Severe Tropical Cyclone Seroja

3-12 April 2021


Severe Tropical Cyclone Seroja was a notable event for several reasons. Firstly, during its initial formation, Seroja producing torrential rainfall and devastating floods in parts of Indonesia and Timor Leste. Later, it interacted with Tropical Cyclone Odette via the Fujiwara effect, a phenomenon rarely observed in the Australian region. Finally, it strengthened into a category 3 tropical cyclone producing a severe impact in the Mid-West region of Western Australia, unusually far south for a coastal crossing of a Severe Tropical Cyclone.

Seroja began as a slow-moving tropical low near the Indonesian Island of Rote on 2 April. It produced sustained heavy rainfall, which caused extensive flooding and landslides on Rote and neighbouring islands, particularly Timor. Widespread and devastating damage was reported by Indonesia's disaster agency Badan Nasional Penanggulangan Bencan (BNPB), including more than 150 fatalities.

The tropical low intensified and on 5 April it was named Seroja by Jakarta Tropical Cyclone Warning Centre (TCWC). It started moving west and quickly intensified into a category 2 tropical cyclone. Then, during 6 and 7 April, Seroja weakened back into a tropical low as it moved southwest. During 8 and 9 April it began to interact with another tropical low, that briefly intensified into Tropical Cyclone Odette. Over a period of approximately 36-48 hours the two tropical cyclones interacted via the Fujiwara effect.

The interaction with Odette is likely to have been a factor in maintaining Seroja's track towards the southwest, rather than recurving into the west Pilbara coast. As Odette circled around to the north and then the east of Seroja, it also made conditions more favourable for intensification by replacing the dry air that had been limiting Seroja's intensity with moist air that could fuel its intensification. The increased moisture combined with lower vertical wind shear resulted in Seroja re-intensifying into a category 2 tropical cyclone while Odette weakened and eventually dissipated.

During 10 April, Seroja took a sharp turn towards the southeast and began to accelerate towards the Western Australian coast. Seroja further intensified into a severe category 3 tropical cyclone on 11 April and maintained this intensity through to its coastal crossing just south of Kalbarri around 8pm Australian Western Standard Time (AWST).

Impacts at Kalbarri and the nearby town of Northampton were severe with many buildings sustaining significant damage, mostly consisting of lost roofs but other structures were destroyed. Many locations recorded maximum wind gusts greater than 125km/h with the highest being 170km/h from Meanarra Tower near Kalbarri. Seroja weakened as it moved further inland, though due to its rapid motion destructive winds extended a long way inland before it eventually weakened below tropical cyclone intensity early on the morning of 12 April near the town of Dalwallinu.

Widespread power outages were experienced through Western Australia's Mid-West region due to fallen trees and power lines.

For more information see the TC Seroja Report (pdf).

Track and Intensity

Best Track of TC Seroja