CHENNAI: Why is the north-east monsoon, the lifeline for Tamil Nadu, becoming erratic and spawning strong and deadly tropical cyclones? Extended dry periods and extreme rainfall events are the new normal. What is causing this change?
The answer is rapid warming of Indo-Pacific Ocean and near the two-fold expansion of "warm pool" covering most part of Bay of Bengal.
This is part of changing global rainfall pattern, reports a study led by Roxy Mathew Koll of the Indian Institute of Tropical Meteorology (IITM), Ministry of Earth Sciences, Pune.
In a study published in the science journal Nature, the researchers report a two-fold expansion of the Indo-Pacific warm pool—the largest expanse of the warmest ocean temperatures on Earth.
They find that the expansion of this warm pool has altered the most dominant mode of weather fluctuation originating in the tropics, known as the Madden Julian Oscillation (MJO).
This Indo-Pacific warm pool has been warming rapidly and expanding during the recent decades in response to increasing carbon emissions.
"The warm pool expanded double its size, from an area of 2.2 × 107 km2 during 1900–1980, to an area of 4 × 107 km2 during 1981–2018. The rate of expansion is 4 × 105 km2, an area equal to the size of Japan, every year.
Just ahead of the commencement of north-east monsoon in October, entire Bay of Bengal becomes part of this giant warm pool aiding rapid intensification of cyclones," Koll told Express, quoting examples of recent devastating Cyclone Fani and Ockhi, which were a nightmare for the meteorological department in predicting their landfall and intensification.
Though the study does not talk about north-east monsoon in specific, Koll said the study period (November-April) corresponds with north-east monsoon rainfall activity.
"We have not explored the details region-wise. But, the study reveals, in general, the rainfall will reduce and extreme weather events to increase due to changes in the MJO behaviour."
The MJO is characterised by a band of rain clouds moving eastward over the tropics. The MJO regulates tropical cyclones, the monsoons, and the El Niño cycle—and occasionally contributes to severe weather events over Asia, Australia, Africa, Europe and the Americas.
The MJO travel a stretch of 12,000–20,000 kms over the tropical oceans, mainly over the Indo-Pacific warm pool, which has ocean temperatures generally warmer than 28°C.
Michael McPhaden, Senior Scientist at the US National Oceanic and Atmospheric Administration (NOAA), who participated in the study, said, “There are coordinated international efforts underway to extend the range of accurate weather forecasts. Climate model simulations indicate that continued warming of the Indo-Pacific Ocean is highly likely, which may further intensify these changes in global rainfall patterns in the future.”
Koll said: "We need to enhance our ocean observational arrays to monitor these changes accurately and update our climate models to skillfully predict the challenges presented by a warming world.”
The study is part of an Indo-US collaboration, between the Ministry of Earth Sciences (India) and the NOAA, facilitated by the U.S. National Academy of Sciences. Other co-authors of the study are Panini Dasgupta (IITM), Deahyun Kim (University of Washington) and Tamaki Suematsu (University of Tokyo).
1. Tamil Nadu is currently reeling under 11 per cent deficit rainfall.
2. Chennai, despite getting a good start, is caught in the extended dry period of nearly 20 days.
3. The city is facing a 39 per cent rainfall deficit.
4. November was supposed to be the wettest month, but this November has been nothing short of a disaster for Chennai.
5. Hopes are now pinned on December rains.
6. 24 districts in the State need good rains to bridge the deficit rainfall.
7. Met officials said light to moderate rain is likely to occur at a few places over coastal Tamilnadu, Pondicherry and Karaikal, light rain is likely over interior Tamil Nadu till December 1.