EI Niño/La Niña Status
Updated on 29 March 2018
The sea surface temperature (SST) cold anomalies in the tropical Pacific Ocean remained within weak La Niña conditions in February 2018 (Figure A). Atmospheric indicators of El Niño/La Niña are either weakening or no longer showing patterns indicative of La Niña conditions. The 1-month Nino3.4 value for February 2018 and the 3-month average (December 2017 to February 2018) Nino3.4 were both at -1.0ºC (Figure B). Partial data available for March 2018 shows weekly SST anomalies plateauing.
Models indicate that the tropical Pacific Ocean anomalies will weaken from April 2018 onwards (Figure C). There is around 60% chance for La Niña conditions to last up to the February-April 2018 season and then return to neutral conditions in the March-May 2018 season (Figure D).
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 (Figure E and Figure F). 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. For example during the Northeast Monsoon season (December to early March), the impact on rainfall from El Niño/La Niña is less pronounced (Figure E and Figure F).
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 had more impact on rainfall during the Southwest Monsoon season than the stronger events, but this could be contributed by other factors as well (Figure G).
For El Niño/La Niña updates, NEA assesses the information provided by the World Meteorological Organization (WMO) and various international climate centres. The centres include 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 consolidates model outputs from various other centres around the world.
Figure A: Sea-surface temperature (SST) anomalies for February 2018 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 cooler than average in February 2018. The western Indian Ocean, WTIO (solid black box, 50°E-70°E and 10°S-10°N) was slightly warmer compared to the south-eastern Indian Ocean, SETIO (dotted black box, 90°E-110°E and 10°S-0°N). Therefore, the Indian Ocean Dipole Mode index (WTIO minus SETIO) was positive but still within neutral values. Data source: ERSSTv4 from NOAA.
Figure B: The Nino3.4 index using the three-month running means 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 (≥ +0.5) correspond to El Niño conditions while cold anomalies (≤ -0.5) correspond to La Niña conditions; otherwise neutral (> -0.5 and < +0.5). 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 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 negative anomaly to weaken and return to neutral conditions by the 2nd quarter of 2018 (image credit: IRI-CPC).
Figure D: Probability of El Niño (red), La Niña (blue) and neutral conditions (grey) for 2018. Neutral conditions are predicted to be more likely from March-May 2018 season onwards (image credit: IRI-CPC).
Figure E: 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 F: 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 G: Singapore rainfall anomalies for June-September (as a 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.