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Download Download Monsoons: an introduc^on... Monsoons: an introduc.on Josephine Brown - Bureau of Meteorology, Research & Development With material from Ma. Wheeler Main reference: Introduc.on to Tropical Meteorology, Chapter 3.5 Outline 1. The global monsoon system & physical mechanisms 2. Regional monsoons – South Asian & East Asian – West African, American – Australian-MariDme ConDnent 3. The Australian monsoon: a more detailed view 4. Variability of the monsoon – Interannual variability: TBO, ENSO and IOD – Intraseasonal variability: bursts & breaks, MJO 5. Monsoons and climate change 1. The global monsoon system & physical mechanisms Early observa.ons of the monsoon § The word “monsoon” comes from the Arabic word “mausim” meaning season. It was first used to refer to the seasonal reversal of winds over Southern Asia and the Indian Ocean. § Halley (1686) proposed that the monsoon winds were driven by differenDal heaDng over land and ocean. § Hadley (1735) suggested that the monsoon winds were deflected by the Earth’s rotaDon, leading to their observed direcDons. Map of the trade winds, Edmond Halley, 1686. Observed austral summer circula.on JANUARY Figure 3.14(a)§ In January, westerly winds extend over northern Australia. North-east trade winds flow from South Asia towards the equatorial Indian Ocean. § Heat lows develop over northern Australia, helping to drive onshore flow. § The ITCZ is displaced southwards. Observed austral winter circula.on JULY Figure 3.14(b)§ In July, there are south-westerly winds towards India, and south-easterly trades towards northern Australia. § The NH conDnents warm, generaDng low surface pressures,

WEP SST bias (K) WEP SST change (K) Warmer in western Eq. Pac.DrierWetter§ Australian summer monsoon rainfall change is negaDvely correlated with the amount of SST warming in the Western Equatorial Pacific (WEP). § Monsoon rainfall change is also weakly correlated with the model bias or error in the SSTs in this same region, so models with a larger cold bias tend to project larger decreases in monsoon rainfall (i.e. models that show large drying are less reliable) Brown et al. (2016), Journal of Climate Further reading General references: § “IntroducDon to Tropical Meteorology”, Chapter 3, SecDon 5 and many references given at the end of Chapter. § Webster, P., 1987: The elementary monsoon. Monsoons, J. S. Fein and P. L. Stephens, Eds., John Wiley & Sons, 3-32. The Asian Monsoon: § “The Asian Monsoon”, Bin Wang, Springer, 2006. The Australian Monsoon: § “IntroducDon to Tropical Meteorology”, Chapter 9, Focus SecDon 1. Monsoons and Climate Change: § IPCC Fiah Assessment Report, “Climate Change 2013: The Physical Science Basis”, Chapter 14, SecDon 2. § “The Monsoons and Climate Change: ObservaDons and Modeling”, Editors: Carvalho and Jones, Springer, 2016. Indices of monsoon variability A (large) number of indices have been proposed to measure regional monsoon intensity: § Webster and Yang index (WYI) – broad-scale South Asian summer monsoon: verDcal shear of zonal wind anomalies between 850mb and 200mb, (40-110°E, 0-20°N), JJA; § Monsoon Hadley index (MH) – South Asian summer monsoon: verDcal shear of meridional wind anomalies between 850mb and 200mb, (70-110°E,

Pacific monsoon is a sub-system of the East Asian monsoon. Figure 3.34(c) Asian Summer Monsoon § The South Asian and East Asian monsoons differ due to different land-ocean distribuDon and topography. § The South Asian (Indian) monsoon is strongly forced by north-south temperature gradient and enhanced heaDng over the Tibetan Plateau § The East Asian monsoon is forced by both meridional and zonal temperature gradients, and hence is weaker, with addiDonal rainfall along subtropical frontal zones. Figure 3.37 Asian Winter Monsoon § The winter monsoon is stronger over East Asia than over the Indian SubconDnent because of the contrast between the very cold Asian landmass and the warm North Pacific Ocean. § Cross-equatorial flow to the Australian-Indonesia monsoon is enhanced by the difference between hot Australia and the relaDvely cool north Pacific. § The north-south contrast is reduced over the Indian subconDnent because the Tibetan Plateau blocks the cold Siberian air mass. Figure 3.43 West African Monsoon Figure 3.45 § The West African Monsoon closely matches the classical monsoon criteria, with moist, cool southwesterly winds during the summer monsoon and dry, warm, dusty northeasterly “Harmaian” winds during the winter. § The Intertropical Front (ITF) or Intertropical disconDnuity (ITD) is the boundary between the moist southwesterly monsoon flow and the hot, dry northeasterly wind from the Sahara. American Monsoons Figure 3.49 § The American monsoons do not match the classical criteria for monsoon as there is no opposiDonal wind shia during the winter (but reversal in the wind anomalies compared to

