The 2020 La Nina in tropical Pacific has officially drawn to a close and could revert to the neutral phase (neither La Nina nor El Nino) while nearer home its twin, the Indian Ocean Dipole (IOD) index, is also expected to be neutral going forward setting the essential background for India’s 2021 South-West monsoon.
The India Meteorological Department (IMD) is expected to come out soon with its eagerly awaited long-term monsoon forecast for the June to September season while private forecaster Skymet Weather will present its forecast as early as tomorrow (Tuesday), after a break during the last (2020) season.
Seasonal accuracy issue
According to the Australian Bureau of Meteorology (BoM) that issued the latest climate updates, model accuracy is generally lower at this time of the year than at other times. So, the current outlooks on both the tropical Pacific (El Nino/La Nina) and the Indian Ocean (positive/negative IOD) should be viewed with caution.
The El Nino-La Nina, or for that matter the IOD, represents seesawing of SSTs at the farther ends of both the oceans. A warmer than normal oceanic region brings in lower air pressure, rising air, cloud formation, storminess and rain while these are reversed in the case of a colder ocean surface (higher pressure, sinking air, suppression of cloud formation, nil cloud or rain).
No direct relationship
Though there is no direct cause-effect relationship, the monsoon has thrived on the back of a La Nina (warmer West Pacific) and a positive IOD (warmer West Indian Ocean). These were in ample evidence in year 2020 (moderate to strong La Nina) and 2019 (a record-breaking positive IOD phase). Both these years delivered a surplus monsoon for India.
By the same coin, colder seas (El Nino in the Pacific and negative IOD in the Indian Ocean) usually signal dryness. Year 2019 was marked by a weak El Nino that followed a weak La Nina in 2018-19 but the strong positive IOD drove the monsoon to an extended peak during the latter part of the season.
The BoM quoted almost all international climate models surveyed by it as indicating that the Pacific would remain neutral until at least the end of the Australian winter (June to August). Only a very few indicated a return to La Nina conditions later in 2021, while an El Nino is ‘very unlikely.’
IOD neutral for now
As for IOD, all but one of the five models surveyed expects it to remain neutral through March to May autumn, with one touching on the negative threshold value during May. July values reach the negative threshold in three of the five models, indicating potential for negative IOD to develop.
IOD events are typically unable to form between December and April since the monsoon trough (that drives the seasonal monsoon from India in the northern hemisphere to Australian in the southern hemisphere) shifts south over the tropical Indian Ocean and alters wind patterns, preventing the IOD pattern from being able to form.
Since 1960, when reliable records of the IOD began, to 2016 there have been 11 negative and 10 positive events. All blank IOD years are IOD-neutral years.
*Long Period Average – average rainfall received for a 50-year period (1951-2000)
**Source: IMD, Australian Bureau of Meteorology
***Forecast conditions for the season