The Indian Ocean Dipole (IOD), or the Indian Nino, is an ocean-atmosphere interaction that happens in the Indian ocean. It is similar to the El Nino fluctuations occurring in the Pacific Ocean. The Indian summer monsoon rainfall is influenced by a system of oscillating sea surface temperatures, and is termed the IOD. IOD is a weaker system than El Nino, and thus has relatively limited impacts. IOD is similar in nature to the El Nino Southern Oscillation (ENSO) phenomenon in the Pacific Ocean but on a smaller scale.
The El Nino, La Nina and IOD
The eastern side of the Pacific Ocean, near the north-western coast of South America, remains cooler than the western side near the islands of Philippines and Indonesia in a normal year. This occurs due to the existing wind systems that move from east to west, which sweep the warmer surface waters towards the Indonesian coast. Consequently, the displaced water is replaced by the relatively cooler waters from below. This phenomenon is called El Nino, which is the result of the weakening of wind systems that leads to lesser displacement of warmer waters when the eastern side of the Pacific Ocean becomes warmer than usual.
During the La Nina, the opposite happens. Both these conditions, together called as ENSO, affect weather events across the world. The El Nino leads to the suppression of monsoon rainfall in India.
The IOD is a similar phenomenon. IOD is the difference in temperature between two poles in the western Indian ocean and the eastern Indian ocean. This is played out in the relatively smaller area of the Indian Ocean between the Indonesian and Malaysian coastline in the east and the African coastline near Somalia in the west. Along the equator, one side of the ocean gets warmer than the other. The IOD is said to be in the positive phase when the western side of the Indian Ocean, near the Somalia coast becomes warmer than the eastern side of the Indian Ocean. The IOD is said to be in the negative phase when the western side of the Indian Ocean is cooler than the eastern side. There is a third neutral phase as well in which the sea surface temperatures remain roughly in line with long-term averages.
ENSO and IOD
ENSO is commonly referred to as an abnormal warming of surface waters in the equatorial Pacific Ocean. It is known to supress monsoon rainfall. The opposite phase, La Nina, which is the abnormal cooling of the sea surface waters in the same region, is known to aid rainfall over India. In its neutral phase, there is no El Nino or La Nina and the sea surface temperatures remain close to average. These three phases in the Pacific Ocean are collectively referred to as ENSO.
The IOD was identified as an independent system only in 1999. The air circulation in the Indian Ocean basin moves from west to east, from the African coast towards the Indonesian islands, near the surface, and in the opposite direction at the upper levels. This implies that the surface waters in the Indian Ocean gets pushed from west to east. Indian Ocean gets slightly warmer as the warmer waters in the western Pacific near Indonesia crosses over into the Indian Ocean during normal years. This causes the air to rise and helps the prevailing air circulation.
However, in those years when the air circulation becomes stronger, warmer surface from the African coast are pushed towards the Indonesian Islands. This phenomenon makes the region warmer than usual. This causes hot air to rise and the cycle reinforces itself, which is called the negative state of the IOD.
In the years when the air circulation becomes slightly weaker than the normal, the air circulation even reverses its direction. Consequently, the African coast becomes warmer while the Indonesian coastline becomes cooler, which is considered the positive state of IOD.
A positive IOD event is often seen developing at times of an El Nino, while a negative IOD is sometimes associated with La Nina. During El Nino, the Pacific side of Indonesia is cooler than normal because of which the Indian Ocean side also gets cooler. That helps the development of a positive IOD. Many studies suggest that IOD events are actually induced by ENSO. When both IOD and ENSO are strong, their circulation patterns could impact each other.
The Impacts of IOD on Indian Monsoon
A positive IOD helps rainfall along the African coastline and also over the Indian sub-continent. Further, this would supress the rainfall over Indonesia, southeast Asia, and Australia. In other words, during a positive IOD event, the western Indian Ocean experiences higher-than-normal rainfall, while the eastern Indian Ocean experiences drier-than-normal conditions. This could lead to increased rainfall over India and could offset the effects of El Nino, which usually brings drier-than-normal conditions to India.
In 2023, after the past four years of normal rainfall, India is possibly bracing itself against a deficient monsoon. This is because of the El Nino, which is caused by unusual warming of the ocean water along central and east-central Equatorial Pacific region. El Nino has had significant impact on the rainfall and weather patterns across countries. It typically recurs within two to seven years, and lasts for nine to 24 months, depending on the intensity of the phenomenon. The last El Nino condition began in 2014 and lasted till 2016.
Due to India being a rain-fed country, the south-west monsoon is very critical for the country’s agriculture. It is the predominant monsoon season that contributes to nearly 85 per cent of the rainfall in the country. However, certain factors could offset the impact of El Nino for India. These include the accumulation of snowfall over Eurasia, which cools the atmosphere and creates a high-pressure system and a positive IOD. This system could strengthen monsoon winds over India. Till July 2023, the monsoon has progressed over the entire India. However, its erratic distribution, coupled with progression of El Nino and a likely IOD, could disrupt the monsoon rainfall pattern in August and September. 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
India being an agricultural country, it is important for the distribution of rainfall to be even for agriculture to thrive. The distribution of rainfall during the sowing season and excessive rainfall during the flowering or harvest season may be detrimental to the crop. Hence, the quantity and timing of the rains are very important for agriculture. If monsoon rains continue to remain insufficient over the next two months, this could risk the country’s macroeconomic stability. Inflation, particularly of food items, has a direct relationship with monsoon. The importance of monsoon could not be underestimated as it has consequences on interest rates, consumption, and economic growth. Hence, it is important to watch the progress of the south-west monsoon over August and September 2023.
Some meteorologists have predicted a neutral IOD condition. This means that there is no significant sea surface temperature gradient across the Indian Ocean. A neutral IOD condition could have a mixed effect on India’s monsoon. This neutral condition could have a mixed effect on India’s monsoon season. On the positive side, a neutral IOD condition could create a more stable atmosphere over the Indian Ocean, supporting the development and strengthening of the monsoon winds. This could lead to increased rainfall over India and help offset the negative impact of any El Nino conditions. However, the negative aspect of a neutral IOD condition is that it could lead to below-average rainfall over the Indian subcontinent when factors such as El Nino or La Nina are unfavourable.
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