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 .