EI Niño/La Niña Status
Updated on 13 December 2019
The El Niño Southern Oscillation (ENSO) monitoring system is in the “Neutral” state. Sea-surface temperatures (SSTs) over the Nino3.4 region are neutral. Atmospheric indicators of ENSO (e.g. OLR and wind anomalies) are in neutral state as well. The Nino3.4 index was 0.28°C for October 2019 and close to zero for the August-October 2019 three-month average. Model outlooks from international centres are predicting Nino3.4 SST anomalies to remain within neutral values into the first quarter of 2020.
Currently, the Indian Ocean Dipole (IOD) is positive. It reached its peak in October 2019 and has since been showing a weakening trend. It is likely to return to neutral state early next year. A positive IOD typically contributes to below-normal rainfall conditions over Singapore and the nearby region during the Southwest Monsoon season, as had happened in the last few months. Its influence typically decays from then onwards.
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 sea-surface temperatures (SSTs) over the central and eastern Pacific remained in the neutral range in October 2019 (Figure 1). Over the Indian Ocean, the SSTs in the west continue to be warmer than average, while the SSTs in the east are colder than average. The difference in SST anomalies between the western and eastern Indian Ocean is monitored through the Indian Ocean Dipole (IOD) index, which is currently positive. It reached its peak in October 2019 and has shown a weakening trend since. It is likely to return to neutral state early next year. A positive IOD typically contributes to below-normal rainfall conditions over Singapore and the nearby region during the Southwest Monsoon season, as had happened in the last few months. Its influence typically decays from then onwards.
Figure 1: Detrended SST anomalies for October 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 red. The Indian Ocean Dipole index is the difference between average SST anomalies over the western Indian Ocean (black solid box) and the eastern Indian Ocean (black dotted box).
Looking at the Nino3.4 index over the past year or so (Figure 2), after the 2017-2018 La Niña, the 1-month Nino3.4 value continued warming. It peaked 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 had weakened and remained within neutral values since. 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) indicate Nino3.4 SST to be within a wide spread of possible outcomes ranging from weak El Niño to weak La Niña until the end of the year and into the first quarter of 2020. The values however are expected to remain largely within neutral thresholds.
Figure 3: Forecasts of Nino3.4 index’s strength for the rest of 2019 and into the first quarter 2020 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 the Northeast Monsoon months from November to January, the correlation of ENSO with rainfall over Singapore is weak (Figure 5). Around this time of the year, Singapore’s rainfall is strongly influenced more by monsoon wind variability than El Niño or La Niña events.
As ENSO’s influence on Singapore’s rainfall during the Northeast Monsoon season is limited, there is a large variability in rainfall patterns across the three ENSO states during this period (Figure 6).
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 November to January (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).