EI Niño/La Niña Status
Updated on 24 January 2017
In November 2016, the cool anomalies of the equatorial Pacific Ocean’s sea-surface temperature (SST) Niño3.4 region weakened further and the value remained neutral for that month (Figure A). Atmospheric variables, such as trade winds and cloudiness, over the equatorial Pacific showed either weakening La Niña or neutral conditions. The Niño3.4 index for December 2016 was -0.31 (Figure B). The latest 3-month average (October-December) is -0.46, which is within the neutral threshold. 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 December 2016 (Figure C) are consistent with rainfall anomaly patterns during La Niña events.
Climate models favour neutral conditions from the December-February (DJF) 2016-17 season onwards (Figure D) and remaining cool SST anomalies over the Niño3.4 region are predicted to fade by April 2017 (Figure E).
Impact of El Niño/La Niña on Singapore
Singapore would normally experience wetter and cooler conditions during La Niña events, especially during the Southwest Monsoon period (June – September), including October. The opposite, i.e. drier conditions over Singapore, usually occurs during El Niño events. Outside this season, the impact of El Niño/La Niña is less significant for Singapore (Figure F and Figure G). For example during the Northeast Monsoon season (December to early March), the impact on rainfall from El Niño/La Niña is less pronounced.
No two El Niño events or two La Niña events are alike in terms of their impact on Singapore’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 during the Southwest Monsoon season than the stronger events (Figure H).
For El Niño/La Niña updates, MSS 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.
Figure A: Warm shades show regions of relative warming, while cool shades show regions of relative cooling with respect to 1971-2000 climatology for December. 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 had weakened 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 Jan 2016 to Dec 2016 (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 December 2016 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 late 2016 and much of 2017. Neutral conditions are favoured from Dec-Feb 2016-2017 season onwards (image credit: IRI-CPC).
Figure E: Forecasts of Niño3.4 index’s strength for the late 2016 and much of 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 (image credit: IRI-CPC).
Figure F: Correlation between total monthly rainfall (averaged over 28 Singapore stations) and Niño3.4 index from 1980-2013. It shows statistically significant (red) negative correlations between local rainfall and Niño3.4 in July, September and October, which suggest that warmer temperatures in the Niño3.4 region lead to significantly less rainfall over Singapore and vice versa. In other months, where the correlations are weaker or insignificant, the relationship is not as established.
Figure G: Correlation between total seasonal rainfall (averaged over 28 Singapore stations) and seasonal Niño3.4 index (also known as Oceanic Niño Index, ONI) from 1980-2013. It shows statistically significant (red) negative correlations between local rainfall and the ONI during JAS and ASO, which suggest that warmer temperatures in the Niño3.4 region lead to significantly less rainfall over Singapore and vice versa during these seasons. In other seasons, where the correlations are weaker or insignificant, the relationship is not as established.
Figure H: Singapore rainfall anomalies for June-September (as percentage of departure from long-term rainfall average) arranged in the order from strong La Niña (left) to strong El Niño (right). Warm shades denote El Niño years, cool shades denote La Niña years (La Niña is the opposite of El Niño) and white denotes neutral years. WL, ML and SL refer to weak, moderate and strong La Niña respectively, while WE, ME and SE refer to weak, moderate and strong El Niño respectively.