  Measurements of the C/O ratio in brown dwarfs are lacking, in part due to
past models adopting solar C/O only. We have expanded the ATMO 2020 atmosphere
model grid to include non-solar metallicities and C/O ratios in the T dwarf
regime. We change the C/O ratio by altering either the carbon or oxygen
elemental abundances, and we find that non-solar abundances of these elements
can be distinguished based on the shapes of the $H$- and $K$- bands. We compare
these new models with medium-resolution ($R\approx1700$), near-infrared
($0.8-2.4\,\mu$m) Gemini/GNIRS spectra of three benchmark late-T dwarfs, GJ
570D, HD 3651B, and Ross 458C. We find solar C/O ratios and best-fitting
parameters ($T_\mathrm{eff}$, $\log(g)$, $Z$) broadly consistent with other
analyses in the literature based on low-resolution ($R\sim100$) data. The
model-data discrepancies in the near-infrared spectra are consistent across all
three objects. These discrepancies are alleviated when fitting the Y, J, H and
K bands individually, but the resulting best-fit parameters are inconsistent
and disagree with the results from the full-spectrum. By examining the model
atmosphere properties we find this is due to the interplay of gravity and
metallicity on $\mathrm{H_2-H_2}$ collisionally induced absorption. We
therefore conclude that there are no significant issues with the molecular
opacity tables used in the models at this spectral resolution. Instead,
deficiencies are more likely to lie in the model assumptions regarding the
thermal structures. Finally, we find a discrepancy between the GNIRS, SpeX, and
other near-infrared spectra in the literature of Ross 458C, indicating
potential spectroscopic variability.
