We present a three dimensional velocity analysis of Milky Way disk kinematics using LAMOST K giant stars and the GPS1 proper motion catalogue .
We find that Galactic disk stars near the anticenter direction ( in the range of Galactocentric distance between R = 8 and 13 kpc and vertical position between Z = - 2 and 2 kpc ) exhibit asymmetrical motions in the Galactocentric radial , azimuthal , and vertical components .
Radial motions are not zero , thus departing from circularity in the orbits ; they increase outwards within R \lesssim 12 kpc , show some oscillation in the northern ( 0 < Z < 2 kpc ) stars , and have north-south asymmetry in the region corresponding to a well-known nearby northern structure in the velocity field .
There is a clear vertical gradient in azimuthal velocity , and also an asymmetry that shifts from a larger azimuthal velocity above the plane near the solar radius to faster rotation below the plane at radii of 11-12 kpc .
Stars both above and below the plane at R \gtrsim 9 kpc exhibit net upward vertical motions .
We discuss some possible mechanisms that might create the asymmetrical motions , such as external perturbations due to dwarf galaxy minor mergers or dark matter sub-halos , warp dynamics , internal processes due to spiral arms or the Galactic bar , and ( most likely ) a combination of some or all of these components .