  We investigate the interplay between hydrodynamic fluctuations and initial
longitudinal fluctuations for their effects on the rapidity decorrelation of
collective flow in high-energy nuclear collisions. We use a (3+1)-dimensional
integrated dynamical model in which we combine initial conditions with
longitudinal fluctuations, fluctuating hydrodynamics and hadronic cascades. We
analyse the factorisation ratio in the longitudinal direction to study the
effect of these fluctuations on the rapidity decorrelation. We find an
essential difference between the effects of the hydrodynamic fluctuations and
the initial longitudinal fluctuations in the centrality dependence of the
factorisation ratios. A combination of the hydrodynamic fluctuations and the
initial longitudinal fluctuations leads to reproduction of the centrality
dependence of the second-order factorisation ratio,
$r_2(\eta_\mathrm{p}^\mathrm{a},\eta_\mathrm{p}^\mathrm{b})$, measured by the
CMS Collaboration. Our model also qualitatively describes the centrality
dependence of the third-order factorisation ratio,
$r_3(\eta_\mathrm{p}^\mathrm{a},\eta_\mathrm{p}^\mathrm{b})$. These results
demonstrate the importance of the hydrodynamic fluctuations, as well as the
initial longitudinal fluctuations, in understanding the longitudinal dynamics
of high-energy nuclear collision reactions.
