northern sea; northern ocean
General term for a sea or ocean in the north. Can refer to any northern sea, not a specific proper name.
The ship is sailing the northern seas.
North Sea
Specifically the North Sea between Great Britain and continental Europe.
The North Sea oil fields are important for Europe's energy supply.
Abbreviation of 北海道. Used in compound words like 北海タイムス or as a prefix in names.
See also: 北海道
The Hokkai Times is a newspaper in Hokkaido.