"southeast" = “东南边" "southeastern" = "东南边的" "south" = "南边" "southerner" = "南边人"

In American English southeast and southeastern can both be used as adjectives, but southeast can also be used as a noun, likewise north/northern, east/eastern, etcetera. In many cases there is no clear reason why one or the other is used. For example, I live on the East Coast of the USA, but the shoreline along the East Coast is called the Eastern Seaboard. In my home state of New Jersey, we divide the state into North Jersey and South Jersey OR northern New Jersey and southern New Jersey, but NOT North New Jersey/South New Jersey. Meanwhile, in California the division is Southern California and Northern California, never North California and South California. South Africa is the name of a country within the continent of Africa, while southern Africa refers to the southern portion of the continent. In an example of using North and South as nouns, we talk about the US Civil War as being between the North and the South. Southerners are people who reside in the southern part of a country and in the US are considered a distinct cultural group. Jura's article goes into more detail about the South and southerners.

Here is a detailed explanation http://xroads.virginia.edu/~DRBR/REED/tears.html