Ahead of every monsoon, meteorologists track, with a degree of nervousness, temperatures in the central and eastern Pacific Ocean. Six in ten years, a half degree or more rise — an El Nino — corresponds to diminished rainfall in India. The converse, or a La Nina, is linked to increased rain. A study last week however suggests that this cyclical swing — called the El Nino Southern Oscillation (ENSO) — affects vast regions of India differently.
Since 1981, the study published this week in Scientific Reports notes, monsoon rainfall over Central India — known as the monsoon core zone and where agriculture is largely rainfed — is increasingly getting disassociated from the ENSO with only 10% of droughts or excess rains linked to ENSO fluctuations. On the other hand, the ENSO link to North India was strengthening, with 70% of rainfall fluctuations linked to the ENSO cycle. In southern India, the relationship has remained largely stable.
While past research into monsoon patterns have suggested a “weakening” of the relationship between ENSO and monsoon, the latest suggests that this too has varied since 1901.
“We notice that the ENSO–ISMR inverse relationship started getting stronger from 1901 to 1940, became stable from 1941 to 1980 and then the relationship has weakened in the recent epoch (1981 onwards),” the authors note in their study.
Monsoon rainfall, which accounts for 80% of India’s annual rainfall, is influenced by two broad factors: The external one is the impact of ENSO which influences the trade winds and their ability to carry warm, moist air towards India around monsoon. The other, internal, is the ‘monsoon trough — an elongated low-pressure area which extends from over Pakistan to the Bay of Bengal. This trough swings between north and south India through the monsoon bringing rain wherever it is active and is fed on moisture brought in from the Bay of Bengal (and the Arabian Sea to a lesser extent) in the form of low-level cyclones called ‘depressions.’
In the last few decades, the role of climate change has dramatically increased ocean temperatures in the Indian Ocean, said Roxy Mathew Koll, scientist at the Indian Institute of Tropical Meteorology, Pune and one of the study authors. This was influencing the number of depressions that formed during the season and consequently rain over Central India. However, these depressions weren’t reaching out as strongly towards North India as it was in the past.
“Consequently, this has left it more vulnerable to the impact from El Nino, which is known to affect the trade winds and the monsoon circulation over India,” he told The Hindu. For the rainfall over south India, the influence of ENSO and strength of monsoon trough have been consistent over the entire period, he added.
Accounting for the regional variation in the effect of El Nino would improve the accuracy of monsoon forecasts. “The ENSO dominance over the core monsoon zone is weak, which means that seasonal prediction over this region has become less predictable in the recent decades. Other factors like Indian Ocean warming should be monitored for the core monsoon zone, due to its impact on the strength of the monsoon trough and the depressions,” he added.
Currently an El Nino forming in the Pacific is likely to strengthen in the coming months and influence monsoon rainfall in August and September.