2021 South Asia monsoon forecast
By
Jason Nicholls, AccuWeather senior meteorologist
Updated Apr 19, 2021 7:58 PM EDT
The Southwest monsoon in India runs from June through September and rainfall during the Southwest monsoon accounts for about 70% of India’s total annual rainfall. The monsoon is also an integral part of India’s economy since farming makes up 15% of India’s GDP and employs more than half of the population. In addition to rainfall determining yields for crops such as rice, wheat, sugarcane and oil seeds, it also helps replenish reservoirs and ground water for irrigation, as well as boosts hydropower production.
So what does the 2021 Southwest monsoon season hold in store for India? First we will take a quick look at the revised onset and withdrawal dates from the India Meteorological Department. The normal onset date in Kerala is still around June 1, but the monsoon is now set to clear all of India nearly a week earlier. The biggest change is the withdrawal starting over two weeks later under the revised dates.
Before getting to the forecast we will look at the analog years that were used for the monsoon season. The following image shows the composite rainfall for the top analog years (top) and all analog years (bottom). The overall pattern for both is not too dissimilar. However, there is a more pronounced dry anomaly in northwest India and more of a wet anomaly in far southern India for our top analog years.
A quick look at several climate models are below. Most show a wet monsoon across much of India, but may be a little overdone in some areas. The biggest issue with some of the modeling is that they show above-normal rainfall across nearly all of India, which is rare even in flood years. Climatology would suggest that parts of India will likely not be as wet as the modeling suggests.
So taking a mix of several climate models and factoring in teleconnections as well as analog years has led us to our outlook for the 2021 Southwest monsoon season. Overall, for India and Sri Lanka we are going with rainfall of 100-105% of normal with an error of + or - 5%. The rainfall forecast for Pakistan is a bit more pessimistic with rainfall expected to be 95-100% of normal for the nation as a whole with the same margin of +/- 5%. The driest parts of Pakistan should tend to be across the central districts.
Some highlights of the forecast include…
1. Onset in Kerala around the normal onset date of June 1 with a margin of +/- 4 days with onset in Sri Lanka by the end of May.
2. Monsoon advance should be relatively steady with a few of the usual surges and stalls, but should clear most of India and enter Pakistan by mid-July.
3. Rainfall during June should be near to above normal in many areas, but it can be drier than normal in northwest India.
4. Areas of dryness can develop in Kerala, Karnataka and parts of northeast India in July or early August. We are also concerned for dryness becoming an issue in portions of northwest India, Tamil Nadu, Bangladesh and Sri Lanka late in the season.
Possible risks for the monsoon forecast are as follows…
1. A negative IOD developing late in the season as expected should result in dryness in northwest India as forecast. However, if the IOD remains neutral, then some teleconnections suggest a wetter pattern in northwest India, especially late in the season. This could result in rainfall closer to normal or perhaps even above normal in northwest India. The ECMWF model even hints at a weakly positive IOD which could also mean wetter than forecast conditions in northwest India.
2. If ENSO trends back toward a classic La Nina in late summer/autumn, then rainfall across India could be greater than forecast. La Nina to neutral usually results in rainfall reduced by 4% across India, but back-to-back La Nina's tend to favor above-normal rainfall in many areas. The exception is toward the southern tip of peninsular India.
3. More lows forming in the Bay of Bengal this year versus 2020 could result in better rainfall across northeast India and Bangladesh than currently forecast.
4. Climate modeling suggests a wetter outlook than forecast, which is a risk.
Lastly, a quick look at the tropical northern Indian Ocean which can be a wildcard for the monsoon. We are currently seeing signals that suggest that the Bay of Bengal should have an increase in the number of lows and depressions pre-monsoon and during the monsoon as compared to 2020 when the basin was fairly quiet until late summer and autumn. There is also the risk for a couple of cyclonic storms to develop in the northern Indian Ocean during May and June, with a leaning toward the Bay of Bengal for such activity. However, the Arabian Sea can still have tropical activity pre-monsoon, but is expected to be less active than 2020.
