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
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