Conclusions

• only indirect calculations of CAM from flow containing torsional oscillations;

• modelling of the flow at CMB as tangential geostrophic, steady flow in a drifting frame, toroidal, piecewise steady can explain surface secular variation of the magnetic field;

• correlation between LOD and modelled CAM;

• provides almost no constraint on core dynamics;

• various coupling mechanisms could affect the AM exchange, probably mixture of gravitational, electromagnetic and (topographic) torques;

• best chance of progress comes from studies of core dynamics and consideration of inner core; dynamo codes offer a powerful tool.