Rapid warming of Arabian sea among causes of 3-fold rise in erratic rain in central India: Weather experts

Unlike what was believed earlier — that depressions formed in the Bay of Bengal are responsible for most heavy rainfall between June and September over India — this study has painted a different picture.

THE rain fury that lashed Maharashtra, West Bengal, Bihar and Assam this monsoon, which displaced lives of lakhs of people, could be the new ‘normal’, according to a study.

Scientists have recorded a three-fold rise in such erratic rainfall over central India — from Gujarat in the west up to Assam in the Northeast — and suggested that such rainfall events are going to be far more frequent in the coming years. They have attributed it mainly to the rapid warming of the Arabian Sea.

According to the study, 13 more incidents like these are noticed with the passing of each decade, thus making floods an annual affair for the people in Gujarat, Maharashtra, Jharkhand, Chhattisgarh, Madhya Pradesh, Bihar, West Bengal and Assam.

As part of this joint study, researchers, led by Roxy Mathew Koll from the Pune-based Indian Institute of Tropical Meteorology (IITM), along with experts from IIT-Bombay and University of Maryland, USA, have tracked this rising trend during the 1950-2015 period. Their study found widespread extreme heavy rainfall (150mm or more) over a span of more than 5 lakh sq km area along central India.

Statistics from the International Disaster Data Base show that close to 70,000 people lost their lives and another 8 crore people were rendered homeless in the 268 events of heavy rain, flash floods, torrential rain or landslides recorded over central India in the last six decades.

Sea warming has been noted along the north of the Arabian Sea — the Indo-Pak region — and along regions in the northwest closer to the Gulf. Koll said, “Due to excess warming, there is significant amount of moisture available in the atmosphere. This moisture then gets transported by strong surges of westerly winds over to the Indian mainland during the monsoon, resulting in widespread and heavy rainfall over central India.”

Human activities in the post-industrialisation period have triggered an exponential rise in greenhouse gas emissions and pollutants. In addition, frequency of events such as El Nino, the abnormal warming of central and east-central Pacific Ocean, have been nailed to be the driving forces for the warming of Arabian Sea in recent decades. Incidentally, the number of cyclones originating from the Arabian Sea has increased.

“Its impact on India has been mainly the change brought in the rainfall duration and amounts,” Raghu Murtugudde, senior researcher from University of Maryland, told The Indian Express in an email reply. “On one hand, coastal and peninsular India has experienced rainfall deficits and for the last three years consecutive years, some parts of Karnataka and Maharashtra have remained dry. On the other hand, the Northwest is getting flooded far too often, and this trend is seen extending to the Northeast.”

“All this simply indicates that pre-monsoon heat and humidity have increased, and it has the most severe impact on farmers, preparing their land for the monsoon season.”

Unlike what was believed earlier — that depressions formed in the Bay of Bengal are responsible for most heavy rainfall between June and September over India — this study has painted a different picture. Researchers at IIT found that Arabian Sea holds as high as 36 per cent moisture, followed by land evaporation contributing up to 29 per cent, while at 26 per, moisture pumped in by Bay of Bengal contributes the lowest.