Nine confirmed deaths after Cyclone Gabrielle sweeps along the east coast of New Zealand's North Island.

Nine people have now been confirmed dead after Cyclone Gabrielle swept across the east coast of North Island, New Zealand, on Sunday 112 February 2023. About five thousand more people are missing following the event, and while it is thought that most have simply been cut off in remote areas by the damage that the storm has done to the country's communications network, ten missing people are considered to be at high risk. A week after the event about 62 000 households are still without electricity, 40 000 of them in the Hawkes Bay area, with about 2000 people still staying in evacuation centres.

Flooding in the town of Wairoa on the east coast of North Island, New Zealand, following the passage of Cyclone Gabrielle. Hawkes Bay Civil Defense. 

The storm brought the second highest rainfall ever recorded in a single day in New Zealand, 348 mm in 24 hours at Whangārei in the Northland Region. as well as sealevels 30-40 cm above normal, winds gusting at up to 155 km per hour, and waves as high as 11 m. The storm brought widespread flooding, with numerous landslides sweeping away roads and buildings. Landslides are a common problem after severe weather events, as excess pore water pressure can overcome cohesion in soil and sediments, allowing them to flow like liquids. Approximately 90% of all landslides are caused by heavy rainfall. The flooding was made worse due to a rainstorm which hit North Island at the end of January, which has already left the ground waterlogged and drainage systems at capacity across much of the affected area, with between 10 and 20% of the floodwaters unleashed by Gabrielle thought to have been linked to this earlier event.

Water pouring from a storm drain into a street at Te Awanga to the southeast of Auckland on Tuesday 14 February 2023. Warren Buckland/AP.

Tropical storms, called Cyclones in the Indian Ocean and South Pacific, are caused by solar energy heating the air above the oceans, which causes the air to rise leading to an inrush of air. If this happens over a large enough area the inrushing air will start to circulate, as the rotation of the Earth causes the winds closer to the equator to move eastwards compared to those further away (the Coriolis Effect). This leads to tropical storms rotating clockwise in the southern hemisphere and anticlockwise in the northern hemisphere. These storms tend to grow in strength as they move across the ocean and lose it as they pass over land (this is not completely true: many tropical storms peter out without reaching land due to wider atmospheric patterns), since the land tends to absorb solar energy while the sea reflects it..

The formation of a tropical cyclone. Natural Disaster Management.

Cyclone Gabrielle initially formed over the Coral Sea, to the east of Queensland, where the Australian Bureau of Meteorology recorded sea surface temperatures of 30°C earlier this month, around 5°C hotter than the threshold generally assumed for the formation of tropical storms. This came amid high sea temperatures across the Western Pacific Warm Pool region driven by three consecutive years of La Niña conditions in the region, which has led to a huge build up of thermal energy in the southern Pacific, with temperatures 4°C above normal recorded 150 m below the surface.

Position of the Warm Pool in the western Pacific under La Niña conditions. Bell et al. (2011).

The La Niña weather system is the opposite of the El Niño weather system, in which unusually cold surface temperatures spread across the equatorial Pacific from the upwelling zone on the South American coast. This traps warm water from the western Pacific, preventing it from spreading east and warming the central Pacific. This leads to lower evaporation over the (cooler) east Pacific, leading to low rainfall on the west coast of South America, and higher evaporation over the (warmer) west Pacific, leading to higher rainfall over East and Southeast Asia and northern Australia.

The effects of a La Niña weather system in December-February. NOAA.

Despite the obvious danger of winds of this speed, which can physically blow people, and other large objects, away as well as damaging buildings and uprooting trees, the real danger from these storms comes from the flooding they bring. Each drop millibar drop in air-pressure leads to an approximate 1 cm rise in sea level, with big tropical storms capable of causing a storm surge of several meters. This is always accompanied by heavy rainfall, since warm air over the ocean leads to evaporation of sea water, which is then carried with the storm. These combined often lead to catastrophic flooding in areas hit by tropical storms. 

The formation and impact of a storm surge. eSchoolToday.

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Large pumice raft observed floating towards Australia's Great Barrier Reef.

A large pumice raft, covering an area of about 400 km², has been observed drifting towards Australia's Great Barrier Reef. The raft was first observed on 9 August 2019, when NASA's Terra Satellite observed it near Late Island in the Kingdom of Tonga. It was recorded again on 13 August by the Operational Land Imager on Landsat 8, having drifted some way to the southwest. On 15 August the Catamaran ROAM encountered a raft of pumice rocks from a few centimetres to tens of centimetres in size, that completely covered the sea so that the water could not be seen. The vessel inadvertently entered the raft at night, and not being able to see the other side, slowed to about 1 km per hour and reversed out. 

