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El Nino – Southern Oscillation

El Nino

  • El Niño is the name given to the occasional development of warm ocean surface waters along the coast of Ecuador and Peru.
  • When this warming occurs the usual upwelling of cold, nutrient rich deep ocean water is significantly reduced.
  • El Niño normally occurs around Christmas and usually lasts for a few weeks to a few months.
  • Sometimes an extremely warm event can develop that lasts for much longer time periods. In the 1990s, strong El Niños developed in 1991 and lasted until 1995, and from fall 1997 to spring 1998.

Normal Conditions   

  • In a normal year, a surface low pressure develops in the region of northern Australia and Indonesia and a high pressure system over the coast of Peru. As a result, the trade winds over the Pacific Ocean move strongly from east to west.
  • The easterly flow of the trade winds carries warm surface waters westward, bringing convective storms (thunderstorms) to Indonesia and coastal Australia. Along the coast of Peru, cold bottom cold nutrient rich water wells up to the surface to replace the warm water that is pulled to the west.



Walker Circulation   

  • The Walker circulation (walker cell) is caused by the pressure gradient force that results from a high pressure system over the eastern Pacific ocean, and a low pressure system over Indonesia.



El Nino Conditions            

  • In an El Niño year, air pressure drops over large areas of the central Pacific and along the coast of South America.
  • The normal low pressure system is replaced by a weak high in the western Pacific (the southern oscillation). This change in pressure pattern causes the trade winds to be reduced == Weak Walker Cell. Sometimes Walker Cell might even get reversed.
  • This reduction allows the equatorial counter current (current along doldrums) to accumulate warm ocean water along the coastlines of Peru and Ecuador.


Effect of El Nino               

  • This accumulation of warm water causes the thermocline to drop in the eastern part of Pacific Ocean which cuts off the upwelling of cold deep ocean water along the coast of Peru.
  • Climatically, the development of an El Niño brings drought to the western Pacific, rains to the equatorial coast of South America, and convective storms and hurricanes to the central Pacific.

Effects of El Nino              

  • The warmer waters had a devastating effect on marine life existing off the coast of Peru and Ecuador.
  • Fish catches off the coast of South America were lower than in the normal year (Because there is no upwelling).
  • Severe droughts occur in Australia, Indonesia, India and southern Africa.
  • Heavy rains in California, Ecuador, and the Gulf of Mexico.


Southern Oscillation    

  • The formation of an El Niño [Circulation of Water] is linked with Pacific Ocean circulation pattern known as the southern oscillation [circulation of atmospheric pressure]
  • Southern Oscillation, in oceanography and climatology, is a coherent interannual fluctuation of atmospheric pressure over the tropical Indo-Pacific region.
  • El Nino and Southern Oscillation coincide most of the times hence their combination is called ENSO – El Nino Southern Oscillation.

Climatic Regions

Equatorial Hot and Wet Climate   


Equatorial Hot and Wet Climate         


  • Mostly between 5° N and S of

Equator. [little or no Coriolis Force

= no tropical cyclones]

  • Its greatest extent is found in the lowlands of the Amazon, the Congo, Malaysia and the East Indies.


  • Temperature is uniform throughout the year.
  • The mean monthly temperatures are always around

27° C with very little variation.

  • There is no winter. [Typical to Equatorial Rainforest Climate]
  • Cloudiness and heavy precipitation moderate the daily temperature.
  • Regular land and sea breezes assist in maintaining a truly equable climate.
  • The diurnal range of temperature is small, and so is the annual range.



  • Precipitation is heavy and well distributed throughout the year.
  • Annual average is always above 150 cm. In some regions the annual average may be as high as 250 – 300 cm.
  • There is no month without rain (distinct dry season is absent). The monthly average is above 6 cm most of the times.
  • There are two periods of maximum rainfall, April and October. [shortly after the equinox]. Least rainfall occurs in June and December [solstice].
  • The double rainfall peaks coinciding with the equinoxes are a characteristic feature of equatorial climates not found in any other type of climate.
  • There is much evaporation and convection air currents are set up, followed by heavy thunderstorms in the afternoons.


