The high-Reynolds-number axisymmetric wake behind a disk has been studied from $x/D\,{=}\,30$ to $x/D\,{=}\,150$ using the proper orthogonal decomposition (POD) applied to measurements of the streamwise fluctuating velocity. It was found that the energetic structure of the axisymmetric wake can very efficiently be described in terms of POD modes. The first radial (or lowest-order) POD mode has 56 % of the energy. Two major features dominate the eigenspectra, manifested as two major peaks. The first peak is an azimuthal mode-1 peak at a frequency corresponding to the Strouhal number of the near wake. The second is an azimuthal mode-2 peak at near-zero frequency. The mode-1 peak dies out faster than the mode-2 peak, so that the far wake is dominated by the latter.

This evolution from azimuthal mode-1 dominance in the near wake to mode-2 dominance in the far wake corresponds closely to the approach to equilibrium similarity. Once azimuthal mode-2 becomes equally important as azimuthal mode-1 (after $x/D\,{=}\,30$ or $x/\theta\,{=}\,110$), the ratio of turbulence intensity to centreline velocity deficit is constant, the mean deficit and turbulence intensity collapse in similarity variables, and the wake grows as $x^{1/3}$.