EI Niño/La Niña Status
Updated on 8 September 2021
The El Niño Southern Oscillation (ENSO) monitoring system state is “Neutral”. The 1-month Nino3.4 sea surface temperatures cooled slightly in July but were within the neutral range. In August, there have been some signs of La Niña-like conditions developing in the ocean (surface and subsurface temperature cooling). Atmospheric indicators (cloudiness and wind anomalies) over the tropical Pacific Ocean remain generally consistent with neutral conditions. The Nino3.4 index was -0.47°C for July 2021 and -0.47°C for the May – July 2021 three-month average.
For the rest of the year, the prediction is uncertain, either neutral conditions will prevail or La Niña conditions could develop. Some models suggest that La Niña conditions will develop, while others predict that neutral conditions are more likely, or that the La Niña-like conditions will only be temporary.
Another key driver that may 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) index, is in the negative phase and models are forecasting the IOD to return to neutral in November or December. A negative IOD can lead to above-average rainfall 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 sea surface temperatures (SSTs) over the central and eastern tropical Pacific overall represented cooler than average but neutral conditions in July 2021 (Figure 1). Across the Indian Ocean, there are some signs of a negative dipole, with slightly below-average sea surface temperatures in the western Indian Ocean (solid black box, Figure 1) while the eastern tropical ocean was above-average (dotted black box, Figure 1). Models predict that either ENSO-neutral or La Niña conditions will be present for the rest of 2021, while the negative IOD is predicted to return to neutral in November or December.
Figure 1: Detrended SST anomalies for July 2021 with respect to 1976-2014 climatology using ERSST v5 data. Red (blue) shades show regions of relative warming (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 in Figure 2, prior to August 2020, the 1-month Nino3.4 values were within the neutral range. From August 2020 to April 2021, the Nino3.4 index was within the La Niña range, with the largest 1-month anomalies observed in October and November 2020. In recent months, the anomalies have been cooler that average, but within the neutral range.
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), based on the Nino3.4 SST index, some models predict La Niña conditions re-developing, while some models predict neutral conditions until the end of the year. Based on the models, the chance of La Niña conditions re-developing approximately 50%.
Figure 3: Forecasts of Nino3.4 index’s strength until January 2022 from various seasonal prediction models of international climate centres (image credit: Copernicus C3S).
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 for the most recent El Niño and La Niña events 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 Southwest Monsoon months, from June to September, as well as October the correlation of ENSO with rainfall over Singapore is strong (Figure 5), i.e. if a significant ENSO event is present, the rainfall patterns are likely to be influenced by a moderate or strong event. When there is no ENSO, there is a large variability in rainfall patterns across the neutral ENSO state 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 the 95% level are underlined, at 99% level in red.
Figure 6: Singapore rainfall anomalies for June – 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).