How are the Australian Bushfires related to floods in Africa?

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Context: Flooding and landslides in East Africa have killed dozens of people and forced hundreds of thousands from their homes. Meanwhile, thousands of miles away in Australia, a period of hot, dry weather has led to a spate of bushfires.

Relevance:
Prelims: Current events of national and international importance
Mains:

  • GS I- geographical features and their location- changes in critical geographical features (including water bodies and ice-caps) and in flora and fauna and the effects of such changes.

Why in news?

  • Flooding and landslides in East Africa have killed dozens of people and forced hundreds of thousands from their homes. Meanwhile, thousands of miles away in Australia, a period of hot, dry weather has led to a spate of bushfires.
  • Both weather events have been linked to higher-than-usual temperature differences between the two sides of the Indian Ocean – something meteorologists refer to as the Indian Ocean Dipole.
What is the Indian Ocean Dipole?
  • Sustained variations in the difference between tropical western and eastern Indian Ocean surface temperatures are referred to as the Indian Ocean Dipole or IOD.
  • It is also known as the Indian Niño, is an irregular sea – surface temperature oscillation in which the western Indian Ocean alternately becomes warmer and colder than the eastern part of the ocean.

Three phases of Indian Ocean Dipole:

  • The IOD has three  phases such as Neutral, Positive and Negative IOD

Neutral Phase of IOD:

  • During this phase Water flows from the Pacific between Indonesia’s islands, keeping seas warm to the northwest of Australia.
  • Air rises above this area and falls across the western half of the Indian Ocean basin, blowing westerly winds along the equator.

Positive Phase of IOD:

  • During this phase the Westerly winds weaken along the equator, allowing warm water to move to Africa.
  • Changes in the winds also allow a rise of cool water from the deep ocean in the east. This creates a temperature difference across the tropical Indian Ocean with cooler than normal water in the east and warmer than normal water in the west.
  • This event has been found to be beneficial for the monsoon.

Negative Phase of IOD:

  • During this phase westerly winds intensify along the equator,  allowing the concentration of warmer waters near Australia.
  • This creates a temperature difference across the tropical Indian Ocean, with warmer than normal water in the east and cooler than normal water in the west.
  • This event obstructs the progression of monsoon over India.

IOD and El Nino:
  • Due to the movement of warm water during IOD, there would be less impact of El Nino on the Indian Monsoon, Hence, not only El Nino La Nina but also IOD and other such ocean phenomena on which the Indian monsoon is depended upon.
  • An IOD can either aggravate or weaken El Nino’s impact on the Indian monsoon. If there is a positive IOD, despite an El Nino year, it can bring good rains to India. For instance, positive IODs in 1983, 1994 and 1997 facilitated normal or excess rainfall over India, despite an El Nino in those years.
El Niño :
  • 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 (Occurs during Normal Years):

  • 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.

During El Nino year:

  • 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.
  •  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.

Normal Conditions:

  • Eastern Pacific == Coast of Peru and Ecuador == Cold Ocean Water == Good for Fishing.
  • Western Pacific == Indonesia ,India and Australia == Warm Ocean Water == Plenty of rains.

El Nino Condition:

  • Eastern Pacific == Coast of Peru and Ecuador == Warm Ocean Water == Fishing industry takes a hit.
  • Western Pacific == Indonesia,India and Australia == Cold Ocean Water == Drought.

How El Nino impacts monsoon rainfall in India?

  • El Nino and Indian monsoon are inversely related.
  • The most prominent droughts in India – six of them – since 1871 have been El Nino droughts, including the recent ones in 2002 and 2009
  • However, not all El Nino years led to a drought in India. For instance, 1997/98 was a strong El Nino year but there was no drought (Because of IOD).
  • On the other hand, a moderate El Nino in 2002 resulted in one of the worst droughts.
  • El Nino directly impacts India’s agrarian economy as it tends to lower the production of summer crops such as rice, sugarcane, cotton and oilseeds.
  • The ultimate impact is seen in the form of high inflation and low gross domestic product growth as agriculture contributes around 14% of the Indian economy.

