Droughts in India have historically been associated with El Nino, anomalous warming of the equatorial Pacific, but Indian scientists have found some relevance in Rossby Waves.

Q.The determinants of Indian Monsoon are no more limited to the Pacific and the Indian Ocean. Discuss.

El-Nino alone do not cause drought

• The study says that nearly six out of 10 droughts, in non-El Nino years occurred during the Indian summer-monsoon season in the past century.
• They may have been driven by atmospheric disturbances from the North Atlantic region.
• In an El Niño year, abnormally warm equatorial Pacific waters pull moisture-laden clouds away from the subcontinent.
• But the IISc Bangalore study shows that in non-El Nino years, these droughts are a consequence of a sudden and steep drop in rainfall in late August.

Then, how were droughts induced?

• In an El Nino year, the rainfall deficit departure from a long-term average set in early around mid-June and progressively worsen.
• Researchers tried to trace this drought back to a forcing agent or system that influences the behaviour over India.
• They found, the winds that were prevalent in these non-El Niño drought years.

Another factor: The Rossby Waves

• The researchers noted that winds in the upper atmosphere are interacting with a deep cyclonic circulation above the abnormally cold North Atlantic waters.
• The resulting wave of air currents called a Rossby wave, curved down from the North Atlantic squeezed in by the Tibetan plateau and hits the subcontinent around mid-August.
• This has a suppressing effect on rainfall and throws off the monsoon that was trying to recover from the June slump.

Now scratch your basics on Planetary Winds. “Go back to the NCERTs !”

What are Rossby Waves?

• They are giant meanders in high-altitude winds that have a major influence on the weather.
• They are influenced by the Coriolis force and pressure gradient.
• The wave’s usual course is to go from west to east, but not towards the equator.

Points to be noted ……

• The Indian Ocean and the Pacific Ocean seem to be at the forefront of all discussions surrounding Indian monsoon droughts.
• Thus beyond looking at the Pacific Ocean it is important to consider other influences on the Indian monsoon from outside the tropics.
• It is perhaps time to focus just as much on mid-latitude influences, which might aid in getting a better handle on enhanced predictability of monsoon variability.

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