ENSO Conditions and Coral Bleaching

The NOAA National Centers for Environmental Prediction's El Niño-Southern Oscillation (ENSO) Alert System status remains (as of February 11, 2021) at a La Niña Advisory. La Niña persisted during January, as indicated by the below-average sea surface temperature (SST) anomalies extending from the western to the east-central Pacific Ocean. SSTs returned to near-average in the eastern Pacific Ocean by the end of the month, as indicated by the latest weekly Niño-3 and Niño-1+2 index values of -0.3 °C and -0.2 °C, respectively. However, the latest weekly Niño index values in the central (Niño-4) and east-central (Niño-3.4) Pacific Ocean were -1.1 °C and -0.7 °C. The below-average SSTs were supported by negative subsurface temperature anomalies, which extended from the surface to at least ~150m below the surface, between 160 °E and 130 °W. Low-level wind anomalies remained easterly from the western to east-central (~140 °W) tropical Pacific, with the largest amplitude near the International Date Line. Upper-level wind anomalies were westerly across most of the tropical Pacific. Tropical convection continued to be suppressed over the western and central Pacific, and enhanced around the Philippines and Indonesia, while both the Southern Oscillation and Equatorial Southern Oscillation remained positive. Overall, the coupled ocean-atmosphere system remains consistent with La Niña.

As you can see in the figure directly above, a majority of the models in the IRI/CPC plume predict a transition to ENSO-neutral during Northern Hemisphere spring 2021. The forecaster consensus is in agreement with this transition; it also predicts a continuation of ENSO-neutral conditions through at least Northern Hemisphere summer. In part, due to the inherent uncertainty in predictions made at this time of year, the forecast for Northern Hemisphere autumn remains split (~50%) between La Niña and a combination of the other two possibilities (El Niño and ENSO-neutral). In summary, there is a ~60% chance of a transition from La Niña to ENSO-neutral during Northern Hemisphere spring 2021.

As an added note, the La Niña is expected to continue to affect temperature and precipitation across the United States during the upcoming months.


This next figure on the left, published February 19, 2021, shows forecasts, made by a set of dynamical and statistical models for SST in the Niño-3.4 region, for nine overlapping three-month periods. Most of the models show La Niña SST conditions are likely to persist until the March-May season, with most transitioning to ENSO-neutral during Northern Hemisphere spring. In the most recent week, the SST anomaly in the Niño-3.4 region was -1.2 °C, indicative of moderate La Niña strength, and -1.05 °C for the month of January 2021. As of mid-February, the subsurface water temperatures in the eastern equatorial Pacific Ocean remain below-average, having been reinforced by the action of the easterly wind anomalies, as warm anomalies shift into the far western equatorial Pacific.

A majority of the dynamical and statistical models predict at least weak La Niña conditions for the February-April season, decreasing to about 55% by the March-May season, and below 50% thereafter. Objective model-based La Niña probabilities are 79% for February-April, dropping to about 40% by the April-June season. ENSO-neutral conditions become the most likely at 62% confidence in the April-June season, but then decay towards climatological odds, according to this suite of models. We will continue to provide updates as the ENSO forecast changes.



As of this initial writing, with the 2015 El Niño continuing to develop and intensify in the Northern Hemisphere, the following is a brief overview of the pattern and timing of heat stress that resulted in widespread severe coral bleaching during 1997-1999. Past reports have estimated that over 15% of the world's coral reefs were effectively lost during the 1997-1999 period (Wilkinson 2000). At that time, NOAA Coral Reef Watch observed widespread, prolonged high temperatures that caused coral bleaching. This was associated with what has been argued to be the largest El Niño on record (1997-1998) followed immediately by a strong La Niña (1998-1999). In general, many areas that are untouched by warming during an El Niño are influenced by warming during a La Niña (see Eakin et al. 2009).

