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The Australian Bureau of Meteorology (BoM) has said that the tropical Pacific, especially water temperatures in the Central Pacific are edging closer to La Nina thresholds. This augurs well for a concurrent Indian monsoon (though in the final stages). Atmospheric signals and international predictions also favour a high likelihood of La Nina over the next three months.
On Tuesday, the India Meteorological Department (IMD) had assessed that the sea surface temperatures (SSTs) and atmospheric conditions over Equatorial Pacific indicated neutral (neither El Nino nor La Nina) conditions. But its Monsoon Mission Climate Forecasting System (MMCFS) model forecast and global models expected SSTs in the region to cool further.
Neutral conditions for now
The IMD said that neutral conditions may continue during the remaining part of the monsoon season (September). Rain forecasts for the month indicates varying below normal, normal and excess precipitation across different parts of the country at a time the monsoon should start withdrawing from North-West and Central India over the next few weeks.
As for conditions in the Indian Ocean, another major determinant of rainfall trend over the mainland, the IMD said that negative Indian Ocean Dipole (IOD) conditions prevail. MMCFS forecast indicates negative IOD conditions will continue during the rest of the season. A negative IOD is inimical to the monsoon at least during the initial months and affects rain spread.
The Australian Bureau said that oceanic indicators have returned to neutral after three weeks in a row over the Indian Ocean where the negative IOD threshold has been exceeded. Roughly half of surveyed climate models continue to forecast a negative IOD developing during the autumn of 2020, which is in agreement with the IMD predictions, too.
Wet monsoon for Australia
La Nina and the negative IOD are both typically associated with above-average rainfall across the Northern Territory of Australia and the state of Queensland during the Southern Hemisphere spring. This is in stark contrast to the situation last year where a prolonged positive IOD delayed the monsoon withdrawal in India and its onset over Australia, leading to raging wild fires.
The influence of La Nina on the northern Australian rainfall normally extends into early summer, the Australian Bureau said, whereas the influence of the IOD typically wanes significantly once (Australian) spring concludes. While a weak La Nina developed in 2017-18, the last significant La Niña was the back-to-back episode in 2010-12.
Australia had its highest two-year rainfall total on record during this time. The last negative IOD was in 2016, and it contributed to well above average winter and spring rainfall. La Nina and the negative IOD usually result in an earlier-than-normal monsoon onset date and also contribute to above-average rainfall totals across northern Australia during the full wet season (October to April).
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