A sine wave or sinusoid is a mathematical curve that describes a smooth periodic oscillation. A sine wave is a continuous wave, it is named after the function sine. It occurs in pure and applied mathematics, as well as physics, signal processing and many other fields, its most basic form as a function of time is: y = A sin = A sin where: A, the peak deviation of the function from zero. F, ordinary frequency, the number of oscillations that occur each second of time. Ω = 2πf, angular frequency, the rate of change of the function argument in units of radians per second φ, specifies where in its cycle the oscillation is at t = 0. When φ is non-zero, the entire waveform appears to be shifted in time by the amount φ /ω seconds. A negative value represents a delay, a positive value represents an advance; the sine wave is important in physics because it retains its wave shape when added to another sine wave of the same frequency and arbitrary phase and magnitude. It is the only periodic waveform; this property makes it acoustically unique.
In general, the function may have: a spatial variable x that represents the position on the dimension on which the wave propagates, a characteristic parameter k called wave number, which represents the proportionality between the angular frequency ω and the linear speed ν. The wavenumber is related to the angular frequency by:. K = ω v = 2 π f v = 2 π λ where λ is the wavelength, f is the frequency, v is the linear speed; this equation gives a sine wave for a single dimension. This could, for example, be considered the value of a wave along a wire. In two or three spatial dimensions, the same equation describes a travelling plane wave if position x and wavenumber k are interpreted as vectors, their product as a dot product. For more complex waves such as the height of a water wave in a pond after a stone has been dropped in, more complex equations are needed; this wave pattern occurs in nature, including wind waves, sound waves, light waves. A cosine wave is said to be sinusoidal, because cos = sin , a sine wave with a phase-shift of π/2 radians.
Because of this head start, it is said that the cosine function leads the sine function or the sine lags the cosine. The human ear can recognize single sine waves as sounding clear because sine waves are representations of a single frequency with no harmonics. To the human ear, a sound, made of more than one sine wave will have perceptible harmonics. Presence of higher harmonics in addition to the fundamental causes variation in the timbre, the reason why the same musical note played on different instruments sounds different. On the other hand, if the sound contains aperiodic waves along with sine waves the sound will be perceived to be noisy, as noise is characterized as being aperiodic or having a non-repetitive pattern. In 1822, French mathematician Joseph Fourier discovered that sinusoidal waves can be used as simple building blocks to describe and approximate any periodic waveform, including square waves. Fourier used it as an analytical tool in the study of waves and heat flow, it is used in signal processing and the statistical analysis of time series.
Since sine waves propagate without changing form in distributed linear systems, they are used to analyze wave propagation. Sine waves traveling in two directions in space can be represented as u = A sin When two waves having the same amplitude and frequency, traveling in opposite directions, superpose each other a standing wave pattern is created. Note that, on a plucked string, the interfering waves are the waves reflected from the fixed end
Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that can be released to fuel the organisms' activities. This chemical energy is stored in carbohydrate molecules, such as sugars, which are synthesized from carbon dioxide and water – hence the name photosynthesis, from the Greek φῶς, phōs, "light", σύνθεσις, synthesis, "putting together". In most cases, oxygen is released as a waste product. Most plants, most algae, cyanobacteria perform photosynthesis. Photosynthesis is responsible for producing and maintaining the oxygen content of the Earth's atmosphere, supplies all of the organic compounds and most of the energy necessary for life on Earth. Although photosynthesis is performed differently by different species, the process always begins when energy from light is absorbed by proteins called reaction centres that contain green chlorophyll pigments. In plants, these proteins are held inside organelles called chloroplasts, which are most abundant in leaf cells, while in bacteria they are embedded in the plasma membrane.
In these light-dependent reactions, some energy is used to strip electrons from suitable substances, such as water, producing oxygen gas. The hydrogen freed by the splitting of water is used in the creation of two further compounds that serve as short-term stores of energy, enabling its transfer to drive other reactions: these compounds are reduced nicotinamide adenine dinucleotide phosphate and adenosine triphosphate, the "energy currency" of cells. In plants and cyanobacteria, long-term energy storage in the form of sugars is produced by a subsequent sequence of light-independent reactions called the Calvin cycle. In the Calvin cycle, atmospheric carbon dioxide is incorporated into existing organic carbon compounds, such as ribulose bisphosphate. Using the ATP and NADPH produced by the light-dependent reactions, the resulting compounds are reduced and removed to form further carbohydrates, such as glucose; the first photosynthetic organisms evolved early in the evolutionary history of life and most used reducing agents such as hydrogen or hydrogen sulfide, rather than water, as sources of electrons.
