettled weather. These storms generally feature significantly lower-than-average temperatures, fierce rain or hail, thunder, and strong winds. The return of

3526-414: The behaviour of the LC during Quaternary at close stratigraphic intervals. The South American summer monsoon (SASM) is known to have become weakened during Dansgaard–Oeschger events. The SASM has been suggested to have been enhanced during Heinrich events. Monsoons were once considered as a large-scale sea breeze caused by higher temperature over land than in the ocean. This is no longer considered as

3608-426: The biogeochemical processes in the area because it leads to upwelling. Upwelling carries nutrient rich, and cold deep-sea water to the euphotic zone, promoting phytoplankton blooms and kickstarting an extremely high-productive environment. Areas of upwelling lead to the promotion of fisheries, in fact nearly half of the world's fish catch comes from areas of upwelling. Ekman suction occurs both along coastlines and in

3690-420: The boral winter and spring, and stronger than normal in the summer and autumn. The Pacific NECC is known to be stronger during classic El Niño events, when there is an anomalous warming of the eastern and central Pacific that peaks in the boreal winter. Klaus Wyrtki was the first to report the connection, in the early 1970s, based on analysis of tide-gauge measurements at Pacific island stations on either side of

3772-619: The cause and the monsoon is now considered a planetary-scale phenomenon involving the annual migration of the Intertropical Convergence Zone between its northern and southern limits. The limits of the ITCZ vary according to the land–sea heating contrast and it is thought that the northern extent of the monsoon in South Asia is influenced by the high Tibetan Plateau. These temperature imbalances happen because oceans and land absorb heat in different ways. Over oceans,

3854-662: The climax of summer heat in June. However, the roads take a battering every year. Often houses and streets are waterlogged and slums are flooded despite drainage systems. A lack of city infrastructure coupled with changing climate patterns causes severe economic loss including damage to property and loss of lives, as evidenced in the 2005 flooding in Mumbai that brought the city to a standstill. Bangladesh and certain regions of India like Assam and West Bengal , also frequently experience heavy floods during this season. Recently, areas in India that used to receive scanty rainfall throughout

3936-424: The coastline. Due to the Coriolis effect , surface water moves at a 90° angle to the wind current. If the wind moves in a direction causing the water to be pulled away from the coast then Ekman suction will occur. On the other hand, if the wind is moving in such a way that surface waters move towards the shoreline then Ekman pumping will take place. The second mechanism of wind currents resulting in Ekman transfer

4018-503: The coasts as well as in the open ocean. Along the Pacific Coast in the Southern Hemisphere northerly winds move parallel to the coastline. Due to the Coriolis effect the surface water gets pulled 90° to the left of the wind current, therefore causing the water to converge along the coast boundary, leading to Ekman pumping. In the open ocean Ekman pumping occurs with gyres. Specifically, in the subtropics, between 20°N and 50°N, there

4100-399: The colder months, the cycle is reversed. Then the land cools faster than the oceans and the air over the land has higher pressure than air over the ocean. This causes the air over the land to flow to the ocean. When humid air rises over the ocean, it cools, and this causes precipitation over the oceans. (The cool air then flows towards the land to complete the cycle.) Most summer monsoons have

4182-406: The current. On the basis of this analysis, Wyrtki hypothesized that such an unusually strong NECC in the western Pacific would lead to an anomalous accumulation of warm water of the coast of Central America and thus an El Niño. Monsoon A monsoon ( / m ɒ n ˈ s uː n / ) is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation but

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4264-500: The east. While the SECC is geostrophic in nature, the physical mechanism for its appearance is less clear than with the NECC; that is, Sverdrup theory does not obviously explain its existence. Additionally, the seasonal cycle of the SECC is not as defined as that of the NECC. The NECC is a direct response to the meridional changes in the coriolis parameter and the wind stress curl near the Intertropical Convergence Zone (ITCZ). In part

4346-459: The hot summers. This causes a low pressure area over the northern and central Indian subcontinent. To fill this void, the moisture-laden winds from the Indian Ocean rush into the subcontinent. These winds, rich in moisture, are drawn towards the Himalayas . The Himalayas act like a high wall, blocking the winds from passing into Central Asia, and forcing them to rise. As the clouds rise, their temperature drops, and precipitation occurs . Some areas of

4428-548: The left (in the South Hemisphere) of the wind currents, and the surface water diverges along these boundaries, resulting in upwelling in order to conserve mass. Ekman pumping is the component of Ekman transport that results in areas of downwelling due to the convergence of water. As discussed above, the concept of mass conservation requires that a pile up of surface water must be pushed downward. This pile up of warm, nutrient-poor surface water gets pumped vertically down

4510-513: The monsoon is not a simple response to heating but a more complex interaction of topography, wind and sea, as demonstrated by its abrupt rather than gradual withdrawal from the region. The Australian monsoon (the "Wet") occurs in the southern summer when the monsoon trough develops over Northern Australia . Over three-quarters of annual rainfall in Northern Australia falls during this time. The European Monsoon (more commonly known as

