Even though they account for 44 per cent of monsoon rainfall, these systems and their counting is not completely understood
There were five low pressure systems (LPS) in the north Bay of Bengal during August 2020, that accounted for 27 low pressure days against a normal of 15 days, according to the India Meteorological Department (IMD).
All the systems moved in a west north-westward direction across central India, reaching Madhya Pradesh and even Rajasthan.
The five LPS in August might have added to the active monsoon season in these central, eastern and some western states of India that led to excess rainfall and subsequent floods in these areas.
Central India received 61 per cent more rainfall than normal in August while the all India excess rainfall was 27 per cent. The previous year with such high rainfall was 1983, with 24 per cent, according to IMD. The highest August rainfall was in 1926, at 33 per cent.
Very high rainfall events defined by IMD as greater than 116 mm of rainfall in a day were also abundant in the central and western Indian states. The highest was in Madhya Pradesh, with 214 such events, according to data from the International Water Management Institute, Colombo, Sri Lanka.
The second and third positions were held by Gujarat and Maharashtra, with 199 and 150 very heavy rainfall events respectively in the month of August.
But how does an LPS influence rainfall during the monsoon season?
An LPS is a region of low atmospheric pressure that generally forms over the sea surface and carries a lot of moisture and winds with it in a roughly cylindrical formation. The system could strengthen into a depression, a deep depression or if it stays over the sea for long enough, even tropical cyclones.
The LPS that form during the monsoon season rarely grow into cyclones but could strengthen to intermediate depressions and deep depressions. The existence of the monsoon trough creates vertical shear winds that disrupt the formation of cyclones during this period.
The LPS that are the most numerous during the season, have a typical life span between three and six days and their horizontal span is 1,000-2,000 kilometres.
They are crucial for monsoon rainfall in many regions in the country, especially the central, north western regions and some parts of eastern India, which are known as the core monsoon zone.
Fifty-seven per cent of seasonal rainfall in the core monsoon regions and 44 per cent across India occurs due to these systems, according to a research paper published in the journal Climate Dynamics in March 2019.
In the calculation, the researchers have assumed that an LPS influences rainfall intensity in a 200-km radius around it since the winds and moisture weaken as one moves towards the edges.
This is an educated estimate and the percentage of monsoon rainfall that can be attributed to LPS may increase or decrease as this radius is increased or decreased. Various other factors have to be also taken into account, making the calculation pretty complicated.
Between 1979 and 2017, scientists have counted 782 LPS in the June-September period that accounts for the normal monsoon period, according to data from the European Centre for Medium Range Weather Forecasts.
They have also counted 109 monsoon depressions. LPS do not act alone in bringing rainfall during the monsoon season.
“Usually, the moist westerlies from the Arabian Sea and the LPS in the Bay of Bengal act together,” Roxy Mathew Koll, a climate scientist at the Indian Institute of Tropical Meteorology, Pune, said.
“LPS in the Bay of Bengal provide the dynamic support and guide the moisture. But the moisture arrives through the westerlies in the Arabian Sea,” he added.
According to Koll, one reason for the heavy rainfall in August was the unusually warm Arabian Sea. In the entire month of August, the northern Arabian Sea was warmer than usual by two-three degrees Celsius (°C).
While the normal sea surface temperatures here are about 28-29°C, the temperatures this August reached up to 29-31°C. “We have found that such high SSTs enhance the monsoon winds and moisture supply over India, resulting in episodes of heavy rainfall,” Koll said.
On the other hand, Raghu Murtugudde from the University of Maryland in the United States pointed out that there might be a problem with the basic definition of an LPS and the way they are counted. LPSs are not monolithic, Murtugudde said.
“You have multiple timescales like synoptic (less than ten days) also known as weather timescale and low-frequency (10-20 and 30-60 days) or active/break timescale. The synoptic timescale LPS have indeed increased,” he noted.
“So, there may be fundamental issue in defining LPS or mixing them up with the so-called Monsoon Intra-seasonal Oscillations that are associated with active / break period timescales of the monsoon season,” he added.
Break periods during the monsoon season are periods when monsoon activity decreases drastically over the core monsoon zone.
Scientists also do not agree if the trend of LPS during the monsoon season is increasing or decreasing. A research paper published in the journal Geophysical Research Letters in 2014 found no statistically significant trend in the number of synoptic scale monsoon depressions, that are formed when an LPS strengthens beyond a certain limit, since the 1990s.
It also found that the IMD’s monsoon depression dataset was unreliable as there might be a lot of missing depressions from the data. One reason for this could be the change in the way monsoon depressions have been measured over the years.
Before computers came in, the storms were analysed and counted from weather charts, making the process less efficient. This could have led the scientists to discount many LPS and monsoon depressions.
“The hydrological importance of synoptic activity in the world’s monsoon regions and the vulnerability of societies in those regions to hydrological change indicates a need for improved monitoring of monsoon depressions and more in-depth study of possible trends in their activity,” the research paper said.
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