  We present a detailed analysis of time variability of two distinct C IV broad
absorption line (BAL) components seen in the spectrum of J162122.54+075808.4
($z_{em}$ = 2.1394) using observations from SDSS, NTT and SALT taken at seven
different epochs spanning about 15 years. The blue-BAL component (with an
ejection velocity, $v_{\rm e}\sim37,500$ kms$^{-1}$) is an emerging absorption
that shows equivalent width variations and kinematic shifts consistent with
acceleration. The red-BAL component ($v_{\rm e} \sim 15,400$ kms$^{-1}$) is a
three component absorption. One of the components is emerging and subsequently
disappearing. The two other components show kinematic shifts consistent with
acceleration coupled with equivalent width variability. Interestingly, we find
the kinematic shifts and equivalent width variability of the blue- and red-BAL
components to be correlated. While the C IV emission line flux varies by more
than 17% during our monitoring period, the available light-curves (covering
rest frame 1300-2300 angstrom do not show more than a 0.1 mag variability in
the continuum. This suggests that the variations in the ionizing flux are
larger than that of the near-UV flux. However, the correlated variability seen
between different BAL components cannot be explained solely by photoionization
models without structural changes. In the framework of disk wind models, any
changes in the radial profiles of density and/or velocity triggered either by
disk instabilities or by changes in the ionizing radiation can explain our
observations. High resolution spectroscopic monitoring of J1621+0758 is
important to understand the physical conditions of the absorbing gas and
thereby to constrain the parameters of disk-wind models.