Geography is an important subject for the UPSC IAS Exam. In this article, you will learn about the topic ‘climate of India’ for the IAS exam. This article covers relevant sub-topics such as seasons of India, the Monsoons, factors determining the climate of India, etc. You can also download a PDF at the end of the article.The Climate of India is “monsoon” type which is found mainly in South-Asia and South-East Asia. The word “monsoon” is derived from the Arabic word “mausim” which means seasons. Originally, the word “monsoon” was used by Arab navigators several centuries ago, to describe a system of seasonal reversals of winds along the shores of the Indian Oceans, especially over the Arabian Sea, in which the winds blow from south-west to north-east during the summer season and from north-east to south-west during the winter season. In other words, monsoons are periodic (seasonal) winds in which there is a complete reversal of the wind direction after every six months.Weather and ClimateWeather refers to the state of the atmosphere over an area at any point in time while Climate refers to the sum total of weather conditions and variations over a large area for a long period of time (more than 30 years).Weather is the momentary state of the atmosphere and it changes quickly (within a day or week) but climate changes imperceptively and may be noted after 50 years or even more.The elements of weather and climate are the same i.e, atmospheric pressure, temperature, humidity, wind and precipitation.On the basis of the generalised monthly atmospheric conditions, the year is divided into seasons e.g, winter, summer or rainy.Though the climate of India is monsoon type, there are regional variations in climatic conditions within the country. These regional variations may be described as sub-types of monsoon climate.Regional variations in Temperature – Churu (Rajasthan) may record a temperature of 50℃ or more on a June day while the mercury hardly touches 19℃ in Tawang (Arunachal Pradesh) on the same day. The temperature may drop down to -45℃ in Drass (Ladakh) while Thiruvananthapuram or Chennai may record 20℃ or 22℃ on the same day.Regional variations in Precipitation and its Amount – It snows in the Himalayan regions while the rest of the country gets rains. Cherrapunji and Mawsynram in the Khasi Hills (Meghalaya) receive rainfall over 1080 cm in a year while Jaisalmer (Rajasthan) rarely gets more than 9 cm

Are the areas with the highest rainfall in India. The frequency at which these depressions visit India, their direction and their intensity determine the rainfall pattern during the southwest monsoon period.Inter Tropical Convergence ZoneThe Inter-Tropical Convergence Zone (ITCZ) is a broad trough of low pressure in equatorial latitudes. Here the north-east and south-east trade winds converge and air tends to ascend. This convergence zone lies almost parallel to the equator but moves north or south with the apparent movement of the sun. In July, ITCZ is located around 20°N – 25° N latitude, over the Gangetic plain and is also called the monsoon trough. This monsoon trough enhances the development of thermal low over north and north-west India. Due to this shift of ITCZ, the trade winds of the Southern Hemisphere cross the equator between 40°E and 60°E longitude and start blowing from south-west to north-east due to the Coriolis force and give rise to the south-west monsoon. The ITCZ moves to the Southern Hemisphere in winter which causes reversal of winds from north-east to south and south-west. These are called north-east monsoons. Indian MonsoonThe monsoons are experienced in the tropical area roughly between 20°N and 20°S. The following facts help in understanding the mechanism of the monsoons in India –The differential heating and cooling of land and water (land heats up and cools down faster than water) creates a pressure difference. The movement is from high pressure to low pressure.The ITCZ positions itself over the Ganga plain in summer (normally 5°N of the equator) and is called the monsoon trough.There is a high-pressure area, east of Madagascar, approximately at 20°S over the Indian Ocean. The position and intensity of this high-pressure area affect the Indian monsoon.During summer, the Tibetan plateau heats up intensely, develops low pressure over it at about 9 km above sea level and results in strong vertical air currents.The movement of westerly jet streams and tropical easterly jet streams also influence the monsoon of India.Southern Oscillations and El-NinoThe change in the pressure conditions over the southern oceans also affect the monsoon.Normally when the tropical eastern south Pacific Ocean experiences high pressure, the tropical eastern Indian Ocean has low pressure over it.But in certain years, there is a reversal in the pressure conditions and the eastern Indian Ocean has high pressure in comparison to the eastern Pacific ocean.This periodic change in pressure conditions is called Southern Oscillations