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| Tropical oceanic-atmospheric phenomenon |
Tropical oceanic-atmospheric phenomenon
La- Nina
El-Nino
Because of the following forces, winds move spirally outwards, away from high pressure areas into low pressure areas, spirally inwards:
- Gravitational Force - compresses the atmosphere
- Pressure Gradient Force - air moves into low-pressure (less compressed) from high-pressure (more compressed)
- Coriolis Force - because Earth spins west to east, the velocity of earth's movement varies from the equator to the poles. This difference in earth's velocity causes and apparent deflection of air in the atmosphere
- Friction Force - is the force that winds have when they drag against the surface of Earth. This slows the surface winds more than the high level winds.
On the equator, which is the hottest part on the earth, the following oceanic-atmospheric phenomena exist, that affect the climate of the tropical regions and sometimes, the climate of the whole planet:
1. ENSO (El Niño-Southern Oscillation) - occurs in the tropical region of the Pacific Ocean.
2. Indian Nino or IOD (Indian Ocean Dipole) - occurs in the tropical region of the Indian Ocean.
3. Atlantic Nino or Atlantic Equatorial Mode - occurs in the tropical region of the Atlantic Ocean
These phenomena occur because
- the surface temperature of ocean water is warm
- the winds blow the warm water from high pressure areas to low pressure areas
- the warm water is replaced by cold water from the lower levels of the ocean
As a result of this wind and water movement, heavy rainfalls occur in high pressure areas and droughts occur in low pressure areas.
ENSO (El Niño-Southern Oscillation)
In the Pacific Ocean, there are three variations:
- Normal- This is characterized by
- High pressure in eastern Pacific (South America)
- Low pressure in western Pacific (Indonesia)
- The winds blow the warm surface air of the ocean from east to west. This is called as "Easterlies"
- The warm water accumulates near Indonesia where it rises up as water vapor causing rainfall in this region
- The Eastern Pacific region is relatively dry, but cool.
- the cooler and nutrient rich water from the deeper levels of the Ocean surfaces on the eastern side of the Pacific which helps in the agriculture.
- El Nino - This occurs when the pressure areas shift and is characterized by
- Easterly trade winds becoming weaker or
- High pressure in western Pacific (Indonesia) and a Low pressure in eastern Pacific (South America)
- The winds blow the warm surface air of the ocean from west to east. This is called as "Westerlies"
- The warm water is spread from Indonesia all the way to South America.
- The warm water rises up as water vapor causing rainfall in this region, but this rainfall is not nutrient rich.
- Droughts and heavy temperature in India, Indonesia, and Australia
- Heavy rains hit East Africa.
El Nino is also called:
- Eastern Pacific (EP) El Nino - the normal El Nino
- Central Pacific (CP) El Nino - is a variation of El Nino that occurs mostly in Central Pacific
- "dateline" El Nino - is a variation of El Nino that occurs near the dateline
- El Nino "Modoki" - is a variation of El Nino that occurs in Bay of Bengal in the Indian Ocean
- La Nina - This is characterized by:
- Higher than normal high pressure in eastern Pacific (South America)
- Lower than normal low pressure in western Pacific (Indonesia)
- Very strong winds blow the warm surface air of the ocean from east to west. This is called as "Easterlies"
- The warm water accumulates near tropical Australia, Papua New-Guinea, and Indonesia where it rises up as water vapor causing rainfall in this region
- The Eastern Pacific region and the central pacific region is cooler because the cooler and nutrient rich water from the deeper levels of the Ocean surfaces on the eastern side of the Pacific. As the winds keep blowing west, there is a build-up of cooler than normal subsurface Ocean waters in tropical Pacific.
As the easterlies strengthen, the surfacing of cold water off Peru and Ecuador intensifies causing the sea-surface temperatures (SSTs) to drop below normal levels. According to records, during the 1988-89 La Niña episode, the SSTs fell by 4°C or 7°F below normal.
- Drought in Peru, Ecuador
- Low temperatures in Eastern Pacific
- Heavy floods in Australia
- High Temperature in Western Pacific, Somalia and Indian Ocean
- Good rains in India
- Drought in East Africa (Somalia Drought of 2011 was linked to it)
La Nina is also called:
- Eastern Pacific (EP) La Niña - the normal La Nina
- Central Pacific (CP) La Niña - is an variation of La Nina that occurs mostly in Central Pacific
- "Dateline" La Niña - is an variation of La Nina that occurs near the dateline
- La Niña "Modoki" - is an variation of La Nina that occurs in Bay of Bengal in the Indian Ocean
Indian Nino or IOD (Indian Ocean Dipole)
It has two phases:
- Positive phase - This is characterized by
- High pressure in eastern Indian Ocean near Indonesia
- Low pressure in western Indian Ocean near Africa
- Movement of warm water on the surface of the ocean to the African coast
- Cooling of waters near Indonesia in the eastern Indian Ocean
- Droughts in adjacent land areas of Indonesia and Australia
- Greater precipitation in the western Indian Ocean region
- Good monsoons in India
- Negative phase - This is characterized by
- High pressure in western Indian Ocean near Africa
- Low pressure in eastern Indian Ocean near Indonesia
- Movement of warm water on the surface of the ocean to the Indonesian coast
- Cooling of waters near Africa in the western Indian Ocean
- Bad monsoons in India
- Greater precipitation and rainfall near Indonesia and Western Australia.
An average of four positive and four negative Indian Nino events have occurred in a 30 year period, with each event lasting for about six months. Since 1980, 12 positive IODs have taken place. Only positive IODs occured from 1992 till 2010. There was a very strong negative IOD event in late 2010.
Atlantic Nino or Atlantic Equatorial Mode
It has two anomalies:
- Warm anomaly - It is characterized by
- High Pressure in western Atlantic Ocean near the Americas
- Low pressure in eastern Atlantic Ocean near Africa
- Relaxed trade winds across the equatorial Atlantic basin
- Warm waters across the tropical Atlantic Ocean
- Cool anomaly - It is characterized by
- High Pressure in western Atlantic Ocean near the Americas
- Low pressure in eastern Atlantic Ocean near Africa
- Easterly winds arising from Africa
- Warm waters closer to Americas
Conclusion
These three weather conditions coexist and are influenced by each other. They are also influenced by other weather and climatic conditions and winds in their respective regions.
El Nino was prevalent and very intense in the last 2 to 3 years. The World Meteorological Organization (WMO) has said, that the worst part of El Nino is over and it is reducing its intensity. Usually, a La Nina follows an El Nino period.
Weather Scientists in India are expecting the La Nina effect to hit India around September. If this is coupled with a positive Indian Nino, India can expect a very good monsoon.
By Pratibh Nanduri
