Spectacular Tropical Cyclone Kyarr peaked on Sunday, October 27, 2019 in the waters west of India as a category 4 storm with 150 mph winds. This made Kyarr the second most intense tropical cyclone on record in the Northern Indian Ocean’s Arabian Sea, according to ratings by the Joint Typhoon Warning Center (note that they originally assessed Kyarr as peaking with 155 mph winds, but later knocked this down to 150 mph). The only stronger storm on record in the Arabian Sea was 2007’s category 5 Tropical Cyclone Gonu.

According to the NOAA Historical Hurricane Tracks database, only four other category 4 or stronger tropical cyclones besides Kyarr have been recorded in the Arabian Sea, where reliable satellite data extends back to 1998:

Gonu, 2007 (165 mph winds, the only cat 5 on record in the Arabian Sea)
Phet, 2010 (145 mph winds)
Chapala, 2015 (140 mph winds)
Nilofar, 2014 (130 mph winds)

Kyarr is the second high-end category 4 cyclone observed this year in the North Indian Ocean. On May 2, Tropical Cyclone Fani, which formed in the Bay of Bengal off the east coast of India, peaked with 155 mph winds. Fani made landfall on May 3 in eastern India with 150 mph winds, killing 89 and causing over $8 billion in damage.

A new record for Accumulated Cyclone Energy in the North Indian Ocean

The North Indian Ocean has seen four named storms so far in 2019. Three formed in the Bay of Bengal (to the east of India)--category 4 Fani, category 2 Vayu, and category 2 Hikaa--and one formed in the Arabian Sea (to the west of India)--category 4 Kyarr. Climatologically, the Bay of Bengal sees about twice as many tropical cyclones as the Arabian Sea. For the 172 tropical cyclones that formed over North Indian Ocean during 1983–2015, 56 formed over Arabian Sea, and 116 formed over the Bay of Bengal.

The four Northern Indian Ocean tropical cyclones of 2019 have combined to produce the most active tropical cyclone season in recorded history for the basin, when measured by Accumulated Cyclone Energy (ACE). Accumulated Cyclone Energy is a measure of the total destructive power of a hurricane season, based on the number of days strong winds are observed. ACE for an individual storm is computed by squaring the maximum sustained winds of the storm at each 6-hourly advisory and summing up over the entire lifetime of the storm. Since 1998, the highest ACE for a North Indian Ocean season was 46.1 in 2007. As of 11 am EDT October 31, 2019, the total ACE for the 2019 North Indian Ocean season was 65.2. That record-breaking total will rise significantly during through early November as a new storm in the Arabian Sea--Tropical Cyclone Maha, which formed on October 30 off the southwest coast of India--follows a path to the northwest and then west across the Arabian Sea, intensifying to near category 3 strength by November 5, according to the 11 am EDT October 31 forecast from the Joint Typhoon Warning Center.

According to the Real-Time Tropical Cyclone Activity page of Colorado State University, the North Indian Ocean has been more active than average in all respects this year. The basin has seen 4 named storms, 4 hurricanes, 2 intense (category 3+) hurricanes, and an ACE index of 65.2. An average season has 3.1 named storms, 0.8 hurricanes, 0.5 intense hurricanes, and an ACE index of 10.7 by October 31. The records are 10 named storms (1992), 5 hurricanes (1998), and 3 intense hurricanes (1999). The North Indian Ocean has had one other named storm in 2019--January’s Tropical Storm Pabuk--but that storm originated in the Northwest Pacific, and is thus not included in the Northern Indian Ocean tallies.

Departure of SST from average
Figure 1. Departure of sea surface temperature (SST) from average for October 24, 2019. Note the cooler-than-average waters in the eastern Indian Ocean, south of the Bay of Bengal, contrasted with the warmer-than-average waters in the Arabian Sea. This pattern is the positive mode of the Indian Ocean Dipole (IOD). Credit: NOAA

Exceptionally warm ocean temperatures helped Kyarr rapidly intensify

Sea surface temperatures (SSTs) across the Arabian Sea have been boosted for months by one of the strongest positive modes on record of the Indian Ocean Dipole (IOD). The IOD is an irregular natural oscillation of SSTs in which the western Indian Ocean becomes alternately warmer and then colder than the eastern part of the ocean. A positive IOD brings warmer-than-average SSTs to the Arabian Sea cooler-than-average SSTs near Indonesia. The current positive IOD event is the strongest in at least 60 years and has boosted SSTs in the region where Kyarr rapidly intensified by about 1°C (1.8°F). There is no long-term trend in the IOD, and it is uncertain how climate change may affect it.

Climate change is increasing the threat of powerful Arabian Sea tropical cyclones

The North Indian Ocean has two tropical cyclone seasons—one centered in May, before the onset of the monsoon, and one centered in October/November, after the monsoon has waned. During the June – September peak of the monsoon, tropical cyclones are uncommon, due to interference from the monsoon circulation. Since the introduction of reliable satellite data in 1998 over the Arabian Sea, there had never been a post-monsoon Arabian Sea tropical cyclone with 3-minute average winds of at least 105 mph (the threshold for extremely severe cyclonic storms, as classified by the India Meteorological Department)--until 2014. But with Kyarr’s formation, we have now seen five since 2014: Nilofar in 2014, Chapala and Megh in 2015, Ockhi in 2017, and now Kyarr in 2019. (There was also one such storm recorded before the onset of reliable satellite data in 1998, though: an unnamed November 1977 cyclone).

This unprecedented shift in tropical cyclone activity led to a 2017 modeling study by Murakami et al. which concluded that human-cause 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 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.