Future of El Niño

El Niño and La Niña are the warm and cold phases of the El Niño–Southern Oscillation (ENSO; the cycle of warm and cold sea surface temperatures in the tropical central and eastern Pacific Ocean). ENSO in a changing climate is an important topic also in light of this week’s COP28 meeting. Climate scientists agree that an El Niño event is already developing and will likely peak in boreal winter 2023, although its maximum strength is still uncertain.

How do we determine the future of climate events such as El Niño? ICTP climate scientists Fred Kucharski and Adnan Abid discuss the future of ENSO.

What is the effect of El Niño on local and global weather?

Fred Kucharski: El Niño is characterized by anomalously warm sea surface temperatures in the eastern Pacific that destabilise the atmosphere, leading to more convective rainfall in the central-eastern Pacific region. The amount of rainfall is proportional to the condensational heating in the atmosphere. But the atmosphere cannot always heat up. It turns out that the most effective way to counteract convective heating in tropical regions is rising air motion, which cools the air because the air is expanding.

At some point, this rising air motion in a large region of the central eastern Pacific will hit the tropopause [the atmospheric boundary that divides the troposphere from the stratosphere] (at about 10–12 km in the tropics), where the air has to move horizontally. This horizontal movement of air perturbs the atmosphere and is eventually responsible for all El Niño impacts through atmospheric wave propagation. This affects the weather and climate in different parts of the globe, in an effect commonly known as teleconnections.

Teleconnections can directly cause tropospheric disturbances.

It's a little like throwing a stone in the middle of a lake, and watching the waves propagating from the initial perturbation in all directions. These teleconnections can directly cause tropospheric disturbances, sometimes indirectly through the stratosphere, especially in the Euro-Atlantic region.

A key property of the wintertime extratropical region is a lot of variation. There's a lot of energy in the temperature gradient change between the tropical and extratropical regions. There are several locations where this occurs, but the strongest ones are in the North Atlantic, such as the east coast of the USA. Cyclones develop there; cyclones and anti cyclones move in that region, carrying a lot of energy. After moving into a phenomenon called the North Atlantic oscillation, when they break, there's a lot of energy released.

Could El Niño produce cyclones?

FK: This is something we’re investigating. El Niño is certainly able to shift storms, what we call storm tracks, where the cycles move with this phenomenon. If the North Atlantic oscillation is in the positive or the negative phase, that will shift the storm track to the north or to the south. By influencing the North Atlantic oscillation, El Niño can also shift the storm track. So El Niño can have an impact in the North Atlantic region, as we see particularly if we divide the data for early and late winter, where the effects may actually be opposite.

El Niño can have an impact in the North Atlantic region.

Adnan Abid: These phenomena become more complex in extratropical regions, because there are different pathways there, such as stratospheric pathways. El Niño also plays some kind of indirect role in this. So things are very complex. This certainly makes it difficult to provide one simple answer.

Are there shifts in El Niño?

FK: El Niño events can be triggered by initially small warm perturbations in the eastern Pacific, or also by small wind perturbations. These perturbations may just occur locally due to random atmospheric perturbations or weather, which, on seasonal timescales may be considered as 'noise'. However, such perturbations can also be generated by remote climate variability in the North and South Pacific, as well as the Indian and Atlantic Oceans.

El Niño impacts global mean surface temperatures.

An El Niño event increases global mean surface temperatures, particularly the year after its peak (e.g., 2024 in this case). This is because El Niño adds overall heat to the atmosphere, and global oceans and land surfaces on average slowly warm up during an El Niño event. This means global warming induced by greenhouse gases is accelerated during El Niño years, and weakened during La Niña years. These impacts are not permanent, because after an El Niño we typically expect a La Niña and vice versa.

It turns out that a series of La Niña events and a lack of strong El Niño events was the main reason for a global warming slow-down. This combination led to an overall cooler eastern Pacific for a decade or so. As soon as the next strong El Niño event came along in 2015/2016, the global warming accelerated again. A similar slow-down of global warming was observed in the last three years because of a long-lasting La Niña event.

What could ENSO be like in the future?

FK: The future of El Niño is still largely uncertain, even though the majority of models indicate a weak increase in magnitude, but there is no consensus on this. A robust change in projections of future climate (using complex numerical models similar to the ones used for seasonal predictions) is that rainfall changes related to ENSO substantially increase.

Another puzzle is that climate models suggest that long-term changes in the Pacific mean state are expected to be El Niño-like (e.g., more warming in the eastern compared to western Pacific). However, current observed long-term changes in the Pacific indicate the opposite. Such differences would lead to drastic changes in the expected precipitation redistribution in any future climate, as well as ENSO characteristics. More research and improved models may be needed to solve these unanswered questions.

How do you see climate modelling developing in the future?

FK: In the changing climate, humanity is facing various natural disasters such as heavy precipitation (e.g., as recently happened in the northwest of Italy), fires (e.g., the Australian and Canadian fires), droughts (east Africa), heat waves (last Spring, a record temperature was noted in South Asia). Some of these are known to be caused by ENSO. It’s challenging for the community to create models able to capture/predict these events at longer timescales. Models can produce reasonable forecasts up to 7-10 days, but going beyond this and understanding details of ENSO teleconnections would be useful.

How do we improve resolution?

FK: Current-generation models cover the whole globe with small 'boxes', various parameters are predicted for each, and information is exchanged with neighbouring boxes. These climate models still have a particular spatial resolution (box sizes of around 10 km) that is far too coarse to resolve many important processes. One process not represented in these models is atmospheric convection, such as thunderstorms. These processes are currently determined using relationships between what the model 'knows' at the 10 km scale, and the much smaller scales in which convective events evolve (e.g., 1 km).

Which techniques could be adopted in climate modelling?

FK: It would be much better to be able to resolve convective processes physically using smaller boxes. However, this would require a substantial increase in computational resources. There are several initiatives in the climate community that push in this direction, and pilot projects are already underway.  Another option is the use of Artificial Intelligence (AI), or Machine Learning (ML) to improve parameterization using all available data.

What are you currently working on?

FK: One line of current research is the impact of ENSO on early summer extreme temperatures in the region surrounding Pakistan. In this case La Niña increases the probability of extremely high temperatures in the pre-monsoon season.

On another topic, we’re currently studying how subtropical and extratropical ENSO impacts are modified by the fact that the Indian and Atlantic Oceans are influenced first during ENSO, and that these impacts change within a season. These impacts may change the way ENSO influences climate in other regions.

For example, if we consider the whole winter period, the ENSO impact on Europe is relatively weak. If, on the other hand, we only look at November to December, we can identify a stronger impact on Europe: during El Niño we find warm and wet conditions in the central European region. This stronger impact is due to changes ENSO causes in the Indian and Atlantic Ocean regions in early winter. Another atmosphere-ocean coupled phenomenon, the Indian Ocean Dipole (IOD) in boreal autumn together with ENSO, is also key for the global climate. Similar changes and impacts are observed in South Asian winter precipitation. We’re currently investigating how such inter-basin connections may change in a future climate.