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

Abstract

Shallow reefs are a major feature of coral assemblages in the Andaman Sea. At Phuket, Thailand sheltered reefs are dominated by massive corals, together with an increasing abundance of branching species during favourable growth conditions. The growth of coral on these reefs is moderated by long‐term increases in sea temperature and relative sea level but fluctuating decadal/intradecadal climate processes of El Niño Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD), which modulate sea level and temperature, are the main drivers of coral cover. In this study, the contribution of these two climate processes was identified and also quantified. Over a 34‐year study of fluctuating coral cover, the three major reductions in cover in 1997, 2010 and 2019 were linked to overlapping positive IOD (pIOD) and El Niños in 1997 and 2019, and with an El Niño alone in 2010. Combined pIOD and El Niño depressed sea level was the major factor in reducing cover in 1997 while El Niño extreme sea temperatures were responsible for large reductions in 2010. In 2019, a bi‐phasic pIOD and El Niño resulted in lowered cover at a time of both decreased sea level and high sea temperature. Under global warming scenarios, it is projected that extreme pIODs, such as those seen in 1997 and 2019, will occur more frequently while El Niño frequencies will continue to increase even after global mean temperature stabilization. In these circumstances, and with steadily rising background sea temperatures, the future risks to the shallow reefs of the Andaman Sea are substantial, despite any temporary respite gained from climate related or land subsidence sea‐level rise. Such findings have wider implications for all reefs affected by climatic‐driven sea‐level depressions, particularly those around Indonesian shores where similar El‐Niño‐related reductions in coral cover have been reported.

Daily timing of low tide drives seasonality in intertidal emersion mortality risk

“…At ABR, emersion mortality was recorded in 2018 (Buckee et al, 2019) following an extended absence of emersion (Figure 9D, ESM 5D). In contrast to West Coast sites, East Coast sites showed a weaker relationship with SOI (Figure 10B), however the association of coral emersion mortality with El Niño conditions in the Western Pacific has been highlighted previously by Ampou et al (2015) and more recently by Dunne et al (2021), and the results of this study generally substantiate those findings. Similarly, Glynn et al (2017) note that catastrophic mortalities of corals can occur due to extreme reef flat exposures during La Niña events that decrease the mean sea level in the Eastern Pacific.…”

“…Although not typically regarded as intertidal organisms, corals are commonly found on, and indeed form, intertidal reef platforms (Yonge, 1940;Davies and Marshall, 1980;Richards et al, 2015) where vertical growth is inherently constrained by an inability to grow in the air (Achituv and Dubinsky, 1990). Temporal variability in sea level is an important, but underreported driver of temporal dynamics in shallow coral reef habitats and has the potential to confound the assessment of coral cover in relation to other factors, including temperature and climate drivers such as the El-Niño Southern Oscillation (ENSO) (Ampou et al, 2017;Zapata, 2017;Buckee et al, 2019;Dunne et al, 2021). The investigation of sea level variability in relation to coral reefs has broad and fundamental implications for all intertidal habitats, including seagrass beds, mangroves, salt marshes and rocky shores.…”

Coral bleaching at Low Isles during the 1928–1929 Great Barrier Reef Expedition

“…Indeed, for the GBR sea-level rise modelling under extreme climatic projections (RCP 8.5) projects that reefs will become submerged at a timescale of 100+ years (Morgan et al 2020). Superimposed on this are fluctuating decadal/intradecadal climatic process such as the Indian Ocean Dipole and El-Niño Southern Oscillation which depress or elevate sea level (as well as sea temperature) in an apparently stochastic manner at scales of months to years and are an important driver of coral cover on shallow reefs (Dunne et al 2021).…”