What’s in news?
The India Meteorological Department (IMD) will issue its first long-range forecast for the south-west monsoon on April 15, 2019 (Monday).
- The forecast of a below average monsoon in 2019 on the back of a prospective El Nino that is often associated with less rainfall has come from a private agency, Skymet.
- The Pacific Ocean has become strongly warmer than average. Even as things may get clearer after the India Meteorological Department’s forecast, Skymet’s Managing Director said they would look into the weather phenomenon called El Nino and its impact on the monsoon.
- According to the latest global forecasts, weak El Nino conditions have developed over equatorial Pacific Ocean and they are likely to persist this summer.
- The south-west monsoon, which makes its onset over India around May-end, is critical for its over 100-million farmer families
- The south-west monsoon, which makes its onset over India around May-end, is critical for its over 100-million farmer families. The four-month rainy season contributes more than 70% of India’s annual showers.
- The forecast will be closely watched as the IMD will also highlight the global impact of El Nino, and its effect on this year’s monsoons in India.
- The phenomenon is associated with below normal monsoon rains and droughts.
- If El Nino retains strength and impacts monsoon rains in June and July, the first two months of the season, it could lead to delay in sowing of rain-fed kharif crops affecting overall crop production.
- Already we faced drought conditions in 2014 and 2015, primarily because of the effects of El Nino, but the monsoon rains improved in 2016 with India receiving normal rainfall in the four months between June and September.
- El Nino is an abnormal warming of water in the central and eastern tropical Pacific Ocean every three to five years and can last up to 18 months.
- Climate records of El Ninogo back millions of years, with evidence of the cycle found in ice cores, deep sea muds, coral, caves and tree rings.
What causes an El Niño?
- Normally, due to the presence of cold Peru Current (Humbolt Current) along the coast of Peru (South America), there is high pressure area in the eastern Pacific Ocean along the Peru Coast.
- Therefore, air moves from the eastern Pacific Ocean to the western Pacific Ocean where there is comparatively a low pressure.
- Because of the warmer oceans this air gets lots of moisture due to evaporation on the way.
- This warm moist air rises to high levels of the atmosphere in the western part of the Pacific Ocean and causes rainfall in Indonesia, eastern and northern Australia etc.
- The rise of warm air finally results in its cooling in the upper atmosphere. A part of this air moves towards the eastern Pacific Ocean in the upper atmosphere and descends over the eastern Pacific Ocean and helps to sustain the higher pressure there and again moves towards the western Pacific Ocean.
- This way we have a complete circulation of air between eastern and western Pacific Ocean. This circulation is called Walker Circulation.
- During El Nino years, due the abnormal warming of water in the central and eastern tropical Pacific Ocean the high pressure area in the eastern Pacific Ocean weakens, thus the wind moving towards the east (called easterly wind) weakens or even reverses its direction.
- Thus there is less rainfall/drought in the western Pacific Ocean (Indonesia, eastern and northern Australia and nearby areas including India etc.) whereas due to the presence of warm water in the eastern Pacific Ocean near Peru cloud formation takes place which causes heavy rainfall/floods in the coastal Peru.
- The atmospheric pressure in the eastern Pacific Ocean is measured at a place called Tahiti whereas the atmospheric pressure in the western Pacific Ocean is measured at a place called Darwin.
- During normal years there is high pressure at Tahiti and low pressure at Darwin. This situation exactly reverses during El Nino years and this phenomenon is called Southern Oscillation.
- There is also an opposite of an El Nino, called La Nina.
- This refers to times when waters of the tropical eastern Pacific are colder than normal and trade winds blow more strongly than usual.
- El Nino and Southern Oscillation together sometimes called ENSO.
- Typically, El Nino occur more frequently than La Nina.
Effect of El Nino:
- During an El Nino, the Pacific’s warmest surface waters occurs offshore of northwestern South America.
- Weakening of prevailing Easterly Trade Winds.
- The location of tropical storms shifts eastward during an El Nino because atmospheric moisture is fuel for thunderstorms, and the greatest amount of evaporation takes place above the ocean’s warmest water.
- Record rainfall often strikes Peru, Chile and Ecuador during an El Nino year.
- Indonesia and northeastern South America tend toward drier-than-normal conditions.