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  • 21 September, 2021

  • 15 Min Read

New Monsoon Forecast Model and Madden Julian Oscillation

New Monsoon Forecast Model

  • The India Meteorological Department (IMD) may introduce new monsoon models this year to better forecast changes in rainfall.
  • The monsoon that concluded in 2020 was unique, in that with monsoon 2019, it was only the third time in a century that India saw back-to-back years of above normal rainfall. In both years — and monsoon 2019 was a 25-year high — the IMD failed to forecast the magnitude of the excess and only indicated that the monsoon would be “above normal”.
  • D.S. Pai, who heads the IMD’s Climate Research Services, Pune, said in his talk there were three different models that could be tested this year. Two of them were dynamical models and one a statistical model.

Computer simulation

  • In the former, the climate on any particular day is simulated on supercomputers and meteorologists observe the changing daily output. The other is the traditional statistical model that equates relationships of physical parameters, such as for instance sea surface temperatures, snowfall, the temperature of landmass etc, with the actual observed rainfall in the past.
  • The three models under consideration are:
  1. 12 global circulation models (dynamical) whose outputs would be combined into a single one;
  2. a model that gauges rainfall based on the sea surface temperature in the tropics (developed by Professor Sumant Nigam, University of Maryland, U.S.) and
  3. the statistical model based on climate variables observed during the pre-monsoon. All of them are ‘ensembles’ meaning smaller models are combined to arrive at an average value.
  • M. Mohapatra, Director General, IMD, told The Hindu that the traditional statistical model would continue to be used this year. “However, we will continue to have discussions and will decide later on what new approach can be added.”

What is Madden Julian Oscillation?

  • It is an oceanic-atmospheric phenomenon which affects weather activities across the globe. It brings major fluctuation in tropical weather on weekly to monthly timescales.
  • 2. The MJO can be defined as an eastward moving 'pulse' of clouds, rainfall, winds and pressure near the equator that typically recurs every 30 to 60 days.
  • 3. It’s a traversing phenomenon and is most prominent over the Indian and Pacific Oceans.

Phases of MJO (Madden Julian Oscillation)

The MJO consists of two parts or phases. Strong MJO activity often dissects the planet into halves. One half within the enhanced convective phase and the other half in the suppressed convective phase.

1. Enhanced rainfall (or convective) phase:

Winds at the surface converge, and the air is pushed up throughout the atmosphere. At the top of the atmosphere, the winds reverse (i.e., diverge). Such rising air motion in the atmosphere tends to increase condensation and rainfall.

2. Suppressed rainfall phase:

  • Winds converge at the top of the atmosphere, forcing air to sink and, later, to diverge at the surface. As air sinks from high altitudes, it warms and dries, which suppresses rainfall.
  • It is this entire dipole structure, that moves west to east with time in the Tropics, causing more cloudiness, rainfall, and even storminess in the enhanced convective phase, and more sunshine and dryness in the suppressed convective phase.

How Does MJO Affect Indian Monsoon?

  • The Indian Ocean Dipole (IOD), El Nino and MJO are all oceanic and atmospheric phenomena, which affect weather on a large scale. IOD only pertains to the Indian Ocean, but the other two affect weather on a global scale-up to the mid-latitudes.
  • IOD and El Nino remain over their respective positions, while MJO is a traversing phenomenon.
  • The journey of MJO goes through eight phases.
  • When it is over the Indian Ocean during the Monsoon season, it brings good rainfall over the Indian subcontinent.
  • On the other hand, when it witnesses a longer cycle and stays over the Pacific Ocean, MJO brings bad news for the Indian Monsoon.
  • It is linked with enhanced and suppressed rainfall activity in the tropics and is very important for the Indian monsoonal rainfall.

Periodicity of MJO:

  • If it is nearly 30 days then it brings good rainfall during the Monsoon season.
  • If it is above 40 days then MJO doesn't give good showers and could even lead to a dry Monsoon.
  • Shorter the cycle of MJO, better the Indian Monsoon. Simply because it then visits the Indian Ocean more often during the four-month-long period.
  • Presence of MJO over the Pacific Ocean along with an El Nino is detrimental for Monsoon rains.

Source: TH


26 Oct,2021

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