The Two Faces of El Niño: How Different Ocean Heat Patterns Rewrite the Map of Droughts and Floods
Context
El Niño and La Niña have long been known as the main conductors of global precipitation, but over the past two decades it has become clear that El Niño comes in two flavors: the classic Eastern Pacific (EP) and the Central Pacific (CP, or Modoki). Most previous studies relied on linear correlations or composite means, which do not capture how the tails of the precipitation distribution behave, that is, very wet and very dry seasons. Our work sits alongside studies that have already shown that El Niño can be predicted a year and a half in advance, and with attempts to revive old weather records, but it differs in that it looks not at average values but at the coincidence of rare events. This allows us to detect signals that linear methods simply miss.
Methods
The authors took monthly GPCC precipitation data at 2.5° × 2.5° resolution for 1951–2016 and aggregated them into seasonal totals for autumn (SON), winter (DJF), and spring (MAM). For each grid cell, seasons with precipitation above the 80th percentile were considered very wet, and those below the 20th percentile very dry. El Niño and La Niña types were taken from the classification of Wiedermann et al. (2016), based on the transitivity index of the climate network. Then, for each cell and each season, the event coincidence analysis method was used to compute the fraction of ENSO periods of a given type that coincided with a very wet or dry season. Statistical significance was assessed analytically under the assumption of random independent event distribution, with a threshold of p < 0.05.
Results
For EP El Niño, known connections were confirmed: droughts in Indonesia, the Philippines, and northern Amazonia in SON; wet conditions in western North America and Ecuador; dry seasons in northeastern Brazil in MAM. For CP El Niño, many of these patterns disappear or weaken, but new ones emerge: droughts on the eastern coast of Australia in SON, wet conditions in southern Chile and Central Asia in DJF, and dry seasons in southeastern Africa in MAM. For La Niña, the EP type gives extensive coherent anomalies (wet in Australia and Indonesia, dry in southern Europe and southern Brazil), whereas the CP type yields only isolated patches: wet in Australia in SON, dry in Florida in DJF, and dry in the UK, Ireland, and western Kazakhstan in MAM. Overall, CP periods produce statistically significant coincidences less often and over smaller areas than EP periods, and this holds for both ENSO phases.
Implications
The work shows that the distinction between EP and CP types matters not only for average precipitation but also for the probability of extreme seasons. This strengthens the case for splitting La Niña into two types as well, although earlier this was controversial based on sea surface temperature correlations. Compared to neighboring studies that use linear methods or composites, this is the first application of an event-based approach to a global precipitation grid, allowing signals invisible to mean values to be seen. This is not just a confirmation of old connections but a new risk map tied to specific ENSO types.
Future development
The method can be extended to other climate variables, such as air temperature, soil moisture, river runoff, as well as crop yields and water availability. If the type of a developing El Niño can be determined early enough, it becomes possible to provide advance warnings of likely droughts or floods in specific regions. It is also worth checking whether these patterns are reproduced in CMIP6 climate models; discrepancies would indicate processes that are not well described.
Impact
Agriculture, water resource management, natural disaster forecasting, and epidemiological surveillance in ENSO-sensitive regions.
Next steps
Check the reproducibility of the patterns in CMIP6 models and extend the analysis to other variables and longer time series to accumulate more events for each type.
Key open problems
For climatology, the question remains open as to how stable the links between ENSO types and extreme precipitation are over time under climate change. It is also unclear whether CP El Niño can be divided into subtypes that affect precipitation differently in the Indo-Pacific region, and to what extent this division applies to La Niña.
The term "Modoki" for Central Pacific El Niño comes from a Japanese word meaning "similar but different," which accurately reflects its character: it looks like classic El Niño on the surface but behaves differently.
Key numbers
- Number of EP El Niño events: 7
- Number of CP El Niño events: 17
- Number of EP La Niña events: 8
- Number of CP La Niña events: 12
- Threshold for very wet season: 80th percentile
- Threshold for very dry season: 20th percentile
- Spatial resolution of data: 2.5° × 2.5°