Mekong Delta will continue to be at risk for severe flooding

Mekong River Delta
Photo Credit: Tsuyoshi Watanabe

Scientific Frontline: Extended "At a Glance" Summary: ENSO Impacts on Mekong River Delta Flooding

The Core Concept: Reef coral growth rings from Con Dao Island provide a high-resolution paleoclimatic record linking the El Niño Southern Oscillation (ENSO) to historical rainfall, droughts, and severe flooding in the Mekong River Delta.

Key Distinction/Mechanism: Unlike standard instrumental weather records spanning only recent decades, coral skeletal salinity records use annual growth rings—similar to tree rings—to reconstruct water salinity and extend hydrological data over a century.

Origin/History: Led by Tsuyoshi Watanabe at Hokkaido University, researchers analyzed reef coral core samples from Con Dao Island, successfully extending rainfall data back to 1924 to evaluate long-term ENSO trends.

Major Frameworks/Components:

• El Niño Southern Oscillation (ENSO) dynamics in the central and eastern tropical Pacific Ocean.
• Coral sclerochronology analyzing monthly water salinity variations through skeletal growth rings.
• Historical correlation between water salinity records and annual maximum rainfall data.

Branch of Science: Climatology, Earth Science, Hydrology, Marine Biology.

Future Application: Advanced predictive modeling for extreme weather events, enhanced regional risk assessments, and proactive agricultural planning.

Why It Matters: Improved long-term climate forecasting is essential for mitigating severe flooding and droughts, protecting vulnerable populations, and ensuring food security in Southeast Asia's agricultural heartland.

(Left) A map showing the amount of rainfall across South East Asia in October from 1980 to 2005 ). (right) A satellite photo of the Mekong River Delta 
Illustration Credit: Takaaki K. Watanabe, Tung Thanh Phan, et al. Scientific Reports. December 7, 2022
Satellite Photo Credit: Google Earth

Reef corals provide an accurate, high-resolution record of the influence of the El Niño Southern Oscillation on rainfall, flooding and droughts in the Mekong River Delta, Vietnam.

The Mekong River Delta is the agricultural heartland of Vietnam; it is affected by droughts and flooding, which have become more severe in recent years. If severe weather events can be more accurately predicted, risk assessments in the regions can be improved. This, in turn, will reduce the negative effects of floods and droughts in the region.

A team led by Tsuyoshi Watanabe at Hokkaido University has revealed the clearest picture yet of how the El Niño Southern Oscillation (ENSO) affected rainfall in the Mekong Delta over the last hundred years. Their findings were published in the journal Scientific Reports. They correlated water salinity data from reef coral samples with historical weather records and uncovered that the ENSO has caused seasons of heavy and light rainfall, resulting in patterns of both flooding and droughts, respectively.

The ENSO occurs in the central and Eastern tropical Pacific Ocean, in irregular cycles of two to seven years. It consists of the El Niño (warming of the ocean surface), La Niña (cooling of the ocean surface) and neutral (neither warming, nor cooling).

“We found that heavy rains and flooding have become more severe in recent years, driven by changes in the ENSO due to global warming,” said Watanabe, corresponding author of the study. “The Mekong River Delta will continue to be faced with a higher risk of severe flooding.”

El Niño and La Niña have significant effects on weather patterns and extreme weather events in these regions. Understanding the weather patterns caused by ENSO in the Mekong River Delta (MRD) is critical to ensuring a stable food supply.

The team headed by Watanabe discovered a reef coral on Con Dao Island, 90 km south of the MRD. From a core sample of this coral, they were able to determine monthly fluctuations in the salinity of the surrounding water. “Reef corals have growth rings, similar to trees,” says Watanabe. “These rings preserve a record of the salinity of the surrounding water in each year. The reef coral we sampled was on Con Dao Island, close to the MRD; as a result, the salinity records can be associated with increased or decreased rainfall in the MRD—and hence with flooding or droughts.”

The growth rings of the reef coral on Con Dao Island investigated in this study, stretching from 2005 back to 1923
Photo Credit: Takaaki K. Watanabe, Tung Thanh Phan, et al. Scientific Reports. December 7, 2022

Combining the salinity data with historical annual rainfall maximums allowed the team to extend rainfall data back to 1924.

Analysis of this rainfall data showed that the hydrological changes in the MRD are affected by El Niño and La Niña in the Central Pacific Ocean — associated with light rain and heavy rain in the MRD respectively. In particular, heavy rain in the MRD is strongly associated with La Niña in the recent past, while the association between light rain and El Niño has been consistent during the last century.

This improved understanding of the effects of the ENSO in the Mekong River Delta will allow for more accurate weather predictions in the future. This should also enable better preparation for extreme weather events, increasing both the safety of the population and food security.

Members of the Coral Reef Environmental Earth Sciences Laboratory including key members of the research team. From the left, Atsuko Yamazaki (co-author), Kevin Garas, Kaeru Takeuchi, Tung Thanh Phan (co-first author), Tsuyoshi Watanabe (corresponding author), Takaaki K. Watanabe (co-first author) and Motoya Odajima
Photo Credit: Tsuyoshi Watanabe

Additional information: This press release is dedicated to the memory of Dr. Tung Thanh Phan, co-first author, who unexpectedly passed away on October 28, 2022. This study is partially based on the results of his doctoral research.

Reference material: What Is: El Niño, La Niña, and a Climate in Flux

Published in journal: Scientific Reports

Title: Nonstationary footprints of ENSO in the Mekong River Delta hydrology

Authors: Takaaki K. Watanabe, Tung Thanh Phan, Atsuko Yamazaki, Hong-Wei Chiang, Chuan-Chou Shen, Lam Dinh Doan, and Tsuyoshi Watanabe

Source/Credit: Hokkaido University

Edited by: Scientific Frontline

Reference Number: en120822_01

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