Effect Of El Nino And La Nina Is Visible In India Relation With Monsoon Changed

The researchers found that while the El Niño-monsoon relationship remained moderately strong and stable over southern India, it became unexpectedly stronger over northern India during the same period. Moreover, this relationship has become remarkably weak and non-existent in the regions of central India (core monsoon zone) in recent decades.

What is El-Nino and La-Nina

During normal conditions, the trade winds blow westward along the equator, carrying warm ocean waters from South America toward Asia. To normalize that warm water, cold water comes up from the depths. This natural process gives rise to two opposite climate patterns like El-Nino and La-Nina. When these trade winds become strong, they pull more warm water towards Asia. To cool it, cooler water from the depths of the ocean is pulled up. Scientists call it by the name of El-Nino. Due to this a cooling effect is created in the atmosphere. On the contrary, when these trade winds are weak, they carry a small amount of warm water from South America towards Asia. Due to this, cold water does not rise from the depths of the ocean and this gives rise to a warming effect, which is known as La Nina.

Relationship between Monsoon and El Nino

The Indian monsoon fluctuates over time. Year-to-year fluctuations of the monsoon are largely controlled by fluctuations in ocean temperatures in the Pacific. In the Pacific Ocean, these oscillations are dominated by phases of warm and cold waters, namely El Niño and La Niña, in the central-eastern Pacific. These are known as the El Niño Southern Oscillation (ENSO). Generally, the trade winds blowing in the Pacific region are weak due to the El Nino effect. These winds are associated with the moisture-laden monsoon winds in India. In this way the monsoon also slows down, reducing the rainfall in the Indian subcontinent. Historically, at least half of El Niño years have been monsoonally dry.

How often has the color of this relationship changed

The ENSO effect over the Indian subcontinent is not uniform everywhere. The relationship between ENSO and Monsoon has not been the same since 1901. The researchers found that the relationship between ENSO and the monsoon got stronger from 1901 to 1940. It remained stable from 1941 to 1980 and then weakened in the recent period (1981 and onwards).

This change in the relationship between ENSO and the monsoon is not uniform at the regional level. There is no significant change in the ENSO-monsoon relationship over the region of South India. At the same time, this relationship is getting stronger in recent decades in North India. In contrast, the relationship between ENSO and rainfall over central India has weakened significantly in recent years. Monsoon rains are also affected by the strength of the monsoon trough and the changes in the monsoon depression caused by them. Monsoon trough and pressure variability have emerged as the main reason for variability in rainfall patterns over central India and have overtaken the dominance of ENSO.

The influence of ENSO on rainfall over South India and the intensity of monsoon trough and depression remained consistent throughout the period. Variation in rainfall patterns over northern India has become more dependent on ENSO than ever before. At the same time, the role of monsoon trough and pressure is decreasing. This decline in monsoon intensity may be due to the warming of the Indian Ocean and the weakening of the monsoon depression’s reach over the northern Indian region in recent decades.

Seema Javed, author is a senior columnist who writes on environmental issues.