Extreme temperatures in the Tasman Sea interior

Gabriela G Pilo¹, Neil N Holbrook¹ 

1. University of Tasmania, Hobart, TASMANIA, Australia

Extreme temperatures can be defined as those exceeding the 90^thpercentile threshold relative to climatology. In the Tasman Sea, between Tasmania and New Zealand, extreme sea surface temperatures were observed for ~250 subsequent days from September 2015 to May 2016. Regional surface waters during this ‘marine heatwave’ event reached up to 3^oC above climatology, impacting local biological communities, fishing and harvesting practices, and marine aquaculture. While several studies characterise extreme temperatures in the ocean surface, extreme temperatures at subsurface are poorly understood. This knowledge gap is due, in large part, to limited subsurface observations. Here, we aim to characterise extreme temperature events in the Tasman Sea interior. We use output from the Ocean Forecasting Australia Model version 3 to identify and characterise temperature extremes between the surface and 2000 m depths. We show that extreme subsurface temperatures are characterised by different intensities, durations, and depth penetration. These differences might be linked to different drivers. We found that eddy and current-driven extreme temperatures tend to occur off Australia’s east coast, extending from the surface to 1500 m depth, are up to 3^oC warmer than climatology at mid-depth (100-500 m), and typically last for up to 60 days. Atmospheric-driven extreme temperatures tend to occur over the entire Tasman sea. These extremes may extend from the surface to 150 m depth, are up to 3^oC warmer than climatology, and typically last for 30 days. Extreme temperatures are also evident between 500 and 2000 m depths, in the centre of the Tasman Sea. At these depths, extreme temperatures are ~0.2^oC warmer than climatology and can persist for over 400 days. The causes for these deep extreme temperatures are being explored. Characterising deep extreme temperatures is the first step to understand their drivers and potential impacts.

Marine heatwaves and their drivers: a global assessment

Neil J Holbrook¹, Hillary A Scannell², Alexander Sen Gupta³, Jessica A Benthuysen⁴, Ming Feng⁵, Eric CJ Oliver⁶, Lisa V Alexander³, Michael T Burrows⁷, Markus G Donat⁸, Alistair J Hobday⁹, Pippa J Moore¹⁰, Sarah E Perkins-Kirkpatrick³, Dan A Smale¹¹, Sandra C Straub¹², Thomas Wernberg¹² 

1. University of Tasmania, Hobart, TASMANIA, Australia
2. School of Oceanography, University of Washington, Seattle, WA, USA
3. Climate Change Research Centre, The University of New South Wales, Sydney, NSW, Australia
4. Australian Institute of Marine Science, Townsville, QLD, Australia
5. Indian Ocean Marine Research Centre, CSIRO Oceans and Atmosphere, Crawley, WA, Australia
6. Department of Oceanography, Dalhousie University, Halifax, Nova Scotia, Canada
7. Scottish Association for Marine Science, Scottish Marine Institute, Oban, Argyll, Scotland, UK
8. Barcelona Supercomputing Center, Barcelona, Spain
9. CSIRO Oceans and Atmosphere, Hobart, Tasmania, Australia
10. Institute of Biological, Environmental and Rural Sciences, Aberystwyth University, Aberystwyth, UK
11. The Laboratory, Citadel Hill, Marine Biological Association of the United Kingdom, Plymouth, UK
12. UWA Oceans Institute and School of Biological Sciences, The University of Western Australia, Crawley, WA, Australia

Marine heatwave (MHW) events can cause devastating impacts on marine ecosystems and species. However, knowledge of their causes is largely based on individual regional case study investigations of high impact MHW events. Here we (1) undertake a systematic and comprehensive search of observed MHW events reported in the peer-reviewed literature since 1950, (2) synthesise and critically assess reported details of the characteristic drivers and oceanographic processes that caused them, classified by ocean climate region and time scale under a common framework, and (3) perform a statistical analysis of these events using a hierarchical MHW definition applied to satellite sea surface temperature records – providing unified estimates of MHW event intensity, duration and spatial extent. Our global assessment presents a unified picture of the important relationships between MHWs and their local and remote drivers.

