Polish scientists get their heads around Sumatran floods

Resorting to posts on Twitter, a Polish-led group of scientists discovered that convectively conjugated oceanic or atmospheric waves (also known as Kelvin waves) are behind the abundance of floods that trouble the western Indonesian island of Sumatra.

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“If it is the dry season, it rains 26 days a month in Sumatra. If it is the rainy season, precipitation occurs 29 days per month on average,” tropical meteorologist Dariusz Baranowski of the Geophysics Institute of the Polish Academy of Sciences (PAN) explains, adding that every now and then heavy rainfall hits the Indonesian island bringing about floods and landslides.

The scientists found that convectively conjugated Kelvin waves play a major role in triggering the disasters. Although the waves are par for the course for this equatorial oceanic phenomena, their influence on Sumatra has been greatly underestimated until the group’s discovery.

Having analysed satellite data, numerical models and, due to the lack of consistent local data, posts on Twitter from 2014-2018, the scientists argued that Kelvin waves were the sole cause of 30 percent of Sumatran floods and played a major role in 65 percent of the rest of the inundations. Therefore, to predict floods better, the scientists suggest focusing on the forecasting and monitoring of Kelvin waves.

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Kelvin waves are a product of disruption in atmospheric pressure. They are characteristic for the equatorial latitude and can be observed both in water and air. Moving from west to east at 43 km/h, the waves appear every 5 - 20 days and maintain their shape. The most characteristic feature of the waves is that they are non-dispersive. Annually round 130 Kelvin waves are reported worldwide.

Ahead of the wave and behind it, strong winds blow in opposing directions. The resulting torrents within the Kelvin wave generate storm clouds and rain. It is the precipitation from these clouds that hits Sumatra and causes the floods.

The research has been published in "Nature Communications", which is a peer-reviewed open access scientific journal published by Nature Research since 2010. It covers the natural sciences, including physics, chemistry, earth sciences, medicine, and biology.

The Polish-led group of scientists consisted of Polish scientists Dariusz Baranowski (PAN), Katarzyna Barabasz (Collegium Civitas), Michał Labuz and Beata Latos (PAN); American Maria K. Flatau, Jerome Schmidt, Professor Piotr J. Flatau; and Indonesian Dwikorita Karnawati, Jaka A. I. Paski and Marzuki.