The El Niño Southern Oscillation (ENSO) monitoring system state is in “La Niña Watch”. The Nino3.4 index was approaching the La Niña threshold in October, and cool subsurface temperature anomalies in the central and eastern Pacific indicated La Niña-like conditions. However, westerly wind anomalies in the eastern tropical Pacific region during early November likely slowed the cooling process of the sea surface temperatures. The Nino3.4 index was -0.58°C for October 2024 and -0.52°C for the August – October 2024 three-month average.
Most models predict La Niña conditions are likely from December 2024 to March 2025. However, some models predict sea surface temperatures to exceed the La Niña threshold for only two to three months, not long enough to declare a La Niña event. If La Niña conditions do not fully develop, ENSO neutral conditions are expected.
Short note on the Indian Ocean Dipole: While there were signs of a negative Indian Ocean Dipole (IOD) in October, these conditions are predicted to be short lived, with the IOD index predicted to return to neutral by December 2024.
Further Information on ENSO
ENSO conditions are monitored by analysing Pacific sea surface temperatures (SSTs), low level winds, cloudiness (using outgoing longwave radiation), and sub-surface temperatures. Special attention is given to SSTs, as they are one of the key indicators used to monitor ENSO. Here, three different datasets are used: HadISST, ERSSTv5, and COBE datasets. As globally, SSTs have gradually warmed over the last century under the influence of climate change, the SST values over the Nino3.4 will increasingly be magnified with time, and hence appear warmer than they should be. Therefore, this background trend is removed from the SST datasets (Turkington, Timbal, & Rahmat, 2018), before calculating SST anomalies using the climatology period 1976-2014. So far, there has been no noticeable background trend in the low-level winds or cloudiness.
El Niño (La Niña) conditions are associated with warmer (colder) SSTs in the central and eastern Pacific. The threshold for an El Niño (La Niña) in the Nino3.4 region is above 0.65°C (below -0.65°C). El Niño (La Niña) conditions also correspond to an increase (decrease) in cloudiness around or to the east of the international dateline (180°), with a decrease (increase) in cloudiness in the west. There is also a decrease (increase) in the trade winds in the eastern Pacific. Sub-surface temperatures in the eastern Pacific should also be warmer (colder) than average, to sustain the El Niño (La Niña) conditions.
For ENSO outlooks, information from the World Meteorological Organization (WMO) and international climate centres are assessed. The centres include the Climate Prediction Center (CPC) USA, the Bureau of Meteorology (BoM) Australia, as well as information from the International Research Institute for Climate and Society (IRI) which consolidates model outputs from other centres around the world. Each centre uses different criteria, including different SST thresholds. Therefore, variations between centres on the current ENSO state should be expected, especially when conditions are borderline.
The sea surface temperatures (SSTs) over the tropical Pacific in October 2024 were below average in the central and eastern regions and near to above average elsewhere (Figure 1). The below average anomalies in the eastern Pacific showed a similar pattern to those in September 2024, still not cool enough to indicate La Niña conditions. Overall, for the Nino3.4 region (red box), SSTs were negative, but within the neutral range. Across the Indian Ocean, tropical SSTs were generally warm. The western portion was near-to above-average (solid black box), with warmer than average temperatures in the eastern portion (dashed black box), showing signs of a negative Indian Ocean Dipole pattern (cooler SSTs in the west compared to the east). However, typically in a negative Indian Ocean Dipole, the SSTs are below- to near-average in the western portion.
Figure 1: Detrended SST anomalies for October 2024 with respect to 1976-2014 climatology using ERSST v5 data. Red (blue) shades show regions of relative warming (cooling). The tropical Pacific Ocean Nino3.4 Region is outlined in red. The Indian Ocean Dipole index is the difference between average SST anomalies over the western Indian Ocean (black solid box) and the eastern Indian Ocean (black dotted box).
Looking at the Nino3.4 index in Figure 2, in May and June 2023, the index was within the ENSO-neutral range, but was warming. In July 2023, the Nino3.4 exceeded the El Niño threshold, peaking around November 2023. After this time, there was a gradual cooling, with the Nino3.4 index within the neutral range by May 2024. Since May, the Nino3.4 index has been within the neutral range, with a gradual cooling of the index and approaching the La Niña threshold in September and October. For La Niña conditions to be present, 1-month cold SST anomalies (observed or forecast) should persist for at least four months below the threshold, with at least one of the months observed along supporting atmospheric observations.
