|Above: A rare fourplex of tropical systems in the western Indian Ocean: Tropical Cyclone Ambali (lower right), Tropical Cyclone Belna (lower left), Tropical Storm Pawan (upper left) and Invest 92B (upper right). This image is from 0Z Friday, 6 December (7 pm EDT Thursday), when Ambali’s top sustained winds were peaking at 155 mph, according to JTWC. Image credit: Meteosat, via RAMMB/CIRA/CSU.|
Barely a tropical storm on Wednesday night EST, Tropical Cyclone Ambali astounded weather watchers on Thursday as it pole-vaulted to the brink of Category 5 strength in the southwest Indian Ocean, counter to nearly all expectations. Ambali’s top sustained winds, as assessed from satellite data by the Joint Typhoon Warning Center, jumped from 35 knots (40 mph) at 0Z Thursday to 135 knots (155 mph) at 0Z Friday. This increase of 100 knots is the largest 24-hour intensification on record in the Southern Hemisphere, breaking the record of 95 knots (110 mph) from Cyclone Ernie in 2017. It falls just short of the world record set by Hurricane Patricia in 2015 of 105 knots (120 mph).
Ambali was ramping down on Friday almost as quickly as it cranked up. As of 12Z Friday, Ambali’s top winds were already back down to 90 knots (105 mph), making it a Category 2 storm. JTWC predicted that Ambali will weaken to a tropical storm by 12Z Saturday, then slowly dissipate several hundred north of La Reunion and Mauritius.
There was no sign a couple of days ago that Ambali was destined for such notoriety. JTWC had only begun tracking Ambali as a minimal tropical storm at 18Z Wednesday. Even the ace high-resolution intensity model HWRF predicted as late as Wednesday night that Ambali would peak as a Category 1 cyclone. Instead, Ambali leapt from initial tracking to 155-mph beast mode in just 30 hours, a rate that likely has few if any counterparts in JTWC records.
How did this happen? Ambali benefited from a powerful outflow jet that pushed storm-top air toward the south in dramatic fashion. Light wind shear (less than 10 knots) and unusually warm water temperatures of around 30°C (86°F)—roughly 1-2°C above average—surely helped. It also mattered greatly that Ambali had such a tiny core. As of Friday morning, tropical-storm-force winds extended out less than 100 miles and hurricane-force winds extended less than 20 miles from Ambali’s center. Smaller tropical cyclones can both strengthen and weaken far more quickly than large systems.
The #NOAA20 captured this image of 3 tropical #cyclones churning over the #IndianOcean on Dec. 5, 2019. #TropicalCycloneAmbali set a record for the most rapid intensification in 24 hours in the Southern Hemisphere since 1980. Learn more: https://t.co/v1hFRyFaYx #Ambali pic.twitter.com/RCUUX9itwD— NOAA Satellites (@NOAASatellites) December 6, 2019
There’s also a bigger context in play that gave Ambali a boost. The cyclone is just one part of a remarkable burst of activity in the western Indian Ocean. On Friday morning, Ambali was one of four tropical systems splayed across the western Indian Ocean—two north of the equator and two south of it.
—Tropical Storm Pawan was nearing landfall on the coast of eastern Somalia early Saturday local time with top sustained winds of 45 knots (55 mph) as of 18Z Friday. Wind impacts should be modest, but rainfall could exceed 3” in parts of southern Somalia as Pawan moves inland. The rains will add to an autumn of misery across East Africa, where some 250 people have been killed in floods and mudslides. More than half a million Somalians have been displaced by heavy rains and flooding even before Pawan’s arrival.
—Tropical Cyclone Belna became a Category 1 storm on Friday afternoon, with top winds estimated by JTWC at 65 knots (75 mph) as of 18Z Friday. Belna is heading toward the northwest coast of Madagascar, where it is expected to make landfall on Monday local time, perhaps as a Category 2 or 3 storm. Belna is wrapped in a large shield of moisture and is gradually strengthening, passing over slightly-warmer-than-average SSTs of 29-30°C (84-86°F) amid light to moderate wind shear of around 10 knots. Belna may take advantage of a strong outflow channel to its south and intensify more rapidly than predicted. The 18Z Friday HWRF suggests that a period of Category 4 strength is not out of the question.
