South korea’s climate is subtropical in some parts because of

the surrounding seas
seasonal winds
its rolling landscape
its mountainous landscape

seasonal winds

Explanation:

Seoul has a wet and dry humid climate and in the summer seasons and os thus has cold and dry weather during the winter season and the spring is windy and autumn is mid but is short duration etc. Thus the mean temperature in Seoul is about 10 to 15 ° C. This shows the Korean peninsula is falling in a subtropical region with the temperatures that reach up to a 25 degrees. This is also affected by the East Asian monsoon, and has heavier rainfall in the summer seasons and is also called as the jangma.

Ca mega city if i’m not mistaken