The maintenance of the sun's differential rotation and its temporal variations

We use the horizontal momentum balance equation to infer the strength of the meridional circulation (MC) and Reynolds stresses (RS) at the Sun's surface from the observed properties of the differential rotation (DR). Both MC and RS are important for maintaining the equatorial acceleration. The results indicate that the average value of MC is about 1.1 m s-1, with circulation directed towards the poles in both the hemispheres, and the average value of RS is 3.6 10(7) cm2 s-2, with transport of angular momentum directed towards the equator in both hemispheres, this latter in good agreement with observations. With the above values of MC and RS, we integrate the momentum equation in time, starting from a state of rigid rotation, to investigate the competitive role of MC and RS in producing the presently observed average DR. Our results show that DR is consistent with observations only if both MC and RS have opposite effects, with the strengths given above, in order to balance the viscous torque. Assuming that RS and MC are periodic functions of time, which is observed in the RS case, we also obtained the observed yearly variations of the equatorial angular velocity omega(0). The results show that the simulated time behaviour of omega(0) is compatible with observations.