  We study the Maxwell quasinormal spectrum on asymptotically anti-de Sitter
black holes with a set of two Robin type boundary conditions, by requiring the
energy flux to vanish at asymptotic infinity. Focusing, for illustrative
purposes, on Schwarzschild-anti-de Sitter black holes both without and with a
global monopole, we unveil that, on the one hand, the Maxwell quasinormal
spectrum bifurcates as the black hole radius increases for both boundary
conditions, which is termed the mode split effect; while on the other hand,
with an appropriate fixed black hole radius but increasing the monopole
parameter, the first (second) boundary condition may trigger (terminate) the
mode split effect.
