Magazine

English Hindi

Index

100 Expected Questions

Prelims Capsule

Geography

IMD’s 2019 Monsoon Forecast

IMD’s 2019 Monsoon Forecast

Relevance:

  • GS 1 || Geography || Indian Physical Geography || Monsoons

Why in news?

  • In its first long-range forecast for the season, the Indian Meteorological Department (IMD) Monday predicted a “near normal” southwest monsoon.

Highlights

  • IMD projected that rainfall was expected to be 96% of Long Period Average (LPA).
  • This is a more optimistic assessment from the one by private weather forecasting agency, Skymet, which earlier this month, warned of ‘below normal’ rains June-September.
  • However, uncertainty clouds this optimism as the agency’s own assessment suggests a significant probability for rains falling in the ‘below normal’ category.
  • The IMD issues its first monsoon forecast in April and then updates it in June with details on how the monsoon will perform in various geographical regions.
    • The April forecast can be unreliable. Last year, the IMD forecast ‘normal rains’ (97% of LPA) and India saw below normal (91% of LPA) rains.
    • This was attributed by the IMD, to an unexpected weakening of the monsoon rains in North Eastern India.

Rationale

  • Weakening El Nino: The IMD’s optimism stems from global climate models projecting a ‘weakening El Nino.’
    • The El Nino, a cyclic warming of the Central and Eastern Pacific region, has historically been linked to a weakening of monsoon rain.
    • A temperature rise greater than 1° C for three months is considered a ‘strong’ El Nino (and threatening to the monsoon).
    • A 0.5°C -1°C rise is called ‘weak El Nino conditions.’
    • Currently the El Nino is 0.9 C.
  • The IMD’s models in March, expect the El Nino to peak around May and then recede for the rest of the monsoon months.
  • Globally too, other models that track El Nino expect it to recede after June or July.
  • All these reflect in IMD’s forecast (of a normal as opposed to below normal monsoon rains). In any given year, the odds of ‘below normal’ rains are 17%.
  • Positive IOD: Another factor, called a positive Indian Ocean Dipole (IOD) (which refers to a warming in the western Arabian ocean) could neutralise the potential negative impact from the El Nino.

Calculation

  • The India Meteorological Department (IMD) expresses the projected rainfall in terms of Long Period Average (LPA).
  • The LPA is the average rainfall recorded during the months from June to September, calculated during the 50-year period, and is kept as a benchmark while forecasting the quantitative rainfall for the monsoon season every year.
  • Like the countrywide figure, IMD maintains an independent LPA for every homogeneous region of the country, which ranges from 71.6 cm to 143.83 cm.
  • The region-wise LPA figures are:
    • 83 cm for East and Northeast India,
    • 55 cm for Central India,
    • 61 cm for South Peninsular India, and
    • 50 for Northwest India,
    • which put together bring the all-India figure to 88.75 cm.
  • The monthly LPA figures for the season are 16.36 cm for June, 28.92 cm for July, 26.13 cm for August and 17.34 cm for September.
  • IMD maintains five rainfall distribution categories on an all-India scale. These are:
    • Normal or Near Normal: When per cent departure of actual rainfall is +/-10% of LPA, that is, between 96-104% of LPA
    • Below normal: When departure of actual rainfall is less than 10% of LPA, that is 90-96% of LPA
    • Above normal: When actual rainfall is 104-110% of LPA
    • Deficient: When departure of actual rainfall is less than 90% of LPA
    • Excess: When departure of actual rainfall is more than 110% of LPA

Mains question

  • What is El Nino? Analyze its impact on Indian monsoon. How IMD calculates monsoon level?