  We present results from a two-night R~4000 0.9-2.5 micron spectroscopic
monitoring campaign of Luhman 16AB (L7.5 + T0.5). We assess the variability
amplitude as a function of pressure level in the atmosphere of Luhman 16B: the
more variable of the two components. The amplitude decreases monotonically with
decreasing pressure, indicating that the source of variability - most likely
patchy clouds - lies in the lower atmosphere. An unexpected result is that the
strength of the K I absorption is higher in the faint state of Luhman 16B and
lower in the bright state. We conclude that either the abundance of K I
increases when the clouds roll in, potentially because of additional K I in the
cloud itself, or that the temperature-pressure profile changes. We reproduce
the change in K I absorption strengths with combinations of spectral templates
to represent the bright and the faint variability states. These are dominated
by a warmer L8 or L9 component, with a smaller contribution from a cooler T1 or
T2 component. The success of this approach argues that the mechanism
responsible for brown dwarf variability is also behind the diverse spectral
morphology across the L-to-T transition. We further suggest that the L9-T1 part
of the sequence represents a narrow but random ordering of effective
temperatures and cloud fractions, obscured by the monotonic progression in
methane absorption strength.
