  The climate of a terrestrial exoplanet is controlled by the type of host
star, the orbital configuration and the characteristics of the atmosphere and
the surface. Many rocky exoplanets have higher eccentricities than those in the
Solar System, and about 18% of planets with masses $< 10 \mathrm{M}_{\oplus}$
have $e>0.1$. Underexplored are the implications of such high eccentricities on
the atmosphere, climate, and potential habitability on such planets. We use
WACCM6, a state-of-the-art fully-coupled Earth-system model, to simulate the
climates of two Earth-like planets; one in a circular orbit ($e=0$), and one in
an eccentric orbit ($e=0.4$) with the same mean insolation. We quantify the
effects of eccentricity on the atmospheric water abundance and loss given the
importance of liquid water for habitability. The asymmetric temperature
response in the eccentric orbit results in a water vapour mixing ratio in the
stratosphere ($> 20$ ppmv) that is approximately five times greater than that
for circular orbit ($\sim 4$ ppmv). This leads to at most $\sim 3$ times
increases in both the atmospheric hydrogen loss rate and the ocean loss rate
compared with the circular case. Using the Planetary Spectrum Generator, we
simulate the idealised transmission spectra for both cases. We find that the
water absorption features are stronger at all wavelengths for the $e=0.4$
spectrum than for the circular case. Hence, highly-eccentric Earth-like
exoplanets may be prime targets for future transmission spectroscopy
observations to confirm, or otherwise, the presence of atmospheric water
vapour.
