Thiruvananthapuram: The coastal areas in Kerala and the southern parts of Tamil Nadu are likely to experience swell waves, or 'Kalla Kadal phenomenon', till 11.30 pm on Monday, warned the Indian National Centre for Ocean Information Services (INCOIS).

During this time, sudden swell of the seas cause rough waves and coastal areas experience severe sea incursions. "As the rough seas are likely to intensify, stay away from the danger zone areas as per the instructions of the authorities,"  INCOIS said in its alert.

INCOIS, the agency that issues weather warnings for fishermen in the country, advised people to safely moor fishing vessels in the harbour. "Keeping a safe distance between boats can avoid the risk of collision. The safety of fishing equipment should be ensured," it said in a statement. INCOIS also advised people to avoid trips to the beach and activities at sea completely.

What is Kalla Kadal phenomenon?
The term 'Kalla Kadal' literally means a sea that comes suddenly like a thief. INCOIS has said that the swell surge is the result of strong winds in the southern part of the Indian Ocean at certain times occurring suddenly without any particular indications or warning, hence the name "Kalla Kadal."

According to weather experts, the phenomenon is triggered by storms as far away as the Antarctic region. After travelling thousands of kilometres across the ocean basin, the swell intensifies when it encounters a coastal current, a phenomenon known as remote forcing. 'Kalla Kadal' usually occur along the southern coast of India, mainly during the pre-monsoon period, in April and May, during clear weather conditions.