Unusual Rainfall During October- November

Unusual Rainfall During October- November

Subject: Geography

Section: Climatology

Context- The catastrophe unleashed during October and November 2021 defies logic as none of the two rain-bearing weather systems was active during the period.

Concept-

• The south-west monsoon began withdrawing on October 6. Though it should have completely withdrawn from the country by October 15, the process continued till October 26, during which the wind system remained weak.
• The delayed withdrawal of the southwest monsoon also pushed the entry of the northeast monsoon, which brings rainfall over the southern peninsular region, towards the end of the month.

Then what caused the incessant rainfall?

• Analysts link the sustained rains to the interplay of a few unique weather events at a time when the atmospheric temperature was unusually high.
• The entire northern hemisphere, including the oceans, has remained warm from January through October in 2021, with the only exception of the Siberian High.
• The Arctic sea-ice loss this year led to high sea-level pressure over western Europe and northeastern China, which steered planetary waves southeastward instead of their eastward trajectory.
  • These waves, which produce circulation anomalies in the upper atmosphere, entered India late in the season and delayed the monsoon withdrawal.
• Then La Niña kicked in on October 14. This cooling phase of the El Niño Southern Oscillation (enso) phenomenon, during which the sea surface temperatures over the eastern and central Pacific Ocean remain cooler than average, affects the trade winds which carry this weather disturbance across the world.
• In India, La Niña generally triggers the formation of low-pressure areas, which contribute to increased rainfall.
  • Thus, a moisture-laden atmosphere combined with La Niña conditions is likely to have created conducive conditions for the formation of more low- pressure areas, and rainfall events.
• The other reason for the unusual rains is the warming up of the Arabian Sea over the past decade.
  • This causes higher evaporation which increases the chances of the formation of a low-pressure area.
  • Traditionally, the sea surface temperature of the Arabian Sea was 1.5-2 per cent lower than that of the Bay of Bengal. But the temperature has increased recently,
• Another weather anomaly that brought more than usual showers is the way low- pressure areas moved north and interacted with the Western Disturbances—extratropical storms in the upper layers of the atmosphere that travel from the Mediterranean region and bring most of the rainfall and snowfall to the north and northwestern India during the winter season.