This is the 9793rd most frequent Russian word.
южный
Southern.
The word 'южных' is the plural form of the adjective 'южный', meaning 'southern', and in this sentence, it modifies 'побережья' to describe its geographic orientation.
Я люблю отдыхать на южных побережьях.
I like vacationing on southern coasts.
The word 'южной' denotes the singular locative form of 'южный', describing the origin of the wind as being from the south.
Ветер дул с южной стороны.
The wind blew from the southern side.
Here, 'южный' is the nominative singular form of the adjective 'southern', modifying 'регион' to specify its location among regions.