Cyanobacteria appeared later. Today, the average rate of energy capture by photosynthesis globally is 130 terawatts, about eight times the current power consumption of human civilization. Photosynthetic organisms convert around 100–115 billion tonnes of carbon into biomass per year. Photosynthetic organisms are photoautotrophs, which means that they are able to synthesize food directly from carbon dioxide and water using energy from light. However, not all organisms use carbon dioxide as a source of carbon atoms to carry out photosynthesis. In plants and cyanobacteria, photosynthesis releases oxygen; this is called oxygenic photosynthesis and is by far the most common type of photosynthesis used by living organisms. Although there are some differences between oxygenic photosynthesis in plants and cyanobacteria, the overall process is quite similar in these organisms. There are many varieties of anoxygenic photosynthesis, used by certain types of bacteria, which consume carbon dioxide but do not release oxygen.
Carbon dioxide is converted into sugars in a process called carbon fixation. Carbon fixation is an endothermic redox reaction. In general outline, photosynthesis is the opposite of cellular respiration: while photosynthesis is a process of reduction of carbon dioxide to carbohydrate, cellular respiration is the oxidation of carbohydrate or other nutrients to carbon dioxide. Nutrients used in cellular respiration include amino acids and fatty acids; these nutrients are oxidized to produce carbon dioxide and water, to release chemical energy to drive the organism's metabolism. Photosynthesis and cellular respiration are distinct processes, as they take place through different sequences of chemical reactions and in different cellular compartments; the general equation for photosynthesis as first proposed by Cornelis van Niel is therefore: CO2carbondioxide + 2H2Aelectron donor + photonslight energy → carbohydrate + 2Aoxidizedelectrondonor + H2OwaterSince water is used as the electron donor in oxygenic photosynthesis, the equation for this process is: CO2carbondioxide + 2H2Owater + photonslight energy → carbohydrate + O2oxygen + H2OwaterThis equation emphasizes that water is both a reactant in the light-dependent reaction and a product of the light-independent reaction, but canceling n water molecules from each side gives the net equation: CO2carbondioxide + H2O water + photonslight energy → carbohydrate + O2 oxygen Other processes substitute other compounds for water in the electron-supply role.
In the first stage, light-dependent reactions or light reactions capture the energy of light and use it to make the energy-storage molecules ATP and NADPH. During the second stage, the light-independent reactions use these products to capture and reduce carbon dioxid
An ice age is a long period of reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers. Earth is in the Quaternary glaciation, known in popular terminology as the Ice Age. Individual pulses of cold climate are termed "glacial periods", intermittent warm periods are called "interglacials", with both climatic pulses part of the Quaternary or other periods in Earth's history. In the terminology of glaciology, ice age implies the presence of extensive ice sheets in both northern and southern hemispheres. By this definition, we are in an interglacial period—the Holocene; the amount of heat trapping gases emitted into Earth's Oceans and atmosphere will prevent the next ice age, which otherwise would begin in around 50,000 years, more glacial cycles. In 1742, Pierre Martel, an engineer and geographer living in Geneva, visited the valley of Chamonix in the Alps of Savoy. Two years he published an account of his journey.
He reported that the inhabitants of that valley attributed the dispersal of erratic boulders to the glaciers, saying that they had once extended much farther. Similar explanations were reported from other regions of the Alps. In 1815 the carpenter and chamois hunter Jean-Pierre Perraudin explained erratic boulders in the Val de Bagnes in the Swiss canton of Valais as being due to glaciers extending further. An unknown woodcutter from Meiringen in the Bernese Oberland advocated a similar idea in a discussion with the Swiss-German geologist Jean de Charpentier in 1834. Comparable explanations are known from the Val de Ferret in the Valais and the Seeland in western Switzerland and in Goethe's scientific work; such explanations could be found in other parts of the world. When the Bavarian naturalist Ernst von Bibra visited the Chilean Andes in 1849–1850, the natives attributed fossil moraines to the former action of glaciers. Meanwhile, European scholars had begun to wonder. From the middle of the 18th century, some discussed ice as a means of transport.
