If you have ever wondered why rare marine creatures, from tiger sharks to round ribbon-tail ray and Omura whales, have been sighted around the Similan Islands national park in the past several months, the answer is a phenomenon called the Indian Ocean Dipole (IOD).

The IOD, also known as the Indian Niño, is an irregular oscillation of sea surface temperatures in which the western Indian Ocean becomes alternately warmer (positive phase) and then colder (negative phase) than the eastern part of the ocean.

The positive phase sees higher than average surface temperatures and increased precipitation in the western Indian Ocean. The negative phase causes the opposite conditions, with warmer water and greater precipitation in the eastern Indian Ocean, and cooler and drier conditions in the west.

The cold water from the deep is rich with nutrients and, when this is driven to the surface, various marine creatures, including some species rarely sighted around the Similan Islands, are drawn to the area to feed.

Despite the merits of IOD, the cold water also has negative effects on young corals and Coralline algae and a positive IOD is also associated with droughts in Southeast Asia and Australia.