  We study the kinematics of the AS 209 disk using the J=2-1 transitions of
$^{12}$CO, $^{13}$CO, and C$^{18}$O. We derive the radial, azimuthal, and
vertical velocity of the gas, taking into account the lowered emission surface
near the annular gap at ~1.7 (200 au) within which a candidate circumplanetary
disk-hosting planet has been reported previously. In $^{12}$CO and $^{13}$CO,
we find a coherent upward flow arising from the gap. The upward gas flow is as
fast as $150~{\rm m~s}^{-1}$ in the regions traced by $^{12}$CO emission, which
corresponds to about 50% of the local sound speed or $6\%$ of the local
Keplerian speed. Such an upward gas flow is difficult to reconcile with an
embedded planet alone. Instead, we propose that magnetically driven winds via
ambipolar diffusion are triggered by the low gas density within the
planet-carved gap, dominating the kinematics of the gap region. We estimate the
ambipolar Elsasser number, Am, using the HCO$^+$ column density as a proxy for
ion density and find that Am is ~0.1 at the radial location of the upward flow.
This value is broadly consistent with the value at which numerical simulations
find ambipolar diffusion drives strong winds. We hypothesize the activation of
magnetically-driven winds in a planet-carved gap can control the growth of the
embedded planet. We provide a scaling relationship which describes the
wind-regulated terminal mass: adopting parameters relevant to 100 au from a
solar-mass star, we find the wind-regulated terminal mass is about one Jupiter
mass, which may help explain the dearth of directly imaged super-Jovian-mass
planets.
