  Polar vortices are common in the atmospheres of rapidly rotating planets
[1-4]. On Earth and Mars they are tied to the surface and their existence
follows the seasonal insolation cycle [1-3]. Venus is a slowly rotating planet
but it is also known to have vortices at both poles at the edge of a
superrotating atmosphere [5-8]. However, their nature and long-term properties
have not been constrained so far impeding precise modeling. Here we report
cloud motions at two altitude levels (about 42 km and 63 km above the surface)
using infrared images from the VIRTIS instrument onboard Venus Express that
show that the south polar vortex is a permanent but erratic and unpredictable
feature. We find that the centers of rotation of the vortex at these levels
rarely coincide and both wander erratically around the pole with speeds of up
to 16 m s-1. The cloud morphology and vorticity patches are uncorrelated and
change continuously developing transient areas of small vertical motions. Venus
south polar vortex is a continuously evolving structure immersed in a
baroclinic environment laying at altitude levels that have variable vertical
and meridional wind shears, extending at least 20 km in height through a
quasi-convective turbulent region.
