Updated on 23 October 2017

For September 2017, sea-surface temperature (SST) anomalies over the equatorial Pacific Ocean show a La Niña pattern (Figure A) with a 1-month Nino3.4 of -0.67. The 3-month average (Jul to Sep) Nino3.4 value however is at -0.39 which is still within neutral values of between ‑0.5 and 0.5 (Figure B). Atmospheric conditions, such as trade winds and cloudiness, over the equatorial Pacific developed patterns that are also currently suggestive of weak La Niña conditions, although the trend towards a La Niña eased slightly during the start of October.

For the tropical Pacific, models indicate that the SSTs anomalies will continue to be negative for the remaining part of 2017 (Figure C). For this period, experts’ consensus favours weak La Niña conditions to continue. On-going La Niña conditions are expected to be short-lived and to return to neutral conditions by first quarter of 2018 (Figure D).

Impact of El Niño/La Niña on Southeast Asia

Typically the impact from La Niña for Southeast Asia is wetter-than-normal rainfall conditions, especially during the Southwest Monsoon period (June – September), including October (Figure E) and especially over the Maritime Continent. During El Niño events the opposite, i.e. drier-than-normal conditions, normally occurs. Locally-specific impact differs from place to place and for different seasons.

No two La Niña events or two El Niño 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.

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    Figure A: Sea-surface temperature (SST) anomalies for September 2017 with respect to 1981-2010 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) was negative in September 2017 and has reached La Niña values (1-month average SST). The western Indian Ocean, WTIO (solid black box, 50°E-70°E and 10°S-10°N) was warmer relative to the south-eastern Indian Ocean, SETIO (dotted black box, 90°E-110°E and 10°S-0°N), which made the Indian Ocean Dipole Mode index (WTIO minus SETIO) slightly positive but still within neutral levels. Data source: ERSSTv4 from NOAA.

 

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    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.

 

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    Figure C: Forecasts of Nino3.4 index’s strength for the rest of 2017 and first half of 2018 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 remain near the La Niña threshold for the rest of 2017 on average and return to neutral by the first quarter of 2018. (image credit: IRI-CPC).

 

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    Figure D: Probability of El Niño (red), La Niña (blue) and neutral conditions (green) for the rest of 2017 and first half of 2018. La Niña (weak) conditions are favoured over neutral and El Niño for the rest of 2017, while a return to neutral conditions are expected by the first quarter of 2018 (image credit: IRI-CPC).

 

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    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.