We present evidence that the allowed periods of equatorially trapped Rossby wave modes on the Sun coincide closely with the 88 day period and 176 day sub harmonic period of Mercury and evidence of Rossby waves on the Sun at the same periods. To test the hypothesis that Rossby waves trigger the emergence of sunspots we use band pass filtering to obtain the 88 day period and 176 day period components of hemispherical sunspot area and compare the variations to the tidal height variation on the surface of the Sun due to Mercury. We find that the two components of hemispherical sunspot area occur in several episodes or activations of duration 2 to 6 years during each solar cycle. When the activations are discrete the variation of the 88 day and 176 day period components are phase coherent with the tidal height variation and a 180 degree phase change is evident between successive activations. We use this result to demonstrate that Rieger type quasi-periodicities in sunspot activity are, in most reported cases, periodicities associated with sidebands of the 88 day and 176 day period variation. We find that, in variables that arise from activity in both solar hemispheres, phase coherence with the Mercury tidal effect occurs at infrequent intervals but, when it does occur, it is possible to observe phase coherence of variables such as sunspot number, solar flares, solar wind speed and cosmic ray flux with the Mercury tidal effect. Sensitivity of a range of variables to the tidal effect is calculated for such intervals in solar cycle 23.