  Turbulent Rayleigh-B\'enard convection in slender cylindrical cells exhibits
rich dynamics of the large-scale circulation (LSC), with several rolls stacked
on top of each other. We propose that the elliptical instability is the
mechanism which causes the twisting and breaking of the LSC into multiple rolls
and that the volume-averaged heat and momentum transport, represented by the
Nusselt number and Reynolds number, is generally weaker for larger number $n$
of the LSC rolls. This is supported by direct numerical simulations for
$Ra=5\times10^5$, $Pr=0.1$, $H=5D$ and $1\leq~n\leq4$.
