El Niño: FAQs for the anticipatory action community

The US National Weather Service has declared that El Niño conditions are now present and expected to strengthen in the Pacific Ocean. In this blog, we explore what this means for the anticipatory action community.

What is ENSO?

The El Niño Southern Oscillation, or ENSO, is a cycle of warming and cooling events in the area of the Pacific Ocean along the equator and the atmosphere above it. El Niño is the warming part of that cycle, where a decrease in the upwelling of cool waters near South America leads to an increase in sea surface temperatures across the Pacific, which warms the atmosphere above it. La Niña is the opposite, or cooling, part of the cycle and has the opposite effect around the world.

El Niño / La Niña events occur every two to seven years on average, but ‘strong’ events occur less frequently.

What influence does ENSO have on weather around the world?

ENSO has far-reaching impacts, called ‘teleconnections’, because the warmer / cooler waters in the Pacific change the way that air and moisture move around the globe, affecting seasonal precipitation and temperature patterns.

By studying many past ENSO events, we can understand, on average, when and which areas are more likely to be wetter or drier during El Niño or La Niña. This analysis tells us where there is an increased likelihood of weather-related hazards.

However, no two El Niño or La Niña events are the same. There are other modes of climate variability which influence the impacts experienced during El Niño; for example, the Indian Ocean Dipole plays an important role in East Africa and parts of Asia-Pacific. In some areas, these other modes dominate any influence of El Niño. There are also many regions where ENSO has no detectable influence on rainfall variability.

On average, what can we expect from an El Niño event?

Figure 1 shows the typical influence of El Niño on seasonal rainfall anomalies. This map is produced based on analysis of past ENSO events, but, as stated, these are just the ’typical’ conditions, and every El Niño event is different.

For example, during an El Niño event, we would expect East Africa to be wetter than average during the short rainy season from October to January. This would be welcome news, alleviating the current drought in the Horn of Africa, but we must be careful: during the 2015/2016 event, parts of Ethiopia suffered from drought. We would also expect El Niño to lead to drier than average conditions in southern Africa, and Zimbabwe did indeed suffer from drought in 2015/2016.

Figures 2a and 2b illustrate the stark contrasts across Africa seen in different El Niño years, showing the Standardized Precipitation Evaporation Index (SPEI), an indicator of drought conditions.

We also know that El Niño leads to a lower number of Atlantic hurricanes affecting the Caribbean and the Gulf of Mexico, and may result in more intense tropical cyclones in the western Pacific. Yet we must be careful not to use this information alone to deprioritize anticipatory and preparedness actions; it only takes one tropical cyclone to cause significant damage.

As every El Niño event is different, what are the key sources of information we should look at?

Forecasts of El Niño alone may serve as an early indicator of what we might expect in the year ahead. Many of the same models which predict the strength of an El Niño or La Niña event are, however, also considering all the other conditions around the world which combine to drive temperature and precipitation anomalies. As a result, it is often better to look to these seasonal forecast models to determine likely conditions in specific areas. As with any forecasting system, it is important to consider the skill of forecasts - a measure of how often we expect them to be correct, based on past performance - before committing resources based on them.

So what is going to happen in 2023? While all forecasts point to a potential El Niño event, this will peak around December / January. As we are still in July, we cannot yet be confident of how strong it will be, or what its impacts will be across the world. There are many regional climate outlook forums which bring together all the available information from different forecast models, which will form a consensus forecast on the season ahead.

What might be different about this year’s El Niño event?

We are already aware of certain factors that will influence how the impacts of this El Niño event will play out. For example:

• While there is an expectation of an end to the drought in the Horn of Africa, it can take some time for rain to filter down into the soil to support deep-rooted plants. And, of course, the impacts of malnutrition will continue for some time.
• While El Niño conditions usually inhibit the growth of tropical cyclones in the North Atlantic, this effect may be balanced out by the anomalously high sea surface temperatures currently being observed in the region where such storms form.
• In Ecuador and Peru, an outbreak of dengue following flooding earlier this year could potentially be exacerbated by the expected El Niño rains in early 2024. In southern Africa, it remains to be seen whether the cholera situation will be improved by the anticipated drier conditions.

In general, climate change is leading to warmer sea surface temperatures, and there is some evidence to suggest that this is affecting how El Niño and La Niña events influence weather patterns around the world. For example, it is hypothesized that La Niña is having less influence over rainfall anomalies in South Asia during the monsoon than it used to, because of the warmer Indian Ocean (see this paper for a more detailed explanation of this).

How is the humanitarian sector preparing for El Niño events?

The humanitarian community is better prepared than ever to address the risks that El Niño poses. Following a series of El Niño and La Niña events in the 2000s and early 2010s, the UN Inter-Agency Standing Committee developed standard operating procedures outlining the timeline and actions in response to the emerging risks of an event. These serve as a baseline for developing more tailored measures in specific organizations and countries.

Within the Red Cross Red Crescent Movement, Early Action Protocols (EAPs) have been developed to address the increased risk posed by El Niño events. In Ecuador and Peru, for example, triggers have been developed to address the increased likelihood of flooding in the rainy season from January to April. In Central America, EAPs cover the increased likelihood of drought from June to August.

Many of the forecasts used in these EAPs take a multi-stage approach, with seasonal forecasts triggering preparedness activities, sub-seasonal activities triggering pre-positioning, and short-term forecasts (e.g., of flooding) triggering distributions to communities. These approaches need to be hazard and context specific.

What action should we take now?

It is still too early to say how strong this El Niño event will be, and what influence it will have on extreme weather events around the world. However, there are many low-cost actions that can support future preparedness activities.

If there is no existing EAP:

• If not undertaken already, identify appropriate and skillful forecasts, and decide on threshold levels which could trigger the next level of actions or anticipatory financing.
• It is better to pre-define these thresholds, because in the past, people have delayed decisions which need to be made under uncertainty, hoping that there will be more certainty if left a little longer. However, this time is needed to take appropriate actions.

If there is an EAP:

• Understand what has changed or is different since the last event. Are reservoirs full? Has vulnerability changed? Has there been deforestation? Is there an ongoing outbreak of disease?
• Familiarize and refresh existing EAPs based on this new information.