a)Left: quasi-full-disc map of the horizontal divergence (color contours) of the 30-min-averaged horizontal surface velocity field at 14:00:00 UTC, apodized beyond 60° from the disc center. Right: local map at the disc center of the divergence field convolved by horizontal-flow Lagrangian trajectories using a line-integral convolution visualization technique. The map in the right panel is a zoom on the square patch in the left panel. b) Left: quasi-full-disc map of the vertical vorticity (color contours) of the 30-min-averaged horizontal surface velocity field at 14:00:00 UTC, apodized beyond 60° from the disc center. Right: local map at the disc center of the same vorticity field convolved by Lagrangian trajectories computed from the vortical part of the horizontal flow only (the same line-integral convolution visualization technique is used). The map in the right panel is a zoom on the square patch in the left panel.