Nonstationary footprints of ENSO in the Mekong River Delta hydrology

The Mekong River Delta (MRD) is an essential agricultural area for the worldwide rice supply. Floods and droughts triggered by El Niño southern oscillation (ENSO) have been threatening sustenance in the MRD. Sustainable food supplies require understanding the response of the MRD hydrology to the changing ENSO behaviour in recent decades. Here, we reconstructed the annual rainfall maxima in the MRD using the oceanic paleoclimate proxy from coral skeletons and compared them with ENSO indexes. Annual minima of coral-based seawater oxygen isotope (δ¹⁸O_sw) correlated with annual rainfall maxima, which allowed to extend rainfall data from 1924 to the recent. The annual rainfall maxima based on δ¹⁸O_sw negatively correlated with the central Pacific El Niño index. This suggested that La Niña and central Pacific El Niño events lead to heavy and light rainy seasons. The heavy rainy season had more serious impacts in recent decades, which likely increases the flood risk. In contrast, the frequency and rainfall amount of the light rainy season has not changed significantly, although a catastrophic drought has hit the MRD. Our finding concludes that the impact of the ENSO event on MRD hydrology is inconsistent in the past century.