(a)
(b)
(c)
(d)
(e)
(f)
Figure 1: Spatial distributions of the 38-year (1979–2016) potential vorticity (units: PVU) climatology on 550 K isentropic surface in (a) May, (b) June, (c) July, (d) August, (e) September, and (f) October. The black solid based on criterion PV contour (−28.8 PVU) represents the vortex edge.