Updated on 27 November 2017

Sea surface temperature (SST) over the tropical Pacific Ocean is currently cool (negative anomaly) with La Niña-like conditions (Figure A). Atmospheric indicators of El Niño/La Niña (e.g. trade winds and the Southern Oscillation Index, SOI) are broadly within neutral at the moment. The 1-month Nino3.4 value eased (warmed) a little to -0.37 compared to last month’s value. The 3-month average (Aug to Oct) Nino3.4 value is at -0.46 which also within neutral values of between -0.5 and 0.5 (Figure B). From partial data in November 2017, the SST and atmospheric variables are showing reinforcing patterns suggestive of weak La Niña conditions.

Models suggest the tropical Pacific Ocean will continue to cool (Figure C). There is chance of a weak La Niña forming by December 2017 of at least 60%, around double the normal likelihood for La Niña (Figure D). If it forms, the event is not expected to last beyond March 2018.

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. For November – January season, the impact of El Niño/La Niña is less coherent for some part of the region example over mainland Southeast Asia, Borneo, Southern Sumatra and 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 induced more significant changes in rainfall than the stronger events.

    Figure A: Sea-surface temperature (SST) anomalies for October 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 October 2017. 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.

 

    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 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 within weak La Niña threshold for up to the first quarter of 2018 and return to neutral conditions beyond that (image credit: IRI-CPC).

 

    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 up to the first quarter of 2018, while a return to neutral conditions are expected beyond the first quarter of 2018 (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).

 

    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 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.