El Nino & La Nina, Walker Circulation, Madden Julian Oscillation and Indian Ocean Dipole
Walker Circulation -
- Near the South American coast, the trade winds blow offshore and blows the surface water westward away from the continent. To replace this water cold water from the bottom upwells.
- The air above this cold water is stabilised and hence convection is suppressed.
- The water flows westward as south-east trade winds blowing towards the western Pacific region.
- There it is heated and rises due to the convection and flows eastward in the upper troposphere.
- On reaching the eastern Pacific ocean, it starts to descend completing the cell.
- This results in high pressure and low temperature in eastern Pacific ocean and low pressure and high temperature in western Pacific ocean.
- The resulting convecting cell is known as Walker Circulation.
- In this normal condition, the eastern Pacific coast experiences draught and western Pacific coast have good precipitation. For example - Atacama desert on the west coast of South America.
El Nino
El Nino Means Christ Child as it starts during December
Normal condition - ITCZ rarely crosses equator and is usually above the equator
but once in 7-8 years - ITCZ shifts southwards of the equator (reason is not clearly known)
Impact of this southward shift of ITCZ -
Weakens the Trade winds in the region 🠊 cold water upwelling is no more (as the wind no more pushed away the surface water from the coast) 🠊 Cold water accumulated near to the coast
Also because of the weakening of the Trade winds, the usual direction of flow of water reversed. Now the water moves from west to the east 🠊 reduces upwelling
Warm Equatorial water starts to move east and accumulates there (near South America Coast) 🠊 El Nino
Warm water 🠊 Convection 🠊 Precipitation (i.e., situation reversed in comparison to normal walker circulation)
Normal walker circulation - convection in west and subsidence in east
El Nino - Subsidence in west and convection in east
The entire condition oscillated 🠊 That's why it is also known as ENSO (El Nino Southern Oscillation)
After 2-3 years, the Hadley cell becomes bit intensified 🠊 Trade wind intensified 🠊 which brings the normal condition (walker circulation) 🠊 Thus, El Nino ends.
Effects of El Nino -
1. El Nino 🠊 reduction in the precipitation in west Pacific leading to draught condition in India and Australia. Whereas it leads to high precipitation in east Pacific leading to good precipitation in Peru and Chile.
2. It had negative impact on fishing industry of Peru and Chile as it restricts the upwelling. (El Nino 🠊 reduces upwelling 🠊 nutrients from the bottom is not coming up & also warm water does not support the growth of plankton 🠊 fishes start to die and these dying fishes releases Hydrogen Sulfides (H2S) which makes the water dark and water becomes toxic.
3. El Nino 🠊 subsidence along Australia, India and also along Caribbean islands 🠊 reduction in cyclonic disturbances.
El Nino -
- Once in 7 to 8 years in October-November, the ITCZ moves too much to the south of the equator resulting in weakening of trade winds and reduction of upwelling.
- The warm tropical surface water which was earlier moving towards the western side flows back eastward and causes further reduction in upwelling.
- This results in warm surface water accumulating off the coast of Central and South America which is known as El Nino.
- The appearance of El Nino causes the air pressure to drop over the eastern Pacific coast and rise over western Pacific. This see-saw variation of the air pressure is known as El Nino Southern Oscillation.
- This causes heavy precipitation along the eastern Pacific and draught conditions along the western Pacific.
- After 1-2 years, the enhanced Hadley circulation strengthens the trade winds and increases the upwelling bringing back the normal condition (Walker circulation).
Effects of El Nino
- It results in draught condition in India and Australia.
- It causes heavy precipitation along the desert region of Peru and Chile.
- Warm water inhibits the growth of algae and suppresses upwelling, devastating the marine ecosystem, killing fishes which release hydrogen sulfides (H2S) causing dark water and making it toxic.
- El Nino causes the global climatic imbalance.
- During El Nino, the high pressure condition along India and Australia as well as along Caribbean sea results in lesser events of Cyclones.
- El Nino creates dry and warm condition along Australia and may leads to Bushfire in Australia.
La Nina
La Nina means 'Little Jin'.
It is same as Walker circulation but in intensified form.
El Nino is usually followed by La Nina.
