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M. Dikpati, P. Charbonneau (HAO/NCAR)
We present here how the stochastic fluctuation in meridional circulation can cause variation in amplitude and phase of the solar cycle as often observed as a long-term variability in the butterfly diagram. It has been shown by various authors that a kinematic, flux-transport dynamo with meridional circulation is capable of reproducing a majority of surface magnetic features. The scaling law, proposed on the basis of detail parameter space survey, suggests that the time-period of cycle is largely governed by flow speed. Since meridional circulation is a weak flow, vigorous convections could perturb this flow randomly by causing a stochastic fluctuation in its magnitude. Surface observations suggest variation of flow speed with time, while simulations suggest up to 100% fluctuation in amplitude of the flow in deep convection zone also. Assuming a correlation length in the streamlines comparable to dimension of granules and treating the correlation time (\tauc) between two fluctuating flows and the amplitude of the fluctuating component as parameters, we show that the long-term variability in the amplitude of the solar cycle is most sensitive to the amplitude of the stochastic fluctuation in meridional circulation, but not so sensitive to \tauc. On the other hand, the longer \tauc affects the time-period, though not violating the so-called "phase locking" of the solar cycle, since we always find that whenever there occur a few short cycles, a few compensating longer cycles also occur to adjust the solar clock again.
The National Center for Atmospheric Research is sponsored by the National Science Foundation.
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