Last week, unprecedented heavy floods ravaged large parts of Southeast Asia. From Indonesia to Thailand and Sri Lanka, many areas were affected by heavy rainfall, resulting in over 1250 deaths and hundreds missing at the time of writing. What caused all this rain?
Three Cyclones at the Same Time
Last week, a very unusual situation occurred in the Indian Ocean and the Western Pacific Ocean: three tropical storms were present simultaneously in the area. Near Sri Lanka was cyclone Ditwah, above Malaysia cyclone Senyar had formed, and off the coast of Vietnam was typhoon Koto.
Cyclones can reach wind speeds of up to 300 kilometers per hour. Depending on the area where they form, they are called hurricanes (Caribbean) or typhoons (Pacific). Read more about tropical cyclones here.
The Wrong Direction
Cyclone Senyar was a very special storm. Cyclones usually do not form near the equator but at latitudes north or south of more than 5 degrees. Senyar formed at 4.7 degrees north latitude and even moved further south. Additionally, Senyar moved in the wrong direction: cyclones in tropical areas usually move from east to west, but Senyar moved from west to east over Malaysia.
This remarkable course was due to the influence of typhoon Koto, which was nearby. When two cyclones are relatively close, they can affect each others intensity and path. This is called the Fujiwhara effect and was recently seen with hurricanes Humberto and Imelda. In this case, Koto pulled Senyar eastward, causing the storm to pass over Malaysia and Indonesia, leading to large-scale flooding.
La Niña
The simultaneous formation of these three cyclones is exceptional. By looking at changes in large-scale atmospheric movements, we can better understand why this happened. One factor meteorologists examine is whether we are in an El Niño, La Niña, or neutral condition. During El Niño, sea water near Peru is warmer than average; during La Niña (image 2), the water is colder than average. The name El Niño comes from the Christ child, a name given by Peruvian fishermen hundreds of years ago to the abnormally warm sea water around Christmas. El Niño and La Niña peak around Christmas, but the effects on atmospheric temperature fluctuations can last for months afterward.
El Niño and La Niña strongly influence large-scale atmospheric movements in the tropical region. This movement is called the Walker circulation and describes where upward and downward airflows occur in the atmosphere. Currently, we are in a La Niña phase. During La Niña, there is a strong upward airflow near Southeast Asia. This upward airflow gives storms forming here an extra push from the atmosphere. It is therefore easier for storms to form in this region now, which helps explain why we saw three cyclones in this area last week.
Additionally, La Niña pushes warmer sea water toward the western part of the Pacific Ocean. A cyclone needs warm sea water of at least 27 degrees Celsius to form and strengthen. This warm sea water in the western Pacific Ocean provided extra energy for cyclones/typhoons trying to form in this area.
The Indian Ocean Dipole
Besides La Niña, another factor made conditions more favorable for these storms to form: the Indian Ocean Dipole (IOD). The IOD can be seen as the little sister of La Niña and occurs in the northern Indian Ocean. When the IOD is positive, sea water in the Arabian Sea (west of the northern Indian Ocean) is warmer than average, and in the Bay of Bengal (east) it is colder than average. The negative phase of the IOD shows the exact opposite pattern, with colder than average sea water in the Arabian Sea and warmer water in the Bay of Bengal.
Currently, we are in a negative phase of the IOD, meaning warmer water is on the west side of Malaysia. Add to that that La Niña causes warmer water on the east side of Malaysia and that the atmosphere in this area helps forming storms due to La Niña. These factors together created very favorable conditions for cyclones Ditwah and Senyar and typhoon Koto to form.
Climate Change
Human-induced climate change causes the atmosphere and oceans to warm. Warmer sea water provides more energy for cyclones to form and intensify. As a result, it is expected that we will see stronger cyclones in the future. Additionally, warm air can hold more moisture. This makes rain showers heavier, and cyclones bring more precipitation, for example.
