Understanding the collusion between El Nino and Monsoon

June 24, 2014

Indian Meteorological Department (IMD) is anticipating an El Nino developing in the Pacific and so there is considerable worry over its impact on this year’s monsoon. These concerns seem to escalate with the delayed arrival of rains over Kerala and a retarded monsoon progression towards north.
Around a century back Sir Gilbert Walker, then Director-General of Observatories in India, found indications that changes in the Pacific affects the monsoon. With more studies, scientific understanding of the ‘El Nino Southern Oscillation’ (ENSO) has increased significantly in recent decades.
It must be noted that not every El Nino hinders the monsoon. But when the Pacific Ocean becomes exceptionally warm the probability of monsoon turning deficient rises sharply. When the Pacific is neither too warm nor cool, there is only a 16% chance of a monsoon ending in a drought. As per rainfall data for 126 years, the possibility of a drought soars to over 40% when there is an El Nino.
Some cases of El Nino’s impact:

1997 was a year of one of the most powerful El Nino events in the last century. The waters of the equatorial Pacific Ocean had warmed sharply even before the onset of monsoon. As such a phenomenon typically stymies monsoon rains over India, a severe drought was widely predicted. However, the monsoon that year ended with above average rains.
In 2002, a moderate El Nino surprisingly broke down the monsoon which resulted into a massive drought.

The above instances clearly show how challenging it is to accurately forecast how an El Nino will advance and what impact it will make on monsoon.
In 1997, the eastern Pacific had become unusually warm, thereby restricting the atmospheric circulation changes that negatively affected the monsoon. It was when the sea surface temperature abnormalities were highest in the central Pacific that an El Nino had drought-producing effects in India. Central Pacific El Ninos were witnessed in 2002 as well as in 2004 and 2009, with all three years ending in drought.
There are several factors which can influence the effect of El Nino on the monsoon. For example:

Part of Pacific that warms, Eastern or Central
Indian Ocean Diapole (IOD), Positive or Negative

How the Indian Ocean Dipole’ (IOD) affects the monsoon?
IOD which is also known as the Indian Nino is an irregular oscillation of sea-surface temperatures in which the western Indian Ocean becomes alternately warmer and then colder than the eastern part of the ocean.
Positive IOD (Indian Ocean Diapole IOD):
In this case, the eastern equatorial Indian Ocean off Sumatra in Indonesia becomes colder than normal while the western tropical part of the ocean near the African coast becomes unusually warm. Such an event has been found to be beneficial for the monsoon.
Role of EQUINOO (Equatorial Indian Ocean Oscillation):
During the positive phase of the ‘Equatorial Indian Ocean Oscillation (EQUINOO),’ there is enhanced cloud formation and rainfall in western part of the equatorial ocean near the African coast while such activity is suppressed near Sumatra. This phase produces good rains over India.
Negative IOD (Indian Ocean Diapole IOD):
In this case the opposite of the above mentioned case occurs. The eastern equatorial Indian Ocean off Sumatra in Indonesia becomes abnormally warm while the western tropical part of the ocean near the African coast becomes relatively colder. This effect obstructs the progression of monsoon over India.
Negative EQUINOO (Equatorial Indian Ocean Oscillation):
A negative EQUINOO causes heightened cloud formation and rainfall near Indonesia thus retards rains over India.
Although EQUINOO and IOD cooperate during strong positive IOD events, as happened in 1994 and 1997, they do not always shape up so. For instance, the severe drought of 2002 occurred when a moderate El Nino as well as strong negative EQUINOO together took a toll on the monsoon. The IOD that year was somewhat positive.
Interplay between IOD and El Nino:
An IOD can weaken or aggravate an El Nino’s impact on the monsoon.
A positive IOD had facilitated normal or excess rainfall over India in 1983, 1994 and 1997 despite an El Nino in those years. But during years such as 1992, a negative IOD and El Nino had cooperatively produced deficient rainfall.
The positive IOD in 2007 appeared together with La Niña which is a very rare phenomenon that has happened only once in the available historical data (in 1967).
Latest Forecast:
At present, it is difficult to say which type of El Nino would evolve this year. The current generation of climate models does not have the capacity to differentiate whether a central or eastern Pacific El Nino will manifest. As per Japan Agency for Marine-Earth Science and Technology (JAMSTEC) group, there is a ‘very high’ chance of a negative IOD turning up in 2014.