EI Niño/La Niña Status
Updated on 25 May 2018
The sea surface temperature (SST) in the tropical Pacific Ocean were within neutral values in April 2018 over the Nino3.4 region (Figure A). Most of the atmospheric indicators of El Niño/La Niña were also indicating neutral conditions. The 1-month Nino3.4 value for April 2018 was -0.3 and the 3-month average (February to April 2018) Nino3.4 was -0.5 (Figure B). Partial data in May 2018 showed further weakening of the SST cold anomalies towards climatological values.
Models indicate that the tropical Pacific Ocean SST anomalies will continue to warm (Figure C) with more than 60% chance of neutral conditions for the May-July season in progress and up to the Jun-August season (Figure D). Some models indicate the possibility of a weak and late 2018 El Niño developing by year’s end but others do not. At present, the probability of an El Niño event occurring late in the year is hardly larger than the system remaining in a neutral state.
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 E and 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 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, information provided by the World Meteorological Organization (WMO) and various international climate centres are assessed. 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 April 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 still colder than average in April 2018 but the cold anomalies are weakening. 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). While the Indian Ocean Dipole Mode index (WTIO minus SETIO) was positive it is 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 and early 2019 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 be within neutral values in the May-July season in progress and with a range of possible outcomes by year’s end including weak, late El Niño developing (image credit: IRI-CPC).
Figure D: Probability of El Niño (red), La Niña (blue) and neutral conditions (grey) for 2018. Neutral conditions are likely for the May-July 2018 season in progress until late in 2018 when El Niño conditions are slightly more favoured (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.