Fourier transform of pressure work, gravity work, and kinetic energy rates. The blue curve indicates the Fourier transform of the work done by pressure gradients per unit time, before the onset of the instability. The red curve represents the Fourier transform of the work done by gravity force per unit time, before the onset of the instability. Both curves show a peak around T ≈ 365 s (identified by eye). The green curve depicts the Fourier transform of the kinetic energy before the onset of the instability. It shows a peak around 180 s.