The Swedish mining expert Daniel Tilas was, in 1742, the first person to suggest drifting sea ice in order to explain the presence of erratic boulders in the Scandinavian and Baltic regions. In 1795, the Scottish philosopher and gentleman naturalist, James Hutton, explained erratic boulders in the Alps by the action of glaciers. Two decades in 1818, the Swedish botanist Göran Wahlenberg published his theory of a glaciation of the Scandinavian peninsula, he regarded glaciation as a regional phenomenon. Only a few years the Danish-Norwegian geologist Jens Esmark argued a sequence of worldwide ice ages. In a paper published in 1824, Esmark proposed changes in climate as the cause of those glaciations, he attempted to show. During the following years, Esmark's ideas were discussed and taken over in parts by Swedish and German scientists. At the University of Edinburgh Robert Jameson seemed to be open to Esmark's ideas, as reviewed by Norwegian professor of glaciology Bjørn G. Andersen. Jameson's remarks about ancient glaciers in Scotland were most prompted by Esmark.
In Germany, Albrecht Reinhard Bernhardi, a geologist and professor of forestry at an academy in Dreissigacker, since incorporated in the southern Thuringian city of Meiningen, adopted Esmark's theory. In a paper published in 1832, Bernhardi speculated about former polar ice caps reaching as far as the temperate zones of the globe. In 1829, independently of these debates, the Swiss civil engineer Ignaz Venetz explained the dispersal of erratic boulders in the Alps, the nearby Jura Mountains, the North German Plain as being due to huge glaciers; when he read his paper before the Schweizerische Naturforschende Gesellschaft, most scientists remained sceptical. Venetz convinced his friend Jean de Charpentier. De Charpentier transformed Venetz's idea into a theory with a glaciation limited to the Alps, his thoughts resembled Wahlenberg's theory. In fact, both men shared the same volcanistic, or in de Charpentier's case rather plutonistic assumptions, about the Earth's history. In 1834, de Charpentier presented his paper before the Schweizerische Naturforschende Gesellschaft.
In the meantime, the German botanist Karl Friedrich Schimper was studying mosses which were growing on erratic boulders in the alpine upland of Bavaria. He began to wonder. During the summer of 1835 he made some excursions to the Bavarian Alps. Schimper came to the conclusion that ice must have been the means of transport for the boulders in the alpine upland. In the winter of 1835 to 1836 he held. Schimper assumed that there must have been global times of obliteration with a cold climate and frozen water. Schimper spent the summer months of 1836 at Devens, near Bex, in the Swiss Alps with his former university friend Louis Agassiz and Jean de Charpentier. Schimper, de Charpentier and Venetz convinced Agassiz that there had been a time of glaciation. During the winter of 1836/37, Agassiz and Schimper developed the theory of a sequence of glaciations, they drew upon the preceding works of Venetz, de Charpentier and on their own fieldwork. Agassiz appears to have been familiar with Bernhardi's paper at that time.
At the beginning of 1837, Schimper coined the term "ice age" for the period of the glaciers. In July 1837 Ag
In astronomy, axial tilt known as obliquity, is the angle between an object's rotational axis and its orbital axis, or, the angle between its equatorial plane and orbital plane. It differs from orbital inclination. At an obliquity of 0 degrees, the two axes point in the same direction. Earth's obliquity oscillates between 24.5 degrees on a 41,000-year cycle. Over the course of an orbital period, the obliquity does not change and the orientation of the axis remains the same relative to the background of stars; this causes one pole to be directed more toward the Sun on one side of the orbit, the other pole on the other side—the cause of the seasons on Earth. There are two standard methods of specifying tilt; the International Astronomical Union defines the north pole of a planet as that which lies on Earth's north side of the invariable plane of the Solar System. The IAU uses the right-hand rule to define a positive pole for the purpose of determining orientation. Using this convention, Venus is tilted 177°.
