Updated on 26 February 2017

In January 2017, the cool anomalies of the equatorial Pacific Ocean’s sea-surface temperature (SST) further weakened in January 2017 (Figure A). The Niño3.4 index for January 2016 was -0.27  (Figure B) and the latest 3-month average (Nov-Jan) weakened from -0.46 to -0.33. Most of the atmospheric conditions over the equatorial Pacific were in near average conditions by mid-February, except for central and western tropical Pacific’s cloudiness and rainfall which remained indicative of a weak La Niña condition. The Niño3.4 index above is based on non-detrended SST data that includes a long-term trend over the ocean due to on-going global warming. This partly reduces the magnitude of the SST cold anomalies during a La Niña episode (while increasing the anomalies during an El Niño episode).  In that respect, the presence of large scale wetter-than-normal rainfall conditions for the region in January 2016 (Figure C) are consistent with rainfall anomaly patterns during La Niña events.

Most models indicate the tropical Pacific will continue warming gradually from this month onwards. Given the observations and outlook, an extension of La Niña is the least likely scenario until mid-2017 (Figure D). A few of the models suggest that the warming may reach weak El Niño levels again in 3Q 2017 (Figure E). However, it is worth noting that most models tend to overestimate the warm anomalies in the wake of negative anomalies and furthermore model outlooks made at this time are not as accurate as those made during the second half of the year. More confident assessment of El Niño risk will be available from May onwards.

Impact of El Niño/La Niña on Southeast Asia

Typically the impact from La Niña for Southeast Asia is wetter-than-normal rainfall conditions, except over much of Borneo and southern Sumatra during November to January (Figure F). 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 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: Warm shades show regions of relative warming, while cool shades show regions of relative cooling with respect to 1971-2000 climatology for January. On average, the tropical Pacific Ocean Niño3.4 region (red box, 120°W-170°W and 5°S-5°N) was still slightly cooler than normal but started to weaken to neutral from November 2016 (image credit: IRI Map Room).

 

    Figure B: Monthly sea-surface temperature (SST) anomaly over the Niño3.4 region (120°W-170°W and 5°S-5°N) of the tropical Pacific Ocean from Feb 2016 to Jan 2017 (image credit: IRI Map Room). Both SST and atmospheric responses over the tropical Pacific Ocean indicate either weakening La Niña or neutral conditions.

 

    Figure C: Spatial rainfall anomaly patterns in the region for January 2017 showing large scale, wetter-than-normal conditions for the region (image credit: IRI Map Room). Brown (green) shades show drier (wetter) than the average climatological rainfall for October (1970 – 2009). Quantitative anomaly values are only indicative due to limitations in the data source.

 

    Figure D: Probability of El Niño (red), La Niña (blue) and neutral conditions (green) for 2017. Neutral conditions are favoured up to mid-2017 and following that some chance of El Niño developing. However, it is worth noting that most models tend to overestimate the warm anomalies in the wake of negative anomalies and furthermore model outlooks made at this time of the year are not as accurate as those made during the second half of the year. (image credit: IRI-CPC).

 

    Figure E: Forecasts of Niño3.4 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 cool anomalies to weaken further and emergence of notable warmer conditions from mid-2017 (image credit: IRI-CPC).

 

    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 was generated using limited number 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, 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. For more information on El Niño/La Niña, please refer to the FAQs website.