The Indian Ocean Dipole and El Niño Southern Oscillation as major drivers of coral cover on shallow reefs in the Andaman Sea

Area of Science:

• Marine Biology
• Climate Science
• Ecology

Background:

• Shallow reefs in the Andaman Sea, particularly at Phuket, Thailand, are characterized by massive and branching coral species.
• Coral growth is influenced by long-term sea temperature and sea level rise, but short-term climate fluctuations are primary drivers of coral cover dynamics.

Purpose of the Study:

• To identify and quantify the impact of El Niño Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) on coral cover in the Andaman Sea.
• To assess the specific roles of sea level and sea temperature changes driven by ENSO and IOD in coral cover reduction.

Main Methods:

• Analysis of 34 years of fluctuating coral cover data.
• Correlation of coral cover reductions with specific ENSO and IOD events (1997, 2010, 2019).
• Quantification of the influence of sea level and sea temperature anomalies during these climate events.

Main Results:

• Major coral cover reductions in 1997, 2010, and 2019 were linked to positive Indian Ocean Dipole (pIOD) and El Niño events.
• Combined pIOD and El Niño-induced sea level depression was critical in 1997; extreme El Niño sea temperatures drove reductions in 2010.
• A bi-phasic pIOD and El Niño event in 2019 caused reduced coral cover through both sea level decrease and high sea temperatures.

Conclusions:

• Increased frequency of extreme pIOD and El Niño events under global warming poses substantial future risks to Andaman Sea shallow reefs.
• Rising background sea temperatures exacerbate the threat, outweighing potential temporary reliefs from climate-driven sea-level changes.
• Findings have broader implications for reefs globally affected by climate-driven sea-level fluctuations, including those in Indonesia.