  We investigate modons on tidally synchronised extrasolar planets. Modons are
highly dynamic, coherent flow structures composed of a pair of storms with
opposite signs of vorticity. They are important because they divert flows on
the large-scale; and, powered by the intense irradiation from the host star,
they are planetary-scale sized and exhibit quasi-periodic life-cycles --
chaotically moving around the planet, breaking and reforming many times over
long durations (e.g. thousands of planet days). Additionally, modons transport
and mix planetary-scale patches of hot and cold air around the planet, leading
to high-amplitude and quasi-periodic signatures in the disc-averaged
temperature flux. Hence, they induce variations of the "hot spot" longitude to
either side of the planet's sub-stellar point -- consistent with observations
at different epoch. The variability behaviour in our simulations broadly
underscores the importance of accurately capturing vortex dynamics in
extrasolar planet atmosphere modelling, particularly in understanding current
observations.
