EI Niño/La Niña Status
Updated on 10 June 2021
The El Niño Southern Oscillation (ENSO) monitoring system state is now back to “Neutral”. The 1-month Nino3.4 sea surface temperature index during April and May 2021 continued to warm and showed a transitioning from weak La Niña to ENSO-neutral conditions. Sub-surface conditions no longer support La Niña conditions. Atmospheric indicators (cloudiness and wind anomalies) are now generally consistent with neutral conditions. The Nino3.4 index was -0.80°C for April 2021 and -0.95°C for the February – April 2021 three-month average. Preliminary assessment of the May 2021 values indicated that the monthly Nino3.4 index has returned to neutral.
Models are predicting ENSO-neutral conditions to continue during July and August.
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 weakening La Niña conditions in April 2021 (Figure 1). Across the Indian Ocean, no Indian Ocean Dipole was present. Models predict that the La Niña conditions are ending and that ENSO-neutral conditions will be present in July and August. The Indian Ocean Dipole is expected to remain in the neutral state for the next few months (June – August).
Figure 1: Detrended SST anomalies for April 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 value were within the neutral range. Since August 2020, the Nino3.4 index has been within the La Niña range, with the largest 1-month anomalies observed in October and November 2020. In recent months, the anomalies have warmed and were close to the La Niña threshold in April. La Niña conditions are considered to be present when the 1-month cold SST anomalies (observed or forecast) persist for at least four months below the threshold, along with supporting atmospheric observations. These La Niña conditions are considered to have ended when the 1-month SST anomalies are no longer above the threshold, as well as atmospheric conditions no longer indicate La Niña conditions.
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 that based on the Nino3.4 SST index, the La Niña conditions are ending (either near or warmer than the La Niña threshold). During July and August, most of the predictions are within the neutral range. After August, the spread is large, with most predictions indicating neutral conditions or La Niña conditions re-developing.
Figure 3: Forecasts of Nino3.4 index’s strength until October 2021 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 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 Southwest Monsoon months from June to September 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.
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).