Long waves are produced by large-scale heat exchange between high and low latitudes. As a long-wave trough passes through East China, a significant cold surge arrives in the South China Coast. They are only small perturbations in mid-latitude westerlies.
Short waves can do constructive or destructive interferences with long waves.
A simple intuition suggests heat will generally flow from low latitude to high latitude. Long waves are produced by large-scale heat exchange between high and low latitudes. As a long-wave trough passes through East China, a significant cold surge arrives in the South China Coast. They are only small perturbations in mid-latitude westerlies. Short waves can do constructive or destructive interferences with long waves.