Sea-surface temperatures (SSTs) over the equatorial Pacific Ocean is warmer than average (Figure A) and the Nino3.4 index was 0.64 for May 2017. The 3-month average (March to May) Nino3.4 value was 0.41 (Figure B), which is still within neutral threshold. Atmospheric conditions, such as trade winds and cloudiness, over the equatorial Pacific are also neutral. For regional SSTs, gradual cooling of the SSTs in the eastern Indian Ocean, off the coasts of Sumatra and Java islands, is observed (Figure A). This development which is associated to a phenomenon known as the positive Indian Ocean Dipole (IOD), is known to lead to drier conditions in the region. Like the El Niño event, its impact on rainfall is dependent on its strength. In the latest outlook, only half of the models assessed predict continued development of significant positive IOD in the coming months.
For the tropical Pacific, most models indicate that it will continue to warm gradually in the second half of 2017 (Figure C). For June-August 2017 season, latest experts’ consensus favours neutral conditions over El Niño (Figure D).
Impact of El Niño/La Niña on Southeast Asia
Typically the impact from El Niño for Southeast Asia is drier-than-normal rainfall conditions, especially during the Southwest Monsoon period (June – September), including October (Figure E). During La Niña events the opposite, i.e. wetter-than-normal conditions, normally occurs. 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 induced more significant changes in rainfall than the stronger events.
- Figure A: Sea-surface temperature (SST) anomalies for May 2017 with respect to 1981-2010 climatology. Warm shades show regions of relative warming, while cool shades show regions of relative cooling. On average, the tropical Pacific Ocean Nino3.4 region (solid red box, 120°W-170°W and 5°S-5°N) was warmer than normal in May. Closer to the region, the western Indian Ocean, WTIO (solid black box, 50°E-70°E and 10°S-10°N) is warmer relative to the south-eastern Indian Ocean, SETIO (dotted black box, 90°E-110°E and 10°S-0°N), which makes the Indian Ocean Dipole Mode index (WTIO minus SETIO) positive. Data source: ERSSTv4 from NOAA.
- Figure B: The Nino3.4 index using three-month running mean of SST anomalies (against 1981-2010 base period) in the Nino3.4 region bounded by 5°N to 5°S and 170°W to 120°W. Warm anomalies (red line) correspond to El Niño conditions while cold anomalies (blue line) correspond to La Niña conditions; otherwise neutral (grey line). 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: ERSSTv4 from NOAA.
- Figure C: Forecasts of Nino3.4 index’s strength for 2017 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 warming to continue and for El Niño conditions to emerge by the season JJA 2017. However, based on conditions in other parts of the tropical Pacific, experts assess that El Niño conditions is less favoured (see Figure D) than neutral conditions (image credit: IRI-CPC).
- Figure D: Probability of El Niño (red), La Niña (blue) and neutral conditions (green) for 2017. Neutral conditions are favoured over El Niño for JJA 2017 with decreasing chance of El Niño developing for the rest of 2017 (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 limited number El Niño/La Niña occurrences between 1979 and 2016 and therefore should be interpreted with caution (data: NOAA CPC CAMS_OPI).
El Niño/La Niña
For El Niño/La Niña updates, ASMC assesses information provided by the World Meteorological Organization (WMO) and various international climate centres, such as the Climate Prediction Center (CPC) US, the Bureau of Meteorology (BoM) Australia, as well information from the International Research Institute for Climate and Society (IRI) which contains model outputs from various other centres around the world.
Frequently Asked Question
What is El Niño/La Niña and how do they affect weather in South East Asia?
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.