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The Great Barrier Reef (GBR) has experienced increasingly frequent coral bleaching events (CBEs) under
present day global warming (Hughes etal.,2017; Stuart-Smith etal.,2018). Evidence for a dominant role of
sustained elevated water temperature in the bleaching process, specifically thermal CBEs, is unequivocal
(Ainsworth etal.,2016; Barnett etal.,2005; Donner etal.,2005; Lough etal.,2018). For the GBR, unusually
warm sea surface temperatures (SSTs) have caused significant thermal CBEs several times in recent dec-
ades, particularly in the summers of 1998, 2002, 2006, 2016 and 2017 (AIMS,2017; Berkelmans etal.,2004;
Great Barrier Reef Marine Park Authority,2006; Hughes etal.,2017,2018). For this study, CBEs refer only
to thermal coral bleaching events.
Previous studies have shown that reef growth and decline across the Pacific is strongly modulated by El
Niño-Southern Oscillation (ENSO) variability (Leonard etal.,2016; Toth etal., 2012). Some of the most
devastating mass CBEs, including those reported over the GBR (McGowan & Theobald,2017), have oc-
curred during El Niño events, where bleaching was reported to be a direct result of increased SSTs (Baker
etal.,2008; Glynn etal.,2001; Kleypas etal.,2015). There are, however, other El Niño events (e.g., 2002–
2003) that have not led to reports of significant CBEs across the GBR. Indeed, El Niño itself does not lead to
the rise of SSTs in all regions. For example, the Western Pacific typically has below average SSTs due to the
eastward shift of the Walker Circulation during El Niño. Thus, attributing the CBEs in the western tropical
Pacific Ocean directly to the increased regional SSTs caused by El Niño is not consistent with the well-es-
tablished understanding of ENSO SST variability across the tropical Pacific Ocean (Baker etal.,2008). It is
Abstract Thermal coral bleaching events (CBEs) over the Pacific, including those over the Great
Barrier Reef (GBR), have commonly been linked to the El Niño–Southern Oscillation (ENSO), with
bleaching reported to be a direct result of sea surface temperature (SST) anomalies driven by El Niño.
However, such a relationship cannot explain CBEs that occurred during La Niña or the neutral phase
of the ENSO. Here, we show that the GBR is characterized by a significant negative correlation between
total cloud cover anomaly (TCCA) and lagged SST anomaly (SSTA) whose magnitude and spatial extent
are greater than the SSTA-ENSO correlation. This significant negative TCCA-SSTA (lagged) correlation
prevails over two-thirds of the study domain even after the ENSO signal is removed, which suggests that
local-scale reduced cloud cover is a key component of the regional warm shallow water formation over the
GBR and the occurrence of thermal CBEs.
Plain Language Summary Thermal coral bleaching events (CBEs) over the Pacific, including
those over the Great Barrier Reef (GBR), have commonly been linked to the El Niño–Southern Oscillation
(ENSO), with bleaching reported to be a direct result of warmer sea surface temperatures (SSTs) driven by
El Niño (positive phase of ENSO). However, CBEs have also been reported over the GBR during La Niña
and the neutral phase of the ENSO, when large-scale SSTs may be cooler than normal. Here, we show
that the SST anomaly over the GBR is more highly correlated with local cloud cover than with ENSO.
This significant relationship between local cloud cover and SST can be found over two-thirds of the study
domain even when the ENSO impact is ignored. Accordingly, we conclude that local-scale reduced cloud
cover plays an important role in regional warming of the shallow water over the GBR, regardless of the
large-scale ENSO impact.
ZHAO ET AL.
© 2021. American Geophysical Union.
All Rights Reserved.
The Role of Clouds in Coral Bleaching Events Over the
Great Barrier Reef
Wenhui Zhao1,2 , Yi Huang2,3 , Steven Siems1,2, and Michael Manton1
1Monash University, Melbourne, VIC, Australia, 2ARC Centre of Excellence for Climate Extreme, Melbourne, VIC,
Australia, 3The University of Melbourne, Melbourne, VIC, Australia
• Lagged regional sea surface
temperature (SST) is correlated
with total cloud cover across the
Great Barrier Reef (GBR) and direct
shortwave cloud radiative forcing
• SST over the GBR is more highly
correlated with the overhead cloud
cover than the large-scale El Niño–
Southern Oscillation signal
• Local-scale reduced cloud cover
plays a crucial role in the shallow
water warming over the GBR and
the occurrence of thermal coral
Supporting Information may be found
in the online version of this article.
Zhao, W., Huang, Y., Siems, S., &
Manton, M. (2021). The role of clouds
in coral bleaching events over the Great
Barrier Reef. Geophysical Research
Letters, 48, e2021GL093936. https://doi.
Received 18 APR 2021
Accepted 1 JUL 2021
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