Pumice raft near Late Island, Tonga, on 13 August 2019. Landsat 8/NASA/Earth Observatory.

Pumice forms when hot lava from submarine volcanic eruptions encounters seawater and cools rapidly, simultaneously crystallising and degassing to form a lightweight volcanic rock with many gas filled vesicles (bubbles) within it, which often floats on the sea surface. Big submarine eruptions can produce large volumes of pumice, forming rafts of pumice that cover hundreds of square kilometres, and drift on the ocean surface for months before dissipating or washing ashore.

 Crew member Larissa Brill on the Catamaran ROAM with samples of pumice collected from a raft near Tonga earlier this month. Sail Surf ROAM/Facebook.

On this occasion the pumice is thought to have been erupted from unnamed submarine volcano near Tonga at 18.325° South, 174.365° West. The raft is drifting towards the Great Barrier Reef on the east coast of Australia, but it not thought to present any threat to the reef, but instead likely to bring new colonising reef organisms with it, as it is predicted to pass reefs off the coast of New Caledonia and Vanuatu, as well as other Coral reef areas in the eastern Coral Sea, during the annual spawning season, during which the organisms of the reefs produce millions of tiny larvae, each looking for a new place on the reef to attach, which are capable of colonising pumice rafts and thereby reaching more remote locations. This input of new organisms could prove highly beneficial to the Great Barrier Reef, which has been damaged in recent years by pollution, heat stress and blooms of the aggressive Crown of Thorns Starfish, Acanthaster solaris, though such new arrivals cannot protect the reef against future damage from the same causes, none of which seem likely to abate.

Reef organisms colonising pumice stone: Two pieces of pumice bound together by Cyanobacteria (photosynthetic filament-forming Bacteria, principally Rivularia sp.) and Macroalgae (Seaweed, Caulerpa sp.) collected from Broadbeach in southeastern Queensland on 27 December 2007. Also present are two Cauliflower Corals (Pocillopora sp.), a Colonial Scyphozoan (Order Coronatae, the benthic larval stage of a Crown Jellyfish), Goose Barnacles (Lepas anserifera) and a Pearl Oyster (Pinctada sp.) Bryan et al. (2012).

The islands of Tonga lie along the boundary between the Pacific and Australian Tectonic Plates. The Pacific Plate is being subducted beneath the Australian Plate along the Tonga Trench, which forms the boundary between these two plates, with the volcanic islands that make up the archipelago being formed as the subducting plate is melted by the heat of the planet's interior, so that lighter minerals rise up through the overlying plate as liquid magma, which fuels the volcanoes that build the islands.

 Diagram showing subduction along the Tonga Trench, and how this feeds the volcanoes of the Tonga Volcanic Arc. York University.

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Flooding as draught breaks in North Queensland.

Many people have been forced to flee their homes, with some needing to be evacuated by emergency services, after flooding hit the North Queensland region of Australia this week. Several people have fallen ill with gastric conditions, possibly caused by contamination of water supplies, but there are no reports of any fatalities associated with the flooding. The worst effected area is around Ingham, Innisfail and Halifax, where around 225 homes have been inundated by waters, though the cities of Townsville and Cairns have also been hit.

Flooding in the town of Ingham in the North Queensland Region of Australia this week. Courier Mail.

North Queensland has a tropical climate, with a long dry season that lasts from mid-April to Mid-December, and a short rainy season that lasts from mid-December to mid-April, during which most areas receive around 400 mm of rain per month. However this year the area has been hit by a draught, driven by unseasonably hot weather over the Coral Sea, which leads to prevailing winds flowing from the Australian Interior out to sea rather than the other way round, with almost no rain falling in December, January and February, a weather pattern that has prevailed in several recent years. 

 Flooding near Cairns this week. Stewart McLean/Gold Coast Bulletin.

This pattern appears to have been broken by the movement of Cyclone Hola over the Coral Sea, which while remote from the Australian coast has pushed water-laden air inland, leading to over 600 mm of rainfall in the Ingahm area in three days. Flooding is a common problem following periods of severe draught, as protracted periods of dry weather can cause topsoil to dry out completely, making it vulnerable to being blown away by the wind. When rain does arrive it then falls on exposed bedrock, which is much less absorbent, triggering flash flooding as the water escapes over the surface of the ground rather than sinking into it.

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