The tropical Pacific Ocean’s sea surface temperature (SST) anomalies were within the neutral range in September 2018 over the Nino3.4 region (Figure A). Even as the Nino3.4 region has been gradually warming in the past months, the SST anomaly pattern in September over the surrounding tropical Pacific Ocean has yet to resemble typical El Niño development. Likewise, most of the atmospheric indicators of El Niño/La Niña indicated neutral conditions. The 1-month Nino3.4 value for September 2018 was -0.3 and the 3-month (July to September 2018) average Nino3.4 was 0.2 (Figure B).
Models indicate that the tropical Pacific Ocean SST anomalies will continue to warm in the coming months (Figure C) and predict a range of possible El Niño intensities from weak to moderate. However, a weak El Niño developing from October-December 2018 season onwards is assessed to be more likely (Figure D).
Impact of El Niño/La Niña on Southeast Asia
Typically the impact of El Niño on Southeast Asia is drier-than-normal rainfall conditions, especially during the period June and October (Figure E), and over the Maritime Continent. Warmer temperature conditions typically follow drier periods. The impact is more significantly felt during strong or moderate intensity events. During La Niña events the opposite, i.e. wetter-than-normal conditions, normally occurs. For November – January season, the impact of El Niño/La Niña is less coherent for some parts of the region, including mainland Southeast Asia, Borneo, southern Sumatra, and the Malay Peninsula (Figure F). Locally-specific impact differs from place to place and for different seasons.
No two El Niño events or two La Niña events are alike in terms of their impact on the region’s rainfall and temperature. Furthermore, the strength of events and the corresponding impact do not always scale. For example, there were years where relatively weaker El Niño/La Niña events had more impact on rainfall during the June to October period than the stronger events, but this could be contributed by other factors as well.
- Figure A: Sea-surface temperature (SST) anomalies for September 2018 with respect to 1977-2016 climatology. Warm shades show regions of relative warming, while cool shades show regions of relative cooling. The tropical Pacific Ocean Nino3.4 Region (solid red box, 120°W-170°W and 5°S-5°N) show warm anomalies in September 2018. The western Indian Ocean, WTIO (solid black box, 50°E-70°E and 10°S-10°N) was warmer compared to the south-eastern Indian Ocean, SETIO (dotted black box, 90°E-110°E and 10°S-0°N). The Indian Ocean Dipole Mode index (WTIO minus SETIO) although positive, was still bordering on neutral values. Data source: ERSSTv5 from NOAA.
- Figure B: The Nino3.4 index using the three-month running means of SST anomalies (against 1977-2016 base period) in the Nino3.4 region bounded by 5°N to 5°S and 170°W to 120°W. Warm anomalies (≥ +0.5) correspond to El Niño conditions while cold anomalies (≤ -0.5) correspond to La Niña conditions; otherwise neutral (> -0.5 and < +0.5). The horizontal axis is labelled with the first letters of the 3-month seasons, e.g. JFM refers to January, February, and March seasonal average. Data source: ERSSTv5 from NOAA.
- Figure C: Forecasts of Nino3.4 index’s strength for the second half of 2018 and first half of 2019 from various seasonal prediction models of international climate centres. Values above +0.5°C indicate El Niño conditions, below -0.5°C indicate La Niña conditions, and in between indicate neutral conditions, i.e. neither El Niño nor La Niña. Models predict the Nino3.4 index to cross the neutral value from the September-November season onwards, with a possibility of a weak El Niño developing thereon (image credit: IRI-CPC).
- Figure D: Probability of El Niño (red), La Niña (blue) and neutral conditions (grey) for the second half 2018 and first half of 2019. Models significantly favour El Niño conditions over neutral and La Niña from the October-December 2018 season onwards (image credit: IRI-CPC).
- Figure E: June to October rainfall anomaly composite for El Niño years minus La Niña years. Brown shades show regions where El Niño induce drier conditions and La Niña induce wetter conditions, while regions in green shades show the opposite effect, i.e. El Niño inducing wetter conditions and La Niña inducing drier conditions (image credit: IRI Data Library). Note that this anomaly composite was generated using a limited number of El Niño/La Niña occurrences between 1979 and 2016 and therefore should be interpreted with caution (data: NOAA CPC CAMS_OPI).
- Figure F: November to January rainfall anomaly composite for La Niña years minus El Niño years. Green shades show regions where La Niña induce wetter conditions and El Niño induce drier conditions, while regions in brown shades show the opposite effect, i.e. La Niña inducing drier conditions and El Niño inducing wetter conditions (image credit: IRI Data Library). In general, La Niña events tend to induce wetter conditions for many parts of the region, except for Borneo and Southern Sumatra. Note that this anomaly composite has been generated using a limited number of El Niño/La Niña occurrences between 1979 and 2016 and therefore should be interpreted with caution.
El Niño/La Niña
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.
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.
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.