EI Niño/La Niña Status
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 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 induced more significant changes in rainfall during the Southwest Monsoon season than the stronger events (Figure G).
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 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: 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 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.