What happens in the ocean when two cyclones collide?

In April 2021, two tropical cyclones, Serroha and Odette, collided in the Indian Ocean off northwestern Australia. Two researchers from Oldenburg University are currently studying how this unusual phenomenon affects the ocean.

According to their case study, this merger caused an unusual cooling of surface waters and an abrupt change in the direction of the storm complex. With the frequency and intensity of tropical cyclones increasing as a result of global warming, such encounters, and thus more extreme air-sea interactions, may become more frequent in the future, the study says. They concluded:

Tropical cyclones (TCs) not only stir up air masses in the atmosphere, but also water masses in regions of the ocean in their path. As Professor Oliver Wahl and Dr. Jens Mayer-Jurgens from the University of Oldenburg report in a paper published in the journal Tellus A: Dynamic Meteorology, when two cyclones collide and merge, the ocean and atmosphere Interactions can become significantly more intense. Oceanography.

Two researchers analyzed the encounter of two relatively weak tropical cyclones, TC Seroja and TC Odette, that formed in the Indian Ocean in 2021, and found that effects previously observed only in stronger cyclones occurred. I discovered what I did. This type of convergence and the resulting extreme interactions between air and ocean are likely to become more frequent in the future, as the frequency and intensity of tropical cyclones is increasing as a result of global warming. The study concludes that there is.

Two tropical cyclones, Seroja and Odette, approached north-west of Australia in April 2021. To investigate the effects of this unusual encounter on the ocean, Wahl and Mayer-Jurgens combined satellite data and measurements from ARGO floats and autonomous drifting vessels with numerical modeling.

These sources provided researchers with information on factors such as salinity and temperature, as well as data on upward and downward (vertical) current velocities from the ocean surface to depths of up to 2,000 meters. In addition to these data, we used numerical model data to analyze upward and downward (vertical) flow velocities.

The encounter between the two cyclones lasted about a week. On April 6, the two sides came within about 1,600 kilometers of each other. “Seroya first stalled the small cyclone Odette and merged with it three days later,” said Wahl, who heads the research group “Ocean Interface Processes and Sensing” at the Institute of Marine Environmental Chemistry and Biology at Oldenburg University in Wilhelmshaven. It is reported that. .

TC Seroha suddenly changed course by 90 degrees on April 9 after the two cyclones merged. “This series of events not only influenced weather patterns, but also triggered previously unobserved interactions on the ocean floor,” Wahl explains.

The analysis showed that the combined effect of the cyclones caused sea surface temperatures to drop by 3 degrees Celsius, stirring deep, cold water masses from 200 meters down to the surface in a process known as “upwelling.” Ta. . The researchers observed that the cooling effect was “very high” compared to the strength of the cyclone.

Maximum wind speeds reached around 130 kilometers per hour on April 11, after the cyclones merged, making it a Category 1 hurricane on the hurricane scale. On the other hand, the observed cooling and upwelling depths were on the scale observed in Category 4 or 5 hurricanes.

Wahl and Mayer-Jurgens were particularly surprised by the strength of the upwelling. At one point, deep water bodies rose to sea level at a rate of up to 30 meters per day. By comparison, the ocean’s typical rate of rise is only 1 to 5 meters per day.

In this particular case, a downward velocity of the ocean was observed just before the cyclones merged. “Thanks to satellite technology and the autonomous deep-sea ARGO float, we were able to demonstrate how rotating cyclones transport cold water from the deep ocean to the surface,” said marine scientist Meyer-Jurgens. Masu.

Tropical cyclones have historically been rare during their one- to two-week lifetimes, but climate models suggest that the number and intensity of tropical cyclones are likely to increase as a result of global warming. , and even a complete cyclone is more likely to form. -A blown away hurricane-sized cyclone collides.

This could lead to “the most extreme interactions between the ocean and atmosphere,” the authors wrote. The fact that the merging of two cyclones can cause a sudden change in course also makes predicting subsequent cyclone behavior even more difficult.

Wahl also pointed to another important finding. “As a result of cyclone-ocean interaction and the upwelling of cold deep water, the ocean absorbs additional heat from the air and transports it to higher latitudes. This is an important process and affects climate around the world. influence.”

In addition, cyclones convert thermal energy into mechanical energy and transport it to higher latitudes as they advance. Next year, the two scientists will take part in an expedition by the research vessel Meteor in the Mediterranean and subtropical Atlantic, during which they will further investigate the relationship between these interactions and extreme weather events.