District-wise rainfall departure between June 1 and August 29, 2021. Districts receiving large-deficient rainfall are shown in yellow, those receiving deficient rainfall are shown in red, those receiving normal rainfall are shown in green, whereas those receiving excess and large-excess rainfall are shown in blue and dark blue colours. See the legend in the figure for thresholds.
(Hydromet Division, IMD)
The all-India performance of the southwest monsoon (hereafter referred to as the monsoon) has not been that impressive so far. The country has witnessed a significant monsoon rainfall deficit of 10% until 29 August, triggering concerns over widening shortfall.
Monsoon 2021 so far…
Intense pre-monsoon thunderstorms and monsoon rainfall (hereafter referred to as rainfall) activities in some parts of the country resulted in 10% above average rainfall in June. However, overall rainfall activities remained subdued for around a month between mid-June and around mid-July, resulting in the onset of below-average total rain since early July. Despite enhanced rainfall activities in the second fortnight of July, the all-India rainfall departure could not turn positive (i.e., above-average rainfall).
Since then, and as of August 29, the rainfall departure has widened to -10%, with 21 out of 36 meteorological subdivisions witnessing below-average rainfall conditions. The rainfall situation is particularly grave in Gujarat—the state has received only half of the average rainfall so far, with all districts witnessing either deficient or large-deficient rainfall conditions. Likewise, to list a few, Odisha, Jammu and Kashmir, Ladakh, Lakshadweep, Kerala and the northeastern states have also received deficient rainfall. As a result, the monsoon’s performance in September, or rather its recovery, will be crucial for determining whether India will witness a ‘normal’ or ‘below normal’ monsoon season this year.
Historical records of the rainfall recovery in September
A scatter plot showing the relationship between June–August rainfall departure (percentage) and June–September rainfall departure (percentage) in India during 1901–2020. The period between 1961 and 2010 is considered as the baseline for calculating rainfall departures. Each black dot shows each year’s situation. A red line, which is the line of best fit, suggests a very strong positive linear correlation between the two shown variables.
(IMD)
If we analyse the 120-year (1901–2020) observed rainfall dataset, it becomes clear that there were 48 years when India witnessed below average total rainfall between June and August. Out of these 48 years, the seasonal (June–September) rainfall was above the long period (1961–2010) average during only eight years — it was mostly near the average value. In 2019, however, the seasonal rainfall was around 10% above the average.
A scatter plot (see Figure 2) shows the linear relationship between June–August rainfall departure (on the horizontal x-axis) and June–September rainfall departure (on the vertical y-axis). The very strong and statistically significant positive linear relationship between the two variables stands out, suggesting that in the majority of cases, India has witnessed above-average seasonal rainfall only when June–August rainfall was above the average. In fact, seasonal rainfall during all 15 years, when June–August rainfall was at least 10% below the average (see Table below), ended being largely below the average, ranging between -10% (rounded value) in 1902 and 1915, and -22% in 1972.
The wettest September on record is 1917, whereas the driest one is 1907. For 15 worst June–August rainfall years, most rainfall recovery in September (in terms of percentage) was observed in 1902 when the rainfall departure improved from -16.44% (June–August) to -9.58% (June–September). Therefore, from a purely statistical perspective, it appears very likely that this year’s all-India seasonal rainfall will remain below the long-period average.
A comparison of rainfall departures during June–August with those during June–September for 15 maximum June–August rainfall deficit years during 1901–2020. The period between 1961 and 2010 is considered as the baseline for calculating rainfall departures.
(IMD)
Forecasts for September 2021
The current forecasts suggest that the first fortnight of September will be wetter than the second one for most of the country due to an active phase of the monsoon intraseasonal oscillation. For brevity, this oscillation can be considered to be a northward moving band of atmospheric conditions that boost or suppress rainfall depending on its current location. This would also help trigger a few monsoon low-pressure systems in this period, as our research suggests.
States such as Maharashtra, Madhya Pradesh, Uttar Pradesh, Andhra Pradesh, Bihar, and parts of northeast India can expect above-normal weekly rainfall until around mid-September. This spell is expected to provide some benefit to parts of Gujarat (i.e., Saurashtra as well as eastern Gujarat) and Odisha. However, given the large rainfall deficit in these states so far, I would be surprised if the overall rainfall situation drastically improves in September.
For Delhi-NCR and parts of north India, weekly rainfall is expected to remain close to the normal, with some tendency of above-normal rain in Uttarakhand and below-normal in Jammu and Kashmir as well as Ladakh in the first fortnight of September. Thus, some improvement in the all-India rainfall scenario is expected in the first fortnight. After that, in the second fortnight, there might be a reduction in overall rainfall activities in many regions of India, possibly resulting in not much contribution to the rainfall recovery. The monsoon normally starts withdrawing from western Rajasthan around 17 September, but this process is expected to be delayed this year as per current forecasts.
In summary, the all-India rainfall deficit can be expected to reduce in the first fortnight of September. However, current extended-range forecasts and the observed behaviour of the monsoon in the past suggest that the southwest monsoon rainfall in India this year will remain below the long-period average.
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Akshay Deoras is a PhD student in the Department of Meteorology, University of Reading, UK. He works on understanding predictions and characteristics of Indian monsoon low-pressure systems. He is also an independent meteorologist. Email: a.s.deoras@pgr.reading.ac.uk; Twitter: @akshaydeoras
This article is a guest column reflecting the author’s opinions and does not necessarily represent the official views of The Weather Channel.
