ENSO Conditions and Coral Bleaching

The NOAA National Centers for Environmental Prediction's El Niño-Southern Oscillation (ENSO) Alert System has issued, as of May 13, 2021, its Final La Niña Advisory. During April, the tropical Pacific Ocean returned to ENSO-neutral conditions as the coupling between the atmosphere and ocean weakened. Sea surface temperatures (SSTs) were near-to-below average across most of the equatorial Pacific Ocean in the past month. The El Niño indices have generally trended toward normal during the last several months, except for the easternmost Niño-1+2 region, which was -0.7 °C in the past week. Subsurface temperature anomalies continued to increase due to a downwelling Kelvin wave, which reinforced the positive temperature anomalies along the thermocline. Low-level easterly wind anomalies were weakly present in the east-central Pacific, but were westerly in the far western Pacific Ocean. Upper-level wind anomalies remained westerly across the central and east-central tropical Pacific. Tropical convection became near-average around the International Date Line, with suppressed convection evident over Indonesia. Overall, the ocean and atmosphere system reflected a return to ENSO-neutral.

Per the figure to the left, a majority of the models in the IRI/CPC plume predict a continuing of ENSO-neutral conditions through Northern Hemisphere summer 2021. The forecaster consensus agrees with this set of models through the summer, and then begins hedging toward cooler conditions with the approach of Northern Hemisphere fall. La Niña chances are around 50-55% during the late fall and winter, which aligns with forecasts from the NOAA National Centers for Enivronmental Prediction's Climate Forecast System (CFS) and North American Multi-model Ensemble. However, there is typically large uncertainty with forecasts made in the spring; so confidence in ENSO-neutral for the coming seasons is highest. In summary, La Niña has ended, with ENSO-neutral likely to continue through Northern Hemisphere summer (67% chance in June-August 2021).

This next figure on the left, published April 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 remain for the March-May season, with most transitioning to ENSO-neutral during Northern Hemisphere spring. For the month of January 2021, the SST anomaly in the Niño-3.4 region was -1.2 °C, indicative of moderate La Niña strength, and -1.05 °C. As of mid-February, the subsurface water temperatures in the eastern equatorial Pacific Ocean were below-average, having been reinforced by the action of the easterly wind anomalies, as warm anomalies shifted into the far western equatorial Pacific.

A majority of the dynamical and statistical models (about 55%) predict at least weak La Niña conditions for the March-May season, decreasing to below 50% thereafter. Objective model-based La Niña probabilities are at 54% for March-May, dropping to 21% by the April-June season. ENSO-neutral conditions become the most likely at 79% confidence in the April-June season, but then start decaying towards climatological odds, according to this suite of models. 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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