The unusually-heavy rains over Tamil Nadu and other parts of southern peninsula may have been caused by La Nina and the positive phase of southern oscillation (SO), global scale circulation features.
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/la-nina-and-positive-so-cause-unusual-rain
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/hindu
[3] http://admin.indiaenvironmentportal.org.in/category/thesaurus/el-nina
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/natural-disasters
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/environmental-science
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/tamil-nadu
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/rainfall