The El Niño–Southern Oscillation (ENSO) continues at neutral levels. Climate model outlooks currently indicate this neutral phase will last at least until October. Oceanic indicators of ENSO persist at neutral levels, with Pacific sea surface temperatures close to the long-term average across most of the equatorial region. Beneath the surface, temperatures are near average, with slightly warmer than average waters across much of the sub-surface. Atmospheric indicators such as the Southern Oscillation Index (SOI) and cloud patterns are also close to average. Trade winds have been stronger than average in the far west, but near average elsewhere [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 April 2020 to April 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 16.5 in January to 11.5 in Feburary, then decreased to -0.3 in March, then increased to 2 in April.

The SST map (Figure 5.9) for April shows cooler-than-average SSTs along most of the eastern half of the equator in the Pacific Ocean, extending into the tropics along the coastline of South America. These cool anomalies were generally stronger in April than during March in the east of the basin, while in the central and western equatorial Pacific SSTs have returned to near average temperatures. April values of the three key NINO indices were: NINO3 −0.4 °C, NINO3.4 −0.3 °C, and NINO4 −0.1 °C.  

Figure 5.7 Monthly SOI-BOM time series from April 2020 to April 2021 (Source: http://www.bom.gov.au/climate/current/soi2.shtml)

Figure 5.8 Map of NINO Region (Source: https://www.climate.gov/sites/default/files/Fig3_ENSOindices_SST_large.png)

Figure 5.9 April 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)