The dipole factor in summer monsoon rainfall

The Indian summer monsoon rainfall is influenced by a system of oscillating sea surface temperatures known as the Indian Ocean Dipole (IOD) in which the western Indian Ocean becomes alternately warmer and then colder than the eastern part of the ocean. While the existence of three types of IOD is well known, a recent study published in the journal Natural Hazards attempts to determine the effects on monsoon rainfall of each of the three types. A positive IOD occurs when the sea surface temperatures are greater than normal in the Arabian Sea and less than normal in the tropical eastern Indian Ocean. When the reverse is the case, a negative IOD is said to have developed.

A positive IOD leads to greater monsoon rainfall and more active (above normal rainfall) monsoon days while negative IOD leads to less rainfall and more monsoon break days (no rainfall).

The study aims to determine the role of the positive phase of the three IODs on summer monsoon rainfall. The three types are: normal IOD, early IOD and prolonged IOD. The study finds that an early IOD, which peaks in the mid-monsoon months (July and August), plays a significant role in enhancing monsoon rainfall even though its intensity is medium compared to other IODs.

Dr. Ramesh Kumar, Chief Scientist and Coordinator AcSIR, Physical Oceanography Division, National Institute of Oceanography, Goa, who is the lead author of the paper noted in an email to this Correspondent: “The point we want to stress is that the early IOD peaks in the peak monsoon months (July and August) and thus plays a vital role in the monsoon rainfall over the Indian subcontinent. The normal IOD and prolonged IOD peak in September, October, November and hence play a lesser role in the monsoon rainfall.”