According to the Australian Government Bureau of Meteorology, the 2021–22 La Niña continues in the tropical Pacific, with little change in strength in the past few weeks. Several indicators of La Niña, including tropical Pacific Sea surface temperatures, cloudiness near the Date Line, and the Southern Oscillation Index (SOI), have maintained or slightly increased their strength over the past fortnight. However, beneath the surface of the tropical Pacific, waters have warmed closer to neutral El Niño–Southern Oscillation (ENSO) levels [1]. Below is specific information about these indicators and their trends.

Figure 5.7 illustrates the behavior of the standard Southern Oscillation Index (SOI) for the period from April 2021 to April 2022. The SOI increased from 4.1 in January until 22.6 in April, indicating a typical and increasing La Niña 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 ONIs for April 2022 were: NINO3 −0.5 °C, NINO3.4 −0.7 °C, and NINO4 −0.6 °C. It implies that the average sea surface temperature in all three regions is lower than the historical average. Moreover, the three key NINO indices decreased 0.1°C compared to March. This is an indication that the La Niña slightly increased its strength over the past month.

Sea surface temperature (SSTs) for April 2022 (Figure 5.9) show cool SST anomalies across the central to eastern equatorial Pacific and along the coastline of South America, and mostly weak warm SST anomalies over parts of the Maritime Continent. Compared to March, cool anomalies in the central to eastern tropical Pacific have strengthened, while in the west SST anomalies are closer to average than they were during March.

In conclusion, from January to April, La Niña continued to be active in the tropical Pacific and its atmospheric indicators were stronger than the oceanic ones. This resulted in a continued impact of La Niña on global weather and climate during the monitoring period. For example, La Niña conditions increased the chances of above average rainfall for much of eastern Australia and caused relatively rare widespread cold and snowy weather in northeastern China. Autumn in the Southern Hemisphere is the usual time of the year in which ENSO events decay and return to neutral. However, no such empirical trend of decay has been observed so far.

Figure 5.7 Monthly SOI-BOM time series from April 2021 to April 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 April 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