Extratropical cyclones in the south-east Pacific underwent the Fujiwhara effect in June 2019.
By Hans-Peter Roesli (Switzerland)
In the early 1920s, the Japanese meteorologist Sakuhei Fujiwhara was the first to describe the effect where binary interaction of smaller circulations can cause the development of a larger cyclone, or cause two cyclones to merge into one (see related content below). Cyclones merging or morphing is now known as the ‘Fujiwhara effect’.
There are not many case and scientific studies of the phenomenon. Those that do exist mainly discuss tropical-cyclone binaries. For extra-tropical systems there is the paper Rotation of mid-latitude binary cyclones: a potential vorticity approach, by B. Ziv and P. Alpert. There is some agreement in the literature that, while interaction between tropical binaries becomes significant at distances below 1400 km, extra-tropical cyclones already interact below 2000 km.
Between 3 and 5 June 2019 a group of mid-latitude cyclones circulated in the south-east Pacific, off the Chilean coast. Not just one, but up to three minor cyclones rotated cyclonically around a larger one. Their positions and relative distances on 2 June at 12:00UTC can be seen in Figure 1 (top right, click to expand). All three distances were well within the 2000 km cited as the limiting interaction radius.
Streamlines in the planetary boundary layer and at 300 hPa (Figure 2) match the cloud systems retrieved from band IR10.3 of GOES-E. It also proves that the cyclones were deep disturbances, with their axes close to vertical.
The GOES-16 infrared and Airmass RGB animation (Figure 3) shows the evolution of the system from 2–5 June at 20-minute intervals, with the false-colour IR10.3 band and the Airmass RGB shown side-by-side.
Figure 3: GOES-16 IR10.3 and Airmass RGB animation, 2 June 09:00 UTC–5 June 08:00 UTC
The cyclonic rotation of the minor cyclones around the larger one, and them being engulfed one after the other, are well depicted. The sequence of six-hour fields of MSLP and 300 hPa geopotential height in the animated gif (Figure 4), serve as further proof of the Fujiwhara effect repeatedly occuring.