La Nina is suspected to have caused recent floods in Australia. How is La Nina different from El Nino?

La Nina is characterized by unusually cold ocean temperature in equatorial Indian Ocean whereas El Nino is characterized by unusually warm ocean temperature in the equatorial Pacific Ocean.

El Nino has adverse effect on south-west monsoon of India, but La Nina has no effect on monsoon climate.

Which of the statements given above is/are correct?

Question

La Nina is suspected to have caused recent floods in Australia. How is La Nina different from El Nino?

La Nina is characterized by unusually cold ocean temperature in equatorial Indian Ocean whereas El Nino is characterized by unusually warm ocean temperature in the equatorial Pacific Ocean.
El Nino has adverse effect on south-west monsoon of India, but La Nina has no effect on monsoon climate.

Which of the statements given above is/are correct?

Accepted Answer

Neither 1 nor 2 La Niña is actually characterized by unusually cold ocean temperatures in the tropical central and eastern Pacific Ocean and is linked to stronger monsoon rains and above-average rainfall in India. In contrast, El Niño is characterized by unusually warm ocean temperature in the tropical eastern Pacific Ocean and is associated with weaker monsoon rains and drought conditions in India. Normal Conditions (The Walker Circulation):   Under the normal conditions in the Pacific ocean, trade winds blow west along the equator, dragging the warm surface water from the tropical east Pacific ocean toward the tropical western Pacific ocean. The place of this warm surface ocean water in the tropical east Pacific ocean is taken by the cold and nutrient-rich water that wells up from below the ocean surface because the cold Peru or Humboldt current flows from south to north along the South American coast. Along with the subsidence of air from above, this upwelling of cold ocean water from below produces high pressure and dry weather over the tropical east Pacific ocean. On the other hand, the rise of air from the warm seas surface produces low pressure and rainfall on the tropical western Pacific ocean. This east-west pressure gradient generates east-west circulation of trade winds over the ocean surface while there is reverse circulation of upper air (i.e., from west to east), thereby completing a convective cell of air circulation called Walker circulation. However, this normal climatic condition is frequently changed by two opposite oceanic phenomena – La Nina and El Nino – thereby influencing global atmospheric circulation and causing anomalies in the normal temperature and rainfall patterns in the tropical Pacific and Indian Oceans. These climatic anomalies are important for India because they affects the Indian monsoon too. El Niño and the Southern Oscillation (ENSO):   Once every 2-5 years, the normal condition of east-west pressure gradient is reversed (i.e., pressure gradient becomes west to east). By October-November, the low pressure of tropical western Pacific ocean shifts to the tropical east Pacific ocean off the coasts of Ecuador and Peru. As a result, the air over the tropical east Pacific ocean rises, cools above and causes heavy rainfall there between January and March. This air turns westward in the upper troposphere and finally descends in the tropical west Pacific ocean, giving rise to high pressure and dry weather there. This high pressure in turn drives the warm and surface ocean water toward the tropical east Pacific ocean, again completing a convective cell of air circulation. This oscillation (or reversal) in the normal east-west pressure gradient and air circulation in the tropical Pacific ocean is called Southern Oscillation (SO). Following the work of Sir Gilbert Walker in the 1930s, the climatologists determined that El Nino occurs simultaneously with the Southern Oscillation, and defined these inter-linked phenomena as El Nino-Southern Oscillation (ENSO). The Mechanics and Impact of El Niño:   In Spanish, the term ‘El Nino’ means the Christ Child and was used by Spanish immigrants to refer to the unusual warming of Peruvian coastal water because the phenomenon happened around Christmas. In fact, El Nino is a narrow, warm ocean current that flows from north to south between 3º S and 36º S latitudes along the coast of Peru in December. This El Nino current is generated by the southward shift of warm Counter Equatorial Ocean Current during southern summer. This warm current is more intense than usual. A strong El Nino event stops the upwelling of cold water from below (Peruvian cold ocean current), turns the tropical central and east Pacific ocean water from being cold to being warm, reduces the atmospheric pressure over that part of the ocean, weakens the Walker circulation, and contributes to the Southern Oscillation. Therefore, a strong El Nino event not only causes 4-6 times more rainfall (than normal) along the Peruvian coast, but it also causes death of marine life especially fish due to the disappearance of Peruvian cold current and thus planktons associated with it. Simultaneously, dry conditions prevail over the tropical western Pacific ocean causing drought in Indonesia, South Asia and parts of Australia. There is also a strong, though not one-to-one, association between El Nino and droughts in India. Historical data of 135 years from 1880 to 2014 show that about 90% of all evolving El Nino years have led to below normal rainfall and 65% of evolving El Nino years have brought droughts. The Mechanics and Impact of La Niña: Also known as the cold phase of ENSO, La Nina refers to the periodic, and more-than-normal cooling of ocean surface temperatures in the tropical central and east Pacific. This is caused when trade winds are even stronger than usual, push more warm water toward the tropical western Pacific ocean, and increase the upwelling of cold water from below in the tropical east Pacific ocean. This is nothing but the strengthening of Walker circulation and associated normal climatic conditions in the Pacific. In other words, the high pressure and dry conditions in the tropical east Pacific ocean as well as the low pressure and wet conditions in the tropical west Pacific ocean are reinforced. During a year of strong La Nina, Indonesia and northern Australia get increased rainfall and risk of floods. South Asia gets more-than-normal rainfall from the summer monsoon. Thus, La Niña event produces the opposite effect of El Niño on global climate. Frequency of Occurrence: El Niño and La Niña typically develop in northern hemisphere’s spring-summer (March-June) and peak in the late autumn or winter (November-February). The oscillation between the ENSO’s warm phase (El Niño) and its normal or cold (La Niña) conditions occurs every three to five years, while also ranging from two to seven years. El Niño can last up to 18 months and La Niña up to three years. The last multi-year La Niña event, began in September 2020 and prolonged into early 2023. As it lasted for three consecutive Northern winters, it was called the first ‘triple dip La Nina’ of the 21st century. It caused floods in Maharashtra, Gujarat, and Odisha in the 2022 monsoon season.