If there are fewer monsoon depressions in the Bay of Bengal then what is driving these rainfall extremes? A new study finds some answers.
In the middle of a monsoon season that has produced floods in the northwest and northeast India with deficit rainfall over peninsular India for the third year in a row, a new academic paper submitted to Nature Communications journal says rainfall extremes have increased threefold over the last three years and now extend over all of central India, from Gujarat to Odisha.
The floods of 2017 are quite consistent with this, in terms of their moisture supply deriving from northern Arabian Sea and not from depressions originating from the Bay of Bengal, as one would usually expect.
The paper, written by Roxy Mathew Koll of the Indian Institute of Tropical Meteorology in Pune and by me, also says early monsoon onset tends to bring bountiful rainfall whereas delayed onset is almost never associated with better than average rainfall, when you look at the total rainfall over the entire monsoon season from June to September.
The onset of the monsoon has been delayed almost every year since 1976, when there was a regime shift in climate around the world. This shift was from a weak to a strong El Niño period and was manifest all around the globe. El Niño is the disturbance off the Chilean coast that affects the global climate.
Since 1976, the monsoon withdrawal has also moved up by almost a week from the end of September. So, the length of the rainy season has been compressed. This is in addition to the approximately 10% downward trend in All India Monsoon Rainfall since 1950. The spatial variability of monsoon rainfall has also increased since the 1950s.
Climate change effect
During the monsoon season, there are so-called break periods, when there is hardly any rainfall. These periods tend to be random. They are associated with systems moving northwards from the equatorial region.
All available data and models blended with data (known as reanalysis) indicate that global warming is shortening the length of the active periods, when it does rain, while lengthening the break periods. They also indicate that climate change is decreasing the extremes in the active periods while increasing them in the break periods.
Just about everything about the monsoon is changing – rainfall intensity, duration, frequency and spatial distribution.
We cannot be entirely sure if all this is in response to global warming – in which case it can be permanent and accelerate — or if the monsoon system will revert to a more normal state. Till we have many more years of data and reanalysis, a complete separation of the global warming impact from natural climate variability such as due to El Niño may not be possible.
The key question right now is whether the shortened length of the rainy season can by itself lead to increased monsoon extremes by trying to squeeze similar amount of rainfall into a shorter period; or whether global warming is causing shifts in duration, intensity and frequency of rainfall.
Analysing the monsoon
Analysis the South Asian monsoon has been difficult. There is a rich vein of long-term data that yield great insights but also raise new questions and pose serious challenges to climate models old and new.
However, now we do know that the rainfall extremes during the monsoon are unrelated to local warming. We also know that the long-term trends we are witnessing – decreasing mean rainfall, increase in spatial variability of rainfall and a threefold rise in rainfall extremes – are associated with a weakening monsoon wind circulation and a decrease in the number of monsoon depressions from the Bay of Bengal. Historically, these depressions used to be responsible for about half the rainfall during the entire monsoon.
If there are fewer monsoon depressions in the Bay of Bengal, what is driving these rainfall extremes? The new study submitted to Nature Communications finds that warming of the land in northwest India and Pakistan creates a pressure force that drives strong near-surface winds from the northern Arabian Sea to central India and brings enough moisture to more than compensate for the weakening monsoon circulation and the decreasing monsoon depressions from the Bay of Bengal.
This also explains the threefold increase in rainfall extremes despite the decreasing trend in the overall monsoon rainfall.
Other factors at play
Other factors influencing the monsoon – the lifeline of South Asia, which brings over 80% of the annual rainfall to the region – include the impact of El Niño and its counterpart La Niña. Then there are the warming and cooling of the Indian and Atlantic oceans – the Indian Ocean Dipole and the Atlantic Niño. All these play a role in the monsoon, but it is not always clear who affects whom and how.
Despite these changes and the decreasing trend, rainfall at the end of most seasons remains within 10% of the long period average. But with the variations across space increasing, that means little to farmers dependent on rain-fed agriculture. Some regions such as northern Karnataka and central Maharashtra have recently had deficits of close to 50%.
The long-term rainfall decline is worrying, especially if it is due to oceans warming up more as a result of climate change. This can alter the land-ocean temperature contrast and reduce the moisture demand over land during the monsoon.
Henry Blanford, director of the India Meteorological Department in the late 19th century, referred to the ephemeral behaviour of the monsoon as its vicissitudes. These have clearly increased now, and made the work of scientists more difficult. But there is a silver lining. We now know that non-local influences are more important; it is easier to track them and improve our forecasting to the point where authorities have sufficient time to act before a flood or a storm.
Another conclusion – since the total rainfall has remained within 10% of the long period average – is that large water reservoirs are unnecessary. The real large-scale plan should be about capturing rainwater on the ground through steps such as agroforestry and rainwater harvesting.
This article was originally published here.
Categories: South Asia