Erratic rainfall trends in 2019
After an extremely dry June, which saw a rain deficiency of 33 per cent, the monsoon brought generous rainfall in July, August and September, each subsequent month exceeding the normal by a higher deviation. In fact, September produced rainfall that was 152 per cent of normal, and this was the second highest rainfall ever recorded in this month. The only higher deviation during September was way back in 1917, when the rainfall was 165 per cent of the then normal for the month.
August and September together produced 130 per cent of normal rainfall, and this was the highest since 1983. And this was the first time since 1931 that the monsoon ended up producing more than 100 per cent rainfall after having a 30 per cent or more deficiency at the end of the first month.
Extreme rainfall events
There is no standard definition of an extreme rainfall event. The India Meteorological Department (IMD) uses different expressions to classify rainfall according to its intensity. More than 12 cm rainfall within a 24-hour period is classified as very heavy rainfall while rainfall in excess of 25 cm within 24 hours is categorised as extremely heavy.
IMD has recorded more than 560 extreme rainfall events this year. This is much more than what has been observed in recent years. Last year, for example, had 321 extreme rainfall events, and the years prior to that had fewer.
The number of very heavy rainfall events, which includes instances of extremely heavy rainfall, was more than 2,600 this year, compared to 2,181 last year, and much fewer in the previous years.
September marks the beginning of the withdrawal of the monsoon. This year, however, withdrawal has seen a record delay. So far, the longest delay happened in 1961 when the withdrawal started on October 1. This year, IMD said, the withdrawal is likely to begin only after October 10.
It is said that extreme events are increasing all over the world, not just in India, because of climate change.
For the 18th time in 19 years, the Northeast had rainfall that was less than 100 per cent. In fact, the East and Northeast region, clubbed together as one of the four geographically homogeneous region for purposes of monsoon rainfall, saw a deficiency of 12 per cent.
The IMD says this indicates that the Northeast region is passing through a “below normal epoch”. Apart from exhibiting year-to-year variability, the monsoon is also supposed to have a 30-year variability cycle. During one cycle, it receives below normal rainfall, and in the next it gets above normal. Of course, within this period it exhibits intra-seasonal variations as well. But overall, it tends to follow this pattern.
Possible reasons of disturbance in rainfall pattern:
Scientists point out that the last time September produced so much rain, 1917, happened to be a La Niña year. La Niña, the phenomenon in the equatorial Pacific Ocean in which the sea surface temperatures turn unusually cold, is known to strengthen rainfall over the Indian sub-continent during the monsoon months. This year, there is no La Niña. In fact, it started with a weak El Niño, the opposite phenomenon in Pacific Ocean that has a negative impact on Indian monsoon, before the situation turned neutral.
- Scientists said though there was no La Niña, a similar phenomenon much closer home, called the Indian Ocean Dipole, could have contributed to enhanced rainfall. There was a cooling of the eastern equatorial Indian Ocean, below Sumatra, and that could have some role to play in the kind of rainfall that we have seen this year.
What is Indian Ocean Dipole?
The Indian Ocean Dipole is a phenomenon similar to the ENSO condition observed in the Pacific Ocean which creates the El Niño and La Niña events. The sea surface temperatures in the Indian Ocean gets warmer and cooler than normal, and this deviation influences regional atmospheric and weather patterns, notably the Indian monsoon.
But there is one major difference from ENSO. While the Pacific Ocean only has an El Niño or a La Niña condition at a time, the Indian Ocean experiences both warm and cold conditions at the same time – hence, a dipole. One of these poles is located in the Arabian Sea while the other is in the Indian Ocean, south of Indonesia.
The Indian Ocean Dipole is said to be positive when the western pole is warmer than the eastern one, and negative when it is cooler. The Indian Ocean Dipole and ENSO are not unrelated. Positive Indian Ocean Dipole events are often associated with El Niño impact and negative Indian Ocean Dipole with La Niña. When the Indian Ocean Dipole and ENSO happen at the same time, the Dipole is known to strengthen the impacts of the ENSO condition.
- Many scientists like to describe the monsoon in terms of the movement of the Intertropical Convergence Zone, or ITCZ, a region near the Equator where the trade winds of the northern and southern hemispheres come together. The intense Sun and the warm waters of the ocean heat up the air in this region, and increase its moisture content. As the air rises, it cools, and releases the accumulated moisture, thus bringing rainfall. During the monsoon season, this ITCZ is located over the Indian subcontinent. By September, as the temperature begins to go down, the ITCZ starts moving southwards of the Indian landmass, towards the equator, and further into the southern hemisphere. This year, this process has not yet started.
In September this year, the northern hemisphere was much warmer than the southern hemisphere, and that could be one reason why the ITCZ has remained longer than usual over the northern hemisphere.
Source: The Indian Express