Mapping the Cloud-driven Atmospheric Dynamics and Chemistry of an Isolated Exoplanet Analog with Harmonic Signatures

Abstract Young planetary-mass objects and brown dwarfs near the L–T spectral transition exhibit enhanced spectrophotometric variability over field brown dwarfs. Patchy clouds, auroral processes, stratospheric hot spots, and complex carbon chemistry have all been proposed as potential sources of this variability. Using time-resolved, low- to mid-resolution spectroscopy collected with the JWST/NIRISS and NIRSpec instruments, we apply harmonic analysis to SIMP J013656.5+093347.3, a highly variable, young, isolated planetary-mass object. Odd harmonics ( k = 3) at pressure levels (≳1 bar) corresponding to iron (Fe) and forsterite (Mg 2 SiO 4 ) cloud formation suggest a potential north–south hemispheric asymmetry in the cloudy, and likely equatorial, regions. We use the inferred harmonics, along with 1D substellar atmospheric models, to map the flux variability by atmospheric pressure level. These vertical maps demonstrate robust interaction between deep convective weather layers and the overlying stratified and radiative atmosphere. We identify distinct time-varying structures in the near-infrared that we interpret as planetary-scale wave-associated (e.g., Rossby or Kelvin) cloud modulation. We detect deviations from bulk (composite) variability in water (maximum signal-to-noise ratio ( S / N max ) = 14.0 ), carbon monoxide ( S / N max = 13.0 ), and methane ( S / N max = 14.9 ) molecular signatures. Forsterite cloud modulation is anticorrelated with the overlying carbon monoxide and water abundances and correlated with deep methane absorption, suggesting complex interaction between cloud formation, atmospheric chemistry, and temperature structure. Furthermore, we identify distinct harmonic behavior between methane and carbon monoxide absorption bands, providing evidence for time-resolved disequilibrium carbon chemistry. At the lowest pressures (≲100 mbar), we find that the mapped methane lines transition from absorption to emission, supporting evidence of high-altitude auroral heating via electron precipitation.