Phy Geo (Unit 3 -Topic 5) El Nino, La Nino and ENSO Modoki
Physical Geography (Topic 5-Unit 3)
El Nino, La Nino and ENSO Modoki
El Nino is an oceanic phenomena observed in the Southern Pacific ocean which emerges after a gap of 3 to 7 years and is associated with abnormally high rainfall in the otherwise dry condition along the coast of Peru. This phenomena results in floods in Peru while Pacific coast of Australia and Indonesia have abnormally dry conditions that forces collapse of Agriculture with instances of Forest fire. During El Niño there are on average fewer hurricanes over the Atlantic Ocean, the Caribbean Sea and the Gulf of Mexico. In the central Pacific Ocean El Niño brings more typhoons, both north and south of the equator. Their more easterly genesis makes that fewer of these tropical cyclones reach Australia. In the northern Pacific Ocean the area with typhoons also shifts east. There are no effects on the number of cyclones over the Indian Ocean. The phenomenon is not well understood but it is related to Ocean current and trade winds. In this, Sea Surface Temperature (SST) increases above normal by .5 degree.
In normal years the trade winds are fairly consistent and strong. The equatorial current of Pacific are well developed which leads to development of warm Western Pacific Pool resulting in a zone of low pressure off Australian and Indonesian Coast with rising limb of air causing rain.
Fig. Warm Western Pacific Pool and trade winds
At the same time, Peruvian coast has cold current (Humboldt current) and upwelling which brings cold current along the coast of Peru resulting in High Pressure with descending air current. This is favorable to Peruvian economy as Upwelling brought up nutrients at surface which flourishes fishing population. This low Pressure at Australian Coast and High Pressure at Peruvian coast form a vertical cell called as Walker cell. (This cell is exception to otherwise general pattern of Air circulation e.g. Trade winds, westerlies and Polar circulation and tri-cellular meridional circulation. Here East-West Zonal Pattern is found).
During El-Nino years, for reasons not yet known the trade winds of Pacific weakens, resulting in weak equatorial current. The Warm Pacific Pool waters flows back strengthening the Counter Equatorial current. This reverses water flows southward towards Peruvian coast and this current is known as El Nino current. (It is a warm surface current appears at the coast of Peru during December flowing from equator towards Pole) which can stop the upwelling Process.
This results in reversing the pressure cells (Now Peruvian coast have low pressure and Australian-Indonesian coast having relative High Pressure)and with this Walker Cell too is reversed i.e. rising limb of air at Peruvian coast and descending limb of air along Australian coast which reverses the climatic condition along these coast which results into Peruvian coast having High rainfall and West Pacific coast having dry condition).
The atmospheric component of El Nino/La Nina is called Southern Oscillation. This component is an oscillation in surface air pressure between the tropical western and the eastern Pacific Ocean waters. In context to Monsoon the two cumulatively is called as ENSO. The strength of it (Southern Oscillation) is measured by the Southern Oscillation Index (SOI). It is computed by taking air pressure Ocean difference between Tahiti in Eastern Pacific Ocean and Darwin in Western Pacific. An El Nino year will have negative value of SOI that means eastern pacific has below normal pressure over Tahiti and above normal pressure over Darwin.
La Nina:
This is another climatic extreme and is opposite of El-Nino. It is also called as El Viejo, The Little Girl, anti-El Nino, or simply ‘a cold event’ or ‘a cold episode’ In this Trade winds are abnormally very strong , this makes Warm Western Pacific Pool strong, Very strong Peruvian current and an intensified Walker Cell. Consequently there is strong upwelling at the coast of Peru and it support very good fishing but due to oversupply glutting happens. (As heavy supply of fishes drags down the prices which is adverse for Peruvian economy). On opposite of it western Pacific countries have abnormally high rainfall which causes flood.
In the US, winter temperatures are warmer than normal in the south-east and cooler than normal in the north-west during a La Nina year. Temperatures are warmer than normal in the Southeast and cooler than normal in the Northwest.
