Severe Tropical Cyclone Seroja was the third-deadliest tropical cyclone on record in the Australian region, behind Cyclone Mahina in 1899 and the Flores cyclone in 1973. Seroja brought historic flooding and landslides to portions of southern Indonesia and Timor-Leste and later went on to make landfall in Western Australia's Mid West region, becoming the first to do so since Cyclone Elaine in 1999. The twenty-second tropical low, seventh tropical cyclone, and third severe tropical cyclone of the 2020–21 Australian region cyclone season, the precursor of Seroja formed off the south coast of Timor island as Tropical Low 22U at 18:00 UTC on 3 April 2021; its genesis was related to convectively coupled equatorial waves. The tropical low moved very slowly near the island, while the system's thunderstorms increased in organization. The low intensified into Tropical Cyclone Seroja by 4 April, while it was passing north of Rote Island, while continuing its slow strengthening trend.
Due to the presence of Tropical Cyclone Odette in Seroja's vicinity, interaction was anticipated as the storm moved away from Indonesia and Timor-Leste. Its intensity fluctuated as it moved southwest, with its strengthening being highly hindered due to interaction with Odette. This caused the system to weaken as Seroja moved closer to it, due to a phenomenon known as the Fujiwhara effect. Eventually, Seroja began to restrengthen and weaken Odette, with Seroja absorbing Odette into its circulation on 10 April. Due to Odette, Seroja was steered to the southeast towards Australia, before strengthening even further. At around 8 p.m. local time on 11 April, Seroja made landfall on the western coastline of Western Australia as a Category 3 severe tropical cyclone, slightly south of the coastal town of Kalbarri, bringing heavy rain and hurricane-force wind gusts (about 170 km/h, or 110 mph). Later that day, Seroja began accelerating southeastward while weakening. On 12 April, Seroja emerged off the southern coast of Western Australia while beginning to undergo an extratropical transition, before being absorbed into another larger extratropical cyclone to the south. The name Seroja means lotus in Indonesian.
As of May 2021, it is estimated that at least 272 people were killed by the storm, with 183 people in Indonesia, 42 in Timor-Leste, and one in Australia. At least 72 people from Indonesia and 30 from Timor-Leste are still missing. The cyclone damaged or destroyed more than 20,000 houses and five bridges in Indonesia's East Nusa Tenggara province while more than 12,000 people were evacuated to government-owned shelters. Around 9,000 people were displaced on Timor-Leste, while at least 10,000 homes have been submerged. Damage reports from Kalbarri started coming in soon after the storm made landfall. The storm is estimated to have caused over $490.7 million (2021 USD) in damages, a majority across Indonesia. The damage caused by Cyclone Seroja in Indonesia was described as catastrophic.
Efforts to relieve the devastating effects of the cyclone came a short time after Seroja left Indonesia. They included almost a million USD in donations, methods to help fight COVID-19, evacuations, local governments sending aid to their respective areas, and more. Timor-Leste also was given outside assistance from other organizations and nations. The Indonesian and Western Australian government's responses were criticized due to the slow nature of their respective responses; in the case of Indonesia, this resulted in some high-ranking officials being fired.
Meteorological history
On 29 March, the BoM began to mention that Tropical Low 22U was developing to the south of Timor. By 3 April, the tropical low fully developed as it was within an active trough. The low was located in a generally favorable environment with deep moisture, low vertical wind shear, and defined outflow. The low drifted close to the coast of Timor very slowly with persistent spiraling bands of convection occupying the storm's circulation, producing prolific rainfall in the surrounding regions on 3–4 April. At this time, the low pressure system was located inside the Area of Responsibility (AoR) of TCWC Jakarta. By the early morning of 4 April, the presentation of its structure had improved with spiral bands of deep convection and tight curvature at its center. Although there were fluctuations in central convection, a favorable environment of deep moisture, low vertical wind shear, and good outflow meant further development of the system was expected. Meanwhile, the Joint Typhoon Warning Center (JTWC) issued their first warning on the storm as Tropical Cyclone 26S at 15:00 on UTC 4 April. The tropical low slowly gained strength, intensifying to a Category 1 tropical cyclone, and was given the name Seroja by the Tropical Cyclone Warning Centre (TCWC) Jakarta at 20:00 UTC on 4 April, about 95 km (59 mi) north-northwest of Rote Island.
