EI Niño/La Niña Status
Updated on 19 October 2018
The tropical Pacific Ocean’s sea surface temperature (SST) anomalies were within the neutral range in September 2018 over the Nino3.4 region (Figure A). Even as the Nino3.4 region has been gradually warming in the past months, the SST anomaly pattern in September over the surrounding tropical Pacific Ocean has yet to resemble typical El Niño development. Likewise, most of the atmospheric indicators of El Niño/La Niña indicated neutral conditions. The 1-month Nino3.4 value for September 2018 was -0.3 and the 3-month (July to September 2018) average Nino3.4 was 0.2 (Figure B).
Models indicate that the tropical Pacific Ocean SST anomalies will continue to warm in the coming months (Figure C) and predict a range of possible El Niño intensities from weak to moderate. However, a weak El Niño developing from October-December 2018 season onwards is assessed to be more likely (Figure D).
Impact of El Niño/La Niña on Singapore
Singapore would normally experience drier and warmer conditions during El Niño events, especially during the period between June and October (Figure E and Figure F). The impact is more significantly felt during strong or moderate intensity events. The opposite, i.e. wetter conditions over Singapore, usually occurs during La Niña events. Outside the June to October 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 June to October period 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) USA, the Bureau of Meteorology (BoM) Australia, as well as 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 September 2018 with respect to 1977-2016 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) show warm anomalies in September 2018. The western Indian Ocean, WTIO (solid black box, 50°E-70°E and 10°S-10°N) was warmer compared to the south-eastern Indian Ocean, SETIO (dotted black box, 90°E-110°E and 10°S-0°N). The Indian Ocean Dipole Mode index (WTIO minus SETIO) although positive, was still bordering on neutral values. Data source: ERSSTv5 from NOAA.
Figure B: The Nino3.4 index using the three-month running means of SST anomalies (against 1977-2016 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: ERSSTv5 from NOAA.
Figure C: Forecasts of Nino3.4 index’s strength for the second half of 2018 and first half of 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 cross the neutral value from the September-November season onwards, with a possibility of a weak El Niño developing thereon (image credit: IRI-CPC).
Figure D: Probability of El Niño (red), La Niña (blue) and neutral conditions (grey) for the second half 2018 and first half of 2019. Models significantly favour El Niño conditions over neutral and La Niña from the October-December 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.