Western Pacific - Very low Pressure 🠊 Very high convection 🠊 Very high precipitation 🠊 Impact - floods in Australia and India
Eastern Pacific - Very high Pressure 🠊 Subsidence 🠊 Almost no precipitation 🠊 Draught condition
La Nina -
- El Nino is usually followed by La Nina.
- It is the intensified high pressure condition & low temperature along the eastern Pacific and very low pressure & high temperature along the western Pacific.
- It causes a very heavy precipitation in India and Australia resulting in floods.
El Nino Modoki
When El Nino circulation got divided into two smaller circulations 🠊 El Nino Modoki
Water from both the end of the Pacific starts to move towards each other causing high pressure at the both ends and low pressure at the centre of the Pacific 🠊 draught condition on both (east and west) Pacific and precipitation at the open ocean.
Impact -
El Nino Modoki 🠊 Subsidence in the east Pacific (Peru and Chile) & Convection in Caribbean 🠊 increase in the number of Cyclones in Caribbean sea.
El Nino Modoki 🠊 Subsidence in west Pacific (Bay of Bengal) and Convection in Arabian sea 🠊 Cyclones in Arabia sea
El Nino Modoki -
- While El Nino is characterised by strong warming in the eastern Pacific ocean, El Nino Modoki results in warming of Central Pacific Ocean with cooling of western and eastern Pacific ocean.
- It creates a two cell Walker Circulation over tropical Pacific region with precipitation at the central part and dry condition at eastern and western part.
- While El Nino results in diminished hurricanes in Caribbean sea, El Nino Modoki results in increased incidence of hurricanes.
- During this time cyclones occur more in Arabian sea than in Bay of Bengal.
Madden Julian Oscillation
It is the Walker circulation but the entire circulation moves along the equator.
Impact -
Monsoon + Convecting cell of MJO 🠊 increased precipitation
Monsoon + Subsiding cell of MJO 🠊 decreased precipitation
- Normal monsoon + El Nino 🠊 decreased precipitation
- Normal monsoon + El Nino + Convecting MJO 🠊 Normal monson
- Normal monsoon + El Nino + Subsiding MJO 🠊 below normal monsoon
2019 -
- Monsoon + El Nino + Subsiding MJO in June 🠊 very low precipitation
- Monsoon + El Nino + Convecting MJO in July-August 🠊 very high precipitation in August and normal precipitation in July.
Madden Julian Oscillation
- Unlike El Nino Southern Oscillation which is stationary, MJO is an eastward moving disturbance of clouds, rain, winds and pressure that traverses the planet in tropics and returns to its initial starting point in 30-60 days on an average.
- MJO has two parts -
- The convecting part which enhanced rainfall.
- The subsiding part which suppressed rainfall.
- MJO can modulate the timing and strength of the monsoon as well as the impact of El Nino.
Indian Ocean Dipole
IOD - difference in water temperature in eastern and western Indian Ocean.
Positive IOD - increased temperature in East & decreased temperature in West
Negative IOD - decreased temperature in East & increased temperature in West
Positive IOD is good for Indian monsoon but is bad form Australia and Indonesia.
- Normal monsoon + El Nino + Positive IOD 🠊 Normal precipitation
- Normal monsoon + El Nino + Negative IOD 🠊 Low precipitation
Indian Ocean Dipole
- The difference in sea surface temperature between the western Arabian sea and eastern Indian ocean of Indonesia is called Indian Ocean Dipole (IOD).
- Similar to MJO, it is a coupled ocean and atmospheric phenomena.
Positive Indian Ocean dipole
- The sea surface temperature in the western part is warmer than the eastern part.
- It results in good rainfall in India.
- It causes less rainfall in Indonesia and Australia.
- It reduces the effect of El Nino on India.
Negative Indian Ocean dipole
- The sea surface temperature is warmer in eastern part than the western part.
- It causes more rainfall in Indonesia and Australia.
- It is bad for India's monsoon.
- It increases the impact of El Nino on India.
Previous Article - Upwelling & Downwelling, Thermohaline circulation, Water mass and UNCLOS
Next Article - Tides
Notes on other subjects
Optional Notes
Note - This is my Vision IAS Notes (Vision IAS Class Notes) and Ashutosh Pandey Sir's Public Administration Class notes. I've also added some of the information on my own.
Hope! It will help you to achieve your dream of getting selected in Civil Services Examination 👍
0 Comments