An unusually warm Arabian Sea is likely to have contributed to intense bursts of monsoon rain in parts of India in August, which in particular led to flooding and landslides in many parts of the west coast, scientists said, citing sea surface temperature readings that strengthen concerns of global warming possibly leading to extreme rain events in the future.
Coupled with the consecutive low pressure areas of Bay of Bengal that put the monsoon in its “active” to “vigorous” phase, the warmer waters in the Arabian Sea further compounded the climatology that triggered heavy rain in several parts of the country.
In the vigorous phase, rain increase by 1.5-4 times the normal volumes.
“Arabian Sea has been warming rapidly in recent decades. This makes the air above warmer, humid and unstable. As a result, the monsoon winds are exhibiting more fluctuations than earlier. So occasionally there are episodes where huge amount of moisture is dumped along the west coast of India in a few days’ time. This year again, the northern Arabian Sea was up to 2-3°C warmer than usual in August, and we saw several spurts of monsoon rains across the west coast,” said Roxy Mathew Koll, climate scientist at the Indian Institute of Tropical Meteorology.
According to monitoring by the National Oceanic and Atmospheric Administration, the average monthly SST (sea surface temperature) for August in parts of north Arabian Sea was 29 to 31 degree C and in southern parts was 29 to 30 degree C. The SST anomaly map showed temperatures to be 2 to 3 degree C and 0.5 to 1.5 degree C higher than expected in pockets of north Arabian Sea.
With the formation of a low pressure area over Bay of Bengal on August 4, there were instances of extreme rain along the west coast. For example on August 3, Mumbai’s Dharavi got 38 cm rain; Santacruz and Colaba recorded 26 and 25 cm respectively; Hosanagar and Bhagamandala in Karnataka recorded 21 and 19 cm respectively.
On August 4, Palghar recorded 46 cm; Talasari 39 cm; Dahanu 38 cm Mahabaleshwar 32 cm; Khanvel 39 cm; On August 6, Vaibhavwadi in Sindhudurg recorded 71 cm rain; Avalanchi in Tamil Nadu’s Nilgiris recorded 58 cm; Bhagamandala 49 cm, Mumbai’s Colaba 33 cm according to India Meteorological Department data.
There are several scientific papers that have concluded that the Arabian Sea is becoming warmer as a result of climate change. The Ministry of Earth Sciences (MoES) report titled “Assessment of Climate Change over the Indian Region” which has analyses and data from 1901-2018 and is modeled on the Intergovernmental Panel on Climate Change’s assessment reports, said increased variability of low-level monsoon westerlies and warming of north Arabian sea lead to increased moisture supply and thus enhance extremely heavy rain events.
“Rapid warming in the Arabian Sea has resulted in a rise in widespread extreme rains over Western Ghats and central India, since warming induces increased fluctuations in the monsoon winds, with ensuing episodes of enhanced moisture transport from the Arabian Sea towards the Indian subcontinent. Indian Ocean warming is also found to reduce rainfall over India during the onset phase and increase it during the withdrawal phase,” the report states. Models also indicate that there will be higher SST warming over Arabian Sea compared to Bay of Bengal.
“Monsoon was very active in August and the westerlies were very strong. Warm Arabian Sea could lead to more moisture incursion along the west coast. More moisture availability could lead to extreme rain episodes,” said M Rajeevan, secretary, ministry of earth sciences.
“The SSTs are supposed to drop by 2-3 degrees in August compared to May. Higher SSTs means more evaporation and the moisture gets transported to the west coast with strong westerly winds when monsoon is active. There is more warming being recorded in the equatorial Indian Ocean,” explained R Krishnan, executive director, Centre for Climate Change Studies at the Indian Institute of Tropical Meteorology (IITM). Some scientists cautioned that SSTs may have been one of the factors. Low pressure areas formed over Bay of Bengal which strengthened the monsoon winds are an equally dominant factor, said RK Jenamani, senior scientist at IMD.
IMD in its two week forecast issued on Friday said below normal rains are likely in most parts of the country except over northeastern states, Maharashtra, Goa, Karnataka and Kerala between September 10 and 16. Development of features for monsoon withdrawal from western parts Rajasthan is likely during the same week.