  Correlations are investigated of the outflow strength of quasars, as measured
by the blueshift and asymmetry index (BAI) of the CIV line (Wang et al. 2011),
with intensities and ratios of broad emission lines, based on composite quasar
spectra built from the Sloan Digital Sky Survey. We find that most of the line
ratios of other ions to CIV prominently increases with BAI. These behaviors can
be well understood in the context of increasing metallicity with BAI. The
strength of dominant coolant, CIV line, decreases and weak collisionally
excited lines increase with gas metallicity as a result of the competition
between different line coolants. Using SiIV+OIV]/CIV as an indicator of gas
metallicity, we present, for the first time, a strong correlation between the
metallicitiy and the outflow strength of quasars over a wide range of 1.7 to
6.9 times solar abundance. Our result implies that the metallicity plays an
important role in the formation of quasar outflows, likely via affecting
outflow acceleration. This effect may have a profound impact on galaxy
evolution via momentum feedback and chemical enrichment.
