EI Niño/La Niña Status
Updated on 12 July 2019
The El Niño Southern Oscillation (ENSO) monitoring system has been downgraded to “Neutral” status this month. While warmer sea-surface temperature (SSTs) remain over the Nino3.4 Region, they have cooled over the past three months and remained in the neutral range. Atmospheric indicators of ENSO, generally, have also remained neutral. The Nino3.4 index was 0.50°C for May 2019 and 0.59°C for March-May 2019 three-month average.
Overall, signs of future El Niño development continue to weaken. Model outlooks from international centres are generally predicting Nino3.4 SST anomalies to continue cooling until around September 2019. Beyond September, however, the model outlooks are uncertain showing a wide spread of possible outcomes ranging from weak La Niña to moderate El Niño. Another key driver that will influence rainfall over Singapore and the nearby region in the next few months is the Indian Ocean sea-surface temperature anomalies. Currently an Indian Ocean-related index monitored, called the Indian Ocean Dipole (IOD), is showing positive values and this is expected to persist in the 3rd quarter of 2019. A positive IOD can lead to below-normal rainfall conditions over Singapore and the nearby region.
Further Information on ENSO
ENSO conditions are monitored by analysing Pacific sea surface temperatures (SSTs), low level winds, cloudiness (using outgoing longwave radiation), and sub-surface temperatures. Special attention is given to SSTs, as they are one of the key indicators used to monitor ENSO. Here, three different datasets are used: HadISST, ERSSTv5, and COBE datasets. As globally, SSTs have gradually warmed over the last century under the influence of climate change, the SST values over the Nino3.4 will increasingly be magnified with time, and hence appear warmer than they should be. Therefore, this background trend is removed from the SST datasets (Turkington, Timbal, & Rahmat, 2018), before calculating SST anomalies using the climatology period 1976-2014. So far, there has been no noticeable background trend in the low-level winds or cloudiness.
El Niño (La Niña) conditions are associated with warmer (colder) SSTs in the central and eastern Pacific. The threshold for an El Niño (La Niña) in the Nino3.4 region is above 0.65°C (below -0.65°C). El Niño (La Niña) conditions also correspond to an increase (decrease) in cloudiness around or to the east of the international dateline (180°), with a decrease (increase) in cloudiness in the west. There is also a decrease (increase) in the trade winds in the eastern Pacific. Sub-surface temperatures in the eastern Pacific should also be warmer (colder) than average, to sustain the El Niño (La Niña) conditions.
For ENSO outlooks, information from the World Meteorological Organization (WMO) and 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 other centres around the world. Each centre uses different criteria, including different SST thresholds. Therefore, variations between centres on the current ENSO state should be expected, especially when conditions are borderline.
The SSTs over the central and eastern Pacific were still warmer than average in May 2019 (Figure 1). The warmer temperatures over the Nino3.4 Region (black box) however are within neutral and have weakened over the past few months. However, the western Indian Ocean SST is warmer than average, while the eastern Indian Ocean SST is colder than average. The difference in SST anomalies between the western and eastern Indian Ocean is monitored through the Indian Ocean Dipole (IOD) index. When the IOD is positive, as is the case now, below-normal rainfall conditions are favoured over the southern and eastern Maritime Continent.
Figure 1: Detrended SST anomalies for May 2019 with respect to 1976-2014 climatology using ERSST v5 data. Warm shades show regions of relative warming, while cool shades show regions of relative cooling. The tropical Pacific Ocean Nino3.4 Region is outlined in black.
Looking at the Nino3.4 index over the past year (Figure 2), after the 2017-2018 La Niña, the 1-month Nino3.4 value continued warming. It reached its peak in November 2018 after having crossed the El Niño threshold (0.65°C) for 2 months (October and November 2018). Subsequently, the Nino3.4 value fluctuated around the threshold until April 2019, when it started to weaken. For El Niño conditions to be present, 1-month warm SST anomalies (observed or forecast) should persist for at least four months above the threshold, with at least one of the months observed along supporting atmospheric observations.
Figure 2: The Nino3.4 index using the 1-month SST anomalies. Warm anomalies (≥ +0.65; brown) correspond to El Niño conditions while cold anomalies (≤ -0.65; blue) correspond to La Niña conditions; otherwise neutral (> -0.65 and < +0.65; grey).
Model outlooks from Copernicus C3S (Figure 3) until July 2019 are still going for Nino3.4 SST to be just above the El Niño threshold on average. Beyond July, however, the outlooks show a wide spread of possible outcomes ranging from weak La Niña to moderate El Niño.
Figure 3: Forecasts of Nino3.4 index’s strength for 2019 from various seasonal prediction models of international climate centres (image credit: Copernicus C3S).
Historical ENSO Variability
To classify a historical El Niño event, the 3-month average Nino3.4 value must be above 0.65°C for 5 or more consecutive months. For La Niña events, the threshold is -0.65°C. Otherwise it is considered neutral. ENSO events with a peak value above 1.5°C (El Niño) or below -1.5°C (La Niña) are considered strong. Otherwise, the events are considered weak to moderate in strength. The following figure (Figure 4) shows the development of the Nino3.4 index in 2015-18 in comparison to other El Niño/La Niña events.
Figure 4: Three-month Nino3.4 index development and retreat of different El Niño (left)/La Niña (right) events since the 1960s. The most recent El Niño and La Niña events are in red and purple, respectively.
Impact of El Niño/La Niña on Singapore
During June to October, there is a strong correlation of ENSO on rainfall (statistically significant at 99% level). With negative correlation, warmer Nino3.4 leads to less rainfall and vice versa (Figure 5). During this period, Singapore would normally experience drier and warmer conditions during El Niño events. The opposite, i.e. wetter conditions over Singapore, usually occurs during La Niña events.
No two El Niño events or two La Niña events are alike in terms of their impacts on Singapore’s rainfall and temperature. Furthermore, the strength of events and the corresponding impact do not always scale. Within each of the three ENSO categories: El Niño, Neutral, and La Niña (Figure 6), there is large variability in the amount of rainfall.
Figure 5: Correlation between total seasonal rainfall (averaged over 5 Singapore stations) and seasonal Nino3.4 index from 1961-2017 centred on the month indicated (e.g. for June’s value it corresponds to season May-June-July). The statistically significant correlations at 95% level are underlined, at 99% level in red.
Figure 6: Singapore rainfall anomalies for June to October (as a percentage of departure from long-term rainfall average) arranged in the order from strong La Niña (left) to strong El Niño (right). Brown bars denote El Niño years’ anomalies, blue bars denote La Niña years’ anomalies, and grey bars denote ENSO neutral years’ anomalies.
References
Turkington, T., Timbal, B., & Rahmat, R. (2018). The impact of global warming on sea surface temperature based El Nino Southern Oscillation monitoring indices. International Journal of Climatology, 39(2).