El Nino and La Nina: Teleconnection to the Indian Monsoon System

Weather phenomena across the world affect our daily lives; from a local-scale weather event impacting our daily life such as commuting to a regional-scale weather that controls precipitation and hence availability of water resources for domestic, agricultural, and industrial demands. The latter is more important for a country like India where 18% of the global population resides with access to only 4% of the global freshwater resources. India receives the majority (~75 to 80%) of freshwater through precipitation due to the Indian Monsoon System. Therefore it is important to know the controls that impact monsoonal precipitation in the country, particularly in times when the climate science community is focused on understanding how changes in atmospheric circulations and systems from around the world are affecting each other. This blog discusses regional scale seasonal atmospheric circulations with a focus on the long distance (tele) connections between the seasonal circulations such as El Nino/La Nina (in Pacific Ocean) and Indian Monsoon System (in Indian Ocean/Subcontinent). For the sake of completeness, the blog begins with introduction of general/primary atmospheric circulations and their brief mechanism at global scale. Further, the timing to discuss this topic is apt as India is facing one of the worst water crises which partly is due to low rainfall during the last monsoon season that was possibly linked to the El Nino event (Fig.1).

Fig. 1 Cumulative rainfall (mm) during the Indian summer monsoon in 2023 in comparison to that of during the time period 1971-2020, and rainfall anomaly during the year 2023 in India. Source www.mausam.imd.gov.in.

El Nino and La Nina are weather/climate phenomena that occur far away in the Pacific Ocean, but why are we discussing them in India?

As per records available on the website of the Indian Meteorological Department( Pune) on, the daily mean rainfall in India during the 2023 monsoon, there have been several days of rainfall deficit. Such days with rainfall deficits are distributed across the monsoon months (June to September). The data also points to several rainfall excess days during the 2023 monsoon season (Fig. 2). By converting rainfall records into a cumulative rainfall record, it is evident that the cumulative rainfall record of India during the 2023 monsoon is 820 mm as against the 868.6 mm cumulative normal rainfall between the years 1971 - 2020. Thus, in India, a rainfall deficit of about 48.6 mm is observed in the monsoonal rains for the year 2023. In other words, monsoon rainfall during 2023 was about 5.6 % less than the average monsoon rainfall between the years 1971 - 2020.

Fig. 2 Daily mean rainfall (mm) and cumulative rainfall over four homogeneous regions of India. Source www.mausam.imd.gov.in.

The distribution of this rainfall deficit varies across the major geographical regions of the country. For example, in Northwest India there was about 1 % rainfall excess whereas in east and northeast India there was about 18% of rainfall deficit. Similarly, in central India there was no change in rainfall whereas in the southern peninsula there was about 8 % rainfall deficit. Thus, the overall rainfall deficit of 5.6% has impacted severely to the population of northeast and south India as compared to the other parts of the country.

Now, we understand that monsoon rainfall is very important and any deficit in it may severely impact water availability in the country. And it is generally believed that El Nino and La Nina are important factors that affect Indian summer monsoon rainfall. El Niño generally suppresses monsoon rainfall; La Niña generally enhances it. But, how well do we understand this relationship between El Niño and poor Indian monsoon rains? Bargaonkar et al., (2010) reported that deficient monsoon rainfall (drought) years (1871-2001) show one-to-one correspondence with El Niño events marked as circles in figure 3.

Fig. 3 The years of monsoon droughts are associated with El Niño since the late eighteenth century (Source: Borgaonkar et al., 2010 in the journal Palaeogeography, Palaeoclimatology, Palaeoecology, 285(1-2), page 74-84)

What happens during El Nino?

General or primary circulations in the atmosphere include trade winds, westerly winds, and polar winds mainly controlled by global pressure belts. These are besides several other variants of general atmospheric circulations such as jet stream, walker circulation, and southern oscillation. Normally, tropical (trade) winds flow towards the equator from NE to SW on the northern hemisphere and from SE to NW on the southern hemisphere. Now this general circulation of tropical winds may become a variant, and begin to flow as east-west zonal circulation. This typical east-west circulation of tropical winds is called Walker Circulation (after G. T. Walker in 1922-23). On a temporal scale, it is observed that after two-three years this general circulation of east-west reverses to become west-east circulation. Thus, there are oscillations of air circulation after an interval of two-three years. Walkar called such oscillations as Southern Oscillations. Both the Walker Circulation and Southern Oscillations are driven by the sea surface temperature gradient caused by the warm ocean currents in the eastern Pacific Ocean along the South American continent. Let’s understand this interaction between atmospheric and oceanic circulations. El Nino is a weather phenomenon that occurs at the western coast of the South American continent. A subsurface warm ocean current, known as El Nino, flows from 3° S to 36° S latitude about 180 km off along the Peruvian coast. The southward shifting of the counter equatorial warm current during southern winter gives birth to El Nino current.

