  Young brown dwarfs are analogs to giant exoplanets, as they share effective
temperatures, near-infrared colors, and surface gravities. Thus, the detailed
characterization of young brown dwarfs might shed light on the study of giant
exoplanets, that we are currently unable to observe with sufficient
signal-to-noise to allow precise characterization of their atmospheres. 2MASS
J22081363+2921215 is a young L3 brown dwarf, member of the beta-Pictoris young
moving group (23 +/-3 Myr), that shares its effective temperature and mass with
the beta Pictoris b giant exoplanet. We performed a ~2.5 hr spectro-photometric
J-band monitoring of 2MASS J22081363+2921215 with the MOSFIRE multi-object
spectrograph, installed at the Keck I telescope. We measured a minimum
variability amplitude of 3.22 +/- 0.42 % for its J-band light curve. The ratio
between the maximum and the minimum flux spectra of 2MASS J22081363+2921215
shows a weak wavelength dependence and a potential enhanced variability
amplitude in the alkali lines. Further analysis suggests that the variability
amplitude on the alkali lines is higher than the overall variability amplitude
(4.5-11 %, depending on the lines). The variability amplitudes in these lines
are lower if we degrade the resolution of the original MOSFIRE spectra to
R~100, which explains why this potential enhanced variability in the alkali
lines has not been found previously in HST/WFC3 light curves. Using
radiative-transfer models, we obtained the different cloud layers that might be
introducing the spectro-photometric variability we observe for 2MASS
J22081363+2921215, that further support the measured enhanced variability
amplitude in the alkali lines. We provide an artistic recreation of the
vertical cloud structure of this beta-Pictoris b analog.
