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[abbreviated] We report resolved near-infrared spectroscopic monitoring ofthe nearby L dwarf/T dwarf binary WISE J104915.57-531906.1AB (Luhman 16AB), aspart of a broader campaign to characterize the spectral energy distribution andtemporal variability of this system. A continuous 45-minute sequence oflow-resolution IRTF/SpeX data spanning 0.8-2.4 micron were obtained, concurrentwith combined-light optical photometry with ESO/TRAPPIST. Our spectralobservations confirm the flux reversal of this binary, and we detect awavelength-dependent decline in the relative spectral fluxes of the twocomponents coincident with a decline in the combined-light optical brightnessof the system over the course of the observation. These data are successfullymodeled as a combination of brightness and color variability in the T0.5 Luhman16B, consistent cloud variations; and no significant variability in L7.5 Luhman16A. We estimate a peak-to-peak amplitude of 13.5% at 1.25 micron over the fulllightcurve. Using a two-spot brightness temperature model, we infer an averagecloud covering fraction of ~30-55% for Luhman 16B, varying by 15-30% over arotation period. A Rhines scale interpretation for the size of the variablefeatures explains an apparent correlation between period and amplitude forthree highly variable T dwarfs, and predicts relatively fast winds (1-3 km/s)for Luhman 16B consistent with lightcurve evolution on an advective time scale(1-3 rotation periods). Our observations support the model of a patchydisruption of the mineral cloud layer as a universal feature of the L dwarf/Tdwarf transition.

A Monitoring Campaign for Luhman 16AB. I. Detection of Resolved Near-Infrared Spectroscopic Variability