In normal condition, high pressure develops on the sea surface of the equatorial east Pacific ocean due to upwelling of cold oceanic water. In contrast, low pressure is formed in the equatorial western Pacific ocean due to the warm sea surface. This pressure gradient generates east-west circulation of trade winds on the surface. This east-west air circulation drives the ocean water mass from the western coast of South America towards the east. This further facilitates the upwelling of cold sea water near the coasts of Peru and Equador resulting in further cooling of air, high air pressure and dry weather. Whereas, east-west air circulation becomes warm trade winds in the equatorial west Pacific ocean where it warms up and rises upward, becomes unstable and causes precipitation. After rising to a certain height it turns eastward and descends in the equatorial eastern Pacific ocean to complete the convective cell.

In the El Nino condition, upwelling of cold ocean currents stop resulting in the formation of low air pressure in the east Pacific off the coasts of South America. This reversal in pressure conditions causes a return of warm sea water towards the tropical east Pacific. Consequently, the warm air rises upward and becomes unstable to yield rainfall after condensation. The rising air in the east Pacific cools above and turns westward in the troposphere and ultimately descends in the tropical west Pacific gives rise to high pressure which drives warm air towards the coasts of South America. This forms the complete convective cell.

Fig. 4 Air circulation during Normal and El Nino conditions. Source: Chapter-6 on General atmospheric circulation in the Book Climatology by Savindra Singh (2005).

What happens to the Indian monsoonal precipitation during the El Nino?

When El Nino is extended to the southern end of South America, warm water is pushed eastward to join the South Atlantic westerlies drift which brings warm water in the southern Indian Ocean. Consequently, the high pressure in the Indian Ocean during southern winter is not intensified due to which the south-west summer monsoon is weakened. Thus, strong El Nino brings heavy rainfall exceeding the normal rainfall in the otherwise dry coastal land of Peru. The cold water mass near Peruvian coast becomes warm due to strong El Nino events resulting in heavy rainfall in the first half of the year (January to March). In contrast, the strong El Nino brings dry conditions in the tropical western Pacific resulting in severe drought in Indonesia, Bangladesh, India etc.

What is La Nina and its relation with the Indian monsoonal precipitation?

La Nina is a counter ocean current which becomes effective in the tropical western Pacific when El Nino becomes ineffective in the tropical eastern Pacific. The dry condition in the western Pacific is terminated and wet condition is introduced in the tropical western Pacific by La Nina.

It's time for Trivia!

El Nino is considered a Christ child and La Nina as a younger sister. Normally, cold and dry conditions persist in the eastern Pacific along the Peruvian coast. And the people of Peru used to Pray “Ye God, give us rain and keep drought away”; they realized that every few years the Peruvian coast becomes warm resulting in heavy rainfall in the first half of the year (January to March). Later these events were known as El Nino. However, sometimes these heavy rainfalls become four to six times more than normal, causing mass destruction of marine life. And the people of Peru used to Pray “Ye God, give us rain and El Nino away”. Later these events were known as La Nina. The El Nino event was first noticed in the year 1541. Since then more than a dozen events have been noted (1951-52,53; 1957-58; 1963-64; 1965-66; 1969-70 and so on). The La Nina event was identified and named as La Nina phenomenon in the year 1986 but its occurrence was recorded in 1950-51; 1954-56; 1964-65; 1970-72; 1973-74; 1974-76 and so on.

Further readings:

Savindra Singh (2005): Climatology, Prayag Pustak Bhawan.
D. S. Lal (2009): Physical Geography, Sharada Pustak Bhawan.
William F. Ruddiman (2014): Earth's Climate: Past and Future, W.H.Freeman & Co Ltd.

Useful links:

Dr. Ajit Singh

Principal Researcher

Earth Sciences

03/06/2024

El Nino and La Nina: Teleconnection to the Indian Monsoon System

Related Posts

Comments