  Characterizing the variability of the extragalactic sources used for
calibration in the Atacama Large Millimeter/Sub-millimeter Array (ALMA) is key
to assess the flux scale uncertainty of science observations. To this end, we
model the variability of 39 quasars which have been used by ALMA as secondary
flux calibrators using continuous time stochastic processes. This formalism is
specially adapted to the multi-frequency, quasi-periodic sampling which
characterizes the calibration monitoring of ALMA. We find that simple mixtures
of Ornstein-Uhlenbeck processes can describe well the flux and spectral index
variability of these sources for Bands 3 and 7 (91.5 and 103.5, and 343.5 GHz,
respectively). The spectral shape of the calibrators are characterized by
negative spectral indices, mostly between $-0.35$ and $-0.80$, and with
additional concavity. The model provides forecasts, interpolations, and
uncertainty estimations for the observed fluxes that depend on the intrinsic
variability of the source. These can be of practical use for the ALMA data
calibrator survey and data quality assurance.