Figure 2: The Nino3.4 index using the 1-month SST anomalies. Warm anomalies (≥ +0.65; brown) correspond to El Niño conditions while cold anomalies (≤ -0.65; blue) correspond to La Niña conditions; otherwise neutral (> -0.65 and < +0.65; grey).
Model outlooks from Copernicus C3S (Figure 3), based on the Nino3.4 SST index, show that most models predict La Niña conditions between December 2024 and February 2025. However, the models still predict a small chance of ENSO neutral conditions to persist, or the La Niña-like conditions to be short-lived (i.e. not last five consecutive months to declare a La Niña event). Most models are also predicting a weak- to moderate strength event. For March and April 2025, models predict either ENSO neutral conditions or La Niña conditions to persist.
Figure 3: Forecasts of Nino3.4 index’s strength until April 2025 from various seasonal prediction models from international climate centres (grey lines). The blue and yellow lines note the La Niña and El Niño thresholds, respectively, used by various centres.
Historical ENSO Variability
To classify a historical El Niño event, the 3-month average Nino3.4 value must be above 0.65°C for 5 or more consecutive months. For La Niña events, the threshold is -0.65°C. Otherwise it is considered neutral. ENSO events with a peak value above 1.5°C (El Niño) or below -1.5°C (La Niña) are considered strong. Otherwise, the events are considered weak to moderate in strength. The following figure (Figure 4) shows the development of the Nino3.4 index for the most recent El Niño and La Niña events in comparison to other El Niño/La Niña events.
Figure 4: Three-month Nino3.4 index development and retreat of different El Niño (left)/La Niña (right) events since the 1960s. Recent El Niño and La Niña events are in red and purple, respectively.
Impact of El Niño/La Niña on Southeast Asia
The typical impact of La Niña on Southeast Asia is wetter-than-average rainfall conditions, including for much of the Maritime Continent during December to February (Figure 5, right). More moderate temperature conditions typically follow wetter periods. The opposite conditions for rainfall (and consequently temperature) are observed during El Niño years (Figure 5, left).
The impact on the region’s rainfall and temperature from ENSO events is more significantly felt during strong or moderate-intensity events. Also, no two El Niño events or two La Niña events are exactly alike in terms of their impact on the region.
Figure 5: December to February (DJF) season rainfall anomaly composites (mm/day) for El Niño (left) and La Niña (right) years. Brown (green) shades show regions of drier (wetter) conditions. Note that this anomaly composite was generated using a limited number of El Niño and La Niña occurrences between 1979 and 2017 and therefore should be interpreted with caution (data: NOAA CPC CAMS_OPI).
References
Turkington, T., Timbal, B., & Rahmat, R. (2018). The impact of global warming on sea surface temperature based El Nino Southern Oscillation monitoring indices. International Journal of Climatology, 39(2).
El Niño/La Niña
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For El Niño/La Niña updates, information provided by the World Meteorological Organization (WMO) and various international climate centres are assessed. The centres include the Climate Prediction Center (CPC) USA, the Bureau of Meteorology (BoM) Australia, as well as information from the International Research Institute for Climate and Society (IRI) which consolidates model outputs from various other centres around the world.
Frequently Asked Questions
What is El Niño/La Niña and how do they affect weather in South East Asia?
The El Niño phenomenon is a non-regular occurrence in the tropical pacific region where warmer waters develop over the Eastern Tropical Pacific Ocean along the coast of South America. In South East Asia, this brings drier weather and increases the risk of forest fires and smoke haze. The La Niña phenomenon is the reverse of the El Niño where cooler waters develop over the Eastern Tropical Pacific Ocean along the coast of South America.
In South East Asia, higher than normal rainfall tends to occur during a La Niña episode which may result in an increased occurrence of floods.
Cross section of El Niño, neutral and La Niña conditions across the Tropical Pacific Ocean. (Image credit: NOAA)
The correlation between El Niño/La Niña and its associated weather impacts on South East Asia differ from one place to another and for different seasons.
Distribution of precipitation anomalies over South East Asia (Jun – Oct) for El Niño (left) and La Niña (right) years. (Image credit: IRI Climate)
The image above shows the precipitation anomalies averaged over the El Niño and La Niña years. For instance, the impact of El Niño is typically stronger over the southern and eastern part of South East Asia during the months of Jun – Oct.