Earth's orbital plane is known as the ecliptic plane, Earth's tilt is known to astronomers as the obliquity of the ecliptic, being the angle between the ecliptic and the celestial equator on the celestial sphere. It is denoted by the Greek letter ε. Earth has an axial tilt of about 23.4°. This value remains about the same relative to a stationary orbital plane throughout the cycles of axial precession, but the ecliptic moves due to planetary perturbations, the obliquity of the ecliptic is not a fixed quantity. At present, it is decreasing at a rate of about 47″ per century. Earth's obliquity may have been reasonably measured as early as 1100 BC in India and China; the ancient Greeks had good measurements of the obliquity since about 350 BC, when Pytheas of Marseilles measured the shadow of a gnomon at the summer solstice. About 830 AD, the Caliph Al-Mamun of Baghdad directed his astronomers to measure the obliquity, the result was used in the Arab world for many years. In 1437, Ulugh Beg determined the Earth's axial tilt as 23°30′17″.
It was believed, during the Middle Ages, that both precession and Earth's obliquity oscillated around a mean value, with a period of 672 years, an idea known as trepidation of the equinoxes. The first to realize this was incorrect was Ibn al-Shatir in the fourteenth century and the first to realize that the obliquity is decreasing at a constant rate was Fracastoro in 1538; the first accurate, western observations of the obliquity were those of Tycho Brahe from Denmark, about 1584, although observations by several others, including al-Ma'mun, al-Tusi, Purbach and Walther, could have provided similar information. Earth's axis remains tilted in the same direction with reference to the background stars throughout a year; this means that one pole will be directed away from the Sun at one side of the orbit, half an orbit this pole will be directed towards the Sun. This is the cause of Earth's seasons. Summer occurs in the Northern hemisphere. Variations in Earth's axial tilt can influence the seasons and is a factor in long-term climate change.
The exact angular value of the obliquity is found by observation of the motions of Earth and planets over many years. Astronomers produce new fundamental ephemerides as the accuracy of observation improves and as the understanding of the dynamics increases, from these ephemerides various astronomical values, including the obliquity, are derived. Annual almanacs are published listing the methods of use; until 1983, the Astronomical Almanac's angular value of the mean obliquity for any date was calculated based on the work of Newcomb, who analyzed positions of the planets until about 1895: ε = 23° 27′ 8.26″ − 46.845″ T − 0.0059″ T2 + 0.00181″ T3where ε is the obliquity and T is tropical centuries from B1900.0 to the date in question. From 1984, the Jet Propulsion Laboratory's DE series of computer-generated ephemerides took over as the fundamental ephemeris of the Astronomical Almanac. Obliquity based on DE200, which analyzed observations from 1911 to 1979, was calculated: ε = 23° 26′ 21.448″ − 46.8150″ T − 0.00059″ T2 + 0.001813″ T3where hereafter T is Julian centuries from J2000.0.
JPL's fundamental ephemerides have been continually updated. For instance, the Astronomical Almanac for 2010 specifies: ε = 23° 26′ 21.406″ − 46.836769″ T − 0.0001831″ T2 + 0.00200340″ T3 − 5.76″ × 10−7 T4 − 4.34″ × 10−8 T5These expressions for the obliquity are intended for high precision over a short time span ± several centuries. J. Laskar computed an expression to order T10 good to 0.02″ over 1000 years and several arcseconds over 10,000 years. Ε = 23° 26′ 21.448″ − 4680.93″ t − 1.55″ t2 + 1999.25″ t3 − 51.38″ t4 − 249.67″ t5 − 39.05″ t6 + 7.12″ t7 + 27.87″ t8 + 5.79″ t9 + 2.45″ t10where here t is multiples of 10,000 Julian years from J2000.0. These expressions are for the so-called mean obliquity, that is, the obliquity free from short-term variations. Periodic motions of the Moon and of Earth in its orbit cause much smaller short-period oscillations of the rotation axis of Earth, known as nutation, which add a periodic component to Earth's obliquity; the true or instant
El Niño–Southern Oscillation
El Niño–Southern Oscillation is an irregularly periodic variation in winds and sea surface temperatures over the tropical eastern Pacific Ocean, affecting the climate of much of the tropics and subtropics. The warming phase of the sea temperature is known as the cooling phase as La Niña; the Southern Oscillation is the accompanying atmospheric component, coupled with the sea temperature change: El Niño is accompanied by high air surface pressure in the tropical western Pacific and La Niña with low air surface pressure there. The two periods last several months each and their effects vary in intensity; the two phases relate to the Walker circulation, discovered by Gilbert Walker during the early twentieth century. The Walker circulation is caused by the pressure gradient force that results from a high pressure system over the eastern Pacific Ocean, a low pressure system over Indonesia; when the Walker circulation weakens or reverses, an El Niño results, causing the ocean surface to be warmer than average, as upwelling of cold water occurs less or not at all.
