Why the fate of monsoon in India depends on favourable conditions from El Nino, Indian Ocean Dipole

Northern India has been sizzling under a heatwave and expectant eyes have been turned to the skies for a sign of rains. However, after a promising start, the rain clouds suddenly seem to have disappeared. Weather officials though have said that rainfall across the country in July is most likely to be normal on the whole. So, what affected the advance of the monsoon, and will it get back on track?

What has been the progress of this year's monsoon?

Monsoon this year was declared to have hit Kerala on 3 June. But a couple of days' delay in its onset was more than offset by its rapid march up the peninsula and, by the middle of June, monsoon clouds had covered 80 percent of the country's landmass.

By then, the monsoon was seen as having covered all of Odisha, West Bengal, Jharkhand, Bihar, Jammu and Kashmir, etc. and was deemed to be on course to reach Delhi and adjoining areas by the end of June.

However, as it turned out, its advance abruptly stopped post-19 June and the latest reports say that Delhi, Haryana, parts of west Uttar Pradesh, Punjab and west Rajasthan are yet to see the arrival of the monsoon.

But the weather department in its forecast for July has said the monthly rainfall will be between 94 percent to 106 percent of the Long Period Average (LPA), which is IMD's textbook definition of normal rainfall. The LPA itself is the average data for rainfall recorded across India between 1961 and 2010, which is 88 centimetres.

So, how did its pace slacken?

Among the factors that have affected this year's rainfall, weather officials say, is the climate phenomenon called El Nino, which is known to exert great influence on the career of India's annual southwest monsoon. In a press release on 1 July, IMD had mentioned that it was tracking the "prevailing neutral El Nino-Southern Oscillation (ENSO)... (and) enhanced possibility of development of negative Indian Ocean Dipole (IOD) conditions over the Indian Ocean during July to September 2021".

According to experts, the El Nino phenomenon causes the Pacific Ocean near the equator to warm up more than is usual in the years that it is active, one of the results of which is lower rainfall during the monsoon season in India.

Then there is the IOD, which too impacts the progress of the monsoon. The IOD is also described as the Indian Nino and is essentially an irregular fluctuation of sea surface temperatures by which the western Indian Ocean becomes alternately warmer, known as the positive phase, and colder, or the negative phase, than its eastern part.

Reports say that a positive phase of IOD in 2019 had resulted in good monsoon rainfall.

Private weather forecasting agency Skymet said that the absence of easterly winds blowing across the Indo-Gangetic plains has led to a weakening of the eastern arm of the monsoon, resulting in the stagnation of its Northern Limit. The Northern Limit Of Monsoon (NLM) is the "northern-most boundary of India up to which monsoon rains have advanced on any given day".

Skymet added that a "push" is needed to take the monsoon showers to the northern areas that are still without rain. For this, either a "fresh low-pressure area" has to form over the Bay of Bengal, or "an active western disturbance" should arise.

What is the forecast for this month?

The weather office warned that the northern plains and the likes of Rajasthan are still looking at a wait for monsoon rains to arrive.

Citing prevailing conditions, an IMD bulletin on July 5 said that "no favourable conditions are likely to develop for further advance of southwest monsoon" into parts of Rajasthan, west UP, Haryana, Punjab, UTof Chandigarh and Delhi during the next 4-5 days.

"Hence, subdued rainfall activity is very likely to continue to prevail over Northwest, Central and Western parts of Peninsular India during next 4-5 days," it added. However, the same regions may witness isolated, or scattered, thunderstorms accompanied by lightning and rain.

However, it said that strong and moist southwesterly winds blowing in from the Bay of Bengal to the North East and parts of eastern India, combined with other factors, will likely cause "fairly widespread to widespread rainfall" in Bihar, West Bengal, Jharkhand, and all of the North East, including Sikkim, in the next 4-5 days.