Snow and rain is experienced on the west-coast and unusually cold weather in Alaska. During this period there are higher than normal number of hurricanes in the Atlantic.
Effects of El Niño on India :
ENSO known to have a significant impact on the strength of South-West Monsoon over India with the Monsoon being weak (causing droughts in India) during the El Niño years whereas La Niña years had particularly good Monsoon strength over India but ENSO does not have a one to one correspondence i.e. not all El Niño years have produced drought in India. During El Niño years probability is high that there will be less rainfall in India but this need not be necessary.
El Niño Modoki & La Niño Modoki (ENSO Modoki) :
These are associated with Central Pacific, rather than the Eastern Pacific as the typical El Niño/ La Niño event does. Teleconnection in these are different from teleconnection pattern of conventional El Niño. During El Nino Modoki the sea surface temperature (SST) anomaly in eastern Pacific is not affected, but a warm anomaly arises in the central Pacific flanked by cold anomalies on both sides of the basin .Together with its counterpart La Nina Modoki, when colder central Pacific is flanked by warmer eastern and western Pacific, the new phenomenon is now called as the ENSO Modoki that assumes both warm and cold phases of its behavior. The associated decadal changes in the sea level are shown to affect not only the islands of central Pacific but remote regions off California and southwestern Indian Ocean
Fig. El Nino Modoki and La Nina Modoki the two phases of ENSO Modoki.
Due to El Nino Modoki Central Pacific has higher storm frequency and a greater potential for making landfall along the Gulf coast and the coast of Central America. Also, due to this (El Nino Modoki ) Arabian sea has more number of cyclone during pre-monsoon and post-monsoon seasons as opposed to general phenomena whereas Bay of Bengal has higher number of cyclone during pre-monsoon and post-monsoon.
The reason why El Nino Modoki brings only fewer numbers of cyclones in the Bay of Bengal is because one of the two descending limbs of the Walker Cell is over the western Pacific and Bay of Bengal. The descending limb causes dry conditions not conducive for cyclone formation. The ascending limb of the Walker Cell, on the other hand, brings rain. Also, an El Nino Modoki creates stronger divergence over the western Pacific and Bay of Bengal compared to El Nino. Divergence (opposite of convergence) means surface winds move away from each other and result in low relative vorticity (rotational flow of winds). These conditions are not conducive for cyclones. This explains why Bay of Bengal region (close to western Pacific) has fewer cyclones during an El Nino Modoki. On the other hand, there is large convergence over the Arabian Sea during an El Nino Modoki explaining the large number of cyclones in that region.
In normal years the trade winds are fairly consistent and strong. The equatorial current of Pacific are well developed which leads to development of warm Western Pacific Pool resulting in a zone of low pressure off Australian and Indonesian Coast with rising limb of air causing rain.
Fig. Warm Western Pacific Pool and trade winds
At the same time, Peruvian coast has cold current (Humboldt current) and upwelling which brings cold current along the coast of Peru resulting in High Pressure with descending air current. This is favorable to Peruvian economy as Upwelling brought up nutrients at surface which flourishes fishing population. This low Pressure at Australian Coast and High Pressure at Peruvian coast form a vertical cell called as Walker cell. (This cell is exception to otherwise general pattern of Air circulation e.g. Trade winds, westerlies and Polar circulation and tri-cellular meridional circulation. Here East-West Zonal Pattern is found).
During El-Nino years, for reasons not yet known the trade winds of Pacific weakens, resulting in weak equatorial current. The Warm Pacific Pool waters flows back strengthening the Counter Equatorial current. This reverses water flows southward towards Peruvian coast and this current is known as El Nino current. (It is a warm surface current appears at the coast of Peru during December flowing from equator towards Pole) which can stop the upwelling Process.
This results in reversing the pressure cells (Now Peruvian coast have low pressure and Australian-Indonesian coast having relative High Pressure)and with this Walker Cell too is reversed i.e. rising limb of air at Peruvian coast and descending limb of air along Australian coast which reverses the climatic condition along these coast which results into Peruvian coast having High rainfall and West Pacific coast having dry condition).
