Tropical cyclones that originate near the Equator, while being devastating, have been unusually subdued in recent decades. The last major cyclone of this kind in the Indian neighbourhood was the 2017 Cyclone Okchi which devastated Kerala, Tamil Nadu and Sri Lanka. However, a combination of global warming and a cyclical event called the Pacific Decadal Oscillation (PDO) that repeats every 20-30 years, could make such cyclones more frequent in the coming years, a study published on Monday in the journal Nature Communications said.

The number of such equatorial-origin cyclones was 43% fewer in 1981-2010 compared with 1951-1980, and this was because the PDO was in a  ‘warmer’ or positive phase. A warming of the Central Equatorial Pacific, called an El Nino, frequently corresponds to reduced rainfall over India whereas cooler-than-normal temperatures, or a La Nina, is linked to excessive rainfall. This pattern collectively called the El Nino Southern Oscillation (ENSO) phenomenon, repeats in the Pacific over two-seven years. However, the PDO isn’t an annual occurrence and, on average, 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, whose stage can be determined any year, a ‘positive’ or ‘warmer phase’ of a PDO can be known only after several years of measuring ocean temperatures and their interaction with the atmosphere.

In 2019, the PDO entered a cooler, negative phase and if it remains so, could mean more tropical cyclones in the post-monsoon months that originate near the equator, R.S. Ajayamohan, meteorologist at the Abu Dhabi Polytechnic, United Arab Emirates, and among the authors of the study, told The Hindu. “It’s usually rare for cyclones to form near the Equator but when the waters are warm, they can gain more moisture and rise in intensity,” Dr. Ajayamohan said.

An El Nino is currently developing in the Pacific, the effects of which are already manifested in central and southern India, which have recorded rainfall deficits of 7% and 17% respectively. “PDO as a phenomenon is less studied. ENSO with a positive PDO is generally not good, but when linked with a negative PDO, brings more rain to India,” M. Rajeevan, former Secretary in the Earth Sciences Ministry and a co-author of the study, said.