Beware Kerala, 20cm rain in 4 hours at Kochi reveals global warming's bare fangs

The scary surge of rains in Southern and Central parts of Kerala on October 21, climatologists say, has offered a peek into the havoc that global warming can inflict.

The rainfall intensity nearly resembled a cloudburst, a dangerous phenomenon that can cause landslides and flash floods.

It is called a cloudburst when a region receives 10 centimetres of rain within an hour.

Kochi received nearly 20 cms in about four hours on October 21. Slightly to the south, Mankombu in Alappuzha was buried under 17 cms of rain in three hours.

According to weathermen, such a burst of calamitous rain, which almost threatened to drown the byelections in Ernakulam, has been caused by what in meteorological jargon is called a strong Indian Ocean Dipole (IOD).

Stripped of all scientific complexity, this refers to the sharp temperature difference between the western and eastern parts of the Indian Ocean.

Temperature difference

This time, the western part of the ocean, which is way off the coast of Kerala, was unusually warmer.

“We have just witnessed the strongest IOD in the last 20 years,” said Dr S Abhilash, Associate Director at the Centre of Atmospheric Sciences at Cochin University of Science and Technology.

“The intensity of the IOD is represented by the anomalous sea surface temperature (SST) gradient between the western equatorial Indian Ocean and the south eastern equatorial Indian ocean. This gradient is called Dipole Mode Index,” Abhilash said.

Now the DMI is plus 2. A positive DMI means “abnormal warming” in the western Arabian sea.

As a consequence, warm air rises on the western part of the ocean and cool air from the east rushes in to take the place.

When the rising warm air meets with the horizontally moving easterly winds they lock together in a kind of atmospheric tango creating a low pressure formation characterised by swirling motion.

This formation intensifies into a marked low pressure area and then into a depression that can let loose water-laden hurricanes like the Ockhi along the coast.

Thankfully for Kerala, the Indian Meteorological Department said the low pressure area that had been formed over east and central Arabian sea has moved north westward towards the African coast. That is the reason why the northeast monsoon looked relatively calm on Tuesday. Latest satellite images, both from INSAT and METEOSAT, show rain clouds moving gradually away along the north-northwestwards direction. The IMD has also withdrawn all red alerts.

Cyclical cloud movement

There is yet another reason why the northeast monsoon is so prodigious this time; already, Kerala has received an excess of 40 per cent rainfall. This phenomenon, along with the Indian Ocean Dipole, has brought heavy rains to Kerala and is called the Madden-Julian Oscillation.

This oscillation, however, was not provoked by global warming, and is so named because it was discovered in 1971 by Roland Madden and Paul Julian of the American National Center for Atmospheric Research.

Madden-Julian Oscillation is the cyclical movement of rain-laden clouds from the west to the east over the warm regions of the Indian and Pacific oceans in intervals of 30 to 60 days.

At this point, the MJO happens to be right over the Indian Ocean where it has the potency to shower copious rain on the southern Indian Peninsula.

Skymet Weather reports that 2019 has been a unique year in that all the five subdivisions of the southern peninsula have performed well. South Interior Karnataka is surplus by 70 per cent, Rayalaseema by 26 per cent and Kerala by 40 per cent. Coastal Andhra Pradesh is deficient by three percent while Tamil Nadu is surplus by three per cent.