ArticlePublisher preview available

The Role of Clouds in Coral Bleaching Events Over the Great Barrier Reef

DOI:10.1029/2021GL093936
Authors:
Wenhui ZHAO
Wenhui ZHAO
Wenhui ZHAO
  • This person is not on ResearchGate, or hasn't claimed this research yet.
Yi Huang at Monash University (Australia)
Yi Huang
Steven Siems
Steven Siems
Steven Siems
  • This person is not on ResearchGate, or hasn't claimed this research yet.
Michael Manton
Michael Manton
Michael Manton
  • This person is not on ResearchGate, or hasn't claimed this research yet.
Read publisher preview
Download citation
To read the full-text of this research, you can request a copy directly from the authors.

Abstract and Figures

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.
Study domain definition and locations of five thermal coral bleaching events (CBEs). (a) Region 140–155°E, 10–25°S is defined as the study domain. Yellow shading area indicates the Great Barrier Reef (GBR) general reference map with locations of five past CBEs (red stars). Three parts of GBR general reference map are defined as Deep Tropics, Mid‐tropics, and Subtropics. (b) Bathymetric map of the domain.
Study domain definition and locations of five thermal coral bleaching events (CBEs). (a) Region 140–155°E, 10–25°S is defined as the study domain. Yellow shading area indicates the Great Barrier Reef (GBR) general reference map with locations of five past CBEs (red stars). Three parts of GBR general reference map are defined as Deep Tropics, Mid‐tropics, and Subtropics. (b) Bathymetric map of the domain.
… 
Cloud‐Radiation‐SST anomalies pattern during coral bleaching events (CBEs). Geo‐distribution of sea surface temperature anomaly (SSTA) (a), shortwave radiative flux anomaly (SWRFA), (b) and total cloud cover anomaly (TCCA), and (c) for five CBEs. Base period is 1996–2018. Open stars in (a) indicate the locations of CBEs in each year. Green lines show the Great Barrier Reef (GBR) general reference map which covers the main coral reefs of the GBR (same as in Figure 1a).
Cloud‐Radiation‐SST anomalies pattern during coral bleaching events (CBEs). Geo‐distribution of sea surface temperature anomaly (SSTA) (a), shortwave radiative flux anomaly (SWRFA), (b) and total cloud cover anomaly (TCCA), and (c) for five CBEs. Base period is 1996–2018. Open stars in (a) indicate the locations of CBEs in each year. Green lines show the Great Barrier Reef (GBR) general reference map which covers the main coral reefs of the GBR (same as in Figure 1a).
… 
Cloud‐Radiation‐SST correlations and El Niño–Southern Oscillation (ENSO) impact for coral bleaching months (JFMA). (a) Correlation between shortwave radiative flux anomaly (SWRFA) and total cloud cover anomaly (TCCA) with no time lag. (b and c) Same as (a) for one‐month lagged correlations of between SWRFA and sea surface temperature anomaly (SSTA) and between, TCCA and SSTA. (d, e, and f) Correlations of ENSO (ONI index) against SWRFA, TCCA and SSTA on monthly scale. (g, h, and i) Partial correlations of these variable anomalies when ENSO signal is removed. Note that the correlations are for coral bleaching season (January to April) over the time period of 1996–2018. Black dots indicate the correlations that are statistically significant at a significant level of 95%.
Cloud‐Radiation‐SST correlations and El Niño–Southern Oscillation (ENSO) impact for coral bleaching months (JFMA). (a) Correlation between shortwave radiative flux anomaly (SWRFA) and total cloud cover anomaly (TCCA) with no time lag. (b and c) Same as (a) for one‐month lagged correlations of between SWRFA and sea surface temperature anomaly (SSTA) and between, TCCA and SSTA. (d, e, and f) Correlations of ENSO (ONI index) against SWRFA, TCCA and SSTA on monthly scale. (g, h, and i) Partial correlations of these variable anomalies when ENSO signal is removed. Note that the correlations are for coral bleaching season (January to April) over the time period of 1996–2018. Black dots indicate the correlations that are statistically significant at a significant level of 95%.
