Strange behavior with contours in Cartopy polar stereographic plot

Question

I'm trying to create a contour plot on a North Polar Stereographic map projection using Cartopy. I used add_cyclic_point() to try and get around the problem of having a gap between longitude 0 and longitude 35X and followed an example from the documentation (always_circular_stereographic) to set up the map axes.

When I call plt.contour, I get the following plot. It looks like the contour plotter is getting confused at the transition from 355 to 0 longitude, and sends contour lines around the globe.

Here is my code:

import numpy as np
import cartopy.crs as ccrs
from cartopy.util import add_cyclic_point
import matplotlib.pyplot as plt

def define_map():
    from matplotlib.path import Path

    fig = plt.figure(figsize=(10,10))
    ax = plt.axes(projection=ccrs.NorthPolarStereo())
    ax.coastlines()

    # From example: http://scitools.org.uk/cartopy/docs/latest/examples/always_circular_stereo.html
    theta = np.linspace(0, 2*np.pi, 100)
    center, radius = [0.5, 0.5], 0.5
    verts = np.vstack([np.sin(theta), np.cos(theta)]).T
    circle = Path(verts * radius + center)

    ax.set_boundary(circle, transform=ax.transAxes)
    return(fig, ax)
lats = np.arange(65,91,5)
lons = add_cyclic_point(np.arange(0,359,5))
data = add_cyclic_point(np.random.random((len(lats),len(lons)-1)))


fig, ax = define_map()
plt.contour(lons,lats,data,5,transform=ccrs.PlateCarree(), cmap=plt.cm.Blues)
plt.colorbar(fraction=0.05, shrink=0.9)
plt.show()

How do I do a Cartopy contour plot properly? Also, why do the contours only show up with transform=ccrs.PlateCarree() and not with transform=ccrs.NorthPolarStereo()?

Answer 1

Apparently the add_cyclic_point function is just for the data; the contour routine treats 0 different than 360. So the simple fix is to set

lons = np.arange(0,360,5)