GUWAHATI- A system of oscillating sea surface temperature in the Indian Ocean, known as the Indian Ocean Dipole (IOD), influences the Indian summer monsoon profoundly.While the issue concerning the impact of the El Nino on the summer monsoon rainfall in India is widely discussed, the impact of the IOD on the Indian summer monsoon rainfall so far remains to be discussed in the same air. Interestingly, the impact of the IOD on the Indian summer monsoon rainfall was discovered by a team of Indian meteorologists, led by Assam-origin Prof BN Goswami, a former director of the Pune-based Indian Institute of Tropical Meteorology (IITM).
The IOD is one element of the general cycle of global climate, which interacts with similar phenomena, like the El Nino-Southern Oscillation (ENSO) in the Pacific Ocean.

Dr Rahul Mahanta, associate professor, Interdisciplinary Climate Research Centre (ICRC), Cotton University, said the discovery of IOD has been able to explain as to how the oscillation in sea surface temperatures in the Indian Ocean influences the Indian summer monsoon. The IOD is a system of oscillating sea surface temperatures that make the western Indian Ocean alternately warmer and subsequently colder than its eastern part. There are three types of IOD and each of them influences the Indian summer monsoon rainfall. They are – normal IOD, early IOD and prolonged IOD.

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, said Dr Mahanta.

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

An early IOD, which peaks in July and August (when the monsoon is at its peak), plays a significant role in enhancing monsoon rainfall even though its intensity is medium compared to other IODs. The normal IOD and prolonged IOD peak in September, October, November and hence, play a lesser role in the monsoon rainfall.

During an early IOD, the combined effect of Arabian Sea evaporation and stronger cross-equatorial flow from Indian Ocean – i.e., the winds blowing from the southern tropics to the north across the equator – play an important role in enhancing the monsoon activity over the Indian subcontinent. Also, there are fewer breaks in monsoon conditions during early IOD events.

Dr Mahanta said that ICRC, Cotton University is initiating a research to study the impact of the IOD on rainfall pattern of northeastern part of the south Asian subcontinent, based on data available for the second half of the twentieth century. Literature available shows that during pure early IOD, the number of break days was much less during the above study period. During some of the early IOD years, no break days were observed and the active days during the same early IOD years were much higher. During the pure normal IOD, the number of break days was considerably higher.

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