It is that time of the year when monsoon watchers turn their focus to the Equatorial Pacific Ocean for clues on the seesawing temperature patterns in its western and eastern basins as reflected in an El Nino or a La Nina, which are collectively called the El Nino-Southern Oscillation (ENSO) cycle.
A La Nina (cooler Equatorial and East Pacific and warmer West Pacific) has been associated with a good monsoon in India though without direct cause-effect relationship. An El Nino is the exact reverse, and usually portends a poor monsoon. In 1997, though, the monsoon was good despite a strong El Nino.
No canonical La Nina
Scientists at the Application Laboratory of the Japanese national forecaster Jamstec had in October 2020 said that while a La Nina continued to develop, its SINTEX-F model predicted that it would be more ‘La Nina Modoki’ type which may start decaying from early 2021.
“We need to be careful of its impact as it may be different from that of a canonical La Nina,” Takeshi Doi, a lead scientist with the Application Laboratory had said. Early this month, he said in an update that the La Nina Modoki is decaying and may go on to become a 'weak La Nina' through the summer and into the autumn.
What is a ‘Modoki’ event?
Swadhin Behera, Director, Application Laboratory, wrote to BusinessLine that, "We had a La Nina Modoki-like condition. But if you have seen the latest predictions the La Nina Modoki-like condition is now turning to a weak La Nina-like condition. Usually, that would mean a normal to above normal summer monsoon for India. Rainfall predictions issued so far also indicate that.”
Conventional La Nina and El Nino events have become less frequent since the 1970s when we have been presented with ‘a second’ or ‘the other’ flavor of ENSO. Modoki in Japanese language means ‘similar but different.’ It falls between a full-blown El Nino/La Nina event and its weaker version.
Both El Nino and La Nina Modoki events can occur independently at times when tropical ocean indices may not achieve thresholds in a canonical El Nino or La Nina event. The canonical La Nina involves the cooling of the East Pacific but during a Modoki event, the anomaly shifts to the Central Pacific.
More cyclonic storms
Research showed that a La Nina Modoki increases the frequency of cyclonic storms over the Bay of Bengal while lowering the potential for severe storms in the Indian Ocean overall (Bay of Bengal and Arabian Sea). This is because the Arabian Sea becomes less prone to cyclone genesis during this phase.
The monsoon long-range forecasts issued by the national forecaster India Meteorological Department as well as leading private forecaster Skymet Weather have indicated that a weak La Nina (instead of a La Nina Modoki) will persist in the Pacific mostly through the monsoon months.
It is just as well, since a La Nina Modoki event may not always facilitate a good Indian monsoon. Evidence through recent years bears this out. For instance, La Nina-Modoki years of 1998–1999, 2000–2001, 2008–2009, 2010–2011 and 2016–2017 have mostly produced less than optimal rainfall for the country.