"Fishing in the Desert" – Unraveling the Mechanism of Extreme Coastal El Niño Events in 2023

Have you ever seen fishing in the desert? Affected by upwelling, Peru is one of the driest regions on the planet along the coast of the desert. However, in the spring of 2023, severe flooding along the coasts of Peru and Ecuador caused a huge lake in the desert (Picture 1, top), and experienced fishermen from the surrounding area rushed to the scene to catch fish in the desert (Picture 1, bottom left). Affected by heavy rainfall, flooding flooded the coastal areas of Peru (Figure 1, bottom right) and caused the worst dengue outbreak in Peru's history, resulting in more than 300 deaths. The culprit behind all of this is what we're going to cover today – the extreme coastal El Niño.

Figure 1. The 2023 extreme coastal El Niño event led to the emergence of rare "desert lakes" and the spectacle of fishing in the desert along the coast of Peru, while triggering intense flooding in Peru, Ecuador and other countries.

Traditionally, El Niño has been thought to be a widespread warming phenomenon in the tropical eastern Pacific region, which, when it occurs, will have important implications for the global climate system. However, the latest research has shown that there is a special El Niño – the coastal El Niño phenomenon – in coastal areas such as Peru and Ecuador. This El Niño is usually confined to the southeastern Pacific coast and occurs between February and April, when the rest of the tropical Pacific is usually unusually cold (weak La Niña) (Figure 2). When extreme coastal El Niño occurs, it is accompanied by a strong local air-sea coupling process in the Southeast Pacific region, which has an important impact on the climate, economy and ecology of coastal countries. Compared with the basin-scale El Niño, which we are familiar with, this coastal El Niño has significant differences in physical mechanism, spatiotemporal evolution, and impact. In view of the low number of such events in the observational record, the main physical processes and inducing mechanisms are still controversial in the academic community. Dr. Qihua Peng and Professor Shangping Xie of the Scripps Institution of Oceanography at the University of California, San Diego, used a large number of observational data and numerical model experiments to study the main drivers of the event and the air-sea coupling process, and the results were recently published in the journal Science Advances.

Figure 2. SST (colored), precipitation (contour) and wind field (arrow) anomalies corresponding to extreme coastal El Niño events in 2023.

For the first time, the study found that the interaction between strong MJO signaling and the steep Andes Mountains in South America is an important factor in the inducing of such extreme coastal El Niño events. At the beginning of March 2023, MJO showed its strongest signal in nearly 50 years in its corresponding phase 8 (Figure 3, top left). Strong phase 8 MJO activity induces strong westerly anomalies over the eastern equatorial Pacific, and when these strong westerly anomalous signals encounter steep Andes Mountains, they propagate polarly in the form of Kelvin waves (Figure 3, upper right), thus driving strong northerly wind anomalies along the Peruvian coast (Figure 3, lower row). The results of the ocean model show that both the westerly wind anomaly in the eastern equatorial Pacific Ocean and the northerly wind anomaly off the coast of Peru are the main drivers of El Niño formation along the coast of this extreme coast. On the other hand, these wind disturbances have led to cyclonic wind anomalies in the Southeast Pacific, which will further enhance the anomalies of these wind fields by facilitating the formation of tropical low pressure systems (such as Yaku in 2023), which are extremely rare in the region.

Figure 3. MJO signaling and its interaction with the Andes during the 2023 extreme coastal El Niño event. Top left: MJO index. Bottom left: Precipitation and surface wind distribution from 8 to 10 March 2023. Bottom right: Intraseasonal correlation coefficient (regression coefficient) distribution of V10 (sea surface wind) and the average U10 in the eastern equatorial Pacific.

The results of atmospheric models show that when the wind anomalies associated with the MJO and the tropical depression system cause the coastal SST to exceed the convection threshold, deep convection will be generated over the southeast Pacific region, which will further enhance the intensity of the transequatorial anomalous northerly winds, thereby triggering a positive coastal Bjerknes feedback in the coastal region (Fig. 4). This positive feedback plays a key role in maintaining and enhancing such extreme events, and provides techniques for the prediction of such events by numerical models. In addition, the numerical model results also show that the increase in coastal SST will in turn enhance the disturbance within the atmosphere, especially in favor of the formation of tropical depression systems, which will enhance the wind anomaly in the eastern Pacific region (Fig. 4) and form positive feedback.

Figure 4. Schematic diagram of strong local air-sea coupling during extreme coastal El Niño events in 2023.

This study reveals the main air-sea coupling process and inducing mechanism of extreme coastal El Niño events in 2023, which is helpful to strengthen the systematic understanding of the main characteristics and formation mechanism of such events, and provides an important scientific basis for the prediction and mitigation of such events.