When ocean currents reach critical velocities, whirlpools and eddies form. Eddies then behave independently and even separate from the currents.
The core of an eddy may be colder or warmer than the surrounding water. It is believed that cyclones passing over warm core eddies tend to intensify while cold core eddies reduce cyclone intensity. But now researchers from IIT Bhubaneswar say that this might not always be the case.
They analysed 60 cyclones in the North Indian Ocean from 2001 to 2018 using cyclone track and intensity data from the IMD archive. From satellite altimetry data along the cyclonic tracks, the researchers measured sea surface height, a proxy for heat stored in the ocean. (Heat is significant for cyclone formation and intensification when water temperature is above 26 degree Celsius). By integrating temperature from the surface to the 26 degree Celsius layer, the researchers calculated tropical cyclonic heat potential.
They also identified eddies along the tracks of the cyclones. The team found that, of 24 cyclones with warm core eddies in their path, only 22 intensified after crossing eddies. And only 12 of 18 cyclones weakened after crossing cold core eddies. Thus, 14 per cent of the cyclones did not conform to the expected pattern. In fact, some cyclones continued to intensify even over cold core eddies and some weakened over warm core eddies.
The researchers checked the tropical cyclonic heat potential along the tracks of cyclones that did not encounter eddies. Only 78 per cent of cyclones intensified after passing through a high heat potential region. The researchers attribute this difference in the behaviour of cyclones to their translation speed. A fast moving cyclone may not spend enough time to get influenced by an eddy or the underlying heat potential.
The researchers found that, even in the absence of an eddy or heat potential region, cyclones often intensify in the Bay of Bengal post-monsoon. This, they say, is because of the thick barrier layer formed by fresh water discharge from rivers running into the Bay of Bengal. The fresh water layer on top stratifies the water column and restricts mixing. This confines the energy that feeds the intensity of the cyclone to the upper layer.
Including factors such as eddies and translation speed can make forecasting models more accurate, say the researchers.
Adv. Space Res., 68 (2): 773-786 (2021);
STEAMindiaReports: providing energy for research in India