In April 2021, two tropical cyclones Seroya and Odette collided in the Indian Ocean northwest of Australia. Two researchers from the University of Oldenburg studied how this rare event affects the ocean. According to their example, the encounter caused an unusual cooling of the water surface and a sudden change in the direction of the combined storm. Experts have concluded that it is possible that such encounters, and therefore more extreme air-sea interactions, will become more frequent in the future, as the frequency and intensity of tropical cyclones increases as a result of global warming.

Tropical cyclones (TCs) not only churn up air masses in the atmosphere, but also churn up water bodies in regions of the ocean in their path. Professor Dr. from the University of Oldenburg. Oliver Wuhrl and Dr. When two hurricanes collide and merge, the interaction between the ocean and the atmosphere can greatly increase, Jens Meyerjurgens states in an article published in the journal. Tellus A: Dynamic Meteorology and Oceanography.

Two researchers analyzed the collision of two relatively weak tropical cyclones, TC Seroja and TC Odette, in the Indian Ocean in 2021 and found impacts normally only seen in much stronger cyclones. The study concludes that such convergences and the resulting extreme air-sea interactions may become more common in the future as tropical cyclones increase in frequency and intensity as a result of global warming.

In April 2021, two tropical cyclones Seroy and Odette merged in northwestern Australia. To investigate the impact of this unusual encounter on the ocean, Wuerl and Meyerjurgens combined satellite data and measurements from ARGO buoys and autonomous drifters with numerical simulations.

These sources provided researchers with data on upwelling and downwelling (vertical) current speeds, as well as factors such as salinity and water temperature from the sea surface to depths of up to 2,000 metres. In addition to these data, they analyzed updraft and downdraft (vertical) velocities using data obtained from numerical models.

The meeting between the two hurricanes lasted about a week. On April 6, they came within about 1,600 kilometers of each other. “Seroja first intercepted the smaller hurricane Odette and merged with it three days later,” says Wurl, who leads the research group “Processes and Sensing of Marine Interfaces” at the Institute of Marine Environmental Chemistry and Biology at the University of Oldenburg in Wilhelmshaven. . .

After the merger of two hurricanes on April 9, TC Seroja suddenly changed course by 90 degrees. “This chain of events not only affected the weather, but also triggered a previously unobserved interaction with the underlying ocean,” explains Wuerl.

Discover the latest news in science, technology and space from the more than 100,000 subscribers who rely on Phys.org for daily information. Subscribe to our free newsletter and receive daily or weekly updates on breakthroughs, innovations and important research.

The analysis showed that as a result of the merging of cyclones, sea surface temperatures dropped by 3°C and masses of deep, cold water rose to the surface from a depth of 200 meters in a process known as ‘upwelling.'” The cooling effect was “extraordinarily high” for the intensity of the cyclones, the researchers said.

The highest wind speed of approximately 130 kilometers per hour was reached on April 11 after the cyclones merged and corresponded to the first category on the hurricane scale. The observed cooling and ascent depth was the same magnitude as that seen in Category 4 or 5 hurricanes.

Wurll and Meyerjurgens were particularly surprised by the power of the upwelling: there were periods when deep-sea masses rose to the sea surface at a speed of up to 30 meters per day. By comparison, the typical speed of a rising ocean is only 1 to 5 meters per day.

In this particular case, a downward velocity of the ocean was observed shortly before the cyclones merged. “Thanks to satellite technology and ARGO’s autonomous deep-sea buoys, we were able to show how the rotation of cyclones carries cold water from the deep ocean to the surface,” says oceanographer Meyerjurgens.

Although collisions between tropical cyclones within one to two weeks are rare today, climate models indicate that the number and intensity of tropical cyclones will likely increase as a result of global warming, increasing the likelihood of a full-blown storm as a result. hurricane force cyclone collisions.

This could lead to “the most extreme interactions between the ocean and the atmosphere,” the paper’s authors write. The fact that the merger of two hurricanes can cause a sudden change in route makes it difficult to predict their future behavior.

Wurl also points out another important conclusion: “As a result of the cyclone’s interaction with the ocean and the rise of cold deep water, the ocean absorbs additional heat from the air and then transports it to higher latitudes – an important process that affects the cyclone’s climate around the world.”

Additionally, cyclones convert thermal energy into mechanical energy and carry this energy to higher latitudes as they progress. The two scientists will join an expedition in the Mediterranean and subtropical Atlantic next year aboard the METEOR research vessel, during which they plan to continue studying these interactions and connections to extreme weather events.