“Sino-“ indicates Chinese. By itself it is not a word but is used only in hyphenated expressions. I have seen it used only as an adjective, not as a noun, so “Sino-Japanese” would be the common usage. “North China” and “northern China” have roughly the same meaning, that is, they have the same denotation (意义), although there is a very, very slight different connotation (言外之意) in the mind of the speaker. When I say “north China” I’m thinking only of the area north of the 长江. I am not thinking at all of the other parts of China. When I say “northern China” I’m thinking of the whole country but concentrating only on that section that lies north of the 长江. The difference is so negligible that others may not even see it this way. Although the meanings of the two words are virtually the same, the usage is not. I would say, for instance,, that I’m from the ‘’southern United States” but I would not use the word “south.” If I wished to use the latter, I would say that I’m from the south. Although it’s been a long time since I studied grammar, I think that the “n” in “north China” would be lower case, since it is simply an adjective and not part of the country’s name. (Sometimes American southerners jokingly capitalize “South” to emphasize that they consider it as distinct place, as opposed to “north’’ which they say is only a direction out of the South.”)