Seasonality in the daily timing of low water levels on Australian coral reefs

Jo Buckee¹, Charitha Pattiaratchi², Yasha Hetzel², Jennifer Verduin³

1. Environmental and conservation Sciences, Murdoch University, Perth, Western Australia, Australia
2. Oceans Graduate School & The UWA Oceans Institute , University of Western Australia, Perth, Western Australia, Australia
3. Environmental and conservation Sciences, Murdoch University, Murdoch, WA, Australia

Water levels have a fundamental influence on the intertidal zone of coral reefs, where exposure to air limits the upward growth of corals and explains the flattened summits of reefs.  Injury to corals occurs when extreme low water levels and emersion of corals coincides with environmental stressors such as high solar radiation, low air temperature, rainfall and wind stress.  Most stressors are both diurnally and seasonally variable, and therefore the daily and seasonal timing of low water is a key determinant of coral response to emersion and consequent variability in coral cover in intertidal areas.  

The interannual, seasonal, and daily timing of low water was investigated using a high resolution numerical model sea level hindcast (1958-2016) (http://sealevelx.ems.uwa.edu.au). The analysis focused on twenty representative coral reef locations around the Australian coastline between 1992-2016.  From these data, the daily timing of low water was calculated to demonstrate the regionally distinct seasonality of low water events. The relative contribution of tidal and non-tidal variability to the low water extremes were also investigated.  Seasonal patterns and historical coincidence of selected stressors with water levels at each location was investigated using the nearest Bureau of Meteorology daily solar exposure and air temperature data. 

Critical low water events were found to be the result of both tidal and non-tidal factors operating at range of time scales.  The coincidence of low water levels and environmental stressors varied with tidal regime, broad-scale fluctuations in mean sea level and regional climatic variables. 

The study demonstrates the intrinsic temporal and spatial variability in the timing of extreme low water levels, with implications for the assessment of coral reef health.  In an era of escalating and valid concern about anthropogenic impacts on coral reefs, it is important that fundamental baseline processes continue to inform impact assessment.

The history and future of coastal inundation in Sydney, NSW

Ben Hague^{1, 2}, Shayne McGregor²

1. Bureau of Meteorology, Docklands, VIC, Australia
2. School of Earth Atmosphere and Environment, Monash University, Clayton, VIC, Australia

Coastal inundation occurs due to a combination of tidal, atmospheric and oceanic factors. However, there is a global pattern of regular tidal inundation (i.e. coastal inundation caused solely by tidal factors) emerging as mean sea levels rise and cause high tide levels to surpass inundation thresholds (e.g. Ghanbari et al. 2019, Sweet et al. 2018). These events can cause disruptions to local economies and populations (e.g. Hino et al. 2019). To date, the risk that tidal inundation poses to Australia in the future has not been assessed within an impact-based framework. The high population and high-value coastal assets at risk in Sydney provide clear motivation for this work. In addition, the high spatial and temporal sea level data availability, including the 104-year Fort Denison record, makes Sydney an ideal candidate for a first-pass assessment of future tidal inundation risk in Australia.

By collating many historical and contemporary reports of coastal inundation we define regional impact-based thresholds corresponding to different severity of impacts following the methodology of Hague et al. (2019). Applying these thresholds to the Fort Denison sea level record yields a 104-year history of coastal inundation in the Sydney region. Furthermore, we assess the emergence times and probabilities of regular (e.g. monthly, weekly, daily) coastal inundation in the Sydney region under various sea level rise scenarios and provide motivation for a national assessment of coastal inundation risk under a changing climate.

