  Black hole - accretion disc systems are the central engines of relativistic
jets from stellar to galactic scales. We numerically quantify the unsteady
outgoing Poynting flux through the horizon of a rapidly spinning black hole
endowed with a rotating accretion disc. The disc supports small-scale,
concentric, flux tubes with zero net magnetic flux. Our General Relativistic
force-free electrodynamics simulations follow the accretion onto the black hole
over several hundred dynamical timescales in 3D. For the case of
counter-rotating accretion discs, the average process efficiency reaches up to
$\left\langle\epsilon\right\rangle\approx 0.43$, compared to a stationary
energy extraction by the Blandford/Znajek process. The process efficiency
depends on the cross-sectional area of the loops, i.e. on the product $l\times
h$, where $l$ is the radial loop thickness and $h$ its vertical scale height.
We identify a strong correlation between efficient electromagnetic energy
extraction and the quasi-stationary setting of ideal conditions for the
operation of the Blandford/Znajek process (e.g. optimal field line angular
velocity and fulfillment of the so-called Znajek condition). Remarkably, the
energy extraction operates intermittently (alternating episodes of high and low
efficiency) without imposing any large-scale magnetic field embedding the
central object. Scaling our results to supermassive black holes, we estimate
that the typical variability timescale of the system is of the order of days to
months. Such timescales may account for the longest variability scales of TeV
emission observed, e.g. in M87.
