Mysterious horizontal lines in image

```
import numpy as np
from mayavi import mlab

u, v = np.mgrid[-5:5:800j, -5:5:800j]
x = u
y = v
z = 0.5*(u**2 + v**2)

fig = mlab.figure(size=(1000, 1000))
mlab.mesh(x, y, z, colormap='Blues', figure=fig)
mlab.view(30.0, 80.0, 45.0)

mlab.savefig('new.png', magnification=5)
```
What are the horizontal lines that pass through the surface? By reducing the image size, I can eliminate those lines, but what exactly is the mechanism behind this phenomenon?

I've attached an image.