4592-576: The north between approximately 8,000 and 4,000 BP, and most recently retreated southward once more between 4,000 and 0 BP. The January ITCZ migrated further south to its present location during the Middle Miocene, strengthening the summer monsoon of Australia that had previously been weaker. Five episodes during the Quaternary at 2.22 Ma ( PL-1), 1.83 Ma (PL-2), 0.68 Ma (PL-3), 0.45 Ma (PL-4) and 0.04 Ma (PL-5) were identified which showed

4674-545: The northern hemisphere and left in the southern hemisphere). This is called the Ekman spiral . The layer of water from the surface to the point of dissipation of this spiral is known as the Ekman layer . If all flow over the Ekman layer is integrated, the net transportation is at 90° to the right (left) of the surface wind in the northern (southern) hemisphere. There are three major wind patterns that lead to Ekman suction or pumping. The first are wind patterns that are parallel to

4756-413: The ocean to the land, bringing moist air inland. This moist air rises to a higher altitude over land and then it flows back toward the ocean (thus completing the cycle). However, when the air rises, and while it is still over the land, the air cools . This decreases the air's ability to hold water , and this causes precipitation over the land. This is why summer monsoons cause so much rain over land. In

4838-482: The oceanic mixed layer . Using the larger Pacific basin as an example, the resulting dynamic height pattern consists of a trough at the equator, and ridge near 5° degrees north, a trough at 10°N, and finally a ridge closer to 20°N. From geostrophy (the perfect balance between the mass field and velocity field), the NECC is located between the ridge and trough at 5°N and 10°N, respectively. Sverdrup theory succinctly summarizes this phenomenon mathematically by defining

4920-490: The open ocean, but also occurs along the equator. Along the Pacific coastline of California, Central America, and Peru, as well as along the Atlantic coastline of Africa there are areas of upwelling due to Ekman suction, as the currents move equatorwards. Due to the Coriolis effect the surface water moves 90° to the left (in the South Hemisphere, as it travels toward the equator) of the wind current, therefore causing

5002-442: The order of 300 km. The Atlantic NECC is unique among the equatorial currents in that basin because of its extreme seasonality. The maximum eastward flow is attained in late boreal summer and fall while the countercurrent is replaced by westward flow in late winter and spring. The NECC has maximum transport of approximately 40 Sv (10^6 m3/s) at 38°W. Transport reaches 30 Sv two months per year at 44°W, while farther east at 38°W

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5084-609: The present day. The Indian Summer Monsoon (ISM) underwent several intensifications during the warming following the Last Glacial Maximum, specifically during the time intervals corresponding to 16,100–14,600 BP, 13,600–13,000 BP, and 12,400–10,400 BP as indicated by vegetation changes in the Tibetan Plateau displaying increases in humidity brought by an intensifying ISM. Though the ISM was relatively weak for much of

5166-510: The region. Examples are the formation of a rare low-latitude tropical storm in 2001, Tropical Storm Vamei , and the devastating flood of Jakarta in 2007. The onset of the monsoon over Australia tends to follow the heating maxima down Vietnam and the Malay Peninsula (September), to Sumatra , Borneo and the Philippines (October), to Java , Sulawesi (November), Irian Jaya and northern Australia (December, January). However,

5248-559: The southwest United States by mid-July. It affects Mexico along the Sierra Madre Occidental as well as Arizona , New Mexico , Nevada , Utah , Colorado , West Texas and California . It pushes as far west as the Peninsular Ranges and Transverse Ranges of Southern California, but rarely reaches the coastal strip (a wall of desert thunderstorms only a half-hour's drive away is a common summer sight from

5330-562: The subcontinent receive up to 10,000 mm (390 in) of rain annually. The southwest monsoon is generally expected to begin around the beginning of June and fade away by the end of September. The moisture-laden winds on reaching the southernmost point of the Indian Peninsula , due to its topography, become divided into two parts: the Arabian Sea Branch and the Bay of Bengal Branch . The Arabian Sea Branch of

5412-546: The summer monsoon shifts through a series of dry and rainy phases as the rain belt moves northward, beginning over Indochina and the South China Sea (May), to the Yangtze River Basin and Japan (June) and finally to northern China and Korea (July). When the monsoon ends in August, the rain belt moves back to southern China. The rainy season occurs from September to February and it is a major source of energy for

5494-690: The sunny skies along the coast during the monsoon). The North American monsoon is known to many as the Summer , Southwest , Mexican or Arizona monsoon. It is also sometimes called the Desert monsoon as a large part of the affected area are the Mojave and Sonoran deserts . However, it is controversial whether the North and South American weather patterns with incomplete wind reversal should be counted as true monsoons. The Asian monsoons may be classified into

5576-402: The surface wind. The direction of transport is dependent on the hemisphere: in the northern hemisphere , transport occurs at 90° clockwise from wind direction, while in the southern hemisphere it occurs at 90° anticlockwise. This phenomenon was first noted by Fridtjof Nansen , who recorded that ice transport appeared to occur at an angle to the wind direction during his Arctic expedition of

5658-415: The theoretical state of circulation if water currents were driven only by the transfer of momentum from the wind. In the physical world, this is difficult to observe because of the influences of many simultaneous current driving forces (for example, pressure and density gradients ). Though the following theory technically applies to the idealized situation involving only wind forces, Ekman motion describes

