|Above: Visible Himiwari-8 satellite image of Cyclone Gita just before sunset (0140Z Monday, February 12, 2018). The islands of Tongatapu and ‘Eau can be seen at far left. A satellite loop shows the storm pushing west toward the islands. Image credit: RAMMB/CSU.|
Thousands of Tongans saw their most terrifying storm in living memory on Monday night local time, as the eyewall of Tropical Cyclone Gita ripped across the southern end of the Southwest Pacific archipelago under cover of darkness. The eyewall of Gita passed directly over the largest and most populous island, Tongatapu, which is home to the nation’s capital, Nuku’alofa (population around 24,000).
Tonga is a far-flung collection of 169 islands spanning roughly 500 miles from north to south. On average, about one tropical cyclone a year affects some part of Tonga, but Gita is by far the strongest on record to make a direct hit on Tongapatu. The Joint Typhoon Warning Center (JTWC) pegged Gita’s top winds averaged over a 1-minute period at 145 mph, making it a Category 4 on the Saffir-Simpson scale (or a Category 5 on the alternative scale used in the Southwest Pacific). The central pressure of Gita as it approached Tonga was estimated by the Fiji Meteorological Service to be 930 mb—even lower than Hurricane Harvey when it struck Texas, as noted by Jon Erdman (weather.com).
|Figure 1. Enhanced infrared satellite image of Cyclone Gita just after its closest approach to the islands of Tongatapu and ‘Eua, which can both be seen outlined just to the northeast of Gita’s eye. Gita moved just to the north of due west as it passed just south of the two islands. Image credit: NOAA/NESDIS.|
Gita’s westerly track and its intensification on approach (greater than originally forecast) produced close to a worst-case scenario for Tongapatu. The center of Gita’s eye passed only about 10-20 miles south of the island. A saving grace is that Tongapatu was on the weaker side of Gita’s eyewall, as winds rotate clockwise around tropical cyclones in the Southern Hemisphere. In addition, Nuku’alofa is located on the north side of the island, so its harbor was shielded from any storm surge approaching from the south. Nevertheless, we can expect widespread damage across Tongapatu, given the strength of Gita.
A Facebook page called Tonganow - Online Community for Tongans is sharing initial damage photos, which include downed trees and power lines and roof damage. Newshub (New Zealand) shared Facebook reports of flooding at the Vaiola Hospital on the coast in Nuku'alofa. Update: Tonga's Parliament House was destroyed by the storm.
Another Tongan island, ‘Eua (population 5000), about 15 miles southeast of Tongatapu, may have gotten an even more powerful hit, as the center of Gita passed only a few miles to the south of ‘Eua. The harbor of ‘Eau’s main town, ‘Ohonua, sits on the west coast, so it would have experienced the full force of the westerly winds on the north side of Gita. The hilly terrain of ‘Eau likely worked to limit storm surge, but the island no doubt got a pounding from Gita’s high winds as well as heavy rains. Cat 6 commentor Jeremy L, who visited the island recently, notes that the south side of the island is quite exposed.
Monday’s last observation from the Nuku’alofa airport—at 8:35 pm local time Monday night, several hours before Gita’s closest approach to the island—showed a wind gust to 63 mph. The Tonga Met Service reportedly lost its roof and its access to radar data in the midst of the storm. “From indoors, it’s like a soundtrack to a horror movie out at sea,” tweeted Sulia Makasini, who had taken shelter at a church on the island.
Our church hall #FWCFanga is filled with evacuees. The sense of security is overwhelming. The women and children are huddled in the higher and central parts of the hall. The men keep vigilant watch by the windows. If that isn't love, I don't know what is #proudTongan #Tonga— Sulia Makasini (@suliamakasini) February 12, 2018
|Figure 2. Gita carried out a multiday cyclonic loop on its approach to Tonga, intensifying dramatically over the final 36 hours before reaching Tonga’s southern islands. Gita brought high winds and flooding rains to Samoa and American Samoa as it passed to their south before rapidly intensifying. Image credit: Brendan Moses, @cyclonebiskit.|
Tonga’s cyclone history
Although Tonga’s wide north-south expanse makes it a prime target for westward-moving tropical cyclones, the small size of each island makes it quite unlikely that a major landfall will occur in any given year. Two of the worst hits to Tonga in recent decades passed through the Ha’apai island group, located in the middle of the Tongan archipelago north of more populous Tongatapu.
