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Black hole - accretion disc systems are the central engines of relativisticjets from stellar to galactic scales. We numerically quantify the unsteadyoutgoing Poynting flux through the horizon of a rapidly spinning black holeendowed with a rotating accretion disc. The disc supports small-scale,concentric, flux tubes with zero net magnetic flux. Our General Relativisticforce-free electrodynamics simulations follow the accretion onto the black holeover several hundred dynamical timescales in 3D. For the case ofcounter-rotating accretion discs, the average process efficiency reaches up to$\left\langle\epsilon\right\rangle\approx 0.43$, compared to a stationaryenergy extraction by the Blandford/Znajek process. The process efficiencydepends on the cross-sectional area of the loops, i.e. on the product $l\timesh$, where $l$ is the radial loop thickness and $h$ its vertical scale height.We identify a strong correlation between efficient electromagnetic energyextraction and the quasi-stationary setting of ideal conditions for theoperation of the Blandford/Znajek process (e.g. optimal field line angularvelocity and fulfillment of the so-called Znajek condition). Remarkably, theenergy extraction operates intermittently (alternating episodes of high and lowefficiency) without imposing any large-scale magnetic field embedding thecentral object. Scaling our results to supermassive black holes, we estimatethat the typical variability timescale of the system is of the order of days tomonths. Such timescales may account for the longest variability scales of TeVemission observed, e.g. in M87.

Striped Blandford/Znajek jets from advection of small scale magnetic field