Tropical cyclone of Category-4 Kyarr churning through the Arabian Sea. (NASA)
TC Kyarr helped propel 2019 into the top spot for the most active cyclone season on record in the North Indian Ocean basin and it is the first storm of such intensity in the Arabian Sea in the last 12 years. Kyarr is the seventh named storm in the basin this season, marking the first time more than half a dozen had spun up in a single season since 1998. The Accumulated Cyclone Energy (ACE), which is a measure of how much kinetic energy a cyclone is dissipating through its winds was found to reach 53.9 units for the year 2019, which exceeded the average ACE (~10 units) for the North Indian Ocean. The highest number of tropical cyclone days was recorded to be 5.25 days in 2007 and 1999, but with TC Kyarr, the 2019 tally exceeded this record. Kyarr also set the record for the lowest barometric pressure ever observed in this part of the basin.
Interestingly, the positive phase of the Indian Ocean Dipole (a large overturning circulation in the tropical atmosphere over the Indian Ocean) which implies warmer than normal ocean temperatures in the Arabian Sea and west North Indian Ocean might have helped fuel more intense storms in this region this year. Since cooler sea surface temperatures exist over Australia and the Maritime Continent further east, this causes the rising motion to the west, with subsidence or sinking to the eastern part of the Indian Ocean basin. That rising motion enhances convection, rainfall and thunderstorm activity, with relatively weak upper-level winds therefore allowing convective activity which in turn creates a tropical disturbance. The Indian Ocean Dipole’s positive amplitude has been near-record strength this year, leading to active monsoon season across India as well. On the other end, father east in Australia, prolonged drought has been parching the continent. The Australian Bureau of Meteorology predicts the Dipole amplitude to strengthen in the next 1-2 months.
This unprecedented shift in tropical cyclone activity led to a 2017 modeling study by Murakami et al. which concluded that anthropogenic climate change had increased the probability of powerful post-monsoon tropical cyclones over the Arabian Sea, and that this risk would increase further in the future--with potentially damaging consequences to the nations bordering the Arabian Sea. In a 2018 review paper by 11 hurricane scientists (Tropical Cyclones and Climate Change Assessment: Part I. Detection and Attribution), all 11 authors concluded that the balance of evidence suggests that there was a detectable increase in post-monsoon extremely severe cyclonic storms in the Arabian Sea during the 1998 – 2015 period; 8 of 11 authors concluded that human-caused climate change contributed to the increase.
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© 2019 Oceanographer Daneeja Mawren