  The heat transport in rotating Rayleigh-B\'enard convection (RBC) can be
significantly enhanced for moderate rotation, i.e., for an intermediate range
of Rossby numbers $Ro$, compared to the non-rotating case. At Rayleigh numbers
$Ra\lesssim5\cdot10^8$, the largest heat transport enhancement (HTE) is
achieved when the thicknesses of kinetic and thermal boundary layer are equal.
However, experimental and numerical observations show that, at larger $Ra$
($\gtrsim5\cdot10^8$), the maximal HTE starts to deviate from the expected
optimal boundary layer ratio and its amplitude decreases drastically. We
present data from direct numerical simulations of rotating RBC in a periodic
domain in the range of $10^7\leq Ra\leq10^{10}$ and $0\leq Ro^{-1}\leq40$ for
Prandtl number $Pr=4.38$ and $6.4$ to identify the reason for the transition to
this large $Ra$ regime of HTE. Our analysis reveals that the transition occurs
once the bulk flow at the optimal boundary layer ratio changes to geostrophic
turbulence for large $Ra$. In that flow state, the vertically coherent
vortices, which support HTE by Ekman pumping at smaller $Ra$, dissolve into
vertically decorrelated structures in the bulk, such that the enhancing effect
of Ekman pumping and the influence of the boundary layer ratio become small.
Additionally, more heat leaks out of the Ekman vortices as the fraction of
thermal dissipation in the bulk increases. We find that the rotation induced
shearing at the plates helps to increase the thermal dissipation in the bulk,
and thus acts as a limiting factor for HTE at large $Ra$: beyond a certain
ratio of wall shear stress to vortex strength, the heat transport decreases
irrespectively of the boundary layer ratio. This $Pr$ dependent threshold,
which roughly corresponds to a bulk accounting for $\approx1/3$ of the total
thermal dissipation, sets the maximal HTE and the optimal rotation rate at
large $Ra$.