Cyclone Ian (2014) is the only other Category 4 known to strike Tonga. Ian packed top 1-minute sustained winds of 145 mph as it passed through the Ha’apai islands. Ian decimated electrical and water supplies to the islands, destroyed roughly 500 buildings, caused at least one fatality, and left an estimated $48 million US in damage.
In 1982, Cyclone Isaac passed through the Ha’apai islands about 50 miles northwest of Tongatapu. Winds reached 106 mph at Nuku-alofa, and damage was extensive across all three of Tonga’s island groups. Isaac resulted in six deaths and left around 45,000 people homeless.
Per NOAA's historical database, 8 Cat. 3+ #tropical cyclones have tracked within 200 nm of the #Tonga capital of Nuku'alofa since 1990. #Gita could be closest such strike, Mon. night/Tue. (Note: Winston '16 appears to have been just north of this search radius.) pic.twitter.com/szq8o6RXR2— Jonathan Erdman (@wxjerdman) February 11, 2018
|Figure 3. Sea surface temperatures have been running more than 1°C (1.8°F) above the long-term average over the area near and just south of Tonga. This map shows departures from average SST for this time of year, averaged on a weekly basis (here valid Feb. 11, 2018) and compared to the 1961-1990 climatology. Image credit: NOAA/NWS/NCEP/EMC Marine Modeling and Analysis Branch.|
Unusually warm waters fueling Gita
Sea surface temperatures (SSTs) are typically near their warmest point of the year in February across the Southwest Pacific. Right now these waters are even warmer than usual in many areas (see Figure 3 above). SSTs were in the range of 28-29°C (82-84°F) in the area that Gita traversed as it neared Tonga, which is more than ample to support tropical cyclone intensification.
Gita appeared to be going through an eyewall replacement cycle late Tuesday local time as it continued pushing westward across the Southwest Pacific. As of 1500Z Monday (4 am Tuesday Tonga time), JTWC continued to classify Gita as a Category 4 on the Saffir-Simpson scale, with top winds of 145 mph. Gita is predicted to gradually weaken as it arcs toward the west and eventually southwest. Gita should bypass other major island groups from here on out, but the storm or its remnants could bring very heavy rain to the North Island of New Zealand in about a week.
|Figure 4. Forecast for Gita issued at 1500Z Monday, February 12, 2017 by the Joint Typhoon Warning Center. Image credit: JTWC.|
Tropical Storm Sanba hitting the Philippines
The first named storm of the 2018 typhoon season in the Northwest Pacific is Tropical Storm Sanba, which formed on February 10 in the waters to the east of the Philippines. Sanba peaked with 65 mph sustained winds on Sunday, February 11. On Monday morning (U.S. EST), when Sanba was approaching landfall in the southern island of Mindanao in the Philippines, the system had weakened to a minimal-strength tropical storm with 40 mph winds, thanks to moderate wind shear of 15 – 20 knots. Satellite loops showed that Sanba was disorganized but had a large area of heavy thunderstorms, and the storm will bring dangerous flooding rains to the southern portion of the Philippines through Tuesday.
|Figure 5. Tropical Storm Sanba approaching the Philippines as seen by the VIIRS instrument on NOAA’s Suomi satellite at 05:12 UTC February 12, 2017. Image credit: NASA.|
The development of both Sanba and Gita was aided by the Madden-Julian Oscillation (MJO), a pattern of increased thunderstorm activity near the Equator that moves around the globe in 30 - 60 days. When the area of increased thunderstorms associated with the MJO is located in the Western Pacific, formation odds of a tropical storm increase. The current MJO pulse is starting to wane, but during its peak on January 30, it was the strongest Western Pacific event since MJO records began in 1979. Note that the previous record-strength MJO events in the region, in March 1997 and March 2015, occurred during the lead-up to record-strength El Niño events later in those years. It is possible that this year’s record-strength MJO event could also help lead to an El Niño by the end of 2018.
Sanba the second tropical storm to hit the Philippines in 2018
While Sanba is the first named storm to form in 2018 in the Northwest Pacific, it is the second tropical storm to hit the Philippines this year. The islands were also hit this year by Tropical Storm Bolaven, which made landfall in Mindanao on January 1, killing three people and doing approximately $11 million in damage. Bolaven formed on December 30, 2017, though, and thus belongs to the 2017 typhoon season. According to archives of Northwest Pacific tropical cyclone activity maintained by Dr. Phil Klotzbach (Colorado State), the Northwest Pacific averages one named storm every two years by this point in the season.
Dr. Jeff Masters contributed to this post.