What is 'Kallakkadal Phenomenon' that is likely to affect Kerala, TN today?

Kerala's entire coastline and parts of Tamil Nadu's southern coast are likely to experience the 'Kallakkadal Phenomenon' until 11.30 pm today. The phenomenon is caused by sudden swell of the seas which results in rough waves. Following this, Indian National Centre for Ocean Information Services (INCOIS) warned fishermen to stay away from danger zone areas as the waves could intensify.