EI Niño/La Niña Status
Updated on 14 September 2020
The El Niño Southern Oscillation (ENSO) monitoring system remains in the “La Niña Watch” status this month. The Nino3.4 sea surface temperature index is below average and atmospheric indicators (cloudiness and wind anomalies) over the tropical Pacific Ocean continue to show a tendency towards La Niña-like patterns. The Nino3.4 index was -0.38°C for July 2020 and -0.43°C for the May-July 2020 three-month average.
Models are predicting La Niña conditions from September 2020. However, the spread of possible outcomes from the models is currently still too wide to indicate a consensus on the strength of the La Niña conditions and whether the conditions will be sustained for long enough to be declared a La Niña event.
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) SST anomalies 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 remained in neutral conditions overall in July 2020 (Figure 1) but continue to observe negative values over the Nino3.4 region. Across the Indian Ocean, the SSTs were warmer than average, but the Indian Ocean Dipole (IOD; difference between the western and eastern anomalies) is overall neutral. Models predict the tropical Pacific to experience La Niña conditions from September onwards. Some models predict a negative IOD may develop in September, albeit short-lived.
Figure 1: Detrended SST anomalies for July 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 from last year (Figure 2), the 1-month Nino3.4 value fluctuated around the El Niño threshold until April 2019, when it weakened and remained within neutral values since. Recently, it started to dip to negative values. For La Niña conditions to be present, 1-month cold SST anomalies (observed or forecast) should persist for at least four months below 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 spread of possible outcomes ranging mostly from neutral to strong La Niña up to January 2021. Thus, the models are predicting La Niña conditions in the later part of 2020, but the spread for possible outcomes currently is still too wide to indicate consensus whether the cool SSTs will remain long enough to declare a La Niña event or the exact strength of the La-Niña conditions.
Figure 3: Forecasts of Nino3.4 index’s strength for until January 2021 \xa0from 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 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).