ZAP // Dall-E-2
In April 2021, two tropical cyclones, Seroja and Odette, collided in the Indian Ocean northwest of Australia. Two researchers from the University of Oldenburg have now studied how this rare phenomenon affected the ocean.
Os tropical cyclones not only do they stir up the air masses in the atmosphere, but they also stir up the water masses in the ocean areas in their path.
When two cyclones collide and merge, these interactions between the ocean and the atmosphere may intensify considerably, the study authors write in their paper.
So what happens when two tropical cyclones collide?
In April 2021, two relatively weak cyclones, the TC Seroja and TC Odette collided in the Indian Ocean in the Indian Ocean, northwest of Australia.
Recently, Oliver Wurl e Jens Meyerjurgensresearchers at the University of Oldenburg, conducted a study to analyze the impact this encounter had on the ocean, and found that effects occurred that would otherwise only have been observed with much stronger cyclones.
According to the results of , now published in Tellus A: Dynamic Meteorology and Oceanographythe meeting provoked a unusual cooling of surface waters and a abrupt change in direction of the combined storm.
The analysis showed that the sea surface temperature dropped by 3°C as a consequence of the merger of cyclones and that the masses of cold and deep water were pushed to the surface from a depth of 200 meters, in a process known as “upwelling“.
The cooling effect was “exceptionally high” in relation to cyclone intensity, the researchers noted.
A maximum wind speedof around 130 kilometers per hour, was reached on April 11, after the merger of the cyclones, and corresponded to category 1 of the hurricane scale. The observed cooling and depth of upwelling, on the other hand, were on the scale seen in Category 4 or 5 hurricanes.
Wurl and Meyerjürgens were particularly surprised by the strength of the resurgence: there were periods when masses of deep water rose to the surface of the sea at a speed of up to 30 meters per day. In comparison, the typical ocean rising speed is only 1 to 5 meters per day.
In this specific case, a downward velocity was observed of the ocean just before the cyclones merged. “Thanks to satellite technology and ARGO autonomous deep-sea floats, we were able to demonstrate how the rotation of cyclones transports cold water from the depths of the ocean to the surface,” explains Meyerjürgens, quoted by .
Although encounters between tropical cyclones during their one- to two-week lifetimes have been rare to dateit is possible that such encounters — and therefore more extreme air/sea interactions — will become more frequent, as the frequency and intensity of tropical cyclones are increasing due to global warming, the researchers point out.
This may result in more extreme interactions between the ocean and the atmospherewrite the authors of the article. The fact that the merger of two cyclones can lead to an abrupt change of track also makes their subsequent behavior more difficult to predict.
Wurl also draws attention to another important consequence: “As a result of a cyclone’s interactions with the ocean and the upwelling of cold, deep waters, the ocean absorbs additional heat from the air and transports it to higher latitudes – a crucial process that influences climate around the world“.
Furthermore, cyclones also convert thermal energy into mechanical energywhich they then transport to higher latitudes as they advance.
The study thus shows that the merger of two tropical cyclones, even weak ones, can have a significant impact on the ocean and global climate — and more striking consequences than those caused by a single strong cyclone.
