5.3 Update on El Niño or La Niña

According to the Australian Government Bureau of Meteorology, La Niña continues in the tropical Pacific. Atmospheric and oceanic indicators of the El Niño–Southern Oscillation (ENSO) reflect a mature La Niña, including tropical Pacific sea surface temperatures, the Southern Oscillation Index (SOI), and tropical cloud patterns [1].

Figure 5.7 illustrates the behavior of the standard Southern Oscillation Index (SOI) for the period from October 2021 to October 2022. The SOI has remained positive and high (greater than +7) for the past four months, with a decreasing trend in July and August and a renewed increase in September and October. Much of the persistent positive SOI signal is due to high pressure systems over Tahiti. Overall, the SOI indicates a typical La Niña event during the monitoring period.

Another commonly used measure of El Niño is known as the Oceanic Niño Index (ONI). Figure 5.8 shows several ONIs and their locations. Values of the three key NINO indices for October 2022 were: NINO3 −0.7°C, NINO3.4 −0.7°C, and NINO4 −0.7°C. It implies that the average sea surface temperature in all three regions is significantly lower than the historical average. This indicates that La Niña re-enforces in September and October, consistent with the monthly SOI-BOM time series trend.

Sea surface temperature (SSTs) for October 2022 (Figure 5.9) were cooler than average across the central and eastern tropical Pacific Ocean, extending from around 160°E to the South American coastline and also across a large area south of the equator, particularly in the east of the basin. Warm anomalies extend into the mid-latitudes in the southern Pacific and across the mid to high latitudes in the north—a pattern characteristic of well-developed La Niña.

The southern Pacific Ocean has been locked in its La Niña phase for three winters running which is exceptional and causes extreme drought and flood conditions. It has been recorded just twice before, once in the mid-1970s and again at the turn of the millennium. Its long duration is a problem. The large, persistent mass of cold air and high pressure influences wind patterns known as jetstreams across the Pacific and the Indian Ocean. Where jetstreams that blow over land from the ocean are diverted, that land loses moisture and can suffer from drought. The land to which that jetstream is diverted gets more moisture and can be flooded [2]. Between July and October, La Niña, combined with a warmer planet, causes a series of extreme weather events that vary with geographic region [3].

North America

The southwestern United States has been in a state of drought for three winters running. The jet stream which blows in from the Pacific, carrying moisture with it, has been forced north by La Niña’s high pressure zone in the southern Pacific. That means more rain falls further north too, and less falls in the south. The National Integrated Drought Information System (NIDIS) shows that 70% of Oklahoma is in a state of exceptional or extreme drought, the most serious level that NIDIS reports. September was the driest since 1956, according to Gary McManus, the state’s climatologist.

This La Niña-induced drought is stimulating wildfires in Kansas. The state is now so dusty that it is becoming a safety concern for drivers and cattle. The wheat crop is weaker than usual and the state is concerned that it will be too dry for much of it to survive the winter. Losses to cotton crops due to drought in Texas are estimated to be worth some $2bn. The end of La Niña will bring relief. America’s National Weather Service’s latest prediction gives a 57% likelihood of ENSO returning to its neutral phase between February and April 2023.

Atlantic Hurricanes

While La Nina is a phenomenon of the Pacific, it also impacts hurricanes in the Atlantic during August, September, and October, the heart of storm season. The changing weather patterns cut off a lot of wind shear in the Caribbean Sea and elsewhere across the basin, allowing more Atlantic hurricanes and tropical storms to form and grow stronger.

Australia

La Niña typically increases the chance of above average rainfall for northern and eastern Australia during spring and summer and the chance of warmer days and nights in northern Australia during spring. Torrential rains have inundated large parts of New South Wales, Queensland, and Victoria.

South America

Typically, La Nina leads to hot and dry conditions for Argentina and southern Brazil as well as a shortened wet season in central Brazil. But in this monitoring period, the third La Nina is starting a little benign for southern Brazil because fronts have regularly stalled in the region and produced good rainfall.  In central Brazil, there has yet to be seen an effect from La Nina. In Argentina, corn is planted in two phases, the first in September and October, and the second in December and January. La Nina pushed a larger portion of the crop into the second phase than normal. Moreover, the drought has caused poor conditions for wheat, which will reach maturity in December.

South Asia

Flooding in Pakistan killed at least 1,700 people during the summer of 2022, and left 7.6 million homeless, according to the UN-monitoring organization, ReliefWeb. The country’s climate minister, Sherry Rehman, said that the worst-affected provinces received between five and seven times their average rainfall in August. The Indus River, which runs the length of Pakistan, burst its banks to swamp thousands of square kilometers of land. Shehbaz Sharif, the prime minister, said they were the worst floods in his country’s history (Figure 5.10).

La Niña is partially responsible. Just as the colder southern Pacific equilibrium pushes moisture away from the southern United States, it happens to push moisture right on top of Pakistan. But La Niña is not acting alone. A warmer climate, due to humanity’s carbon dioxide emissions, also means a more flood-prone Pakistan. In a paper published in September, a group of climate scientists estimated that rainfall over the worst-impacted provinces was 75% more intense than it would have been without the 1.2ºC of warming to which the planet has already been subjected.

East Africa

One of the regions most affected by La Niña is East Africa, where a prolonged drought has been experienced. At least one million people in Somalia have been displaced by the worst drought in decades due to climate change, which has also affected the wider Horn of Africa, including Ethiopia and Kenya. Between July and October, when staple food crops such as maize are growing and developing in East Africa, widespread drought brings reduced food production and hunger.

Future

As 2022 shifts to 2023, La Niña is weakening. Its end will bring relief to many places with extreme weather. Unfortunately, climate change will continue to exacerbate the effects of ENSO on local weather conditions in vulnerable regions across the globe.

Figure 5.7 Monthly SOI-BOM time series from October 2021 to October 2022

(Source: http://www.bom.gov.au/climate/enso/soi/)

Figure 5.8 Map of NINO Region

(Source: https://www.ncdc.noaa.gov/teleconnections/enso/sst)

Figure 5.9 Monthly temperature anomalies for October 2022

(Source: http://www.bom.gov.au/climate/enso/index.shtml#tabs=Pacific-Ocean)

Figure 5.10 Heavy monsoon rains flooded residential areas in Dera Allah Yar in Jaffarabad district, Balochistan province, Pakistan, in August.

(source: https://www.bloomberg.com/graphics/2022-la-nina-weather-risk-global-economies/)

Main Sources:

[1] http://www.bom.gov.au/climate/enso/index.shtml#tabs=Overview

[2] https://www.economist.com/interactive/the-world-ahead/2022/11/18/the-weather-system-that-influences-the-world

[3] https://www.bloomberg.com/graphics/2022-la-nina-weather-risk-global-economies/