Variation in monsoon rainfall

The month of June was followed by heavy rainfall in August and September, contrary to the forecasts of meteorologists.

Important point

monsoon
monsoon

  • This year’s monsoon saw a 30% decrease in the normal average rainfall for the month of June, in contrast the average monsoon received 10% more rainfall. This type of incident has been seen for the first time since 1931.
  • The Long Period Average- LPA for September month rainfall was 152%, the highest since 1917 and the LPA for August rainfall at 115%, the highest since 1996. In addition, LPA remained 110% of the overall monsoon rainfall, the highest since 1994.
  • At the end of the first week of September, the Meteorological Department of India had reported that rainfall would be 96–104% of the LPA in this monsoon year.
  • El Nino Southern Oscillation- ENSO of the Pacific was one of the factors influencing the monsoon rainfall in India this year, even after this, meteorological investigation of the causes of unexpected rainfall in India. .

Indian Ocean Bipolar (IOD)

  • The Indian Ocean dipole is a phenomenon of the Ocean – Atmosphere Interaction, similar to El-Nino.
  • IOD arises due to differences in sea surface temperatures in the western Indian Ocean (Arabian Sea) and the eastern Indian Ocean (south of the Indonesian coast).
  • In this phenomenon, the water in the western Indian Ocean is warmer than the eastern Indian Ocean, it is called positive IOD and its opposite state is negative IOD.
  • In the equatorial Pacific Ocean, IOD affects weather and climate events in the same way as El-Nino, although its impact is weaker because the area of ​​the Indian Ocean is less than that of the Pacific Ocean, in addition to the Indian Ocean than the Pacific Ocean. It is also shallow.
  • IOD also affects the Indian monsoon. Monsoon rainfall during positive IOD has adverse effects on monsoon rainfall during positive and negative IOD.

monsoon rainfall
monsoon rainfall

Unusual rainfall of this year:

  • This year, IOD began around June and became stronger after August. This year’s IOD was slightly stronger than usual.
  • IOD records are not very old. According to the Australian Meteorological Bureau its exact measurements have been available since the year 1960. In the current year, IOD was at a level of + 2.15 ° C (positive) which is the strongest since 2001.
  • Thus, this change of IOD is yet to be considered by meteorologists as a possibility for this year’s unpredictable rainfall in India because of the high rainfall in the monsoon in the event of positive IOD in previous years.
  • Positive IOD conditions were observed in the first year of the current year in 1997 and in the year 2006, the southwest monsoon rainfall in India was seen above normal in both the years.

monsoon rainfall in india
monsoon rainfall in india

This year’s forecast:

  • In fact the effect of IOD on the Indian monsoon rainfall has not been fully understood. One reason for this is that the weather forecast is wrong.
  • It is generally believed that the effect of IOD is less on the rainfall of India than in El-Nino, but no specific studies are available regarding this view.
  • IOD usually develops in the latter part of monsoon rainfall i.e. August and September and according to meteorologists, the possibility that the monsoon may play an important role in the origin of IOD as well.
  • This year, sea surface temperatures were lower than normal due to the presence of cold winds on the Sumatra coast (eastern Indian Ocean) but temperatures in the Arabian Sea (western Indian Ocean) were seen to be normal. For this reason also, the forecasts expressed by meteorologists were not accurate.
  • According to the Australian Bureau of Meteorology, there have been only 10 positive positive IOD incidents since 1960. Out of these, a decrease in monsoon rainfall was observed in four years, an increase in four years and a trend of normal rainfall in the remaining two years.

Long term average (LPA)

  • The LPA is a 50-year average rainfall across the country during the southwest monsoon.
  • Currently India’s LPA 89 cm. Is based on the average rainfall of the period 1951–2000.
  • It serves as a criterion on the basis of which rainfall is measured in any monsoon season.
  • According to meteorologists, if LPA is less than 90% of the rainfall in a year in the country, the rainfall is considered to be below average. In contrast, LPA has 110% more rainfall than the average rainfall.
  • Thus, if the percentage of rainfall in a year is more than LPA, then more rainfall and if the percentage of rainfall is less than LPA, less rainfall is considered.
  • The LPA of monsoon rainfall in India is generally between 96–104%.

Importance of LPA:

  • Through this, average rainfall of 50 years is used because there is a lot of variability in India’s annual rainfall.
  • The effect of seasonal events like El-Nino and La-Nina is observed once every three or four years on the monsoon rainfall of India.
  • Severe droughts, floods and hurricanes in India arise due to seasonal events such as El-Nino and La-Nina, derived from the unusual surface temperatures of the Pacific Ocean.
  • The Indian Meteorological Department’s rainfall forecast through LPA also goes wrong at times like normal rainfall was predicted in the year 2013 but actual rainfall remained at 106% above the LPA level which is well above the average for normal rainfall.
  • Many forecasts of rainfall through LPA have proved to be wrong but such forecasts help the government and farmers to formulate better strategies. Through this, the government can make better preparations by adopting a protective approach for drought or flood affected areas. .

Collection of rainfall data by Indian Meteorological Department:

  • The rainfall data collected by the Indian Meteorological Department is based on the actual rainfall recorded at 2,412 sites through 3,500 rain-gauge stations.
  • Based on the daily rainfall data received from these rain-gauge stations, the monsoon data are prepared for administrative areas such as districts, states and countries.
  • These data are released at the level of the overall country after compilation of data from 36 meteorological subdivisions, four broad regions – South Peninsula, North-West India, Central India and North-East India.

rainfall in india
rainfall in india

Area wise details of rainfall:

  • Similar to nationwide statistics, the Indian Meteorological Department issues an independent LPA for each homogeneous regions of the country, averaging 71.6–143.83 cm for these regions. Is between
  • The region-wise LPA figures are as follows:
    • 143.83 cm for East and Northeast India.
    • 97.55 cm for Central India.
    • 71.61 cm for South Peninsular India.
    • 61.50 for North-West India
  • Monthly statistics of LPA during the monsoon season are as follows:
    • June 16.36 cm
    • July 28.92 cm
    • August 26.13 cm
    • September 17.34 cm
  • The average rainfall of a region is compared with the rainfall data of the last 50 years for the average rainfall of an area. According to the output of the data thus obtained, the level of rainfall is categorized.
  • According to the Indian Meteorological Department, the normal rainfall pattern in India is divided into the following categories-
  • Normal or Near Normal: Actual rainfall +/- 10% or 96-104% of normal LPA.
  • Below Normal: Actual rainfall 10% less than normal LPA or 90-96%.
  • Above Normal: Actual rainfall is between 104-110%.
  • Deficient: Actual rainfall less than 90%.
  • Excess: Actual rainfall more than 110%.

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