Two tropical cyclones seen on same longitude

By HansPeter Roesli (Switzerland)

Between 23 April and 3 May 2019 two tropical cyclones, Lorna and Fani, had almost simultaneous life cycles. Lorna evolved south of the equator and Fani was north of the equator. Figure 1 shows the lifecycles of both tropical cyclones from 23 April to 5 May.

Symmetrical and quasi-simultaneous cyclone cross-equator occurrences are rare events, most probably limited to the vast basins of the Pacific and Indian oceans. They usually only happen during the crossover of the late/early cyclone seasons in the southern/northern hemispheres, when conditions such as Sun position and sea surface temperature values weaken/strengthen such as to be favourable for their cyclone activity. In fact, Lorna was the last tropical cyclone of the 2018–19 cyclone season in the south Indian Ocean, and Fani ,was the first tropical cyclone of the 2019 cyclone season in the north Indian Ocean.

The research paper Twin tropical cyclones in the Indian Ocean: the role of equatorial waves explored the role of equatorial waves for two similar twin storms that occurred over the Indian Ocean in late December 2011 and in May 2013.

Frequency statistics depend very much on the criteria of symmetry relative to the equator and synchrony that are applied. In the Earth Science Beta discussion Have there ever been simultaneous cyclones in the same ocean but different hemispheres? a number of criteria are cited, some of which applied in the case of the Lorna-Fani duo:

• Formed within 3.5 days (nine days allowed).
• Initial latitudinal separation was 13.1° (not more than 22°) and got even smaller in time.
• Initial longitudinal East-West difference was -13.1° (well within +9° and -17°).

Thus, Lorna and Fani qualified as ‘cross-equatorial tropical cyclone pairs’ or ‘twin tropical cyclones’, as they are referred to the paper Dynamical Aspects of Twin Tropical Cyclones Associated with the Madden–Julian Oscillation. This paper also looked at the dynamic aspects of such twins, associated with the Madden-Julian Oscillation (MJO) propagating slowly eastward from the eastern Indian Ocean to the central Pacific.

Lorna lasted from 23 April 12:00 UTC to 29 April 12:00 UTC, while Fani had a lifetime from 27 April 2019 00:00 UTC to 3 May 12:00 UTC (image source CIMSS ).

Figure 2 shows the complete tracks of Lorna and Fani at six-hour intervals in red, laid over the Meteosat-8 Natural Colour RGB from 27 April at 12:00 UTC. The concurrent parts are overlaid in orange. Cloud cover can be seen on the RGB image, the date being identified on the tracks by yellow dots.

The two tropical cyclones were simultaneously active between 27 April (birth of Fani) to 29 April 2019 (death of Lorna) (Figure 3). While Lorna dissolved over the sea, Fani strengthened further and made landfall over north-eastern India, see Tropical Cyclone Fani makes landfall in India.