In short, several areas of the tropical oceans experienced thermal stress sufficient for coral bleaching with impacts covering a 14-month period (May 1997-June 1998). Reefs in the central Pacific and eastern tropical Pacific were exposed to thermal stress early after the onset of the El Niño. Thermal stress moved to the Great Barrier Reef and across the islands of the south Pacific during February-April 1998 and the Indian Ocean during March-June 1998. In the Gulf of Mexico and the Caribbean, the worst of the El Niño impacts with the highest thermal exposure culminated in July-October 1998. The mid-1998 onset of La Niña conditions resulted in thermal stress in the northwestern Pacific Ocean from July-October 1998.

The following discussion uses the NOAA Coral Reef Watch SST Anomaly and Bleaching Alert Area products to illustrate the events. The Bleaching Alert Area shows patterns of areas with accumulated thermal stress sufficient to cause coral bleaching. The patterns are a bit different, and more coral-focused, than those visible in the SST Anomaly data. More on these products can be found at: https://coralreefwatch.noaa.gov/satellite/index.php.


The classical El Niño warming pattern formed by May 1997, with initiation of SST warming from the central tropical Pacific eastward to the South American coastline. Prolonged thermal stress with the potential to cause bleaching was seen along the equator, from Howland and Baker Islands east to the Galapagos and the Ecuadorian coastline, during the remainder of 1997. During this time, warming also proceeded northward along the South American coast to Panama. Additionally, warming was seen reaching northeast to Mexico, and some warming was seen along the Central American Coastline. Limited warming was seen in the Caribbean.  



By early 1998, the classic El Niño pattern was fully developed, with broader areas of high temperature in the eastern Tropical Pacific and extending up the Central American coast past Costa Rica, including all of the eastern Tropical Pacific islands; high temperatures in these regions began to dissipate in June. Bleaching levels of warming were seen along the Great Barrier Reef in February-March. Warming also began in the eastern to central Indian Ocean south of the equator, spreading to the eastern Indian Ocean by March, and dissipating after May.  


In May and June, bleaching levels of warming were also seen across the Indian Ocean north of the equator and into Southeast Asia.
With the rapid onset of La Niña conditions in July 1998, warming was observed in the western Pacific Ocean, north of the equator. August-September saw warming in the South China Sea, Philippines, and the Ryukyu Islands, which spread southeastward through Palau and Micronesia, finally dissipating in November.  

Fig. 4.9. Significantly (at 5% level) warmer (red) or cooler (blue) annual maximum SST difference: a El Niño year t, b El Niño year t+1, c La Niña year t, and d La Niña year t+1. Average values calculated for 20 El Niño events and 20 La Niña events, and tested for significant differences from 20 ENSO-neutral years. The groups of years were identified from the Troup (1965) SOI updated by the Australian Bureau of Meteorology.

Bleaching warming in the Western Atlantic/Gulf of Mexico/Caribbean is most commonly seen in the year after the onset of an El Niño (see figure on left from Eakin et al. 2009).

The bleaching warming started to develop in July 1998 and peaked in the Gulf of Mexico in August and in the Caribbean Sea in October.  


By 1999, the worst of the thermal stress was over, from a coral bleaching perspective, but only after major losses of coral reefs worldwide in 1998. Some warming was seen along the Great Barrier Reef in early 1999 but was much weaker than the prior year. Conditions were rather quiescent until August, when warming of the western North Pacific led to low bleaching levels of thermal stress that reached down to the Mariana Islands. Warming was again seen in the Caribbean in August-September 1999 but to a lesser extent than in 1998.


Eakin CM, Lough JM, and Heron SF (2009) Climate Variability and Change: Monitoring Data and Evidence for Increased Coral Bleaching Stress. In Coral Bleaching, vanOppen MJH and Lough JM eds. Ecological Studies 205: 41-67, Springer-Verlag Berlin Heidelberg.

Wilkinson CR (2000) Status of Coral Reefs of the World: 2000. Global Coral Reef Monitoring Network and Australian Institute of Marine Science, Townsville, Australia 363pp.

For more information, please contact coralreefwatch@noaa.gov


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