—Invest 92B is slowly organizing a few hundred miles southwest of India. SSTs are more than warm enough for development, but 92B appears likely to suffer from wind shear, and models bring it to minimal tropical storm strength at best as it drifts westward over the next several days. It could reach Somalia by the middle of next week.
Three cyclones hanging out over the Indian Ocean. Looking at @SONAR_FW , container ships (left-hand side) are naturally staying away from the storms. Higher concentrations of ships indicated by brighter colors. #freight #logistics pic.twitter.com/e5Qn0ZUvZt— Nick Austin (@FreightWeather) December 6, 2019
Ferreting out factors behind the fourplex
Dr. Jeff Masters sent me these comments:
“Experiencing four simultaneous tropical cyclones or tropical disturbances in the Arabian Sea and the mirror image portion of the South Indian Ocean, south of the equator, is certainly a rare occurrence—and one possibly unmatched in the satellite era. We've basically crammed in all of the tropical systems that can possibly fit into that limited area of ocean! We know it is rare to see two simultaneous named tropical cyclones in the northern hemisphere's Arabian Sea. According to the India Meteorological Department, that has occurred only once in recorded history—in October 2019, when Cyclone Kyarr and Cyclone Maha were both active simultaneously.”
There are at least two climate phenomena working in favor of tropical cyclones over the western Indian Ocean, as discussed by Jonathan Belles in a weather.com writeup on Thursday.
One immediate factor is a strong phase of the Madden-Julian Oscillation (MJO). The MJO is a sprawling wave-like atmospheric feature, centered on the tropics, that propagates eastward, circling the globe about every 30 to 60 days, waxing and waning with large variations in its strength and timing. Broadly speaking, the MJO fosters upward motion on one side of the globe and sinking air on the other. An especially strong, focused phase of the MJO has supported rising air over the western Indian Ocean in the last couple of weeks, paving the way for the current fourplex of storms.
Sometimes its difficult to pinpoint #MJO initiation. Not for this event. Its easy to illustrate using @AliciaMBentley's PW+Irr wind plots showing MJO initiation over the Indian Ocean on 26 Nov.— Philippe Papin (@pppapin) December 6, 2019
Trigger mechanism for 4 TCs in the basin + strengthening the waveguide downstream. pic.twitter.com/J0eUpq2kxh
Another factor is the Indian Ocean Dipole (IOD), which has been in a strongly positive mode through the latter half of 2019. Much like the El Niño/Southern Oscillation in the tropical Pacific, the IOD is a semi-cyclic ocean-and-atmosphere phenomenon that tends to push warmer-than-average water toward one side of the Indian Ocean or the other. A positive IOD tends to bring unusually warm seas and enhanced rainfall toward the western Indian Ocean, with cooler-than-average sea surface temperatures (SSTs) and reduced rainfall in the eastern Indian Ocean. The IOD this autumn reached its highest values since 2006.
With the help of this autumn’s strongly positive IOD, the North Indian Ocean has churned out a record amount of cyclone activity. The region’s accumulated cyclone energy reached 88.3 units as of Friday, nearly five times average for the year so far. Much of this has been in the hyperactive Arabian Sea, which has seen two other hurricane-strength cyclones this year, Vayu and Hikaa, in addition to Kyarr and Maha.
“There is no long-term trend in the IOD, and it is uncertain how climate change may affect it,” said Jeff Masters in an October post at ScientificAmerican.com. However, he noted, the authors of a 2018 review paper agreed unanimously that a detectable increase in post-monsoon extremely severe cyclonic storms in the Arabian Sea during the 1998-2015 period was at least partially related to climate change.