Modons On Tidally Synchronised Extrasolar Planets

The relative vorticity (ζ) field from a T682L20 resolution simulation at t= 100 in Mollweide projection. The dissipation order, viscosity coefficient and time-step size are p = 8, ν16 = 2.3×10−48 and ∆t = 2×10−5 , respectively. The p-levels shown, 5.0 × 10−3 (a) and 9.5 × 10−2 (b), correspond to the mid-points of the top and bottom layers of the computational domain, respectively. The flow is dominated by two highly dynamic, planetary-scale modons – a cyclonic modon at the sub-stellar point (at centres of the frames) and a much weaker and larger anti-cyclonic modon at the night side (the sides of the frames). The cyclonic modon straddles an undulating, zonally-asymmetric equatorial jet at both p-levels. The equatorial jet in a) is halted just to the east of the sub-stellar point and breaking throughout its “core” (along the equator): the jet core is where there is a jump in ζ in the meridional direction (near the equator). The equatorial jet in b), in contrast, is rolling up much more prominently at its northern and southern edges. The cyclonic modons in both frames emit large-amplitude gravity waves and generate thousands of small-scale vortices at their peripheries; the anticyclonic modons are barely visible at both p-levels because they are more diffused and lack sharp bounding fronts in these frames. Both cyclonic and anticyclonic modons are highly dynamic and exhibit periodic life-cycles in which they generally (but not always) migrate westward around the planet, while strongly interacting with other flow structures – e.g. storms, jets and waves.

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.

J. W. Skinner, J. Y-K. Cho

Comments: Accepted for publication in MNRAS
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2109.06568 [astro-ph.EP] (or arXiv:2109.06568v1 [astro-ph.EP] for this version)
Submission history
From: Jack Skinner
[v1] Tue, 14 Sep 2021 10:35:20 UTC (8,679 KB)

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