Why the weatherman read the monsoon wrong this year

Negative Indian Ocean Dipole, unfavourable Madden Julian Oscillation and fewer currents of Bay of Bengal disturbed the rains

The uneven nature of the southwest monsoon this season, characterised by unusually long breaks between phases of heavy rain, has put a question mark on the longrange seasonal assessment made by the state-run India Meteorological Department (IMD) and private weather forecaster Skymet.

Both had predicted an extremely good monsoon this year, with cumulative seasonal average rainfall in excess of 100 per cent of the long-period average (LPA).

The cumulative LPA for the monsoon in India is 88 cm.

IMD Director General Mrutyunjay Mohapatra said there were three major factors in this.

“First, the behaviour of the negative Indian Ocean Dipole (IOD) wasn’t anticipated correctly; secondly, the Madden Julian Oscillation (MJO) too wasn’t favourable for much of the monsoon months; and thirdly fewer low-pressure areas (LPAS) formed over the Bay of Bengal this year as compared to normal.”

However, it is difficult to say which factor contributed how much, he said.

The MJO remained in phase 1 for many days in the first three months of the season and that increased conductivity in the air.

If the MJO had moved to phase 2-3-4 or five for many days in the last three months, it could have energised the monsoons because once the MJO crosses phase 3, Bay of Bengal currents get activated, causing rain in Central and North India. This did not happen this year.

Mohapatra said low-pressure systems too did not develop with their normal frequency over the Bay of Bengal in July and August because the number of typhoons over the South China Sea and the North-west Pacific Ocean were fewer this year in August than they usually are.

“Usually, 60-70 per cent of the low-pressure systems that get formed over the Bay of Bengal are due to the remnants of the typhoons from the South-china Sea and N-W Pacific Ocean but this year it did not happen,” Mohapatra said. He said the IMD would look into what went wrong. “No model can be perfect,” Mohapatra said.

Mahesh Palawat, vice-president of Meteorology and Climate Change at Skymet, too said though El Nino was neutral during the monsoon months and was in fact veering towards La Nina, it was the behaviour of the Indian Ocean Dipole (IOD) that flummoxed weathermen.

Usually, El Nino is the more dominant factor in uneven rain over India, but this time it was the IOD, Palawat said.

“The IOD was predicted to be negative during the monsoon months and was expected to be within the threshold limits, but it didn’t happen that way, which caused such big breaks in rain,” Palawat told Business Standard.

What are the IOD and MJO?

The Indian Ocean Dipole, often called the “Indian Niño” because of its similarity to its Pacific equivalent, is classified as difference in sea-surface temperatures in opposite parts of the Indian Ocean.

The IOD affects the climate of Australia and other countries that surround the Indian Ocean Basin (that includes India), and is a significant contributor to rainfall variability in this region. The MJO, on the other hand, is defined as a major fluctuation in tropical weather on weekly to monthly timescales.

The Australian Weather Bureau (AWB) characterises the MJO as an eastward-moving “pulse” of cloud and rainfall near the equator, which typically recurs every 30 to 60 days.