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The tilt angles of sunspot groups represent the poloidal field source inBabcock-Leighton-type models of the solar dynamo and are crucial for thebuild-up and reversals of the polar fields in Surface Flux Transport (SFT)simulations. The evolution of the polar field is a consequence of Hale'spolarity rules, together with the tilt angle distribution which has asystematic component (Joy's law) and a random component (tilt-angle scatter).We determine the scatter using the observed tilt angle data and study theeffects of this scatter on the evolution of the solar surface field using SFTsimulations with flux input based upon the recorded sunspot groups. The tiltangle scatter is described in our simulations by a random component accordingto the observed distributions for different ranges of sunspot group size (totalumbral area). By performing simulations with a number of different realizationsof the scatter we study the effect of the tilt angle scatter on the globalmagnetic field, especially on the evolution of the axial dipole moment. Theaverage axial dipole moment at the end of cycle 17 (a medium-amplitude cycle)from our simulations was 2.73G. The tilt angle scatter leads to an uncertaintyof 0.78 G (standard deviation). We also considered cycle 14 (a weak cycle) andcycle 19 (a strong cycle) and show that the standard deviation of the axialdipole moment is similar for all three cycles. The uncertainty mainly resultsfrom the big sunspot groups which emerge near the equator. In the framework ofBabcock-Leighton dynamo models, the tilt angle scatter therefore constitutes asignificant random factor in the cycle-to-cycle amplitude variability, whichstrongly limits the predictability of solar activity.

Effects of the scatter in sunspot group tilt angles on the large-scale magnetic field at the solar surface