… 
Figures - available from: Geophysical Research Letters
This content is subject to copyright. Terms and conditions apply.
  • A preview of this full-text is provided by Wiley.
  • Learn more
Preview content only
Content available from Geophysical Research Letters
This content is subject to copyright. Terms and conditions apply.
1. Introduction
The Great Barrier Reef (GBR) has experienced increasingly frequent coral bleaching events (CBEs) under
present day global warming (Hughes etal.,2017; Stuart-Smith etal.,2018). Evidence for a dominant role of
sustained elevated water temperature in the bleaching process, specifically thermal CBEs, is unequivocal
(Ainsworth etal.,2016; Barnett etal.,2005; Donner etal.,2005; Lough etal.,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 etal.,2004;
Great Barrier Reef Marine Park Authority,2006; Hughes etal.,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 etal.,2016; Toth etal., 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
etal.,2008; Glynn etal.,2001; Kleypas etal.,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 etal.,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
Key Points:
• 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
bleaching events
Supporting Information:
Supporting Information may be found
in the online version of this article.
Correspondence to:
W. Zhao,
wenhui.zhao@monash.edu
Citation:
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.
org/10.1029/2021GL093936
Received 18 APR 2021
Accepted 1 JUL 2021
10.1029/2021GL093936
RESEARCH LETTER
1 of 10
... However, these warm ocean conditions contributed to northeast Australia experiencing high cloud cover and extreme rainfall (Evans and Boyer-Souchet, 2012;Ummenhofer et al., 2015). This resulted in reduced heat stress on the GBR (Leahy et al., 2013;Zhao et al., 2021), although the resultant flooding contributed to other forms of coral stress (e.g. Jones and Berkelmans, 2014). ...
... The extent and magnitudes of HotSpots and DHWs on GBR satellite reef pixels exceed those observed during this period in any previous year on record . The presence of a La Niña, and more specifically high cloud cover, could provide the GBR with some relief from heat stress (Zhao et al., 2021). Whether that would be enough to avoid bleaching-level heat stress across large sections of the GBR in January/February/March 2022 remains to be seen. ...
Unprecedented early-summer heat stress and forecast of coral bleaching on the Great Barrier Reef, 2021-2022
Article
  • Jun 2022
The Great Barrier Reef (GBR) is predicted to undergo its sixth mass coral bleaching event during the Southern Hemisphere summer of 2021-2022. Coral bleaching-level heat stress over the GBR is forecast to start earlier than any previous year in the satellite record (1985-present). The National Oceanic and Atmospheric Administration (NOAA) Coral Reef Watch (CRW) near real-time satellite-based heat stress products were used to investigate early-summer sea surface temperature (SST) and heat stress conditions on the GBR during late 2021. As of 14 December 2021, values of instantaneous heat stress (Coral Bleaching HotSpots) and accumulated heat stress over a 12-week running window (Degree Heating Weeks) on the GBR were unprecedented in the satellite record. Further, 89% of GBR satellite reef pixels for this date in 2021 had a positive seven-day SST trend of greater than 0.2 degrees Celsius/week. Background temperatures (the minimum temperature over the previous 29 days) were alarmingly high, with 87% of GBR reef pixels on 14 December 2021 being greater than the maximum SST over that same 29-day period for any year from 1985-2020. The GBR is starting the 2021-2022 summer season with more accumulated heat than ever before, which could have disastrous consequences for the health, recovery, and future of this critical reef system.
View
... However, these warm ocean conditions contributed to northeast Australia experiencing high cloud cover and extreme rainfall (Evans and Boyer-Souchet, 2012;Ummenhofer et al., 2015). This resulted in reduced heat stress on the GBR (Leahy et al., 2013;Zhao et al., 2021), although the resultant flooding contributed to other forms of coral stress (e.g. Jones and Berkelmans, 2014). ...
