5.3 Update on El Niño

The Australian Government Bureau of Meteorology's ENSO Outlook remains at La Niña ALERT, meaning around a 70% chance of La Niña forming in the coming months. Several climate drivers are combining to produce the current wet outlook for Australia. International climate models have strengthened their forecast likelihood of La Niña forming before the end of the year. However, atmospheric and oceanic observations have yet to consistently reach La Niña levels. The latest tropical Pacific Ocean temperatures, while cooler than average, are at similar levels to a fortnight ago and do not meet La Niña thresholds. Similarly, in the atmosphere, the Southern Oscillation Index (SOI) has eased back slightly from La Niña levels. Regardless of whether La Niña thresholds are met, a La Niña-like pattern in the Pacific may still increase the chances of above-average rainfall for northern and eastern Australia at times during spring and summer [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 October 2020 to October 2021. 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 decreased from 15.9 in July to 4.6 in August, then increased to 9.3 in September, and then decreased to 6.7 in October.

Figure 5.8 shows several El Niño regions. Persistent NINO3 or NINO3.4 values cooler than −0.8 °C are typical of La Niña, while persistent values warmer than +0.8 °C typically indicate El Niño. Values of the three key NINO indices for October 2021 were: NINO3 −0.4 °C, NINO3.4 −0.6 °C, and NINO4 −0.3 °C. There is no La Niña forming, but the risk remains.

Sea surface temperatures (SSTs) for October 2021 (Figure 5.9) show weak cool SST anomalies were present across much of the central to eastern equatorial Pacific, while generally weak and warmer than average SSTs were present in waters around the north of Australia and the Maritime Continent. 

Figure 5.7 Monthly SOI-BOM time series from October 2020 to October 2021 

(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 October 2021 sea surface temperature degree difference from normal conditions (climatology). 

(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