  The solar magnetic activity cycle has an amplitude that varies within a wide
but limited range of values. This implies that there are nonlinear mechanisms
that prevent runaway solutions. The purpose of this paper is to propose
observable nonlinear mechanisms in the framework of the Babcock-Leighton-type
dynamo. Sunspot emergences show systematic properties that strong cycles tend
to have higher mean latitudes and lower tilt angle coefficients. We use the
surface flux transport model to investigate the effect of these systematic
properties on the expected final total dipolar moment, i.e. cancellation plus
generation of dipole moment by a whole solar cycle. We demonstrate that the
systematic change in latitude has similar nonlinear feedback on the solar cycle
(latitudinal quenching) as tilt does (tilt quenching). Both forms of quenching
lead to the expected final total dipolar moment being enhanced for weak cycles
and being saturated to a nearly constant value for normal and strong cycles.
This explains observed long-term solar cycle variability, e.g., the
Gnevyshev-Ohl rule, which, in turn, justifies the nonlinear mechanisms inherent
in the Babcock-Leighton-type dynamo.