... The extent and magnitudes of HotSpots and DHWs on GBR satellite reef pixels exceed those observed during this period in any previous year on record . The presence of a La Niña, and more specifically high cloud cover, could provide the GBR with some relief from heat stress (Zhao et al., 2021). Whether that would be enough to avoid bleaching-level heat stress across large sections of the GBR in January/February/March 2022 remains to be seen. ...
Unprecedented early-summer heat stress and forecast of coral bleaching on the Great Barrier Reef, 2021-2022
Article
Full-text available
  • Feb 2022
The Great Barrier Reef (GBR) is predicted to undergo its sixth mass coral bleaching event during the Southern Hemisphere summer of 2021-2022. Coral bleaching-level heat stress over the GBR is forecast to start earlier than any previous year in the satellite record (1985-present). The National Oceanic and Atmospheric Administration (NOAA) Coral Reef Watch (CRW) near real-time satellite-based heat stress products were used to investigate early-summer sea surface temperature (SST) and heat stress conditions on the GBR during late 2021. As of 14 December 2021, values of instantaneous heat stress (Coral Bleaching HotSpots) and accumulated heat stress over a 12-week running window (Degree Heating Weeks) on the GBR were unprecedented in the satellite record. Further, 89% of GBR satellite reef pixels for this date in 2021 had a positive seven-day SST trend of greater than 0.2 degrees Celsius/week. Background temperatures (the minimum temperature over the previous 29 days) were alarmingly high, with 87% of GBR reef pixels on 14 December 2021 being greater than the maximum SST over that same 29-day period for any year from 1985-2020. The GBR is starting the 2021-2022 summer season with more accumulated heat than ever before, which could have disastrous consequences for the health, recovery, and future of this critical reef system.
View
... Accordingly, our observations from Heron Reef begin to shed light on the possible role of coral reefs in this cloud formation over the GBR i.e., as coral reefs warm, they initiate convection triggering cloud formation under favourable synoptic conditions such as occur during the summer monsoon. In contrast, under settled summer El Niño conditions when the atmosphere is more stable over the GBR, cloud development may be suppressed resulting in extreme heating of water overlying coral reefs causing bleaching (McGowan and Theobald, 2017;Zhao et al., 2021). ...
Coral Reef Coupling to the Atmospheric Boundary Layer Through Exchanges of Heat, Moisture, and Momentum: Case Studies From Tropical and Desert Fringing Coral Reefs
Article
Full-text available
  • Jul 2022
Coral reefs represent abrupt changes in surface roughness, temperature, and humidity in oceanic and coastal locations. This leads to formation of internal atmospheric boundary layers that grow vertically in response to turbulent mixing which conveys changes in surface properties into the prevailing wind. As a result, coral reefs under favourable conditions couple to the overlying atmosphere. Here we present rare observations of coral reef – atmospheric interactions during summer monsoon conditions on the Great Barrier Reef, Australia, and a desert fringing coral reef in the Gulf of Eilat, Israel. We show that in the hyper-arid location of the Gulf of Eilat where air temperatures are greater than water temperatures, a stable atmospheric boundary inhibits coupling of the reef to the atmosphere. In contrast, under monsoon conditions on the Great Barrier Reef, the coral reef is shown to couple to the overlying atmosphere leading to the formation of a convective internal boundary layer in which convective exchange of heat and moisture influences cloud and possibly precipitation. We conclude that understanding these processes is essential for determining the role of coral reefs in coastal meteorology, and whether through coupling with the overlying atmosphere coral reefs may regulate their meteorology by triggering formation of cloud and/or vertical exchange on aerosols and their precursor gases such as Dimethyl sulphide.