During 5 April, Seroja continued to move at 3 km/h (1.9 mph) in a west-southwest direction, away from the Indonesian coastline. The storm intensified to a Category 2 tropical cyclone at 19:00 UTC, with 10-minute sustained winds of 105 km/h (65 mph) and its central pressure deepening to 982 mb (29.00 inHg). Long term uncertainty in both track and intensity persisted in the forecasting of Seroja's track, due to interaction with Tropical Low 23U, to the far west of the storm. Seroja held this strength for roughly 12 hours, before unexpectedly weakening to a Category 1 tropical cyclone yet again on 6 April, due to southeasterly wind shear, and dry air partially exposing the low level circulation (LLC) and degrading its thunderstorm activity. Seroja fluctuated in strength for several hours, intensifying slightly on 7 April, as it continued in a generally southwesterly direction. Seroja became disorganized on satellite imagery as convection became dislocated to the south on 8 April, while it began to closely interact with Tropical Cyclone Odette (formerly Tropical Low 23U) just to the northwest, as a result of the Fujiwhara effect, complicating the forecasts for the tropical cyclone and causing it to weaken again. Seroja resumed intensification by the next day, re-intensifying into a Category 2 tropical cyclone at 13:16 UTC on 9 April, as the storm began to absorb Odette and as wind shear started to ease off. By 10 April, Seroja had absorbed most of Odette's remaining thunderstorms and began to accelerate to the southeast towards Western Australia, maintaining a relatively organized structure. Later that day, Seroja fully absorbed Odette into its circulation.
Seroja continued to steadily intensify as it approached the Gascoyne Region; the JTWC upgraded Seroja to a Category 1–equivalent tropical cyclone on the Saffir–Simpson scale, with sustained winds of 120 km/h (75 mph), at 18:00 UTC on 10 April. By early on 11 April, Seroja had intensified into a Category 3 severe tropical cyclone, the third severe tropical cyclone of the season, as a ragged eye began to emerge from the cyclone's central dense overcast. Seroja continued rapidly accelerating to the southeast at 53 km/h (33 mph), with its eye becoming more defined on infrared and Doppler weather radar images before making landfall near Gregory, Western Australia, or just north of Geraldton at peak strength with 1-minute sustained winds of 130 km/h (81 mph), 10-minute sustained winds of 120 km/h (75 mph), and a minimum central pressure of 971 hPa (28.76 inHg), with observations of gusts as high as 150 km/h (93 mph). Following landfall, Seroja quickly moved to the southeast over Australia, while gradually weakening. Early on 12 April, Seroja's extratropical remnant entered the Great Australian Bight, while beginning to transition into an extratropical cyclone, before being absorbed into a larger extratropical cyclone to the south later that day.
Preparations
Indonesia
The Indonesian Meteorology, Climatology, and Geophysical Agency declared two days before the cyclone's formation that they were already anticipating the possibility of tropical cyclone development over the Savu Sea. On 4 April 2021, the agency issued an early warning. On 8 April 2021, the agency again issued warnings to other provinces such as Bali, East Java, Central Java, and West Nusa Tenggara.
Australia
The Australian Bureau of Meteorology (BOM) warned on 5 April 2021 that Seroja could approach the West Pilbara or Gascoyne coastline. It was also advised that tourists be ready to change plans, in case the cyclone impacts the area and brings unsafe conditions to go home. Due to the cyclone's uncertain track from its Fujiwhara interaction with Tropical Cyclone Odette, the chance of impacts on Western Australia was uncertain as to where it would go; out to sea or into land.
As the storm approached Australia, the price of Australian commodities such as coals and iron ores were expected to rise. In addition, there was concern that it would disrupt or delay shipments and trades from the region. The BOM also issued a tropical cyclone watch early on 9 April for much of the region between Coral Bay and Lancelin.
Department of Fire and Emergency Services (DFES) Acting Commissioner Craig Waters advised all people in the watch area to 'leave immediately' as many of the homes in the area would be under significant risk with pre-saturated soil allowing flooding to likely occur much more easily. BOM also mentioned that this region was not accustomed to tropical cyclone impacts, making the risk of Seroja's impacts more dangerous than usual with a month's worth of rain expected to fall within a day. A tropical cyclone warning was then issued for Cape Cuvier to Geraldton on 10 April as Seroja neared, albeit strengthening slower than anticipated. All people between Coral Bay and Geraldton were eventually told to evacuate immediately on 11 April as homes were not built to withstand such high winds.
Premier Mark McGowan said that Seroja was "like nothing we have seen before in decades". There was a Red Alert placed along an 800 kilometer stretch of coastline south of Carnarvon and Lancelin. The people under the alert were required to stay in their houses or in an evacuation center. The latter structures were open in Port Denison, Carnarvon and Denham. The cyclone was expected to impact the area until midnight. High tides, intense rainfall, flash flooding, dangerous surf and beach erosion were among the warned dangers of Seroja.
