Japan is cool because of the
Kuril Current, which originates off the east coast of
Kamchatka up near the western tip of the
Aleutian Islands. This current, coupled with a weaker current that circles the
Sea of Okhotsk counterclockwise, brings cold air from
Siberia down to
Hokkaido and the
Tohoku region, and helps to balance out the currents from the
South China Sea that make
Tokyo and
Osaka sweaty in the summertime.
Of course, Okinawa is never cool, being at the same latitude as Florida and the Persian Gulf. Most of Japan is around the same latitude as the South and the Mediterranean Sea.
/me bows for turning this into a factual node