An strong Walker circulation causes a La Niña, resulting in cooler ocean temperatures due to increased upwelling. Mechanisms that cause the oscillation remain under study; the extremes of this climate pattern's oscillations cause extreme weather in many regions of the world. Developing countries dependent upon agriculture and fishing those bordering the Pacific Ocean, are the most affected; the El Niño–Southern Oscillation is a single climate phenomenon that periodically fluctuates between three phases: Neutral, La Niña or El Niño. La Niña and El Niño are opposite phases that require certain changes to take place in both the ocean and the atmosphere, before an event is declared; the northward flowing Humboldt Current brings cold water from the Southern Ocean northwards along South America's west coast to the tropics, where it is enhanced by up-welling taking place along the coast of Peru. Along the equator trade winds cause the ocean currents in the eastern Pacific to draw water from the deeper ocean towards the surface, helping to keep the surface cool.
Under the influence of the equatorial trade winds, this cold water flows westwards along the equator where it is heated by the sun. As a direct result sea surface temperatures in the western Pacific are warmer, by about 8–10 °C than those in the Eastern Pacific; this warmer area of ocean is associated with cloudiness and rainfall. During El Niño years the cold water weakens or disappears as the water in the Central and Eastern Pacific becomes as warm as the Western Pacific; the Walker circulation is caused by the pressure gradient force that results from a high pressure system over the eastern Pacific Ocean, a low pressure system over Indonesia. The Walker circulations of the tropical Indian and Atlantic basins result in westerly surface winds in northern summer in the first basin and easterly winds in the second and third basins; as a result, the temperature structure of the three oceans display dramatic asymmetries. The equatorial Pacific and Atlantic both have cool surface temperatures in northern summer in the east, while cooler surface temperatures prevail only in the western Indian Ocean.
These changes in surface temperature reflect changes in the depth of the thermocline. Changes in the Walker circulation with time occur in conjunction with changes in surface temperature; some of these changes are forced externally, such as the seasonal shift of the sun into the Northern Hemisphere in summer. Other changes appear to be the result of coupled ocean-atmosphere feedback in which, for example, easterly winds cause the sea surface temperature to fall in the east, enhancing the zonal heat contrast and hence intensifying easterly winds across the basin; these anomalous easterlies induce more equatorial upwelling and raise the thermocline in the east, amplifying the initial cooling by the southerlies. This coupled ocean-atmosphere feedback was proposed by Bjerknes. From an oceanographic point of view, the equatorial cold tongue is caused by easterly winds. Were the Earth climate symmetric about the equator, cross-equatorial wind would vanish, the cold tongue would be much weaker and have a different zonal structure than is observed today.
During non-El Niño conditions, the Walker circulation is seen at the surface as easterly trade winds that move water and air warmed by the sun toward the west. This creates ocean upwelling off the coasts of Peru and Ecuador and brings nutrient-rich cold water to the surface, increasing fishing stocks; the western side of the equatorial Pacific is characterized by warm, low-pressure weather as the collected moisture is dumped in the form of typhoons and thunderstorms. The ocean is some 60 cm higher in the western Pacific as the result of this motion. Within the National Oceanic and Atmospheric Administration in the United States, sea surface temperatures in the Niño 3.4 region, which stretches from the 120th to 170th meridians west longitude astride the equator five degrees of latitude on either side, are monitored. This region is 3,000 kilometres to the southeast of Hawaii; the most recent three-month average for the area is computed, if the region is more than 0.5 °C above normal for that period an El Niño is considered in progress.
