Time sequence of the swirling strength distribution. Each panel represents a bi-dimensional histogram normalized to the maximum density fraction at each height level z. The bin sizes are Δz = 10 km and Δλ = 3.62 × 10⁻³ Hz. These histograms refer to stacks of quadratic, plane-parallel cross sections of 1000 km side length centered on the swirl event of Fig. 4. Data points with λ < 2.09 × 10⁻² Hz are excluded. The black arrows point to the upward propagation of a local peak of swirling strength.