Strong influence of El Niño Southern Oscillation on flood risk around the world

Abstract

El Niño Southern Oscillation (ENSO) is the most dominant interannual signal of climate variability and has a strong influence on climate over large parts of the world. In turn, it strongly influences many natural hazards (such as hurricanes and droughts) and their resulting socioeconomic impacts, including economic damage and loss of life. However, although ENSO is known to influence hydrology in many regions of the world, little is known about its influence on the socioeconomic impacts of floods (i.e., flood risk). To address this, we developed a modeling framework to assess ENSO's influence on flood risk at the global scale, expressed in terms of affected population and gross domestic product and economic damages. We show that ENSO exerts strong and widespread influences on both flood hazard and risk. Reliable anomalies of flood risk exist during El Niño or La Niña years, or both, in basins spanning almost half (44%) of Earth's land surface. Our results show that climate variability, especially from ENSO, should be incorporated into disaster-risk analyses and policies. Because ENSO has some predictive skill with lead times of several seasons, the findings suggest the possibility to develop probabilistic flood-risk projections, which could be used for improved disaster planning. The findings are also relevant in the context of climate change. If the frequency and/or magnitude of ENSO events were to change in the future, this finding could imply changes in flood-risk variations across almost half of the world's terrestrial regions.

Keywords: El Niño Southern Oscillation; climate variability; flood hazard; flood risk; global scale.

Fig. 1.

Percentage anomaly in flood volumes with return periods of 100 y during (A) El Niño years and (B) La Niña years (compared with all years). Statistical significance per cell was assessed by bootstrapping (α = 0.05), using 1,000 repetitions. Field significance of the gridded results was assessed using the binomial distribution (26) and found to be highly significant (P < 0.001). Absolute values of the flood anomalies (normalized to area) are shown in Fig. S1. For validation results see SI Discussion, Validation of Hydrological and Hydraulic Models.

Fig. 2.

Percentage anomaly per FPU in annual expected damage in urban areas during (A) El Niño years and (B) La Niña years (compared with all years). Similar results for annual exposed GDP and annual exposed population are shown in Figs. S4 and S5, respectively. For validation results see SI Discussion, Validation of Impact Assessment Results.

Fig. 3.

Percentage of global land area (excluding Antarctica and Greenland) for which there are (A) reliable anomalies in annual expected urban damage during either/both El Niño and La Niña years and (B) reliable anomalies exceeding 25% in annual expected urban damage during either/both El Niño and La Niña years. For example, “positive in 1 phase; negative in other” means there is a reliable anomaly in either the El Niño or La Niña phase and a negative anomaly in the opposite phase.