  • High temperature and abundant rainfall support a luxuriant tropical rainforest.
  • In the Amazon lowlands, the forest is so dense that it is called ‘selvas’. [selvas: A dense tropical rainforest usually having a cloud cover (dense canopy)]
  • Unlike the temperate regions, the growing season here is all the year round-seeding, flowering, fruiting and decaying do not take place in a seasonal pattern.
  • The equatorial vegetation comprises a multitude of evergreen trees that yield tropical hardwood, e.g. mahogany, ebony, dyewoods etc.
  • Many parts of the tropical rain forests have been cleared either for lumbering or shifting cultivation.
  • In the coastal areas and brackish swamps, mangrove forests thrive.
Monsoon Type of Climate


Monsoon Type of Climate

  • Monsoons are land and sea breezes on a much larger scale.
  • Unlike equatorial wet climate, monsoon climate is characterized by distinct wet and dry seasons associated with seasonal reversal of winds.
  • Floods in wet season and droughts in dry season are common.
  • Usually there are three seasons namely summer, winter and rainy season.
  • Occur within 5° to 30° N and S of the equator.
  • On-shore [sea to land] tropical monsoons occur in the summer and off-shore [land to sea] dry monsoons in the winter.
  • They are best developed in the Indian subcontinent, Burma, Thailand, Laos, Cambodia, parts of Vietnam and south China and northern Australia.

Monsoon – Origin    

  • The basic cause of monsoon climates is the difference in the rate of heating and cooling of land and sea (This is old theory. New theory will be explained while studying Indian Climate).
  • In the summer, when the sun is overhead at the Tropic of Cancer, a low pressure is created in Central Asia.
  • The seas, which warm up much slower, remain comparatively at high pressure. At the same time, the southern hemisphere experiences winter, and a region of high pressure is set up in the continental interior of Australia.
  • Winds blow outwards as the South-East Monsoon, to Java, and after crossing the equator are drawn towards the continental low pressure area reaching the Indian sub-continent as the South-West Monsoon (Coriolis force).
  • In the winter, conditions are reversed.


Monsoon Type


  • Monthly mean temperatures above 18 °C.
  • Temperatures range from 30-45° C in summer. Mean summer temperature is about 30°C.
  • In winters, temperature range is 15-30° C with mean temperature around 20-25° C.


  • Annual mean rainfall ranges from 200-250 cm. In some regions it is around 350 cm.
  • Places like Cherrapunji & Mawsynram receive an annual rainfall of about 1000 cm.



  • Broad-leaved hardwood trees. Well developed in southeast Asia.
  • Trees are normally deciduous, because of the marked dry period, during which they shed their leaves to withstand the drought [They shed their leaves to prevent loss water through transpiration].
  • The forests are more open and less luxuriant than the equatorial jungle and there are far fewer species.
  • Where the rainfall is heavy, e.g. in southern Burma, peninsular India, northern Australia and coastal regions with a tropical marine climate, the resultant vegetation is luxuriant.
  • With a decrease in rainfall in summer, the forests thin out into thorny scrubland or savanna with scattered trees and tall grass.
  • In parts of the Indian sub-continent, rainfall is so deficient that semi-desert conditions are found in summer. Monsoonal vegetation is thus most varied, ranging from forests to thickets, and from savanna to scrubland.

Tropical Marine Climate

  • Outside the monsoon zone, the climate is modified by the influence of the on-shore Trade Winds all the year round. This type of climate is called Tropical Marine Climate. Such a climate has a more evenly distributed rainfall.
  • Such a climate is experienced in Central America, West Indies, north-eastern Australia, the Philippines, parts of East Africa, Madagascar, the Guinea Coast and eastern Brazil.
  • The rainfall is both orographic where the moist trades meet upland masses as in eastern Brazil, and convectional due to intense heating during the day and in summer.
  • Its tendency is towards a summer maximum without any distinct dry period.
  • Due to the steady influence of the trades, the Tropical Marine Climate is more Favourable for habitation, but it is prone to severe tropical cyclones, hurricanes or typhoons.
Tropical Desert 
  • The aridity of the hot deserts is mainly due to the effects of off-shore Trade Winds, hence they are also called Trade Wind Deserts.
  • The major hot deserts of the world are located on the western coasts of continents between latitudes 15° and 30°N. and S (Question asked in Previous Mains Exam).
  • They include the biggest Sahara Desert (3.5 million square miles), Great Australian Desert, Arabian Desert, Iranian Desert, Thar Desert, Kalahari and Namib Deserts.
  • In North America, the desert extends from Mexico into U.S.A. and is called by different names at different places, e.g. the Mohave, Sonoran, Californian and Mexican Deserts.
  • In South America, the Atacama or Peruvian Desert (rain shadow effect and off-shore trade winds) is the driest of all deserts with less than 2 cm of rainfall annually.