Impact
Timor-Leste
In Timor-Leste, a landslide and floods displaced more than 8,000 people and was initially claimed to have led to the deaths of 42 people in total. The storm caused power outages in the entirety of Timor-Leste, in addition to damaging the presidential palace. Flood caused by the storm damaged the national laboratory of Guido Valadares National Hospital, the only laboratorium in the country, causing concern that it would affect Timor-Leste's effort to mitigate the COVID-19 pandemic. The floods also seemed to have damaged the country's central pharmacy, where vaccine stocks for East Timor population were stored. Other damaged facilities include two COVID-19 isolation facilities. It is thought that many more people in Timor-Leste have died due to the storm's widespread flooding. At least 30 people remain missing in the country.
Preliminary reports from 7 April 2021 showed that approximately 10,325 people were affected by the storm. 76% of those people were in the Dili municipality. The rains were so heavy that the capital was referred to as a "mud pit". Damage from Seroja in Timor-Leste was expected to exceed US$100 million, according to Prime Minister Taur Matan Ruak.
According to a report published in January 2022 by the United Nations Office for Disaster Risk Reduction (UNDRR), the flooding in Timor-Leste affected 13 municipalities and 30,322 households, destroyed 4,212 houses, took 34 lives, and was "... said to be the worst the country has seen in 50 years ...". The Timor-Leste country coordinator for the Australian Humanitarian Partnership consortium said that it was "anything but typical". The country manager of Australian People for Health, Education and Development Abroad (APHEDA) attributed it to the obstruction of waterways by housing erected along them. She said that such housing blocks water canals, and that the government needed to enforce the country's pre-independence rule against building within 100 m (330 ft) of bodies of water.
Indonesia
As a tropical low, Seroja caused widespread rainfall and thunderstorms in the West Nusa Tenggara and East Nusa Tenggara provinces in Indonesia. In the early hours of 4 April, floods struck the island, hours before people intended to gather for Easter celebrations. In the immediate aftermath, 41 people were killed and 28 were seriously injured. Bridges and roads connecting to Flores Island and Adonara Island were destroyed. The banks of two rivers in Ende district burst, killing two people and resulting in extensive flooding in East Flores. Dozens of homes were buried under mudslides in the village of Lamenele on Adonara island. In the East Flores Regency, fourteen villages were hit by flash flooding as a result of runoff of volcanic materials from Mount Ile Lewotolok. A local chief of a village in East Flores reported that hundreds of hectares of crops were destroyed. Dozens of farm animals were also reportedly swept away by flash floods. A majority of the casualties in East Flores were from the Nelelamadike village in Adonara Island after a massive rockslide struck the village. The village reported 55 deaths, of whom 15 were children. More than 600 residents of the village were left homeless by the cyclone.
In the city of Kupang, 734 houses were damaged, affecting 2,190 people, while in Rote Ndao, around 30 houses were flooded. A bridge connecting the cities of Kupang and Malaka Regency was destroyed, making travel between the two regions extremely difficult. A passenger ship, the Jatra I, sunk on a port in Kupang city, after experiencing a leak. No casualties were reported from the sinking. Another ship, the Namparnos, went adrift and grounded on Kambing Island. Trans-Timor roads were flooded, cutting off access between regencies. The office building of the governor of East Nusa Tenggara, commonly known as "Sasando Building," was also damaged by the cyclone. In East Nusa Tenggara and West Nusa Tenggara, approximately 11,406 people were displaced, while 66,036 homes were damaged.
Five cities and regencies experienced power outages, but electricity was partially restored later. At least 98 base transceiver stations in East Nusa Tenggara province were damaged, although some were restored later by the Ministry of Communication and Information Technology. Four fuel trucks owned by Pertamina were damaged. A dam in East Sumba overflowed, causing a flood in a neighboring city. The cyclone affected areas as far as the city of Bima in West Nusa Tenggara, where it caused floods, submerging 10,000 homes and affecting more than 27,000 people. On Lembata, the rain loosened solidified lava from an eruption in November of Ili Lewotolok volcano. The stone tumbled into several villages, killing at least 11 people. A garden which had belonged to several residents of Sikumana Village in Kupang was severely flooded and turned into a two-hectare wide karst lake, after the composition of limestone and karst rock was eroded. The karst lake was given the name "Tuaknatun Lake" by local residents due to the presence of teak trees nearby.