View
... In that study, cloud cover alone was responsible for up to 32% of the variation in SST, although there was a 3-day lag between a change in cloud cover and a change in SST. Local-scale cloud cover over shallow GBR waters was recently reported to be more highly correlated with SST cooling than the larger-scale regional modulation of cloud cover linked to the El Niño-Southern Oscillation (Zhao et al., 2021). This cloud cover-SST relationship has been proposed to operate as an ocean thermostat in the western Pacific warm pool to the north of Australia where coral bleaching events have been relatively few in number despite the region having the highest average SST of all ocean regions (Kleypas et al., 2008). ...
The Potential for Great Barrier Reef Regional Climate Regulation via Dimethylsulfide Atmospheric Oxidation Products
Article
Full-text available
  • Jun 2022
Research related to the potential for coral reef-derived dimethylsulfide (DMS) oxidation products to regulate the regional climate of the Great Barrier Reef (GBR) according to the CLAW hypothesis is summarized in this mini review. The GBR has been indicated as a region of high DMS production where atmospheric emissions may be increased when corals are subject to environmental stresses associated with low tide. During low wind speeds over aerially exposed coral reefs, plumes of atmospheric DMS and new sulfate-containing nano-particle production under photo-oxidative conditions have been detected on the GBR. Hygroscopic growth of these particles in combination with coagulation and condensation processes could potentially provide a coral-mediated mechanism of new aerosol for seeding low-level stratocumulus clouds. Fine mode aerosol optical depth over GBR coral reefs has been correlated with low wind speeds and a coral stress metric formulated as a function of irradiance, water clarity, and tide height. This correlation has been proposed as a possible mechanism by which the GBR might alter the optical properties of the overlying atmosphere to attenuate local insolation leading to regional climate regulation. However, recent regional-scale aerosol-climate modeling indicates that the potential for GBR regional climate regulation via DMS atmospheric oxidation products is weak under current anthropogenic conditions which have instigated mass coral bleaching events along the entire length of the GBR between 1998 and 2022. This increased bleaching indicates that DMS oxidation products are insufficient to regulate the regional climate of the GBR according to the CLAW hypothesis under current global warming conditions.
View
Unprecedented early-summer heat stress and forecast of coral bleaching on the Great Barrier Reef, 2021-2022
Article
Full-text available
  • Nov 2022
The Great Barrier Reef (GBR) is predicted to undergo its sixth mass coral bleaching event during the Southern Hemisphere summer of 2021-2022. Coral bleaching-level heat stress over the GBR is forecast to start earlier than any previous year in the satellite record (1985-present). The National Oceanic and Atmospheric Administration (NOAA) Coral Reef Watch (CRW) near real-time satellite-based heat stress products were used to investigate early-summer sea surface temperature (SST) and heat stress conditions on the GBR during late 2021. As of 14 December 2021, values of instantaneous heat stress (Coral Bleaching HotSpots) and accumulated heat stress over a 12-week running window (Degree Heating Weeks) on the GBR were unprecedented in the satellite record. Further, 89% of GBR satellite reef pixels for this date in 2021 had a positive seven-day SST trend of greater than 0.2 degrees Celsius/week. Background temperatures (the minimum temperature over the previous 29 days) were alarmingly high, with 87% of GBR reef pixels on 14 December 2021 being greater than the maximum SST over that same 29-day period for any year from 1985-2020. The GBR is starting the 2021-2022 summer season with more accumulated heat than ever before, which could have disastrous consequences for the health, recovery, and future of this critical reef system.
