Warming of Pacific Ocean likely to increase cyclones in India: Study

Tropical cyclones that have been unusually rare in recent decades have formed near the equator. Cyclone Ockchi in 2017, which devastated Kerala, Tamil Nadu, and Sri Lanka, was the last major cyclone in the Indian neighborhood.

Warming, PDO may increase cyclones

A study published in the journal Nature Communications suggests that a combination of global warming and a cyclical phenomenon called the Pacific Decadal Oscillation (PDO), which repeats every 20 to 30 years, could make such cyclones more frequent in the coming years.

Between 1981-2010, equatorial-origin cyclones decreased by 43% compared to 1951-1980 due to the 'warmer' positive phase of PDO. El Nino warms Central Equatorial Pacific, leading to reduced Indian rainfall. Excessive rain relates to cooler-than-normal temperatures or La Nina.

The El Nino Southern Oscillation (ENSO) phenomenon repeats in the Pacific over seven years. This pattern is collectively called the ENSO phenomenon. However, the Pacific Decadal Oscillation (PDO) isn't an annual occurrence and, on average, it corresponds to a warmer than average Western Pacific Ocean and relatively cooler Eastern Pacific, though this plays out over much longer time scales. However, unlike an ENSO, researchers can determine the stage of a positive or warmer phase of a PDO only after several years of measuring ocean temperatures and their interaction with the atmosphere.

The number of such equatorial-origin cyclones was 43 per cent less from 1981 to 2010 than from 1951 to 1980, and this was due to the warm or positive phase of the Pacific Decadal Oscillation (PDO). El Nino warms central equatorial Pacific, causing reduced Indian rain. La Nina brings below-normal temperatures, linked to excessive rain.

ENSO and PDO ocean patterns

This pattern, collectively called the El Niño Southern Oscillation (ENSO) event, is repeated every two to seven years in the Pacific. However, the Pacific Decadal Oscillation (PDO) is not a year-round phenomenon and corresponds to an average warmer western Pacific Ocean and a relatively cooler eastern Pacific Ocean, albeit over a much longer period.

ENSO determines its phase yearly, while knowing the PDO's 'positive' or 'warm phase' necessitates measuring ocean temperatures and atmosphere interactions for years.

M Rajeevan, former secretary of the Ministry of Earth Sciences and co-author of the paper, said, "Cyclone Ockhi (2017) which devastated Kerala and formed near the equator inspired this study. Tropical cyclones (TCs) forming near the equator are very rare. According to this study, the Pacific Decadal Oscillation (PDO), a Pacific climate phenomenon with a 21-year cycle, can influence low-latitude cyclones by increasing the low-level vorticity, which is a dynamical parameter for cyclone genesis. Currently, PDO has entered into a negative phase, which should favour low latitude cyclones."

He added, "The Peninsular coast is theoretically at greater risk of such cyclones, but various other parameters also influence cyclone development."

Warm waters intensify equatorial cyclones

Cyclones are generally rare to form near the equator, but when the waters warm, they can gain more moisture and increase in intensity.

At present, the Pacific Ocean is undergoing El Nino development, which is already affecting central and southern India. The study reports a rainfall deficiency of 7 to 17 percent due to this phenomenon. The Pacific Decadal Oscillation (PDO) has been less studied as a phenomenon.

The El Nino Southern Oscillation (ENSO) with a positive Pacific Decadal Oscillation (PDO) is generally not good, but when combined with a negative PDO, it brings more rain to India.

The authors concluded that the reduced low-level vorticity and increasing vertical wind shear primarily cause the decline in LLC frequency in the recent epoch.

“The paper said that when the natural and anthropogenic forcing changes and they start working synergistically, the risk of severe cyclones in the post-monsoon north Indian Ocean may be amplified. These results may guide planning and mitigating LLC-induced disaster in the Indian subcontinent.”

Ockhi, a recent LLC, was developed in November 2017 and travelled over 2,000 km. It devastated parts of Sri Lanka and India, causing extensive damage to properties worth US$920M and killing 884 persons.

"The occurrence of tropical cyclones over these regions is rare as it is the first very severe cyclone to form over the Lakshadweep Sea since 1925. The study team was curious about investigating near-equatorial cyclones and their frequency, as stated by the authors.

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