The United Kingdom's Met Office uses a several month period to determine ENSO state. When this warming or cooling occurs for only seven to nine months, it is classified as El Niño/La Niña "conditions".
Indian Ocean Dipole
The Indian Ocean Dipole known as the Indian Niño, is an irregular oscillation of sea-surface temperatures in which the western Indian Ocean becomes alternately warmer and colder than the eastern part of the ocean. Monsoon in India is affected by the temperature between bay of Bengal in the east and The Arabian sea in the west; the IOD involves an aperiodic oscillation of sea-surface temperatures, between "positive", "neutral" and "negative" phases. A positive phase sees greater-than-average sea-surface temperatures and greater precipitation in the western Indian Ocean region, with a corresponding cooling of waters in the eastern Indian Ocean—which tends to cause droughts in adjacent land areas of Indonesia and Australia; the negative phase of the IOD brings about the opposite conditions, with warmer water and greater precipitation in the eastern Indian Ocean, cooler and drier conditions in the west. The IOD affects the strength of monsoons over the Indian subcontinent. A significant positive IOD occurred in 1997–98, with another in 2006.
The IOD is one aspect of the general cycle of global climate, interacting with similar phenomena like the El Niño-Southern Oscillation in the Pacific Ocean. The IOD phenomenon was first identified by climate researchers in 1999. An average of four each positive-negative IOD events occur during each 30-year period with each event lasting around six months. However, there have been 12 positive IODs since 1980 and no negative events from 1992 until a strong negative event in late 2010; the occurrence of consecutive positive IOD events is rare with only two such events recorded, 1913–1914 and the three consecutive events from 2006 to 2008 which preceded the Black Saturday bushfires. Modelling suggests that consecutive positive events could be expected to occur twice over a 1,000-year period; the positive IOD in 2007 evolved together with La Niña, a rare phenomenon that has happened only once in the available historical records. A strong negative IOD developed in October 2010, coupled with a strong and concurrent La Niña, caused the 2010–2011 Queensland floods and the 2011 Victorian floods.
In 2008, Nerilie Abram used coral records from the eastern and western Indian Ocean to construct a coral Dipole Mode Index extending back to 1846 AD. This extended perspective on IOD behaviour suggested that positive IOD events increased in strength and frequency during the 20th century. A 2009 study by Ummenhofer et al. at the University of New South Wales Climate Change Research Centre has demonstrated a significant correlation between the IOD and drought in the southern half of Australia, in particular the south-east. Every major southern drought since 1889 has coincided with positive-neutral IOD fluctuations including the 1895–1902, 1937–1945 and the 1995–2009 droughts; the research shows that when the IOD is in its negative phase, with cool Indian Ocean water west of Australia and warm Timor Sea water to the north, winds are generated that pick up moisture from the ocean and sweep down towards southern Australia to deliver higher rainfall. In the IOD-positive phase, the pattern of ocean temperatures is reversed, weakening the winds and reducing the amount of moisture picked up and transported across Australia.
The consequence is that rainfall in the south-east is well below average during periods of a positive IOD. The study shows that the IOD has a much more significant effect on the rainfall patterns in south-east Australia than the El Niño-Southern Oscillation in the Pacific Ocean as shown in several recent studies. A 2018 study by Hameed et al. at the University of Aizu simulated the impact of a positive IOD event on Pacific surface wind and SST variations. They show that IOD-induced surface wind anomalies can produce El Nino like SST anomalies, with the IOD's impact on SST being the strongest in the far-eastern Pacific, they further demonstrated. Arctic dipole anomaly Subtropical Indian Ocean Dipole Monsoon Abram, Nerilie J.. "Seasonal characteristics of the Indian Ocean dipole during the Holocene epoch". Nature. 445: 299–302. Bibcode:2007Natur.445..299A. Doi:10.1038/nature05477. PMID 17230187. Ashok, Karumuri. "Impact of the Indian Ocean Dipole on the Relationship between the Indian Monsoon Rainfall and ENSO".