View
Unprecedented early-summer heat stress and forecast of coral bleaching on the Great Barrier Reef, 2021-2022
Article
  • May 2022
The Great Barrier Reef (GBR) is predicted to undergo its sixth mass coral bleaching event during the Southern Hemisphere summer of 2021-2022. Coral bleaching-level heat stress over the GBR is forecast to start earlier than any previous year in the satellite record (1985-present). The National Oceanic and Atmospheric Administration (NOAA) Coral Reef Watch (CRW) near real-time satellite-based heat stress products were used to investigate early-summer sea surface temperature (SST) and heat stress conditions on the GBR during late 2021. As of 14 December 2021, values of instantaneous heat stress (Coral Bleaching HotSpots) and accumulated heat stress over a 12-week running window (Degree Heating Weeks) on the GBR were unprecedented in the satellite record. Further, 89% of GBR satellite reef pixels for this date in 2021 had a positive seven-day SST trend of greater than 0.2 degrees Celsius/week. Background temperatures (the minimum temperature over the previous 29 days) were alarmingly high, with 87% of GBR reef pixels on 14 December 2021 being greater than the maximum SST over that same 29-day period for any year from 1985-2020. The GBR is starting the 2021-2022 summer season with more accumulated heat than ever before, which could have disastrous consequences for the health, recovery, and future of this critical reef system.
View
A characterization of clouds over the Great Barrier Reef and the role of local forcing
Article
  • Apr 2022
A characterization of cloud properties associated with precipitation in the region around the Great Barrier Reef (GBR) is constructed using decade‐long (2007‐2017) satellite observations from Moderate Resolution Imaging Spectroradiometer (MODIS) and Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) combined with CloudSat. The spatial and vertical distributions of low‐level cloud properties over the region are also investigated and discussed. In general, different cloud types are observed over different regions and vary by season: high clouds are dominant over the low‐latitudes in summer, altocumulus are mainly observed over north‐east Queensland, and low‐level clouds are dominant over the ocean and coast during the winter months at higher latitudes under a trade wind regime. A strong latitudinal dependence of total precipitation across the Greater GBR region is identified with a significant orographic enhancement near Cairns in the wet tropics. MODIS and CloudSat‐CALIPSO cloud observations show good agreement on significant differences in low‐level cloud microphysical properties between the land and the ocean. The largest land‐ocean differences in warm cloud microphysical properties are found over the mid‐latitudes near 18°S, which is strongly associated with orographic forcing, with these enhancements extending further east to the coral reef area. However, the frequency of warm cloud is not enhanced upwind of the mountains in the wet tropics. In addition, no significant differences in warm cloud properties between the actual GBR and the open ocean are identified. These results suggest that low‐level clouds over the GBR do not show any significant response to the reef‐related microphysical perturbations. This article is protected by copyright. All rights reserved.
View
Physical Diagnosis of the 2016 Great Barrier Reef Bleaching Event
Article
Full-text available
Widespread coral bleaching across the Great Barrier Reef (GBR) in 2016 is often reportedly caused by El Niño and/or global warming. However, the GBR is not in a region where it is straightforward to anticipate sea surface temperature (SST) warming during El Niño, and the role of climate change is unclear. This study uses a diverse range of observations to investigate the physical causes of SST anomalies that developed on the GBR in 2016. Warm SST anomalies developed in two stages. Initial warming was caused by El Niño shifting the global‐scale pattern of convection, increasing solar radiation in the Coral Sea. The warm anomaly was extended and amplified near the coast by a terrestrial heat wave propagating across eastern Australia, further warming the GBR through turbulent heat flux. It is concluded that El Niño caused the SST anomaly, and global warming increased its amplitude and extended it by several months.
