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. 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. 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.