  In recent studies on wake stability, it has been observed that a simple
linear stability analysis applied to the mean flow instead of the basic flow,
could give an accurate prediction of the global mode selected frequency,
although these phenomena are strongly non-linear. In this letter, we study the
transient regime between the stationary (so called basic state) and
unstationary solutions of the wake of a circular cylinder at Re=150. We show
that the shift of the global frequency as a function of time due to strong
non-linear effects, can be interpreted by a continuous mean flow correction
induced by the growth of the instability. We show that during this transient
regime, the mean state as a function of time plays the role of an instantaneous
basic state on which the global frequency can be determined linearly.