View
Cloud_cci Advanced Very High Resolution Radiometer post meridiem (AVHRR-PM) dataset version 3: 35-year climatology of global cloud and radiation properties
Article
Full-text available
  • Jan 2020
We present version 3 of the Cloud_cci Advanced Very High Resolution Radiometer post meridiem (AVHRR-PM) dataset, which contains a comprehensive set of cloud and radiative flux properties on a global scale covering the period of 1982 to 2016. The properties were retrieved from AVHRR measurements recorded by the afternoon (post meridiem – PM) satellites of the National Oceanic and Atmospheric Administration (NOAA) Polar Operational Environmental Satellite (POES) missions. The cloud properties in version 3 are of improved quality compared with the precursor dataset version 2, providing better global quality scores for cloud detection, cloud phase and ice water path based on validation results against A-Train sensors. Furthermore, the parameter set was extended by a suite of broadband radiative flux properties. They were calculated by combining the retrieved cloud properties with thermodynamic profiles from reanalysis and surface properties. The flux properties comprise upwelling and downwelling and shortwave and longwave broadband fluxes at the surface (bottom of atmosphere – BOA) and top of atmosphere (TOA). All fluxes were determined at the AVHRR pixel level for all-sky and clear-sky conditions, which will particularly facilitate the assessment of the cloud radiative effect at the BOA and TOA in future studies. Validation of the BOA downwelling fluxes against the Baseline Surface Radiation Network (BSRN) shows a very good agreement. This is supported by comparisons of multi-annual mean maps with NASA's Clouds and the Earth's Radiant Energy System (CERES) products for all fluxes at the BOA and TOA. The Cloud_cci AVHRR-PM version 3 (Cloud_cci AVHRR-PMv3) dataset allows for a large variety of climate applications that build on cloud properties, radiative flux properties and/or the link between them. For the presented Cloud_cci AVHRR-PMv3 dataset a digital object identifier has been issued: https://doi.org/10.5676/DWD/ESA_Cloud_cci/AVHRR-PM/V003 (Stengel et al., 2019).
View
Insights From a Decade of Research on Coral Reef-Atmosphere Energetics
Article
Full-text available
  • Apr 2019
Coral reefs cover approximately 0.10 to 0.25% of the marine environment and yet are home to around 25% of marine species and support the livelihoods of more than 500 million people. They face a wide range of threats, with the impact of global warming gaining most attention due to its frequently claimed causal link to coral bleaching. Here we review a decade of research into the micrometeorology of Heron Reef, a lagoonal platform coral reef in the southern Great Barrier Reef, Australia. Using novel pontoon-mounted eddy covariance systems, we show that often >80% of net radiation is partitioned into heating the water overlying the reef, the reef benthos, and substrate. Significant spatial variability in energy and trace gas exchanges occurs over the reef in response to different geomorphic and hydrodynamic conditions. Synoptic weather patterns that bring light winds, clear skies, and high humidity, result in reef scale meteorology that appears to have a greater influence on coral bleaching events than the background oceanic warming trend. The reef develops its own convective internal boundary layer, with potential to influence cloud development and therefore the surface energy balance. Knowledge of such local effects is lacking, so it is recommended that future research is needed into reef scale processes and how they interact with larger-scale forcing.
View
Ecosystem restructuring along the Great Barrier Reef following mass coral bleaching
Article
Full-text available
Global warming is markedly changing diverse coral reef ecosystems through an increasing frequency and magnitude of mass bleaching events1-3. How local impacts scale up across affected regions depends on numerous factors, including patchiness in coral mortality, metabolic effects of extreme temperatures on populations of reef-dwelling species4 and interactions between taxa. Here we use data from before and after the 2016 mass bleaching event to evaluate ecological changes in corals, algae, fishes and mobile invertebrates at 186 sites along the full latitudinal span of the Great Barrier Reef and western Coral Sea. One year after the bleaching event, reductions in live coral cover of up to 51% were observed on surveyed reefs that experienced extreme temperatures; however, regional patterns of coral mortality were patchy. Consistent declines in coral-feeding fishes were evident at the most heavily affected reefs, whereas few other short-term responses of reef fishes and invertebrates could be attributed directly to changes in coral cover. Nevertheless, substantial region-wide ecological changes occurred that were mostly independent of coral loss, and instead appeared to be linked directly to sea temperatures. Community-wide trophic restructuring was evident, with weakening of strong pre-existing latitudinal gradients in the diversity of fishes, invertebrates and their functional groups. In particular, fishes that scrape algae from reef surfaces, which are considered to be important for recovery after bleaching2, declined on northern reefs, whereas other herbivorous groups increased on southern reefs. The full impact of the 2016 bleaching event may not be realized until dead corals erode during the next decade5,6. However, our short-term observations suggest that the recovery processes, and the ultimate scale of impact, are affected by functional changes in communities, which in turn depend on the thermal affinities of local reef-associated fauna. Such changes will vary geographically, and may be particularly acute at locations where many fishes and invertebrates are close to their thermal distribution limits7.
