A global sweep of westerlies, which has triggered severe flooding across central Europe, fused with the south-west monsoon over north India, causing the deadliest rains in decades, a top Met official has said. Meteorologists investigating the factors behind the devastation said although such extreme weather events were not uncommon in the Himalayan states, such as Uttarakhand, the monsoon could
not have, on its own, resulted in heavy rainfall.
Driven by westerlies, western disturbances, or extra-tropical storms originating over the Mediterranean, swept into the north Indian hilly states around mid-June, where a monsoon-related low-pressure system had already moved in from Bay of Bengal, meteorologists said.
The convergence over Uttarakhand caused severe rainfall, as swollen rivers barreled down gorges and valleys, burying hotels, houses and people.
“When westerlies encounter the monsoon, they don’t leave each other easily,” said S Damodar Pai, the Meteriological department’s chief long-range forecaster.
The monsoon is relatively weaker in the Himalayan region, which is located near the “periphery” or the northern limit of the monsoon that brings up to 80% of India’s annual rainfall.
The westerlies virtually locked on to the monsoon system, Pai said, the two systems feeding moisture into each other and giving rise to intense interaction.
While monsoon currents progress from south to northwest, western disturbances move across north India from west to east, driving up pressure.
In fact, in some Himalayan areas, the monsoon is activated by western disturbances.
This summer, the westerlies have left a trail of deadly flooding in central European nations, such as Germany, Austria and the Czech Republic. Some of these westerlies intruded into India from Afghanistan, said R Krishnan, a senior meteorologist with Indian Institute of Tropical Meteorology.