  We reveal several distinct regimes of the relaxation dynamics of a small
quantum system coupled to an environment within the plane of the dissipation
strength and the reservoir temperature. This is achieved by discriminating
between coherent dynamics with damped oscillatory behavior on all time scales,
partially coherent behavior being nonmonotonic at intermediate times but
monotonic at large ones, and purely monotonic incoherent decay. Surprisingly,
elevated temperature can render the system `more coherent' by inducing a
transition from the partially coherent to the coherent regime. This provides a
refined view on the relaxation dynamics of open quantum systems.