View
The Community Cloud retrieval for CLimate (CC4CL) – Part 1: A framework applied to multiple satellite imaging sensors
Article
Full-text available
We present here the key features of the Community Cloud retrieval for CLimate (CC4CL) processing algorithm. We focus on the novel features of the framework: the optimal estimation approach in general, explicit uncertainty quantification through rigorous propagation of all known error sources into the final product, and the consistency of our long-term, multi-platform time series provided at various resolutions, from 0.5 to 0.02°. By describing all key input data and processing steps, we aim to inform the user about important features of this new retrieval framework and its potential applicability to climate studies. We provide an overview of the retrieved and derived output variables. These are analysed for four, partly very challenging, scenes collocated with CALIOP (Cloud-Aerosol lidar with Orthogonal Polarization) observations in the high latitudes and over the Gulf of Guinea–West Africa. The results show that CC4CL provides very realistic estimates of cloud top height and cover for optically thick clouds but, where optically thin clouds overlap, returns a height between the two layers. CC4CL is a unique, coherent, multi-instrument cloud property retrieval framework applicable to passive sensor data of several EO missions. Through its flexibility, CC4CL offers the opportunity for combining a variety of historic and current EO missions into one dataset, which, compared to single sensor retrievals, is improved in terms of accuracy and temporal sampling.
View
Increasing thermal stress for tropical coral reefs: 1871–2017
Article
Full-text available
  • Apr 2018
Tropical corals live close to their upper thermal limit making them vulnerable to unusually warm summer sea temperatures. The resulting thermal stress can lead to breakdown of the coral-algal symbiosis, essential for the functioning of reefs, and cause coral bleaching. Mass coral bleaching is a modern phenomenon associated with increases in reef temperatures due to recent global warming. Widespread bleaching has typically occurred during El Niño events. We examine the historical level of stress for 100 coral reef locations with robust bleaching histories. The level of thermal stress (based on a degree heating month index, DHMI) at these locations during the 2015-2016 El Niño was unprecedented over the period 1871-2017 and exceeded that of the strong 1997-1998 El Niño. The DHMI was also 5 times the level of thermal stress associated with the 'pre-industrial', 1877-1878, El Niño. Coral reefs have, therefore, already shown their vulnerability to the modest (~0.92 °C) global warming that has occurred to date. Estimates of future levels of thermal stress suggest that even the optimistic 1.5 °C Paris Agreement target is insufficient to prevent more frequent mass bleaching events for the world's reefs. Effectively, reefs of the future will not be the same as those of the past.
View
Encyclopaedia of Mathematics, Supplement III
Book
  • Jan 2002
This is the third supplementary volume to Kluwer's highly acclaimed twelve-volume Encyclopaedia of Mathematics. This additional volume contains nearly 500 new entries written by experts and covers developments and topics not included in the previous volumes. These entries are arranged alphabetically throughout and a detailed index is included. This supplementary volume enhances the existing twelve volumes, and together, these thirteen volumes represent the most authoritative, comprehensive and up-to-date Encyclopaedia of Mathematics available.