Mid-latitude Deserts 

  • These inland basins lie hundreds of miles from the sea, and are sheltered by the high mountains all around them. As a result they are cut off from the rain-bearing winds.
  • Occasionally depressions may penetrate the Asiatic continental mass and bring light rainfall in winter. Due to their coldness and elevation, snow falls in winter.
  • The annual range of temperature is much greater than that of the hot deserts. Continentality accounts for these extremes in temperature.
  • Winters are often severe, freezing lakes and rivers, and strong cold winds blow all the time. When the ice thaws in early summer, floods occur in many places.

Deserts – Rainfall      

  • Deserts, whether hot or mid-latitude have an annual precipitation of less than 25 cm.
  • Atacama (driest place on earth) has practically no rain at all.
  • Rain normally occurs as violent thunderstorms of the convectional type.
  • It ‘bursts’ suddenly and pours continuously for a few hours over small areas.
  • The thunderstorm is so violent, and comes so suddenly that it has disastrous consequences on desert landforms [flash floods].

Deserts – Temperature 

  • There is no cold season in the hot deserts and the average summer temperature is high around 30°C.
  • The highest temperature recorded is 57.77° C in 1922 at Al Azizia, Libya.
  • The reasons for the high temperatures are obvious—a clear, cloudless sky, intense insolation, dry air and a rapid rate of evaporation.
  • Coastal deserts by virtue of their maritime influence and the cooling effect of the cold currents have much lower temperatures.
  • The desert interiors, however, experience much higher summer temperatures and the winter months are rather cold.
  • High diurnal temperature range is a typical feature of hot deserts. Average diurnal range varies from 14 to 25° Celsius.
  • Frosts may occur at night in winter.

Deserts – Vegetation

  • The predominant vegetation of both hot and mid-latitude deserts is xerophytic or droughtresistant.
  • This includes the cacti, thorny bushes, long-rooted wiry grasses and scattered dwarf acacias.
  • Trees are rare except where there is abundant groundwater to support clusters of date palms.
  • Along the western coastal deserts washed by cold currents as in the Atacama Desert, support a thin cover of vegetation.
  • Intense evaporation increases the salinity of the soil so that the dissolved salts tend to accumulate on the surface forming hard pans
  • Most desert shrubs have long roots and are well spaced out to gather moisture, and search for ground water. Plants have few or no leaves and the foliage is either waxy, leathery, hairy or needleshaped to reduce the loss of water through transpiration.
  • The seeds of many species of grasses and herbs have thick, tough skins to protect them while they lie dormant.





Savannah – Rainfall     

  • Mean annual rainfall ranges from 80 – 160 cm [Rainfall decreases with distance from equator].
  • In the northern hemisphere, the rainy season begins in May and lasts till September.
  • In the southern hemisphere, the rainy season is from October to March.

Savannah – Temperature

  • Mean annual temperature is greater than 18° C.
  • The monthly temperature hovers between 20° C and 32° C for lowland stations.
  • Highest temperatures do not coincide with the period of the highest sun (e.g. June in the northern hemisphere) but occur just before the onset of the rainy season, i.e. April in Northern Hemisphere and October in Southern Hemisphere.
  • Days are hot and nights are cold. This extreme diurnal range of temperature is another characteristic feature of the Sudan type of climate.

Savannah – Vegetation     

  • The savanna landscape is typified by tall grass and short trees.
  • The grasslands are also called as ‘bush-veld’.
  • The trees are deciduous, shedding their leaves in the cool, dry season to prevent excessive loss of water through transpiration, e.g. acacias.
  • Trees usually have broad trunks, with water-storing devices to survive through the prolonged drought.
  • Many trees are umbrella shaped, exposing only a narrow edge to the strong winds.
  • In true savanna lands, the grass is tall and coarse, growing 6 to 12 feet high. The elephant grass may attain a height of even 15 feet.
  • Grasses appear greenish and well-nourished in the rainy season but turns yellow and dies down in the dry season that follows.
  • As the rainfall diminishes towards the deserts the savanna merges into thorny scrub.