IOD and Indian Monsoon:
  • The Indian Ocean Dipole (IOD) is defined by the difference in sea surface temperature between two areas (or poles, hence a dipole) – a western pole in the Arabian Sea (western Indian Ocean) and an eastern pole in the eastern Indian Ocean south of Indonesia.
  • IOD develops in the equatorial region of the Indian Ocean from April to May peaking in October.
    • Although ENSO was statistically effective in explaining several past droughts in India, in recent decades the ENSO-Monsoon relationship seemed to weaken in the Indian subcontinent.
      • For e.g. 1997, strong ENSO failed to cause drought in India.
    • However, it was later discovered that just like ENSO was an event in the Pacific Ocean, a similar seesaw ocean-atmosphere system in the Indian Ocean was also at play.
      • It was discovered in 1999 and named the Indian Ocean Dipole (IOD).
        • With a positive IOD winds over the Indian Ocean blow from east to west (from Bay of Bengal towards Arabian Sea). This results in the Arabian Sea (western Indian Ocean near African Coast) being much warmer and eastern Indian Ocean around Indonesia becoming colder and dry.
        • In the negative dipole year (negative IOD), reverse happens making Indonesia much warmer and rainier 
Impact on IOD on Cyclonogeneis in Northern Indian Ocean:
  • Positive IOD (Arabian Sea warmer than the Bay of Bengal) results in more cyclones than usual in the Arabian Sea.
  • Negative IOD results in stronger than usual cyclogenesis (Formation of Tropical Cyclones) in the Bay of Bengal. Cyclogenesis in the Arabian Sea is suppressed.
Impact of IOD Indian Monsoon:
  • The Indian monsoon rainfall is influenced by a system of oscillating sea surface temperatures known as the Indian Ocean Dipole (IOD) in which the western Indian Ocean becomes alternately warmer and then colder than the eastern part of the ocean.
  • A positive IOD leads to greater monsoon rainfall and more active (above-normal rainfall) monsoon days while negative IOD leads to less rainfall and more monsoon break days (no rainfall).
  • It was predicted by some agencies that there will be impact of El Nino on Indian Monsoon this year.
  • Hence in light of this IOD would be the savior factor that is likely to be in the neutral or positive phase during the monsoon.

Impact of IOD on Australia:
  • The impact of a delayed monsoon may have implications for Australian summer crops with the decline in planted area (approx. fall of 28%) in 2019–20.
  • Typically, a positive IOD brings below-average winter-spring rainfall to southern and central Australia and a more severe fire season for South-East Australia.
  • In the tropical Pacific Ocean, the El-Nino Southern Oscillation (ENSO) remains neutral.
  • When ENSO is neutral, it has little effect on Australian and global climate, meaning other influences are more likely to dominate.

Crux:

  • “When an Indian Ocean dipole event occurs, the rainfall tends to move with the warm waters, so you get more rainfall than normal over the East African countries,”
  • “On the other hand, in the east of the Indian Ocean, sea surface temperatures will be colder than normal and that place will get a reduced amount of rainfall.”
  • A negative dipole phase would bring about the opposite conditions – warmer water and greater precipitation in the eastern Indian Ocean, and cooler and drier conditions in the west.
  • A neutral phase would mean sea temperatures were close to average across the Indian Ocean.

 

The dipole has resulted in floods in eastern Africa and bushfires in Australia:
  • Africa:
    • Heavy downpours have devastated parts of East Africa over the last two months, with the Horn of Africa seeing up to 300% above average rainfall between October and mid-November, according to the Famine Early Warning Systems Network.
    • Djibouti, Ethiopia, Kenya, Uganda, Tanzania, Somalia, and South Sudan have been particularly badly affected, with flash floods and landslides hitting communities across the region.

  • Australia:
    • About 100 bushfires are raging in the Australian state of New South Wales (NSW), with the most severe forming into a “mega blaze” north of Sydney.
    • The Australian Bureau of Meteorology has warned communities to prepare for more severe fire danger, with a high chance of warmer than usual days and nights for much of the country throughout summer.

The effects of the dipole could get worse because of climate change:

  • Extreme climate and weather events caused by the dipole are predicted to become more common in the future as greenhouse gas emissions increase.
  • In a 2014 study published in Nature, scientists in Australia, India, China, and Japan modeled the effects of CO2 on extreme Indian Ocean dipoles, such as those in 1961, 1994 and 1997.
  • Assuming emissions continue to go up, they projected that the frequency of extreme positive dipole events would increase this century from one every 17.3 years to one every 6.3 years.
  • “The countries in the west of the Indian Ocean, so on the African coast, are going to see much, much more flooding and heavy rainfall relating to these events,”.



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