5.3 Update on El Niño

The El Niño–Southern Oscillation (ENSO) remains neutral with most oceanic and atmospheric indicators within the neutral range. While pressure patterns show some La Niña-like characteristics, as indicated by the latest Southern Oscillation Index (SOI) 30-day value of +15.1, it is likely that some of this shift in pressure is driven from warm conditions in the eastern Indian Ocean. Most climate model outlooks indicate the central tropical Pacific is likely to cool over the coming months, with three of seven models surveyed by the Bureau indicating this cooling will be enough to reach La Niña thresholds in spring, with the remaining four models staying neutral. Along with the negative Indian Ocean Dipole (IOD), this may be contributing to the wetter than median climate outlooks in Australia [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 July 2020 to July 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 increased from 2.0 in April to 3.6 in March, then decreased to 2.6 in June, then increased to 15.9 in July.

The SST map (Figure 5.9) for July 2021 shows SSTs were close to average across most of the equatorial Pacific Ocean, although warmer than average SSTs continued in the eastern Pacific Ocean close to South America. SSTs were warmer than average in waters around the north and south-east of Australia. Values of the three key NINO indices for July 2021 were: NINO3 +0.1 °C, NINO3.4 +0.0 °C, and NINO4 +0.1 °C.

Figure 5.10 Monthly SOI-BOM time series from July 2020 to July 2021 

(Source:http://www.bom.gov.au/climate/current/soi2.shtml)

Figure 5.11  Map of NINO Region

(Source: https://www.climate.gov/sites/default/files/Fig3_ENSOindices_SST_large.png)

Figure 5.12  July 2021 sea surface temperature departure from the 1961-1990 average 

(Source:http://www.bom.gov.au/climate/enso/wrap-up/archive/20210511.ssta_pacific_monthly.png?popup)

Main Sources:

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