Cyclone is weakening after setting a regional record.

Cyclone Amphan registered winds of 165 miles an hour on Monday, making it the strongest cyclone ever recorded in the Bay of Bengal. By Tuesday a phenomenon called vertical wind shear — the shifting of winds with altitude — had disrupted the storm’s rotational structure, weakening it.

By early Wednesday, Amphan had sustained winds of 115 miles an hour, according to the Joint Typhoon Warning Center, which is operated by the United States Navy. That wind speed would still make it the equivalent of a major hurricane, a Category 3 storm, in the United States. Wind shear was expected to weaken it further before landfall.

Amphan initially grew powerful because the waters it passed over were exceedingly warm, as high as 88 degrees in parts of the Indian Ocean. Warmer water provides more of the energy that fuels such rotating storms.

As a result of climate change, ocean temperatures are rising, but other factors, including natural variability, can play a role. While it is not possible to say whether any one specific storm such as Amphan was made more powerful by climate change, scientists have long expected that tropical storms like it would increase in strength as the world warms.

That expectation was based on the laws of physics and computer climate models and not on studies of actual storms. But earlier this week, researchers in the United States with the National Oceanic and Atmospheric Administration, using observational data, reported that the likelihood of these kinds of cyclonic storms developing into the equivalent of Category 3 storms had increased by about 8 percent per decade since the late 1970s.

What makes a storm a hurricane, a typhoon or a cyclone? It comes down to location. They all refer to tropical cyclones — low-pressure circular storm systems with winds greater than 74 miles per hour that form over warm waters — but different terms are used in different parts of the world.

The word hurricane is used for tropical cyclones that form in the North Atlantic, northeastern Pacific, the Caribbean Sea and the Gulf of Mexico. Typhoons are storms that develop in the northwestern Pacific and usually threaten Asia.

The international date line serves as the Pacific Ocean’s dividing marker, so when a hurricane crosses over it from east to west, it becomes a typhoon instead, and vice versa.

The same storms in the Southern Hemisphere are easier to keep straight. The storm Amphan is moving over the Bay of Bengal, so that makes it simply a cyclone — the same for storms over the Arabian Sea, which is also in the northern Indian Ocean. In the southern Indian Ocean and South Pacific, they are “tropical cyclones” or “severe tropical cyclones.”

All of these cyclonic storms act to regulate the overall climate, moving heat energy from the tropics toward the poles.

Reporting was contributed by Jeffrey Gettleman, Sameer Yasir, Kai Schultz, Henry Fountain and Jennifer Jett.



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