5.3 Update on El Niño

According to the Australian Government Bureau of Meteorology, the ENSO Outlook remains that La Niña has peaked, but its influence will persist until April. Climate models and observations suggest the 2021–22 La Niña has peaked, and will most likely return to neutral El Niño–Southern Oscillation (ENSO) (neither La Niña nor El Niño) during March to May. La Niña increases the chance of tropical cyclones and above average rainfall across northern and eastern Australia from December 2021 to February 2022 and, to a lesser degree, from March to April. As La Niña weakens it can continue to influence global weather and climate. Atmospheric and oceanic indicators remain at La Niña levels, but have likely peaked in strength. While eastern tropical Pacific sea surface temperatures remain cooler than average, beneath the surface, waters in the central and eastern Pacific are now warming. These changes in the sub-surface typically foreshadow a breakdown in a La Niña event, which normally occurs from March to May. In the atmosphere, decreased cloudiness along the Date Line, strengthened trade winds in the western Pacific and a positive Southern Oscillation Index (SOI) reflect a mature La Niña [1].

Figure 5.7 illustrates the behavior of the standard Southern Oscillation Index (SOI) published by the Australian Bureau of Meteorology (BOM) for the period from January 2021 to January 2022. Sustained positive values of the SOI above +7 typically indicate La Niña while sustained negative values below -7 typically indicate El Niño. Values between about +7 and -7 generally indicate neutral conditions. During this monitoring period, SOI increased from 6.7 in October to 12.5 in November, and peaked at 13.8 in December before falling back to 4.1 in January.

Another commonly used measure of El Niño is known as the Oceanic Niño Index (ONI). Figure 5.8 shows several ONIs and their locations. Historically, scientists have classified the intensity of El Niño based on Sea surface temperature (SST) anomalies exceeding a pre-selected threshold in a certain region of the equatorial Pacific. The most commonly used region is the Niño 3.4 region, and the most commonly used threshold is a positive SST departure from normal greater than or equal to +0.5°C. Since this region encompasses the western half of the equatorial cold tongue region, it provides a good measure of important changes in SST and SST gradients that result in changes in the pattern of deep tropical convection and atmospheric circulation. The criteria, that is often used to classify El Niño episodes, is that five consecutive 3-month running mean SST anomalies exceed the threshold. SST values in the Niño 3.4 region may not be the best choice for determining La Niña episodes but, for consistency, the index has been defined by negative anomalies in this area. A better choice might be the Niño 4 region, since that region normally has SSTs at or above the threshold for deep convection throughout the year. Values of the three key NINO indices for January 2022 were: NINO3 −1.0 °C, NINO3.4 −0.7 °C, and NINO4 −0.2 °C. It will most likely return to neutral El Niño–Southern Oscillation.

Sea surface temperature (SSTs) for January 2022 (Figure 5.9) indicate weak cool SST anomalies across most of the eastern half of the equatorial Pacific. Weak warm SST anomalies were observed over parts of the Maritime Continent and the Coral Sea and parts of Queensland's east coast.

Figure 5.7 Monthly SOI-BOM time series from January 2021 to January 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 in the tropical Pacific for January 2022

(Source: http://www.bom.gov.au/climate/enso/wrap-up/#tabs=Sea-surface)

Main Sources:

[1] http://www.bom.gov.au/climate/enso/wrap-up/#tabs=Overview