PWM an AC pump using MOSFET
The circuit is potentially OKish BUT there is no certainty that it will work.   

If PWM frequency is >> mains frequency then You are effectively reducing driving amplitude.
Adding series resistors in the AC line will give you some indication of how effective this will be.  

An alternative would be to miss out some half cycles.
This would be annoying acoustically and pump MAY have some electromechanical resonance which requires a few cycles to get up to pressure. 

If you "subcycled" it and fed a reservoir of suitable size so that you achieved smoothed average flow it would achieve pressure reduction. 

An alternative which would work is to operate the pump continually into a reservoir and then modulate a bleed valve which vented pressure to ambient. By changing the bleed rate you would change available pressure. You can buy small solenoid valves intended for a range of purposes and fabrication of something yourself should be achievable. If "all else fails" the filling solenoids used in most washing machines could be repurposed for this use. Add a suitably small bleed orifice on the exit side, or feed them with a semi clamped flexible tube or ... . These valves are usually operated on/off but may be amenable to partial opening using DC feed. Some will have AC mains coils but others will be low voltage DC powered. Don't forget a reverse diode across the solenoid coil.



ADDED:

Q: > so would a SSR with random turn on mode be okay instead of the MOSFET?

A: Probably not.
As Phil said and as I suggested, the issue of working is more with the pump than the driving method. Read his comments on pump operation and mine re trying a series resistor.
BUT an SSR will probably be less effective than your MOSFET cct because they are almost always TRIAC based and once on will stay on for the rest of a half cycle. If you fire it randomly with trigger rate >> Fmains you will get ~= 100% on and if you fire it randomly at around 2Fmains (half cycle rate) you'll get about 50% on mean period but with a spread of periods  - which your pump may or may not like.Fire randomly at &lt; half cycle rate and you'll get 0 to 1 1/2 cycle on each firing. 

An easy test to start with is to get several power resistors of various values (or a suitable length of Nichrome or similar wire)(taking care not to electrocute yourself) and try adding these in series with the pump and see what effect it has.  This is not a direct substitute for your PWM'd FET but should give an idea of how the pump responds.