2025MNRAS.539.3758C

We present a new epoch of JWST spectroscopic variability monitoring of the benchmark binary brown dwarf WISE 1049AB, the closest, brightest brown dwarfs known. Our 8-h JWST/MIRI low resolution spectroscopy and 7-h JWST/NIRSpec prism observations extended variability measurements for any brown dwarfs beyond 11 $\mu$m for the first time, reaching up to 14 $\mu$m. Combined with the previous epoch in 2023, they set the longest JWST weather monitoring baseline to date. We found that both WISE 1049AB show wavelength-dependent light-curve behaviours. Using a robust k-means clustering algorithm, we identified several clusters of variability behaviours associated with three distinct pressure levels. By comparing to a general circulation model, we identified the possible mechanisms that drive the variability at these pressure levels: patchy clouds rotating in and out of view likely shaped the dramatic light curves in the deepest layers between 1-2.5 $\mu$m, whereas hotspots arising from temperature/chemical variations of molecular species likely dominate the high-altitude levels between 2.5-3.6 $\mu$m and 4.3-8.5 $\mu$m. Small-grain silicates potentially contributed to the variability of WISE 1049A at 8.5-11 $\mu$m. While distinct atmospheric layers are governed by different mechanisms, we confirmed for the first time that each variability mechanism remains consistent within its layer over the long term. Future multiperiod observations will further test the stability of variability mechanisms on this binary, and expanded JWST variability surveys across the L-T-Y sequence will allow us to trace and understand variability mechanisms across a wider population of brown dwarfs and planetary-mass objects.