5740-442: The transport reaches that level five months per year. The magnitude of the NECC weakens substantially east of 38°W due to water being absorbed by the westward equatorial current south of 3°N. While the variability of the Atlantic NECC is dominated by the annual cycle (weak late winter, strong late summer), there is also interannual variability as well. The strength of the Atlantic NECC is notably stronger in years following El Niño in

5822-408: The tropical Pacific, with 1983 and 1987 being notable examples. Physically, this implies that the altered convection in the Pacific Ocean due to El Niño drives changes in the meridional gradient of wind stress curl over the equatorial Atlantic. The Pacific NECC is a major eastward moving surface current that transports more than 20 Sv from the West Pacific warm pool to the cooler east Pacific. In

5904-713: The warm Tsushima Current into the Sea of Japan. Circa 3.0 million years ago, the EAWM became more stable, having previously been more variable and inconsistent, in addition to being enhanced further amidst a period of global cooling and sea level fall. The EASM was weaker during cold intervals of glacial periods such as the Last Glacial Maximum (LGM) and stronger during interglacials and warm intervals of glacial periods. Another EAWM intensification event occurred 2.6 million years ago, followed by yet another one around 1.0 million years ago. During Dansgaard–Oeschger events ,

5986-453: The water column, resulting in areas of downwelling. Ekman pumping has dramatic impacts on the surrounding environments. Downwelling, due to Ekman pumping, leads to nutrient poor waters, therefore reducing the biological productivity of the area. Additionally, it transports heat and dissolved oxygen vertically down the water column as warm oxygen rich surface water is being pumped towards the deep ocean water. Ekman pumping can be found along

6068-740: The water to diverge from the coast boundary, leading to Ekman suction. Additionally, there are areas of upwelling as a consequence of Ekman suction where the Polar Easterlies winds meet the Westerlies in the subpolar regions north of the subtropics, as well as where the Northeast Trade Winds meet the Southeast Trade Winds along the Equator. Similarly, due to the Coriolis effect the surface water moves 90° to

6150-459: The westerlies affects Europe's Northern Atlantic coastline, more precisely Ireland, Great Britain, the Benelux countries , western Germany, northern France and parts of Scandinavia. Ekman transport#Ekman suction Ekman transport is part of Ekman motion theory, first investigated in 1902 by Vagn Walfrid Ekman . Winds are the main source of energy for ocean circulation, and Ekman transport

6232-478: The western Pacific the countercurrent is centred near 5°N while in the central Pacific it is located near 7°N. At the surface, the current is located on the southern slope of the North Equatorial Trough, a region of low sea level which extends from east to west across the Pacific. The low sea level is a result of Ekman suction caused by the increased easterly winds found just to the north of

6314-656: The wind does not cross the Western Ghats. The Bay of Bengal Branch of Southwest Monsoon flows over the Bay of Bengal heading towards north-east India and Bengal , picking up more moisture from the Bay of Bengal. The winds arrive at the Eastern Himalayas with large amounts of rain. Mawsynram , situated on the southern slopes of the Khasi Hills in Meghalaya , India, is one of the wettest places on Earth. After

6396-446: The wind-driven portion of circulation seen in the surface layer. Surface currents flow at a 45° angle to the wind due to a balance between the Coriolis force and the drags generated by the wind and the water. If the ocean is divided vertically into thin layers, the magnitude of the velocity (the speed) decreases from a maximum at the surface until it dissipates. The direction also shifts slightly across each subsequent layer (right in

6478-822: The world consist of the West African , Asian– Australian , the North American , and South American monsoons. The term was first used in English in British India and neighboring countries to refer to the big seasonal winds blowing from the Bay of Bengal and Arabian Sea in the southwest bringing heavy rainfall to the area. The etymology of the word monsoon is not wholly certain. The English monsoon came from Portuguese monção ultimately from Arabic موسم ( mawsim , "season"), "perhaps partly via early modern Dutch monson ". Strengthening of

6560-536: The year, like the Thar Desert , have surprisingly ended up receiving floods due to the prolonged monsoon season. The influence of the Southwest Monsoon is felt as far north as in China's Xinjiang . It is estimated that about 70% of all precipitation in the central part of the Tian Shan Mountains falls during the three summer months, when the region is under the monsoon influence; about 70% of that

6642-429: The zonal and meridional mass transport terms with units of mass per unit time per unit length. Contrarily to common logic, north–south winds cause mass transport in the east–west direction. In order to understand the vertical velocity structure of the water column, equations 1 and 2 can be rewritten in terms of the vertical eddy viscosity term. where A z {\displaystyle A_{z}\,\!}

6724-586: Was associated with an expansion of temperate deciduous forest steppe and temperate mixed forest steppe in northern China. By around 5,000 to 4,500 BP, the East Asian monsoon's strength began to wane, weakening from that point until the present day. A particularly notable weakening took place ~3,000 BP. The location of the EASM shifted multiple times over the course of the Holocene: first, it moved southward between 12,000 and 8,000 BP, followed by an expansion to

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