  We present wide field JHKs photometry of 16 Galactic globular clusters
located towards the Galactic bulge, calibrated on the 2MASS photometric system.
Differential reddening corrections and statistical field star decontamination
are employed for all of these clusters before fitting fiducial sequences to the
cluster red giant branches (RGBs). Observed values and uncertainties are
reported for several photometric features, including the magnitude of the RGB
bump, tip, the horizontal branch (HB) and the slope of the upper RGB. The
latest spectroscopically determined chemical abundances are used to build
distance- and reddening-independent relations between observed photometric
features and cluster metallicity, optimizing the sample size and metallicity
baseline of these relations by supplementing our sample with results from the
literature. We find that the magnitude different between the HB and the RGB
bump can be used to predict metallicities, in terms of both iron abundance
[Fe/H] and global metallicity [M/H], with a precision of better than 0.1 dex in
all three near-IR bandpasses for relative metal-rich ([M/H]$\gtrsim$-1)
clusters. Meanwhile, both the slope of the upper RGB and the magnitude
difference between the RGB tip and bump are useful metallicity indicators over
the entire sampled metallicity range (-2$\lesssim$[M/H]$\lesssim$0) with a
precision of 0.2 dex or better, despite model predictions that the RGB slope
may become unreliable at high (near-solar) metallicities. Our results agree
with previous calibrations in light of the relevant uncertainties, and we
discuss implications for clusters with controversial metallicities as well as
directions for further investigation.
