As a deficient southwest monsoon looms large, scientists are predicting that a developing ‘El Nino’ condition in the Pacific Ocean might cause possible below normal rains this year. During ‘El Nino’, the surface of the eastern and central Pacific Ocean gets unusually warmer as against the normal pattern of western parts of the Pacific Ocean being warmer and the eastern surface remaining cooler.
The occurrence of El Nino is preceded by several changes in the overlying atmosphere and underlying ocean much before it peaks in the northern hemisphere winter season, according to Dr.Francis P.A., climatologist at ESSO-Indian National Centre for Ocean Information Services (INCOIS).
He said El Nino develops as a coupled ocean-atmosphere system called ENSO ( El Nino and Southern Oscillation) as the nature of the Pacific Ocean was generally favourable for a strong ocean-atmosphere interaction. Normally ENSO has a life span of 12-18 months and the general belief was that there was a higher propensity for drought in India during the developing phase of El Nino.
Condition reverses
However, a reverse of this condition leading to excess monsoon occurs during a La Nina event, which was nothing but the opposite of El Nino. But a near normal monsoon during 1997 when the El Nino phenomenon was the strongest ever recorded in the century forced meteorologists to look for other factors that influence the year-to-year variation of the Indian summer, Dr. Francis said and observed that around the same time scientists discovered an El Nino-like phenomenon in the Indian Ocean called the Indian Ocean Dipole (IOD).
It was noticed that during a positive IOD, the eastern part of the equatorial Indian Ocean becomes abnormally cool and the western part remains unusually warm, while the reverse of this pattern occurs during a negative IOD.
However, unlike El Nino, the IOD lasts only six to nine months due to smaller size of the Indian Ocean basin and since then attempts to link Indian monsoon with IOD were not encouraging as there appeared to be an association between the two only during positive phase of IOD.
Dr. Francis said that subsequently it was shown that ‘Equatorial Indian Ocean Oscillation’ (EQUINOO) (which was nothing but an oscillation of atmospheric cloudiness between eastern and western parts of the Indian Ocean) together with ENSO could explain the large variation in the monsoon.
It was generally seen that positive EQUINOO with enhanced cloudiness over western part as compared to eastern region was favourable to Indian monsoon.
During the last 65 years, the monsoon never failed when both the phases of ENSO and EQUINOO were positive. Similarly, the monsoon was never above normal when both theses phenomenon were negative.
Classic example
In fact 1997 was a classic example of a tug-of-war between ENSO and EQUINOO when the latter won and a near-normal monsoon occurred. Dr. Francis, however, said the outlook on the development of ENSO was still being used to give long-term forecasts of Indian monsoon as it has better predictability than EQUINOO. The need of the hour was to vastly improve prediction system of the Indian Ocean phenomena like IOD and EQUINOO for achieving more accurate outcomes, he added.
