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.”
Elaborating further, he said, “During an early IOD, the combined effect of Arabian sea evaporation and stronger cross equatorial flow — 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.”
The study examined rainfall patterns under the three dipole conditions for the period 1961 to 2003. During pure early IOD, the number of break days was much less. In fact, there were no break days during 1976, 1991 and 2003 and there was only one break event of 3 days in 1983. The average number of active days during early IOD was five. Compared to the other two, the number of break days was considerably higher during pure normal IOD.
The study also took into account the combined effect of El Nino and positive IOD on monsoon rainfall. The effect of El Nino in general is to decrease the monsoon rainfall over the Indian subcontinent. This study shows that the IOD on the other hand increases the monsoon activity over India.
The co-occurrence of the different types of El Nino events over the tropical Pacific (weak to very strong) and the result of their interaction with the three types of IOD on monsoon rainfall were studied. It was found that during weak El Nino years (1994 and 2006) there were no break events in the peak monsoon months of July and August (early IOD), whereas in the case of moderate (1987), strong (1972) and very strong El Nino years (1982, 1997) more break events were observed during the mid-monsoon months.
