A system and method for drilling a wellbore with a drill rig by: rotating a drillstring and a drill bit with a drill rig drive system; applying a weight of the drillstring on the drill rig; measuring surface torque oscillations of the drill string via: determining a fundamental oscillation time period; select a time window based on the fundamental oscillation time period; collecting torque present value data of the drill string for the selected time window; determining an amplitude of torque oscillation from the collected torque present value data; determining a reference torque; and dividing the determined amplitude of torque oscillation by the determined reference torque to obtain a surface torque oscillation performance index, whereby the measurement of the surface torque oscillations of the drill string is a fractional value to indicate the magnitude and severity of surface torque fluctuations of the drilling string; and modifying a drilling parameter based on the surface torque oscillation performance index.