  Numerical simulations that reproduce solar-like magnetic cycles can be used
to generate long-term statistics. The variations in N-S hemispheric cycle
synchronicity and amplitude produced in simulations has not been widely
compared to observations. The observed limits on asymmetry show that
hemispheric sunspot area production is no more than 20% asymmetric for cycles
12-23 and phase lags do not exceed 20% (2 yrs) of the total cycle period.
Independent studies have found a long-term trend in phase values as one
hemisphere leads the other for ~four cycles. Such persistence in phase is not
indicative of a stochastic phenomenon. We compare the findings to results from
a numerical simulation of solar convection recently produced with the EULAG-MHD
model. This simulation spans 1600 yrs and generated 40 regular, sunspot-like
cycles. While the simulated cycle length is too long and the toroidal bands
remain at too high of latitudes, some solar-like aspects of hemispheric
asymmetry are reproduced. The model reproduces the synchrony of polarity
inversions and onset of cycle as the simulated phase lags do not exceed 20% of
the cycle period. Simulated amplitude variations between the N and S
hemispheres are larger than observed in the Sun. The simulations show one
hemisphere persistently leads the other for several successive cycles, placing
an upper bound on the efficiency of transequatorial magnetic coupling
mechanisms. These include magnetic diffusion, cross-equatorial mixing within
elongated convective rolls and transequatorial meridional flow cells. One or
more of these processes may lead to magnetic flux cancellation whereby the
oppositely directed fields come in close proximity and cancel each other across
the magnetic equator late in the solar cycle. We discuss the discrepancies
between model and observations and the constraints they pose on possible
mechanisms of hemispheric coupling.