The atmospheric component of El Nino/La Nina is called Southern Oscillation. This component is an oscillation in surface air pressure between the tropical western and the eastern Pacific Ocean waters. In context to Monsoon the two cumulatively is called as ENSO. The strength of it (Southern Oscillation) is measured by the Southern Oscillation Index (SOI). It is computed by taking air pressure Ocean difference between Tahiti in Eastern Pacific Ocean and Darwin in Western Pacific. An El Nino year will have negative value of SOI that means eastern pacific has below normal pressure over Tahiti and above normal pressure over Darwin.
La Nina:
This is another climatic extreme and is opposite of El-Nino. It is also called as El Viejo, The Little Girl, anti-El Nino, or simply ‘a cold event’ or ‘a cold episode’ In this Trade winds are abnormally very strong , this makes Warm Western Pacific Pool strong, Very strong Peruvian current and an intensified Walker Cell. Consequently there is strong upwelling at the coast of Peru and it support very good fishing but due to oversupply glutting happens. (As heavy supply of fishes drags down the prices which is adverse for Peruvian economy). On opposite of it western Pacific countries have abnormally high rainfall which causes flood.
In the US, winter temperatures are warmer than normal in the south-east and cooler than normal in the north-west during a La Nina year. Temperatures are warmer than normal in the Southeast and cooler than normal in the Northwest.
Snow and rain is experienced on the west-coast and unusually cold weather in Alaska. During this period there are higher than normal number of hurricanes in the Atlantic.
Effects of El Niño on India :
ENSO known to have a significant impact on the strength of South-West Monsoon over India with the Monsoon being weak (causing droughts in India) during the El Niño years whereas La Niña years had particularly good Monsoon strength over India but ENSO does not have a one to one correspondence i.e. not all El Niño years have produced drought in India. During El Niño years probability is high that there will be less rainfall in India but this need not be necessary.
El Niño Modoki & La Niño Modoki (ENSO Modoki) :
These are associated with Central Pacific, rather than the Eastern Pacific as the typical El Niño/ La Niño event does. Teleconnection in these are different from teleconnection pattern of conventional El Niño. During El Nino Modoki the sea surface temperature (SST) anomaly in eastern Pacific is not affected, but a warm anomaly arises in the central Pacific flanked by cold anomalies on both sides of the basin .Together with its counterpart La Nina Modoki, when colder central Pacific is flanked by warmer eastern and western Pacific, the new phenomenon is now called as the ENSO Modoki that assumes both warm and cold phases of its behavior. The associated decadal changes in the sea level are shown to affect not only the islands of central Pacific but remote regions off California and southwestern Indian Ocean
Fig. El Nino Modoki and La Nina Modoki the two phases of ENSO Modoki.
Due to El Nino Modoki Central Pacific has higher storm frequency and a greater potential for making landfall along the Gulf coast and the coast of Central America. Also, due to this (El Nino Modoki ) Arabian sea has more number of cyclone during pre-monsoon and post-monsoon seasons as opposed to general phenomena whereas Bay of Bengal has higher number of cyclone during pre-monsoon and post-monsoon.
The reason why El Nino Modoki brings only fewer numbers of cyclones in the Bay of Bengal is because one of the two descending limbs of the Walker Cell is over the western Pacific and Bay of Bengal. The descending limb causes dry conditions not conducive for cyclone formation. The ascending limb of the Walker Cell, on the other hand, brings rain. Also, an El Nino Modoki creates stronger divergence over the western Pacific and Bay of Bengal compared to El Nino. Divergence (opposite of convergence) means surface winds move away from each other and result in low relative vorticity (rotational flow of winds). These conditions are not conducive for cyclones. This explains why Bay of Bengal region (close to western Pacific) has fewer cyclones during an El Nino Modoki. On the other hand, there is large convergence over the Arabian Sea during an El Nino Modoki explaining the large number of cyclones in that region.
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