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.
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).
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.
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.
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.
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