南下 is read as なんか (nanka) and means going south.
なんか
nanka
なんせい
southwest
なんとう
southeast
なんぼく
South Korea and North Korea
みなみ
south
みなみあふりか
South Africa
なんちゅう
culmination
なんなんとう
south-southeast
なんなんせい
south-southwest
みなみにほん
southern Japan (usu. referring to Kyushu and southwards)
なんざんだいがく
Nanzan University
みなみいりのいだいがく
Southern Illinois University
なんごく
southern country