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Weather Blogs / Global weather
2021 South Asia monsoon forecast
By Jason Nicholls, AccuWeather senior meteorologist
Updated Apr 19, 2021 7:58 PM EDT
The Southwest monsoon in India runs from June through September and rainfall during the Southwest monsoon accounts for about 70% of India’s total annual rainfall. The monsoon is also an integral part of India’s economy since farming makes up 15% of India’s GDP and employs more than half of the population. In addition to rainfall determining yields for crops such as rice, wheat, sugarcane and oil seeds, it also helps replenish reservoirs and ground water for irrigation, as well as boosts hydropower production.
So what does the 2021 Southwest monsoon season hold in store for India? First we will take a quick look at the revised onset and withdrawal dates from the India Meteorological Department. The normal onset date in Kerala is still around June 1, but the monsoon is now set to clear all of India nearly a week earlier. The biggest change is the withdrawal starting over two weeks later under the revised dates.
Before getting to the forecast we will look at the analog years that were used for the monsoon season. The following image shows the composite rainfall for the top analog years (top) and all analog years (bottom). The overall pattern for both is not too dissimilar. However, there is a more pronounced dry anomaly in northwest India and more of a wet anomaly in far southern India for our top analog years.
A quick look at several climate models are below. Most show a wet monsoon across much of India, but may be a little overdone in some areas. The biggest issue with some of the modeling is that they show above-normal rainfall across nearly all of India, which is rare even in flood years. Climatology would suggest that parts of India will likely not be as wet as the modeling suggests.
So taking a mix of several climate models and factoring in teleconnections as well as analog years has led us to our outlook for the 2021 Southwest monsoon season. Overall, for India and Sri Lanka we are going with rainfall of 100-105% of normal with an error of + or - 5%. The rainfall forecast for Pakistan is a bit more pessimistic with rainfall expected to be 95-100% of normal for the nation as a whole with the same margin of +/- 5%. The driest parts of Pakistan should tend to be across the central districts.
Some highlights of the forecast include…
1. Onset in Kerala around the normal onset date of June 1 with a margin of +/- 4 days with onset in Sri Lanka by the end of May.
2. Monsoon advance should be relatively steady with a few of the usual surges and stalls, but should clear most of India and enter Pakistan by mid-July.
3. Rainfall during June should be near to above normal in many areas, but it can be drier than normal in northwest India.
4. Areas of dryness can develop in Kerala, Karnataka and parts of northeast India in July or early August. We are also concerned for dryness becoming an issue in portions of northwest India, Tamil Nadu, Bangladesh and Sri Lanka late in the season.
Possible risks for the monsoon forecast are as follows…
1. A negative IOD developing late in the season as expected should result in dryness in northwest India as forecast. However, if the IOD remains neutral, then some teleconnections suggest a wetter pattern in northwest India, especially late in the season. This could result in rainfall closer to normal or perhaps even above normal in northwest India. The ECMWF model even hints at a weakly positive IOD which could also mean wetter than forecast conditions in northwest India.
2. If ENSO trends back toward a classic La Nina in late summer/autumn, then rainfall across India could be greater than forecast. La Nina to neutral usually results in rainfall reduced by 4% across India, but back-to-back La Nina's tend to favor above-normal rainfall in many areas. The exception is toward the southern tip of peninsular India.
3. More lows forming in the Bay of Bengal this year versus 2020 could result in better rainfall across northeast India and Bangladesh than currently forecast.
4. Climate modeling suggests a wetter outlook than forecast, which is a risk.
Lastly, a quick look at the tropical northern Indian Ocean which can be a wildcard for the monsoon. We are currently seeing signals that suggest that the Bay of Bengal should have an increase in the number of lows and depressions pre-monsoon and during the monsoon as compared to 2020 when the basin was fairly quiet until late summer and autumn. There is also the risk for a couple of cyclonic storms to develop in the northern Indian Ocean during May and June, with a leaning toward the Bay of Bengal for such activity. However, the Arabian Sea can still have tropical activity pre-monsoon, but is expected to be less active than 2020.
Report a Typo