Nordic stations came with strong flutter. I was able to recognize them from the fluttery sound of CW. As the left figure shows, the short path goes over the North Pole where the aurora activity is high. This makes ionospheric structure irregular and temporary unstable, so to modulate the signals that passes through. This is why the CW sound becomes fluttery.

On the other hand, the short path from ZL7 to JA is right over the Pacific (white solid line) without any aurora activity gives clear sound of CW. However, they may have flutter due to high activity of Equatorial Ionospheric Anomaly (EIA) in the late at night.