Curious case of dip, rise in Indian seas

Global warming may be inching the oceans higher every year, but researchers studying the seas around India report a paradox. From 1993 to 2003 — the first decade when satellites started consistently tracking the rise and fall of ocean heights and global temperatures soared — the North Indian Ocean (NIO) sea levels fell. The NIO consists of the Arabian Sea, the Bay of Bengal and part of the Indian Ocean up till the 5°S latitude. After 2004, sea levels began an unprecedented, accelerated spike till 2014.

This rise and fall was even as global temperatures steadily climbed and registered their largest two-decadal jump in more than a century.

Previous studies that had measured ocean heights based on traditional tide gauges found that the NIO —like the rest of the world’s seas — continued to rise between 1993 and 2004. While Intergovernmental Panel on Climate Change reports have concluded that while unabated greenhouse gas emissions into the atmosphere would cause oceans to rise every year, there would be years during which some seas could register a fall. Scientists associated with the study said that such a “decadal swing” in the North Indian Ocean was unique and never observed in either the Pacific or Atlantic oceans.

“We were trying to understand why the North Indian Ocean was warming twice as fast as the other oceans after 2004,” lead author M. Ravichandran, Director, National Centre for Antarctic and Ocean Research (NCAOR), told The Hindu.

Melting ice sheets

The NIO went down about 0.3 mm a year and from 2004 gained about 6 mm annually. This was twice the global, annual average of about 3 mm. When temperature and sea level trends in the NIO were mathematically separated out from the other oceans, the fall was even more dramatic: nearly 3 mm per year and the Arabian Sea cooling off rapidly at 4 mm per year. Mr. Ravichandran and his colleagues relied on satellite observations and a global network of floats, which log temperature and salinity, to arrive at their findings. Their work appears in the latest edition of the peer-reviewed Climate Dynamics.

Sea levels primarily rise due to water expanding from atmospheric heat and, more water being added from, for instance, melting ice sheets and glaciers. In this case, said Mr. Ravichandran, 70% of the NIO’s warming could be explained by expansion.

Unlike the Pacific and Atlantic, the NIO was hemmed in all sides, except for an outlet on the southern side. This influenced the rate at which heat was absorbed and flushed out from within the system. According to their calculations, heat was moving out slower during after 2004 than during the 1990s.

Wind flows

“This inter-decadal trend is an extremely important factor,” said Shailesh Nayak, a co-author and former secretary of the Central Earth Sciences department, “and underlines why we need to plan coastal management better.”

Mr. Ravichandran said wind flows, which welled warm water on the Indian Ocean surface, changed directions every decade and probably influenced sea level patterns. “It could be that coming decades — in spite of rapid, rising temperatures — will see a fall in sea levels but that’s still hypothetical,” he added.

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Printable version | Jul 23, 2021 9:15:38 PM | https://www.thehindu.com/sci-tech/energy-and-environment/curious-case-of-dip-rise-in-indian-seas/article18348042.ece

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