拉尼娜 is the cool-phase counterpart of 厄尔尼诺 (È'ěrnínuò, El Niño). Spanish 'La Niña' = 'the girl' (vs. 'El Niño' = 'the boy'). Together they form the ENSO climate cycle.
memory
Pure phonetic loan from Spanish: La-Ni-Ña → 拉-尼-娜. The 娜 (nà) character has the 女 (woman) radical, fitting 'the girl'.