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We report on a search for low-frequency radio variability in 944 bright (>4Jy at 154 MHz) unresolved, extragalactic radio sources monitored monthly forseveral years with the Murchison Widefield Array. In the majority of sources wefind very low levels of variability with typical modulation indices < 5%. Wedetect 15 candidate low frequency variables that show significant long termvariability (>2.8 years) with time-averaged modulation indices M = 3.1 - 7.1%.With 7/15 of these variable sources having peaked spectral energydistributions, and only 5.7% of the overall sample having peaked spectra, wefind an increase in the prevalence of variability in this spectral class. Weconclude that the variability seen in this survey is most probably aconsequence of refractive interstellar scintillation and that these objectsmust have the majority of their flux density contained within angular diametersless than 50 milli-arcsec (which we support with multi-wavelength data). At 154MHz we demonstrate that interstellar scintillation time-scales become long(~decades) and have low modulation indices, whilst synchrotron drivenvariability can only produce dynamic changes on time-scales of hundreds ofyears, with flux density changes less than one milli-jansky (withoutrelativistic boosting). From this work we infer that the low frequencyextra-galactic southern sky, as seen by SKA-Low, will be non-variable ontime-scales shorter than one year.

The Murchison Widefield Array Transients Survey (MWATS). A search for low frequency variability in a bright Southern hemisphere sample