Tropical Cyclone Seroja: A Rare and Destructive Event Near the Equator
On 5 April 2021, East Nusa Tenggara was struck by sudden heavy pouring rain leading to flash floods and landslides. More than 160 people have been killed, at least 22,000 people have been displaced, and about 2,000 homes have been damaged (BNPB, 2021). What is unique about this huge destructive phenomenon, Nusa Tenggara region is known as one of the drylands in Indonesia with climatological yearly precipitation around 1,406 mm/year (compared to the climatological mean of West Java, 2,199 mm/year) (BPS, 2017).
This disaster was generated by a tropical cyclone, named Seroja by Tropical Cyclone Warning Center (TCWC) Jakarta. A tropical cyclone is a large force of spinning storm formed in a low air pressure system in the waters around the tropics. Tropical cyclones form in the ocean and will become extinct or weaken when entering cold water areas or land. This phenomenon generates winds that exceed 119 km/h (74 mph). In some extreme cases, winds may exceed 240 km/h (150 mph), and gusts may surpass 320 km/h (200 mph) along with the intensification of rain and storm surges (Ahrens, 2007).
The convective system of a tropical cyclone needs fuel that is provided by a transfer of water vapor and heat from the warm ocean to the overlying air. Thus, the environment should contain warm waters of at least 26.5 °C (80 °F) or warmer, and this warm layer must be at least 50 m (150 ft) deep. Second, we need force to make the system spin, so there is Coriolis force. Coriolis force is an invisible force that appears to deflect the wind caused by Earth's rotation. The Coriolis force is zero at the equator and maximum in the polar region. That is why tropical cyclones rarely occur in regions under 15° latitude due to lack of Coriolis force. However, Nusa Tenggara is located close to the equator (8–10° South), which makes Seroja a little special. A record by LAPAN Atmospheric Science shows a positive trend of cyclone development near the equator, especially around the Banda Sea. One tropical disturbance (early tropical cyclone stage) was found in the 1980s and the amount doubled in the 1990s. The amount increased dramatically over the last decade: there were six tropical disturbances recorded around the Banda Sea and one of them became Seroja.
The cause of increasing tropical disturbances near the tropical region and their occurrence mechanism remain interesting research topics. But there is a hypothesis about the interaction of the Inter Tropical Convergence Zone (ITCZ), Madden-Julian Oscillation (MJO), and Equatorial Rossby (ER) wave. ITCZ is a band of wind convergence at the equator that also means a humid region with plenty of water vapor. This band seasonally shifts north–south following the seasonal cycle of incoming solar radiation. In April, the ITCZ should be in the northern hemisphere, while this year a double ITCZ was observed that split in the western Indian Ocean. These two bands of ITCZ promote the development of tropical cyclone Seroja in the southern hemisphere and Surigae near the Philippines. The next question is, how can a cyclone survive in a low Coriolis force region? This role is taken by the interaction of the eastward-moving convective system of an MJO disturbance that met a westward ER wave. But still, these hypotheses need further research.
(Read more about possible tropical cyclogenesis mechanisms: Introduction to Tropical Meteorology — The COMET Program.)
There is a more unique yet rare event during Seroja's lifetime called the Fujiwhara effect. It is when two tropical systems get close, interact, and circulate around each other. In this case, Tropical Cyclone Seroja and another tropical low, Odette, off the coast of Western Australia were merging on 10 April. This effect was responsible for tropical cyclone Seroja's intensification after it struck the land of East Nusa Tenggara, then moved southeast to greet the west coastline of Australia after 11 April.
Figure 1. Fujiwhara effect of Tropical Cyclone Seroja and Odette (source: NASA Worldview).
These rare destructive events are evidence that our understanding regarding Earth's climate needs more attention while the climate itself is changing. The warmer sea condition in the tropics makes more favorable conditions for tropical cyclone development than it used to be. Moreover, it brings a realization that weather and climate in the tropics can also be dangerous and highlights the need for severe weather mitigation, not only by the government but by all of society.
Besides this phenomenon bringing disaster on land, we truly remember how Tropical Storm Lex of the tropical Pacific caused the Glomar Java Sea disaster back in 1983 due to communication and weather forecast errors. As time went by, the development of technologies and research allowed improvements to weather forecasting, and it became a primary need for fieldwork safety. Since 2010, BW Geohydromatics has already been taking part in providing weather information and predictions for daily use as well as severe conditions. With the integration of weather information by researchers, government policies regarding mitigation, and public support, we believe we could be better prepared and thus minimize losses in the severe weather conditions to come.
References