View
Large-scale bleaching of corals on the Great Barrier Reef
Article
In 2015-2016, record temperatures triggered a pan-tropical episode of coral bleaching. In the southern hemisphere summer of March-April 2016, we used aerial surveys to measure the level of bleaching on 1,156 individual reefs throughout the 2,300 km length of the Great Barrier Reef, the world's largest coral reef system. The accuracy of the aerial scores was ground-truthed with detailed underwater surveys of bleaching at 260 sites (104 reefs), allowing us to compare aerial and underwater bleaching data with satellite-derived temperatures and with associated model predictions of bleaching. The severity of bleaching on individual reefs in 2016 was tightly correlated with the level of local heat exposure: the southernmost region of the Great Barrier Reef escaped with only minor bleaching because summer temperatures there were close to average. Gradients in nutrients and turbidity from inshore to offshore across the Great Barrier Reef had minimal effect on the severity of bleaching. Similarly, bleaching was equally severe on reefs that are open or closed to fishing, once the level of satellite-derived heat exposure was accounted for. The level of post-bleaching mortality, measured underwater after 7-8 months, was tightly correlated with the aerial scores measured at the peak of bleaching. Similarly, reefs with a high aerial bleaching score also experienced major shifts in species composition due to extensive mortality of heat-sensitive species. Reefs with low bleaching scores did not change in composition, and some showed minor increases in coral cover. Two earlier mass bleaching events occurred on the Great Barrier Reef in 1998 and 2002, that were less severe than 2016. In 2016, less than 9% of scored reefs had no bleaching, compared to 42% in 2002 and 44% in 1998. Conversely, the proportion of reefs that were severely bleached (>60% of corals affected) was four times higher in 2016. The geographic footprint of each of the three events is distinctive, and matches satellite-derived sea surface temperature patterns. Our aerial surveys indicate that past exposure to bleaching in 1998 and 2002 did not lessen the severity of bleaching in 2016. This dataset of aerial bleaching scores provides a historical baseline for comparison with future bleaching events. No copyright restrictions apply to the use of this data set other than citing this publication. This article is protected by copyright. All rights reserved.
View
ENSO Weather and Coral Bleaching on the Great Barrier Reef, Australia
Article
The most devastating mass coral bleaching has occurred during El Niño events, with bleaching reported to be a direct result of increased sea surface temperatures (SSTs). However, El Niño itself does not cause SSTs to rise in all regions that experience bleaching. Nor is the upper-ocean warming trend of 0.11°C per decade since 1971, attributed to global warming, sufficient alone to exceed the thermal tolerance of corals. Here we show that weather patterns during El Niño that result in reduced cloud cover, higher than average air temperatures and higher than average atmospheric pressures play a crucial role in determining the extent and location of coral bleaching on the world's largest coral reef system, the World Heritage Great Barrier Reef (GBR), Australia. Accordingly, synoptic scale weather patterns and local atmosphere-ocean feedbacks related to ENSO and not large scale SST warming due to El Niño alone and/or global warming, are often the cause of coral bleaching on the GBR.
View
Vertical Fluxes of Momentum Sensible Heat and Water Vapour During the Air Mass Transformation Experiment (AMTEX) 1974
Article
  • Jan 1975
Vertical fluxes of momentum, sensible heat and water vapour were determined during the 1974 Air Mass Transformation Experiment at 3 sites using the eddy correlation and spectral density techniques. The paper describes these fluxes in terms of the bulk transfer coefficients during the cold air outbreak occurring between February 23rd and 28th. The evidence suggests an increase of the neutral drag coefficient (referred to 10m) with wind speed, to a value of 2×10⁻³ for wind speed approaching 15m s⁻¹, whilst the ‘neutral’ heat and w.v. transfer coefficients remain approximately constant at 1.5×10.3 over the wind speed range 4-15m s⁻¹. The mean sensible and latent heat fluxes for this 5 day period, based on data from Tarama-jima and Miyakojima were 11 mw cm⁻² and 55 mw cm⁻² (226 langleys day⁻¹ and 1130 langleys day⁻¹) respectively. The implied Bowen ratio of 0.20 is consistent with the measured sea surface temperature of 21°C, in agreement with the work of Priestley and Taylor (1972).
View