Link between Indian ocean dipole and locusts


Desert locusts is a awful threat for Indian agriculture sector. Since 2018, it has been a major problem for Indian farmers. Tens of thousands of hector crops have been ruined across India during last two years. There is a no lasting solution to stem this tragedy.
Several parts of West Asia, Oman Yemen and horn of Africa - Ethiopia, Somalia and Kenya are the main epicenters of locusts breading. Changing of climate in Indian Ocean may be a trigger for the breeding locusts. Positive Indian ocean dipole affects monsoon in South-West parts of Asia and Eastern parts of Africa. So, Indian ocean dipole is a primary cause of this breeding.

Here, I am elaborating the detailed theory of Indian ocean dipole. If you have little knowledge about IOD , then you can skip this segment of article and shift to next part.

What is Indian ocean dipole?

 In simple words, change of temperature between Western and Eastern part of Indian Ocean - between Arabian Sea and Bay of Bengal - is called IOD (Indian ocean dipole). It is an irregular oscillation of sea-surface temperature. This variation of temperature affects monsoon in India. IOD is also known as Indian Nino.

Why does Indian Ocean has a dipole?

Indian Ocean is the warmest ocean among all others and Pacific Ocean follows it. That the Indian Ocean is landlocked in North and doesn't come in contact with the Arctic waters, is the first reason for having its high temperature. The second one is that Pacific water flows between the islands of Indonesia and Australian continent and mixes into India Ocean. This mixing of warm water makes Indian Ocean more  hotter. Easterly winds that blows between the 30° of North hemisphere and 30° of South hemisphere on the earth, stimulates Pacific water to blow towards Indian Ocean at very fast pace.
These are the two main cause which drive Indian Ocean to the most warm ocean. And the irregular varying of temperature between Eastern and Western side of Indian Ocean develops its two dipole.


3 stages of Indian Ocean Dipole
  1. Positive IOD
  2. Negative IOD
  3. Neutral IOD
1. Positive IOD : It happens when Western side of Indian Ocean is more warm than its Eastern side. So, this side receives higher rainfall and the Eastern parts face the severe drought condition in the same time.

Process of formation
  • Warm Pacific Ocean water flows between islands of Indonesia and Australian continent towards the western part of Indian Ocean. Strong easterly winds that is depicted in the above figure, stimulates the intensity of this flow. Due to strong easterlies, hotter water than normal accumulates near the eastern cost of Africa. 
Now, western side of Indian Ocean has more warm water than normal, and its eastern side has more cool water than normal because of the process of thermocline and upwelling in the ocean. Thermocline is a thin and distinct layer into any large fluid body in which temperature declines rapidly from warm water at surface of the ocean to cool water at the depth of ocean.                        Upwelling is a process in which currents bring deep, cold water to the surface of the ocean. Upwelling is a result of winds and the rotation of the Earth.                                  So, due to the process of upwelling, cold and deep water moves towards the eastern part of Indian Ocean and much of warm water remains on the side of western Indian Ocean.                                                         .                       
                                                        
  • Due to the low pressure and high temperature on the western side of Indian Ocean, warm air rises upward. After the certain height, air condense and makes clouds. The formation of clouds on the western side leads to a torrential rainfall on its side. India also receives much of rainfall due the Positive IOD.
  • On the same time, eastern parts of Indian Ocean face severe drought condition owing to high pressure and low temperature. 
  • The blowing pattern of wind above the sea level is western to eastern side, and the same on the surface level follows eastern to western side.

2. Negative IOD :  It happens when Eastern side of Indian Ocean is more warm than its Western side. So, this side receives higher rainfall and the western parts face the severe drought condition in the same time.

Process of formation

  • Due to the general circulation of atmosphere, now, the westerly winds become strong and easterly become weak. Therefore, the warm water of western side blows towards eastern side of Indian Ocean.
  • Due to the process of upwelling and thermocline, eastern side has warm water than normal and  just opposite condition in the western side.
  • Now, the same process of cloud formation happens and this side gets torrential rainfall. Same time, western parts of Indian Ocean, including Indian continent, face a severe drought condition.

3. Neutral IOD : In this phase, temperature is close to normal across the tropical Indian Ocean and hence, Neutral IOD results in little change to Australian as well as Indian climate.
Temperature is close to normal, nevertheless some parts of eastern Indian Ocean receives few spell of rainfall and western side faces normal drought condition.

Indian Ocean Dipole may be a trigger for locusts breeding

The phenomenon called Indian Ocean Dipole, in which western and eastern parts warm differentially, may be the root of locusts breeding. Positive IOD - western part warms more than eastern part and it results to a torrential rainfall in the South-west Asian region and horn of Africa - is the most acceptable stimulus in the locusts reproduction. Last year, in June 2019, South-west Asian nations, including Indian sub-continent, Oman, Yemen and horn of Africa - Somalia, Ethiopia & Kenya receive a heavy rainfall which lasted nearly month more than what is normal. It surpassed the concerns of drought condition across the nations. The forming of positive IOD began in late 2018. Due to the heavy rainfall, these regions dry sand became moisture and it helped to the production of locusts in these traditional epicenters of insects.
Later, these swarms moved towards the traditional grounds of Iran, Afganistan, Pakistan and India with the favourable condition of winds.











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