Geophysical Research Letters. 28: 4499–4502. Bibcode:2001GeoRL..28.4499A. Doi:10.1029/2001GL013294. Li, Tim. "A Theory for the Indian Ocean Dipole–Zonal Mode". Journal of the Atmospheric Sciences. 60: 2119–35. Bibcode:2003JAtS...60.2119L. Doi:10.1175/1520-0469060<2119:ATFTIO>2.0. CO. Rao, S. A.. "Interannual variability in the subsurface Indian Ocean with special emphasis on the Indian Ocean Dipole". Deep-Sea Research Part II. 49: 1549–72. Bibcode:2002DSR....49.1549R. Doi:10.1016/S0967-064500158-8. Saji, N. H.. "A dipole mode in the tropical Indian Ocean". Nature. 401: 360–3. Doi:10.1038/43854. PMID 16862108. Behera, S. K.. "Unusual IOD event of 2007". Geophysical Research Letters. 35: L14S11. Bibcode:2008GeoRL..3514S11B. Doi:10.1029/2008GL034122. IOD home page IOD, ENSO. Indian Ocean causes Big Dry: drought mystery solved. Animation of Indian Ocean Dipole in Victoria, Australia
Monsoon is traditionally defined as a seasonal reversing wind accompanied by corresponding changes in precipitation, but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with the asymmetric heating of land and sea. The term monsoon is used to refer to the rainy phase of a seasonally changing pattern, although technically there is a dry phase; the term is sometimes incorrectly used for locally heavy but short-term rains, although these rains meet the dictionary definition of monsoon. The major monsoon systems of the world consist of the West Asia-Australian monsoons; the inclusion of the North and South American monsoons with incomplete wind reversal has been debated. The term was first used in English in British India and neighbouring 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 English monsoon came from Portuguese monção from Arabic mawsim, "perhaps via early modern Dutch monson."
Strengthening of the Asian monsoon has been linked to the uplift of the Tibetan Plateau after the collision of the Indian sub-continent 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 studies of plant fossils in China and new long-duration sediment records from 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 in strength since this time linked to global climate change the cycle of the Pleistocene ice ages. A study of marine plankton suggested that the Indian Monsoon strengthened around 5 million years ago. During ice periods, the sea level fell and the Indonesian Seaway closed; when this happened, cold waters in 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. Five episodes during the Quaternary at 2.22 Ma, 1.83 Ma, 0.68 Ma, 0.45 Ma and 0.04 Ma were identified which showed a weakening of Leeuwin Current. The weakening of the LC would have an effect on the sea surface temperature field in the Indian Ocean, as the Indonesian through flow warms the Indian Ocean, thus these five intervals could be those of considerable lowering of SST in the Indian Ocean and would have influenced Indian monsoon intensity. During the weak LC, there 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 through flow, thus a better understanding of the possible links between El Niño, Western Pacific Warm Pool, Indonesian Throughflow, wind pattern off western Australia, ice volume expansion and contraction can be obtained by studying the behaviour of the LC during Quaternary at close stratigraphic intervals.
The impact of monsoon on the local weather is different from place to place. In some places there is just a likelihood of having a little less rain. In other places, quasi semi-deserts are turned into vivid green grasslands where all sorts of plants and crops can flourish; the Indian Monsoon turns large parts of India from a kind of semi-desert into green lands. See photos only taken 3 months apart in the Western Ghats. In places like this it is crucial for farmers to have the right timing for putting the seeds on the fields, as it is essential to use all the rain, available for growing crops. Monsoons are large-scale sea breezes which occur when the temperature on land is warmer or cooler than the temperature of the ocean; these temperature imbalances happen. Over oceans, the air temperature remains stable for two reasons: water has a high heat capacity, because both conduction and convection will equilibrate a hot or cold surface with deeper water. In contrast, dirt and rocks have lower heat capacities, they can only transmit heat into the earth by conduction and not by convection.
Therefore, bodies of water stay at a more temperature, while land temperature 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, the air above it retains a higher pressure; this difference in pressure causes sea breezes to blow from the ocean to the land, bringing moist air inland. This moist air rises to a higher altitude over land and it flows back toward the ocean. However, when the air rises, while it is still over the land, the air cools; this decreases the air's ability to hold water, this causes precipitation over the land. This is. In the colder months, the cycle is reversed; 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, this causes precipitation over the oceans.
(The cool air flows towards the land to complete the cy