1. Ghanbari, M., Arabi, M., Obeysekera, J. and Sweet, W. 2019. A coherent statistical model for coastal flood frequency analysis under nonstationary sea level conditions. Earth's Future, 7, 162–177. DOI: 10.1029/2018EF001089
2. Hague, B., Murphy, B., Jones, D. and Taylor, A. 2019. Impact-based thresholds for extreme sea levels and the emergence of tidal inundation in Northern Australia, Oral Presentation, AMOS-ICTMO Conference 2019, Darwin.
3. Hino, M., Belanger, S. T., Field, C. B., Davies, A. R. and Mach, K. J. 2019. High-tide flooding disrupts local economic activity. Sci. Adv., 5, DOI: 10.1126/sciadv.aau2736
4. Sweet, W., Dusek, G., Obeysekera, J. and Marra, J. J. 2018. Patterns and Projections of High Tide Flooding Along the U.S. Coastline Using a Common Impact Threshold. NOAA Technical Report NOS CO-OPS 086, pp.56

Marine heatwave impacts on ocean biogeochemistry

Hakase Hayashida^{1, 2}, Richard Matear³, Pete Strutton^{1, 2}

1. Institute for Marine and Antarctic Studies, University of Tasmania, Battery Point, TASMANIA, Australia
2. Australian Research Council Centre of Excellence in Climate Extremes, University of Tasmania, Hobart, Tasmania, Australia
3. CSIRO Oceans and Atmosphere, Hobart

Ocean temperature extremes such as marine heatwaves are expected to intensify in coming decades due to anthropogenic global warming. Reported ecological and economic impacts of marine heatwaves include coral bleaching, local extinction of mangrove and kelp forests, and elevated mortalities of invertebrates, fishes, seabirds, and marine mammals. In contrast, little is known about the impacts of marine heatwaves on microbes that regulate biogeochemical processes in the ocean. Here we analyse the physical and biogeochemical output of a near-global eddy-resolving ocean circulation model (OFAM3) simulation to elucidate the impacts of marine heatwaves on phytoplankton blooms in the tropics and temperate oceans. The model successfully simulates many of the observed marine heatwave events in recent decades. Our model-based analysis reveals contrasting responses of phytoplankton blooms to marine heatwave events which are dependent on the background nutrient concentration. In nutrient replete conditions, marine heatwaves are associated with elevated phytoplankton blooms, whereas in nutrient limited conditions, they are associated with reduced blooms. We analyse the daily-mean satellite observations of sea surface temperatures, chlorophyll a concentrations, and carbon biomass to assess the validity of the findings of the model-based analysis. Lastly, we discuss the implications of these findings for marine ecosystems and biogeochemistry under future climate.

2013/14 South Atlantic marine heatwave and South American drought triggered by common driver

Regina R. Rodrigues¹, Andréa S. Taschetto², Alex Sen Gupta², Gregory R. Foltz³

1. Dept. Oceanography Florianópolis, Federal University of Santa Catarina, Florianópolis, SC, Brazil
2. Climate Change Research Centre, University of New South Wales, Sydney, NSW, Australia
3. Atlantic Oceanographic and Meteorological Laboratory, NOAA, Miami, Florida, USA

A severe marine heatwave was observed in the southwest Atlantic in summer 2013-14 coinciding with an intense drought over southeast South America. Over land it led to water shortages in São Paulo, the world’s fourth most populated city, and tripled the dengue fever cases with fatalities. Similarly, ocean heatwaves can severely impact local ecosystems. The effects of the 2013-14 event for the South Atlantic marine biology is yet to be quantified, although a significant drop in the chlorophyll suggests a reduction in primary production and fishery in the region. This study investigates the mechanisms that triggered the marine heatwave in the southwest Atlantic. We found that the ocean heatwave was caused by the same mechanism that triggered the drought in southeast South America, i.e. atmospheric blocking events that persisted almost the entire summer over the region. The atmospheric blocking was triggered by convection in the eastern Indian Ocean associated with Madden-Julian Oscillation. The atmospheric blocking-related anticyclone inhibited cloud cover off southeast Brazil coast and increased shortwave radiation as well as weakened surface winds and reduced ocean heat loss thus establishing the marine heatwave in the region. We further show that the atmospheric blocking explains approximately 60% of the marine heatwave events in the southwestern Atlantic. An increase in frequency, duration, intensity and extension of marine heatwave events is seen over the satellite period 1982–2016. By understanding when and how these events form we can start to predict them and provide forewarning to local industries and communities.