Why does it feel like pre- or post-monsoon in the middle of monsoon?

Akshit Sangomla

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The southwest monsoon season has behaved weirdly towards the end of August and early September, with characteristics more like the pre- or post-monsoon season. This adds to the other weird anomalies, including the more than 60,000 lightning strikes over Odisha in a matter of a couple of hours on September 3, that have been observed in the current monsoon season.

The imposition of the ongoing El Nino event in the equatorial Pacific Ocean on a generally warmer planet may be the reason for this strange behaviour of the monsoon.

To begin with the western end of the monsoon trough, the extended low pressure region that causes most of the rainfall over India during the season lies north of its normal position that is a characteristic of the break monsoon phase, while its eastern end lies south of its normal position, according to the India Meteorological Department (IMD). This makes the monsoon pattern quite skewed.

Further, "the monsoon winds at lower levels have been avoiding India since around August 28 and are ongoing right now. You can see (in the graphic below) that the dominant wind pattern over India was from the north / northwest and the monsoon flow was over the Arabian Sea. Then it was going down towards southern India and into the Bay of Bengal. This is the usual pre-monsoon pattern,” Akshay Deoras, a research scientist at the National Centre for Atmospheric Science and Department of Meteorology, University of Reading, United Kingdom, told Down To Earth.

Wind patterns over India on August 31, 2023

“This phenomenon may continue till around September 10 after which active monsoon conditions may begin again. Till then monsoon would be in a retreating monsoon like pattern,” he added.

Meanwhile, a developing low pressure area in the Bay of Bengal is going to increase rainfall over most of eastern and southern India beginning September 4.

El Nino is the warmer than normal phase of the El Nino Southern Oscillation (ENSO) in the equatorial Pacific Ocean and generally has an impact on the southwest monsoon season. One likely impact of the ongoing El Nino event has been the continuous break monsoon phases in August, which made it the driest August for India in the last 123 years.

India experienced 36 per cent less rainfall in the month, according to data from IMD. The deficit was as high as 60 per cent in the southern Peninsula. This was mainly because the monsoon was north of its normal position (known as the break monsoon phase) between August 5 and August 16 and between August 27 and August 31, according to IMD.

Its western end is still in the same phase. The IMD cites El Nino, unfavourable Madden Julian Oscillation, weaker cross equatorial flow from the Arabian Sea, weaker and more northwest Tibetan High as the reasons for the lack of rainfall in August. (It is a high pressure area over Tibet that generally enhances monsoon rainfall when it is strong.) But El Nino backed by global warming could be the major reason among these.

“The first thing to note is that the El Nino is unusual this year in the sense that the western Pacific is not cooler as happens generally during El Ninos and Indonesia is not dry and burning as it does during normal El Ninos. This is because the global warming pattern is superimposing on El Nino anomalies,” Raghu Murtugudde, professor at Indian Institute of Technology, Bombay and emeritus of the University of Maryland, told Down To Earth.

This has led to tremendous warming of the ocean surface due to wind changes. That has led to the accumulated cyclone energy in the western Pacific typhoon region to be much higher than normal. This continued to perturb the monsoon circulation and potentially created a long break, according to Murtugudde.

This came on top of earlier impacts of cyclones and typhoons on the monsoon trough. “The warm Arabian Sea and the trough in a paschimottanasana pose (see graphic above) was responsible for pumping huge moisture into Himachal and causing massive destruction,” explained Murtugudde.

“Typhoons may be calming down for now, allowing the monsoon to swing back into action but we still don’t know what the unusual El Nino pattern has meant for the monsoon or whether it was the monsoon and the typhoons that have produced the unusual El Nino pattern. Predictability of the monsoon is highly non-stationary and we are mostly watching as they keep evolving,” Murtugudde concluded.