ENSO Conditions and Coral Bleaching

The NOAA National Centers for Environmental Prediction's El Niño-Southern Oscillation (ENSO) Alert System status is currently (as of November 14, 2019) Not Active. Over the last month, ENSO-neutral conditions continued. Near-to-above average sea surface temperatures (SSTs) were observed in the east-central tropical Pacific Ocean. In the most recent week, the SST indices in the westernmost Niño-4 and Niño-3.4 regions were +0.7°C and +0.5°C, respectively. Farther east in the Niño-3 and Niño-1+2 regions they were near-to-below average (+0.3°C and -0.6°C) respectively. The subsurface temperature anomalies (averaged across 180°-100°W) were above average during the month, as a downwelling oceanic Kelvin wave that began in September continued progressing eastward into the eastern Pacific. Low-level winds were near average, while easterly upper-level wind anomalies were observed over the eastern Pacific. Finally, tropical convection was suppressed near the International Date Line and also over Indonesia, while somewhat enhanced convection prevailed over the western Pacific, northeast of Papua New Guinea. Overall, despite the recent anomalous warming across the east-central equatorial Pacific, the oceanic and atmospheric system reflected ENSO-neutral. The majority of models continue to favor ENSO-neutral (Niño-3.4 index between -0.5°C and +0.5°C) conditions through Northern Hemisphere spring 2020. Many dynamical forecast models, including the National Centers for Environmental Prediction (NCEP) Climate Forecast System Version 2 (CFSv2), suggest Niño-3.4 SST index values will remain near +0.5°C during November, before decreasing toward zero. Forecasters believe this recent warmth reflects sub-seasonal variability and is not indicative of an evolution toward El Niño. The chances for El Niño are predicted to be near 25% during the winter and spring. The figure on the left also reflects the persistence of ENSO-neutral conditions through Northern Hemisphere winter 2019-20 (70% chance) into spring 2020 (60-65% chance).


This next figure on the left, published October 18, 2019, shows forecasts, made by a set of dynamical and statistical models for SST in the Niño 3.4 region, for nine overlapping 3-month periods. Most (about 85%) of the dynamical and statistical models issued during mid-October show slightly above-average but neutral conditions for the fall, winter and springi 2019-2020. A few models show weak El Niño conditions developing during fall and extending into winter and spring. Only one model predicts La Niña development. In the most recent week, the SST anomaly in the Nino3.4 region was 0.4°C, in the upper portion of the ENSO-neutral range, and -0.18°C for the month in the ENSO-neutral category. Subsurface sea temperature anomalies have warmed to somewhat above average. Objective model-based probabilities are 74% for neutral conditions for October-December 2019. These drop slightly but stay above 65% for November 2019-January 2020 through April-June 2020. Throughout the forecast period, El Niño probabilities exceed La Niña probabilities, but only rise to 30% or more for the last two seasons (May-July and June-August 2020). 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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