  A decade ago the archetypal Seyfert-1 galaxy NGC 5548 was discovered to have
undergone major spectral changes. The soft X-ray flux had dropped by a factor
of 30 while new broad and blueshifted UV absorption lines appeared. This was
explained by the emergence of a new obscuring wind from the accretion disk.
Here we report on the striking long-term variability of the obscuring disk wind
in NGC 5548 including new observations taken in 2021-2022 with the Swift
Observatory and the Hubble Space Telescope's (HST) Cosmic Origins Spectrograph
(COS). The X-ray spectral hardening as a result of obscuration has declined
over the years, reaching its lowest in 2022 at which point we find the broad C
IV UV absorption line to be nearly vanished. The associated narrow
low-ionization UV absorption lines, produced previously when shielded from the
X-rays, are also remarkably diminished in 2022. We find a highly significant
correlation between the variabilities of the X-ray hardening and the equivalent
width of the broad C IV absorption line, demonstrating that X-ray obscuration
is inherently linked to disk winds. We derive for the first time a relation
between the X-ray and UV covering fractions of the obscuring wind using its
long-term evolution. The diminished X-ray obscuration and UV absorption are
likely caused by an increasingly intermittent supply of outflowing streams from
the accretion disk. This results in growing gaps and interstices in the clumpy
disk wind, thereby reducing its covering fractions.
