  We study the saturation effect on broad absorption line (BAL) variability
through a variation phenomenon, which shows significant variation in Si IV BAL
but no, or only small, change in C IV BAL (hereafter Phenomenon I). First, we
explore a typical case showing Phenomenon I, quasar SDSS J153715.74+582933.9
(hereafter J1537+5829). We identify four narrow absorption line (NAL) systems
within its Si IV BAL and two additional NAL systems within its C IV BAL, and
confirm their coordinated weakening. Combining with the obvious strengthening
of the ionizing continuum, we attribute the BAL variability in J1537+5829 to
the ionization changes caused by the continuum variations. Secondly, a
statistical study based on multiobserved quasars from SDSS-I/II/III is
presented. We confirm that (1) the moderate anticorrelation between the
fractional variations of Si IV BALs and the continuum in 74 quasars that show
Phenomenon I and (2) the sample showing BAL variations tends to have larger
ionizing continuum variations. These results reveal the ubiquitous effect of
the continuum variability on Phenomenon I and BAL variation. We attribute the
relative lack of variation of C IV BALs in Phenomenon I to the saturation
effects. Nonetheless, these absorbers are not very optically thick in Si IV and
the ionization changes in response to continuum variations could be the main
driver of their variations. Finally, we find that the saturation effect on BAL
variability can well explain many phenomena of BAL variations that have been
reported before.
