WunderBlog Archive » Category 6™

Category 6 has moved! See the latest from Dr. Jeff Masters and Bob Henson here.

Debbie’s Silver Lining: Cooler Waters and Less Coral Reef Damage

By: Bob Henson and Jeff Masters 4:46 PM GMT on March 28, 2017

Tropical Cyclone Debbie roared into northern Queensland, Australia, early Tuesday local time, hurling giant waves, fierce winds, and a major storm surge into the sparsely populated coast. Landfall occurred near Airlie Beach at about 12:40 pm local time Tuesday. Debbie’s top sustained winds at landfall, averaged over 1 minute (as is standard practice for Atlantic hurricanes), were estimated at 105 knots (120 mph), near the midpoint of the Category 3 range on the Saffir-Simpson scale. Debbie ranked as a Category 4 on Australia’s tropical cyclone rating scale, which uses different thresholds and 10-minute wind averaging.

An anemometer located near the airport on rugged Hamilton Island, near the south eyewall of Debbie as the cyclone approached the coast, recorded peak sustained winds of 99 knots (114 mph) gusting to 142 knots (163 mph) at 10:30 am Tuesday local time. The anemometer is at an elevation of 59.4 meters (195 feet). One would expect slightly higher gusts at this height than at sea level, as the effects of surface friction would be less and the turbulence produced by the coastal topography could allow for stronger winds to mix down from aloft.

As of midday Tuesday EST, only one serious injury had been reported from Debbie, although the toll could rise as relief crews enter the hardest-hit areas on Wednesday local time. Power was knocked out to some 40,000 homes and business, noted weather.com’s Sean Breslin. The cyclone's approach marked the largest pre-deployment ever carried out by the Australian Defense Force ahead of a natural disaster, said Prime Minister Malcolm Turnbull.

Figure 1. Tropical Cyclone Debbie making landfall over Queensland, Australia, as seen by the MODIS instrument at 0342 UTC Tuesday, March 28, 2017 (1:45 pm Tuesday local time). Image credit: NASA.

Figure 2. Storm clouds gather in the town of Ayr in far north Queensland as Cyclone Debbie approaches on March 27, 2017. Image credit: Peter Parks//AFP/Getty Images.

Figure 3. Winds at the airport on Hamilton Island, Queensland, AU, peaked at 142 knots (163 mph) at 10:30 am local time on March 28, 2017, as Tropical Cyclone Debbie approached just to the north. The site experienced sustained winds exceeding hurricane force (65 knots) from 1:00 am to 2:00 pm Tuesday, a span of 13 hours. The anemometer at Hamilton Island is located at a height of 195 feet, which allowed for slightly stronger winds than at sea level. Image credit: Courtesy Michael Theusner.

Debbie’s prolonged storm surge
Debbie’s slow forward motion in the hours prior to landfall allowed surge to pile up from southeast winds pushing water into a concave portion of the coastline about 20-30 miles south of the landfall location. According to storm surge expert Hal Needham, witnesses reported water about one meter (3.3 feet) above ground level In the area near Midge Point. At Laguna Quays, the storm surge was 2.75 meters (9 feet). The surge’s effects were exacerbated by the new moon and by Debbie’s slow forward motion, which allowed at least one meter of storm surge to persist through a full astronomical cycle (12 hours), including a high tide.

Needham reports that the surge from Debbie was the 14th highest on the Queensland coast since 1880, and the highest since Tropical Cyclone Yasi (2011). On top of the storm surge, mammoth waves—again boosted by Debbie’s slow motion—battered the coast. A buoy located about a mile offshore from Hay Point recorded waves exceeding 10 meters (33 feet), topping the previous record at that location of 7 meters in data going back to 1993.

Figure 4. Significant wave height and maximum wave height on March 27-29, 2017, at the Hay Point buoy, located about a mile offshore from Hay Point and about 8 miles southeast of Mackay, Queensland. Image credit: Queensland Government.

