EI Niño/La Niña Status
Updated on 28 Mar 2017
Observations: In February 2017, the cool anomalies of the equatorial Pacific Ocean’s sea-surface temperature (SST) further weakened towards neutral values (Figure A). The Nino3.4 index for February 2017 was -0.2 (Figure B) and the latest 3-month average (Dec-Feb) was -0.5. Most of the atmospheric indicators over the equatorial Pacific are at neutral values since mid-February.
Outlook: Most models indicate the tropical Pacific will continue warming gradually for the next 6 months. Given the observations and outlook, there is an increasing chance for El Niño development (Figure D) in the second half of the year, with a majority of the models indicating weak El Niño strength (Figure E). However, model outlooks made at this time of the year are not as accurate as those made during the second half of the year and furthermore their Nino3.4 anomaly predictions may contain a tropical warming component leading to an overestimation of the effective Nino3.4 values. More confident assessment of El Niño risk will be available from May onwards.
Impact of El Niño/La Niña on Singapore
Singapore would normally experience drier and warmer conditions during El Niño events, especially during the Southwest Monsoon period (June – September), including October (Figure F). The opposite, i.e. wetter conditions over Singapore, usually occurs during La Niña events. Outside this season, the impact of El Niño/La Niña is less significant for Singapore. For example during the Northeast Monsoon season (December to early March), the impact on rainfall from El Niño/La Niña is less pronounced (Figure G).
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).
 From February 2017 onwards, MSS has adopted a new dataset for calculation of the Nino3.4 index which is the latest ERSST v4 (Extended Reconstructed Sea Surface Temperature) version 4 product from US NOAA NCDC.
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.
Figure A: Sea-surface temperature (SST) anomalies for February 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 (red box, 120°W-170°W and 5°S-5°N) was still slightly cooler than normal but is weakening to neutral (image credit: IRI Map Room).
Figure B: The Nino3.4 index based on raw data, 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 an El Niño event (above 0.65°C) while cold anomalies (blue line) correspond to a La Niña event (below -0.65°C); 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: Spatial rainfall anomaly patterns in the region for February 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 February (1970 – 2009). Quantitative anomaly values are only indicative due to limitations in the data source (data: NOAA CPC CAMS_OPI).
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 increasing chance of El Niño developing. However, model outlooks made at this time of the year are not as accurate as those made during the second half of the year and furthermore their Nino3.4 anomaly predictions may contain a tropical warming component leading to an overestimation of the effective Nino3.4 values (image credit: IRI-CPC).
Figure E: 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 from mid-2017. However, model outlooks made at this time of the year are not as accurate as those made during the second half of the year and furthermore their Nino3.4 anomaly predictions may contain a tropical warming component leading to an overestimation of the effective Nino3.4 values (image credit: IRI-CPC).
Figure F: Correlation between total monthly rainfall (averaged over 28 Singapore stations) and Nino3.4 index from 1980-2013. It shows statistically significant (red) negative correlations between local rainfall and Nino3.4 in July, September and October, which suggest that warmer temperatures in the Nino3.4 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 Nino3.4 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 Nino3.4 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.