EI Niño/La Niña Status
Updated on 6 January 2021
The El Niño Southern Oscillation (ENSO) monitoring system is in “La Niña Conditions” in January 2021. The 1-month Nino3.4 sea surface temperature index continues to indicate moderate La Niña conditions and atmospheric indicators (cloudiness and wind anomalies) over the tropical Pacific Ocean remain consistent with La Niña conditions. The Nino3.4 index was -1.58°C for November 2020 and ‑1.38°C for the September-November 2020 three-month average.
Models are predicting moderate La Niña conditions to persist until February 2021 and then weaken during March to May 2021.
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 moderate La Niña conditions in November 2020 (Figure 1) with continued cooling in the central and eastern tropical Pacific Ocean. Across the Indian Ocean, no Indian Ocean Dipole was present. Models predict that the La Niña conditions will continue before weakening during March – May. The Indian Ocean Dipole is expected to remain in the neutral state until at least April 2021.
Figure 1: Detrended SST anomalies for November 2020 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 \xa0Figure 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 further cooling in November. 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.
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 persist (indicating moderate La Niña conditions) up to February 2021. From February, the models predict the La Niña conditions to weaken, although to remain within La Niña levels until at least March 2021.
Figure 3: Forecasts of Nino3.4 index’s strength for until May 2021 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 most of Northeast Monsoon months from December to March, there is little relationship between ENSO and rainfall over Singapore (Figure 5). This poor correlation indicates that the Nino3.4 index is not a good predictor of rainfall amount for Singapore. However, towards the end of the Northeast Monsoon season and into the first inter-monsoon period, ENSO has a small influence on Singapore’s rainfall. Between February and May, La Niña events tend to bring wetter conditions than El Niño events, although there is still a lot of year-to-year variability (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 February – May (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).