Is Indian Ocean Dipole the only measure of this year's below-par monsoon?

Experts say though rains have recovered the past few days, they won't wipe off the deficit and the 2021 monsoon season will end with a shortfall of 5-6% from the normal

Though the southwest monsoon has made a strong revival over several parts of the country during the past few days, it remains to be seen as to whether the rains will be good enough to wipe off all the seasonal deficit, which was 9 per cent below average on August 31.

Most meteorologists and weathermen said the current wet spell will last for the next 2-3 days, following which dry weather will set in most parts of the country.

In fact, monsoon rains in July and August, the two most important months of the four-month southwest monsoon season, have been one of the worst.

India received just about 196 millimeters of rainfall in August, which was the sixth driest August since 1901, according to some experts and the third driest in this century, after 2005 and 2009.

The IMD had predicted a normal monsoon in August at 96-104 per cent of the LPA, while the actual rainfall was just 76 percent of normal.

The uneven and erratic nature of the southwest monsoon, characterised by unusually long breaks in between heavy rainfall, has put a question mark on the seasonal long range assessment made by both the state-run India Meteorological Department (IMD) and private weather forecasters such as Skymet.

Both the agencies 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), which is 89 centimeters.

What did IMD and Skymet forecast for the 2021 monsoon?

The IMD had, in its first forecast this April, said that the monsoon this year would be normal at 98 per cent of the LPA.

Thereafter, in June, the met revised its forecast and upped it to 101 per cent of the LPA, with most parts of the country, except regions of eastern and north-eastern India, expected to get normal to above normal rainfall.

Rainfall between 96-104 per cent of the LPA is considered normal.

Releasing the second stage forecast for the 2021 monsoon season, the met department had said that the forecast has been revised upwards due to prevailing neutral El Nino conditions along with neutral Indian Ocean Dipole, the two weather systems that have a direct bearing on the performance of southwest monsoon in the country.

Both the forecasts came with a model error of plus and minus 5 per cent.

Region-wise, the IMD said that North-West India was expected to get normal rains at 92-108 per cent of the LPA, while Central India was expected to get above normal rainfall at more than 106 per cent of LPA.

Southern India was expected to get rainfall at 93-107 percent of LPA, while North-East India would get below normal rainfall at less than 95 per cent.

Private weather forecasting agency Skymet too said, in its first forecast in April, that the southwest monsoon in 2021 was expected to be ‘normal’ at 103 per cent of the LPA.

The forecast was with a model error of plus and minus 5 percent.

Both the IMD and Skymet hoped that for the third consecutive year, India would have normal to above normal rains during the monsoon months.

In 2020, actual rainfall across India was 109 per cent of the LPA, while in 2019, it was 110 per cent.

Last time India had three straight years of normal monsoon according to Skymet, was between 1996 to 1998--more than two decades back.

What went wrong?

IMD Director General Mrutyunjay Mohapatra said there were three major factors that lead to a lower-than-expected performance of southwest monsoon.

“First the behaviour of 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 number of low-pressure areas (LPAs) were formed over the Bay of Bengal this year as compared to normal,” Mohapatra said.

He said all three factors contributed to actual monsoons differing from the forecast but it is difficult to say how much and to what extent the three factors contributed to the monsoon's below-par performance.

“We will surely analyse all the multi-modal weather models that were considered for arriving at the consensus forecast for 2021 and pin-point which was the weather phenomenon that the models could not capture correctly and reasons for the same. But, it must be remembered that models do have their limitations,” Mohapatra.

Mahesh Palawat, vice president of Meteorology and Climate Change at Skymet, too said that 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) which flummoxed weathermen.

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

IOD along with El Nino Southern Oscillation (ENSO) are the two major factors that govern the inter-annual variation of south west monsoon.

The other contributing factors are North Atlantic Oscillation (NAO) and Pacific Decadal Oscillations (PDO).

“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 the monsoon season, first in June and then again in August, leading to cumulative rains being ‘below normal’ as against the forecast of ‘normal’,” Palawat told Business Standard.

He too said the Madden Julian Oscillation (MJO)--another factor influencing Indian monsoon--wasn't favourable this year.

“Normally, around 4-5 weather systems are formed over the Bay of Bengal during the month of August, but this year hardly 2-3 weather systems were formed,” Palawat said.

He said though the rains have recovered the past few days, they won’t be good enough to wipe off the deficit and the 2021 monsoon season will end with rainfall of about 94 per cent of LPA, which means a deficit of 6-7 per cent from the normal.

“The monsoon season might end with 4-5 per cent lower than normal rains, somewhere around 96 per cent of LPA,” Mohapatra said.

So what is this IOD?

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.

According to the Australian Weather Bureau (AWB), the Indian Ocean Dipole (IOD) is defined by the difference in sea surface temperature between two areas (or poles, hence a dipole)-–a western pole in the Arabian Sea (western Indian Ocean) and an eastern pole in the eastern Indian Ocean south of Indonesia.

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.

IOD is said to be positive when warmer sea surface temperatures in the western Indian Ocean relative to the east easterly wind anomalies across the Indian Ocean and less cloudiness to Australia's northwest less rainfall over southern Australia.

IOD is said to be negative when cooler sea surface temperatures in the western Indian Ocean relative to the east winds become more westerly.