Debbie’s heavy rain to swipe New Zealand
Even as it decays, the circulation of Debbie will continue to dump very heavy rain across parts of Queensland, with the help of precipitable water (moisture in a column above the surface) exceeding 2.75” in some spots, according to satellite-based estimates. Later this week, Debbie’s remnants will flow southeastward and merge with a frontal zone, perhaps dumping additional heavy rain onto the mountains of southern New Zealand.

Figure 5. Rainfall for the six-day period starting at 06Z Tuesday (5:00 pm Tuesday local time in Queensland) could exceed 200 mm (8”) over the higher terrain of Queensland, Australia. Storm totals for Debbie could reach 500 mm (20”) in some spots. A swath of moisture associated with Debbie’s remnants may extend to the west coast of New Zealand’s South Island. Projections shown here are from the 06Z Tuesday run of the GFS model. Image credit: tropicaltidbits.com.

Debbie will slow coral bleaching in portions of the Great Barrier Reef
Though the wave action from Tropical Cyclone Debbie likely caused some destruction of fragile corals along portions of Australia’s Great Barrier Reef, that wondrous natural site is likely to experience a net benefit from the passage of Debbie. The cooling effect of the storm on near record-warm summer waters that were besieging the reef will help reduce coral mortality from the reef’s ongoing coral bleaching event. By raising global temperatures overall, human-produced climate change has led to especially sharp oceanic warming during El Niño events. The result has been three global coral bleaching events: the first in 1998, the second in 2010, and the third beginning in 2015. That event is still under way— the longest, most widespread, and most damaging on record, according to NOAA.

Preliminary satellite data (Figure 5) showed that Debbie’s passage had already cooled waters near the coast of Australia by up to 2°C on Monday, and additional cooling has occurred since that time. This amount of cooling may be enough to stop widespread coral bleaching and die-off along central portions of the Great Barrier Reef.

Figure 6. Departure of sea surface temperature (SST) from average on March 27, 2017. Debbie’s passage had already cooled waters near the coast of Australia by up to 2°C by this time, and additional cooling has occurred since the time of this image. Image credit: NOAA.

Australia’s Great Barrier Reef suffered its greatest coral die-off ever observed in 2016, when about two-thirds of the shallow-water coral on the reef’s previously pristine, 430-mile northern stretch died. The only thing that saved the southern portion of the 1,400-mile long reef was wind, rain and cloud cover from the passage of ex-Cyclone Winston, which churned up the waters and cooled water temperatures by up to 3°C. As discussed in a paper published this month in the journal Nature, ”Global warming and recurrent mass bleaching of corals,” Winston passed over Fiji on February 20, when the southern Great Barrier Reef was only 1 °C cooler than the north. By March 6th, this disparity increased to 4 °C. Corals in the south that had begun to pale in February regained their color in the south in March, whereas bleaching continued to progress in central and northern sectors.

Figure 7. Prior to the arrival of Debbie, the entire Great Barrier Reef was experiencing “Alert Level 2” bleaching stress (associated with widespread coral bleaching and significant mortality), according to the latest four-month forecast from NOAA’s Coral Reef Watch. This is the highest level of thermal stress in NOAA’s warning system for coral reefs.

Hurricanes can alleviate thermal stress on corals by three mechanisms:

1) Hurricanes absorb energy from surface waters through the transfer of latent heat, causing evaporative cooling. The magnitude of the cooling is related to the intensity and extent of the hurricane.

2) Local upwelling brings deeper, cooler water to the surface. The amount of surface cooling resulting from these mixing mechanisms will depend on the hurricane wind speed and how the water temperature varies with depth at each location.

3) Clouds of a hurricane shade the ocean surface from solar heating, allowing the water to cool and reducing light stress.

The benefits of reducing coral reef damage can be economic as well as ecological. Australia relies on the Great Barrier Reef for about 70,000 jobs and billions of dollars annually in tourism revenue, according to a recent New York Times analysis.

We’ll be back with a new post by Wednesday at the latest.

Bob Henson and Jeff Masters

Figure 8. A variety of corals form an outcrop on Flynn Reef, part of the Great Barrier Reef near Cairns, Queensland, Australia. Image credit: Toby Hudson/Wikimedia Commons.

Hurricane Coral Reefs

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.