On the last day of the Southwestern Monsoon calendar, 30 September, Kerala and its coastal waters witnessed extreme rainfall. It is just about the climax, and not curtains, for the region’s largest weather event.
“Southwest Monsoon has been vigorous over Kerala,” India Meteorological Department (IMD) reported on its daily weather report on Friday, 29 September.
Rain occurred at most places in Kerala and Lakshadweep, with heavy (7–11cm in 24 hours) to very heavy (12–20 cm in 24 hours) rainfall in Alappuzha, Thrissur, Kannur, and elsewhere.
Forecasts till 5 October say “rain or thundershower is most likely to occur at many places in Lakshadweep and at a few places in Kerala.”
Extreme rainfall across Kerala
The National Centre for Medium Range Weather Forecasting (NCMRWF) maps for the week show extreme rainfall forecast across Kerala. In these maps, extreme rainfall means rainfall over 24 hours exceeding 95 percentiles, roughly put, the top 5 percent values. That could be a potentially dangerous situation.
Kerala State Disaster Management Authority noted on Saturday, 30 September, that there is a yellow alert (watch and stay updated) for all districts of the state except Kottayam, advising special alert for hillsides where the rains can gain strength.
Overall, however, the season’s rainfall has been deficient in Kerala by over a third (36 percent), that is 1,291 mm against the normal of 2,011.
September rains, however, have removed the prospects of droughts with much of South India, except a moderately dry Andhra Pradesh, showing a near-normal situation in the coming four weeks, according to the NCMRWF drought outlook.
El Nino’s effect
“Monsoon this year was different from the textbook pattern,” said Madhavan Nair Rajeevan, monsoon expert at the National Centre for Earth Science Studies, Thiruvananthapuram. “It is good we are ending with a negative departure but not as a drought.”
“El Niño had its impact, especially in August,” Rajeevan said. “But other factors like Pacific decadal oscillation and favourable phases of Madden Julian Oscillation helped to revive monsoon in September. Therefore, September was crucial.”
The Pacific Decadal Oscillation is a climate index based on variations in the North Pacific sea surface temperature from 1900 to the present. It has a warm phase, and a cool phase based on the positive or negative sign of sea surface temperature anomalies, each lasting decades. A cold phase as it is happening now is characterised by an increase in the monsoon rainfall.
El Niño denotes a large-scale ocean-atmosphere climate phenomenon linked to periodic warming in sea-surface temperatures across the central and east-central equatorial Pacific. Historically it has been associated with droughts in India. Long breaks in the rainfall over August were often attributed to El Niño impact.
Madden Julian Oscillation (MJO) involves a large-scale envelope of cloud clusters and rain systems moving eastward along the equator in cycles of 30–60 days, influencing tropical cyclones and extreme weather.
MJO influences the Indian monsoon by increasing or reducing convection (vertical transport of heat and moisture as in a thunderstorm) in the atmosphere during the active and break periods, respectively. Prolonged MJO activity can lead to bursts of rainfall.
65 percent districts mildly dry
Rajeevan, however, noted that about 65 percent of districts in India are under mild dry conditions or worse. “This year is characterised by large spatial and temporal variability.”
Such changes in the rainfall pattern can affect people in significant ways.
“It can affect not only agriculture but also water utilisation,” said Abhilash S, director of the Advanced Centre for Atmospheric Radar Research (ACARR), Cochin University of Science and Technology.
He added, “Our farming calendar is based on the rainy season, but then the rains are not aligned with that anymore.”
Spatial variability of rains calls for localised forecasts, Abhilash said. He said that extreme rainfall and cloudbursts recorded recently in Kerala were highly localised events.
He further added, “Often rainfall caused by cumulous clouds is limited to 5–6 km areas. When forecasts are issued for a district, there should be information on which part of the district is the most impacted.”
Meanwhile, recent scientific papers led by ACARR scientists have called for localised forecasts. IMD scientists, who collaborate in this research, have also acknowledged the need for localised and impact-based forecasts that tell what the weather event will do rather than what the weather will be.