EI Niño/La Niña Status
Updated on 11 Sep 2024
The El Niño Southern Oscillation (ENSO) monitoring system state is in “La Niña Watch”. The Nino3.4 index and atmospheric indicators (cloudiness and trade winds) are still indicating ENSO neutral conditions. Cool subsurface temperature anomalies in the eastern-central and eastern Pacific support a developing La Niña. The Nino3.4 index was -0.08°C for July 2024 and 0.06°C for the May – July 2024 three-month average.
On average, models predict La Niña conditions to develop in September – October 2024, although the chance of La Niña conditions developing has lowered compared to the previous outlook. If La Niña conditions do not develop, models predict ENSO neutral conditions to persist until the end of the year.
Short note on the Indian Ocean Dipole: The Indian Ocean Dipole (IOD) is currently neutral. Models are predicting either a negative IOD or neutral state in the next few months.
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 tropical Pacific in July 2024 were below average in the east and above average elsewhere (Figure 1). The below average anomalies in the eastern Pacific were starting to resemble a La Niña pattern, although not extensive enough to indicate La Niña conditions. Overall for the Nino3.4 region (red box), SSTs were near average. Across the Indian Ocean, tropical SSTs were generally warm, including much of the western portion (solid black box) and the eastern portion (dashed black box) in line with IOD neutral conditions.
Figure 1: Detrended SST anomalies for July 2024 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, between March and June 2023, the index was within the ENSO-neutral range, but was warming. In July 2023, the Nino3.4 exceeded the El Niño threshold, peaking around November 2023. After this time, there was a gradual cooling, with the Nino3.4 index within the neutral range by May 2024. Since May, the Nino3.4 index has been 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, show that models predict either ENSO-neutral conditions or La Niña conditions until at least January 2025. During September and October 2024, on average models predict La Niña conditions to develop, although some models do predict ENSO-neutral conditions to persist, rather than La Niña conditions to develop. It is unlikely that El Niño conditions will re-develop during this time.
Figure 3: Forecasts of Nino3.4 index’s strength until January 2025 from various seasonal prediction models from international climate centres (grey lines). The solid blue and yellow lines note the La Niña and El Niño thresholds used by MSS, while the dotted lines note the thresholds used by some other international centres.
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 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. 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 an El Niño or La Niña conditions are present, the rainfall patterns are likely to be influenced, particularly by a moderate or strong event. When there are no El Niño or La Niña conditions present (i.e. ENSO-neutral), there is a large variability in rainfall (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).