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 October 8, 2020) under a La Niña Advisory. La Niña conditions continued during September, as evidenced by below-average sea surface temperatures (SSTs) extending from the International Date Line to the eastern Pacific Ocean. The SST indices in the two westernmost Niño regions, Niño-4 and Niño-3.4, cooled throughout the month, and the Niño-3.4 index was -1.1 °C in the past week. The equatorial subsurface temperature anomalies (averaged from 180°-100°W) remained substantially unchanged, and continued to reflect below-average temperatures, from the surface to 200m depth in the eastern Pacific Ocean. The atmospheric circulation anomalies over the tropical Pacific Ocean remained consistent with La Niña. Low-level wind anomalies were easterly across most of the tropical Pacific, and upper-level wind anomalies were westerly over the east-central Pacific. Tropical convection continued to be suppressed, from the western Pacific to the International Date Line, and a slight enhancement of convection emerged over Indonesia. Additionally, both the Southern Oscillation and Equatorial Southern Oscillation indices remained positive. Overall, the coupled ocean-atmosphere system indicates the continuation of La Niña.

As you can see in the figure directly above, a majority of the models in the IRI/CPC plume predict La Niña (Niño-3.4 index less than -0.5 °C) will persist through Northern Hemisphere winter 2020-21, and weaken during spring 2021. The latest forecasts from several models, including the NOAA NCEP Climate Forecast System Version 2 (CFSv2), suggest the likelihood of a moderate or even strong La Niña (Niño-3.4 index values less than -1.0 °C) during the peak November 2020-January 2021 season. The forecaster consensus supports that view in light of significant atmosphere-ocean coupling already in place. In summary, La Niña is likely to continue through Northern Hemisphere winter 2020-21 (~85% chance) and into spring 2021 (~60% chance during February-April).

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

This next figure on the left, published October 19, 2020, 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. Many of the model predictions issued during mid-October 2020 show moderate or even strong La Niña SST conditions for the remainder of 2020, likely dwindling to weak La Niña conditions by late winter, and ENSO-neutral conditions during spring 2021. In the most recent week, the SST anomaly in the Niño-3.4 region was -1.1 °C, in the weaker portion of the moderate La Niña category, and -0.95 °C for the month of September, in the stronger portion of the weak La Niña range. During mid-October, the subsurface water temperatures were moderately below-average, while the SST had cooled somewhat further than the levels observed through most of September.

Approximately 92% of the dynamical and statistical models predict at least weak La Niña conditions for the October-December season, decreasing to 59% by February-April 2021, and below 45% thereafter. Objective model-based La Niña probabilities are 97% for October-December, dropping to 59% by February-April, 39% by March-May 2021, and below 30% thereafter. We will continue to provide updates as the ENSO forecast changes.

1997-1999 ENSO and Patterns of Coral Bleaching

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.

1997:

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.

1998:

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.

1999:

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.

References

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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