I am in search of the basic principles of the autothrust and its different modes/settings.
You can try this:
AIRBUS 320 / FCOM 3 / CHAPTER 22 AUTOFLIGHT
If you are in high altitude cruise, approaching a tropical storm cell with possible turbulences (but not using the "severe turbulences" procedure putting the A/THR off and fixing N1), what would be the selected mode ? Speed mode ?
If you have no other choice than approaching that cell...you might elect to decelerate towards your best penetration speed due to mod/sev turb expectations. On the bus, in most cases, you will be in Mach mode.