According to the Australian Government Bureau of Meteorology, the El Niño-Southern Oscillation Index (ENSO) remains neutral, with sea surface temperatures (SST) in the central equatorial Pacific at ENSO-neutral levels. Atmospheric indices, such as those related to surface air pressure, cloudiness, and trade wind patterns, are largely consistent with a neutral ENSO state. Although there have been some La Niña-like signals in recent months, no sustained atmospheric changes have been observed. Overall, the ocean-atmosphere coupled system reflects a continued neutral state of El Niño.

The following figure shows the changes in the standardized Southern Oscillation Index (SOI) from October 2023 to October 2024. Over the past four months, the SOI values have generally remained within the El Niño-neutral range (-7 to +7), with only August slightly exceeding 7 (7.8), but it subsequently declined and remained within the neutral range.

Figure 5.4 Monthly SOI-BOM time series from October 2023 to October 2024(Source: http://www.bom.gov.au/climate/enso/soi/)

Another commonly used indicator for measuring the El Niño phenomenon is known as the Oceanic Niño Index (ONI). Figure 5.5 shows several ONIs and their locations. Analysis of Table 5.3 reveals that in September and October, the ONI anomalies in all three regions (NINO3, NINO3.4, and NINO4) generally show a declining trend, indicating that the sea surface temperatures are continuously decreasing. This suggests that El Niño continues to manifest under neutral conditions and may possibly transition towards La Niña.

Figure 5.5 Map of NINO Region（Source: https://www.ncdc.noaa.gov/teleconnections/enso/sst）

Sea surface temperatures (SST) for October 2024 are shown in Figure 5.6, with SSTs in the central and eastern equatorial Pacific generally close to the average SST. SSTs in the far western tropical and eastern equatorial Pacific were up to 1.2°C above average; in the central and eastern equatorial Pacific, east of 170°W, they were up to 1.2°C below average. International climate models indicate that SSTs are likely to remain within the ENSO-neutral range (-0.8°C to +0.8°C) throughout the entire forecast period.

Figure 5.6 Monthly temperature anomalies for October 2024(Source: http://www.bom.gov.au/climate/enso/wrap-up/#tabs=Sea-surface)

Table 5.2 ONI (°C) Anomaly Values from September 2024 to October 2024(Source: https://www.cpc.ncep.noaa.gov/data/indices/sstoi.indices)

According to the diagnostics from NOAA's Climate Prediction Center, the coupled ocean-atmosphere system reflects the current ENSO-neutral state. For the future, the latest North American Multi-Model Ensemble (NMME) forecasts the emergence of a weak La Niña. Due to this guidance and the presence of La Niña-like atmospheric circulation anomalies in the tropics, the team predicts that La Niña is most likely to occur from October to December 2024 (with a 57% chance) and is expected to persist until January to March 2025. However, even if La Niña develops, it is expected to be relatively weak (in terms of SST anomalies) and short-lived, with all models predicting a return to ENSO neutrality by March 2025.