Study on Variability of Monsoon (Download PDF)


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The Government of India undertook study on variability of monsoon especially in South Asia. Monsoon is the manifestation of seasonal migration of the intertropical convergence zone (ITCZ). Vertical moist stability is the key parameter in the variation of monthly convection over ocean and land as well as poleward propagations.

Image of Indian Summer Monsoon Rainfall Epochal Variation

Image of Indian Summer Monsoon Rainfall Epochal Variation

Image of Indian Summer Monsoon Rainfall Epochal Variation

The long term mean of the Indian summer monsoon rainfall (ISMR) is about 85cm. The diagram shows interannual (year to year) variation of the anomaly (defined as the actual value in any year minus the mean value) of ISMR as a percentage of the mean value.

  • For Indian monsoon mean seasonal pattern is described
  • It is difficult to simulate monsoons
  • Monsoon varies on all time scales from daily to seasonal, decadal, and centennial.

Key Measures of Variability

  • Rainfall over the Indian region differs in terms of
  • Seasonal mean
  • Dates of onset and retreat differ from the mean dates
  • Within the peak monsoon months of July and August, wet spells with continuous rains for a few days are interspersed with relatively dry spells of varying duration (Monsoon Breaks). There is variation in these monsoon breaks

Facets of the Variation

  • active-weak cycles
  • break monsoon
  • poleward propagations of the ITCZ at intervals of 2–6 weeks

Reasons for Variations

  • Connection between El Nino Southern Oscillation (ENSO), monsoon, and its time varying relationships- current seasonal prediction models currently able to capture these tele-connections.
  • There is sub-seasonal variability of monsoon due to ENSO and Indian Ocean.
  • There is relation between changing mean state of Indian Ocean and its role on possible weakening of ENSO monsoon tele-connections
  • Relationship between South-East Asia rainfall variability with Indian rainfall variability is established.

- Published/Last Modified on: August 12, 2017


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