Ecosystems around the world negatively impacted by marine heatwaves

Dan Smale¹, Thomas Wernberg², Eric Oliver³, Mads Thomsen⁴, Ben Harvey⁵, Sandra Straub², Michael Burrows⁶, Lisa Alexander⁷, Jessica Benthuysen⁸, Markus Donat⁷, Ming Feng⁹, Alistair Hobday¹⁰, Neil Holbrook¹¹, Sarah Perkins-Kirkpatrick⁷, Hilliary Scannell¹², Alex Sen Gupta⁷, Ben Payne⁶, Pippa Moore⁵ 

1. Marine Biological Association of the United Kingdom, UK
2. UWA Oceans Institute and School of Biological Sciences, Perth
3. Department of Oceanography, Dalhousie University, , Halifax, Nova Scotia
4. School of Biological Sciences, University of Canterbury, Christchurch
5. Aberystwyth University, Aberystwyth
6. Department of Ecology, Scottish Association for Marine Science, Scottish Marine Institute, Oban
7. University Of New South Wales, Kensington, NSW, Australia
8. Australian Institute of Marine Science, Crawley
9. CSIRO Oceans and Atmosphere, Perth
10. CSIRO Oceans and Atmosphere, Hobart
11. Institute for Marine and Antarctic Studies, University of Tasmania, Hobart
12. School of Oceanography, University of Washington, Seattle

Recent work has demonstrated that marine heatwaves (MHWs) have become longer and more frequent over recent decades and these trends are likely to accelerate in the future. Here we examined historical trends in MHW characteristics and identified regions where MHW changes are collocated with areas of high diversity value, species living at the upper level of their thermal tolerances, and regions concurrently impacted by non-climatic anthropogenic stressors.

In a global meta-analysis of 116 research papers, we examined responses of organisms, populations and communities to eight distinct MHW events. All events were associated with negative ecological impacts. Sessile species like corals were most heavily impacted while some mobile species experienced no deleterious impacts. Some fish responses to MHWs were positive, largely related to the invasion of tropical species into temperate regions.

A species-level analysis demonstrated that populations living closer to the warm limit of their species distributions are more negatively impacted by MHWs, as they are more likely to experience conditions that exceed their thermal tolerances.

Finally, an examination of three globally important habitat-formers found that increases in the frequency of MHW days was significantly associated with increases in coral bleaching, decreases in seagrass density and deceases kelp biomass, explaining between 30 to 40% of the ecological performance of these taxa.

Our study shows that MHWs cause widespread ecological impacts (i.e. across multiple taxa, regions and processes) and can drive step-wise shifts in ecosystem structure and functioning. Given that they are predicted to intensify with anthropogenic climate change, MHWs are likely to be prominent agents of ecological change in the coming decades.

Indo-Pacific convection drives unprecedented marine heatwave in the South Atlantic Ocean and drought over eastern South America

Regina Rodriges¹, Andréa Taschetto², Alex Sen Gupta² 

1. Coordenadoria de Oceanografia - Universidade Federal de Santa Catarina, Florianópolis
2. University Of New South Wales, Kensington, NSW, Australia

In the summer of 2013-14, an unprecedented marine heatwave occurred in the south-eastern Atlantic. This co-occurred with drought in eastern South America and outbreaks of dengue fever. The combined terrestrial-marine extreme event was driven by a blocking high pressure system which prevented the passage of synoptic systems and the normal development of the South Atlantic Convergence Zone that brings rain to the region during summer. Over the ocean, the high-pressure system was associated with reduced cloud cover - thereby increasing solar radiation - and weaker winds - that reduced the turbulent heat loses from the ocean.

Based on observational data from the last few decades, we show that these atmospheric conidiations typically prevail during MHW events in this region and that the blocking high is part of a planetary wave train triggered by convection associate with the passage of the MJO in the tropical Info Pacific. We also find a consistent reduction in marine primary productivity associated with these regional MHWs.

A next-generation tool for modelling dynamic coastal inundation

Julian O'Grady¹, Kathleen McInnes¹, Raymond Cohen², Mahesh Prakash² 

1. O&A, CSIRO, Aspendale, Victoria, Australia
2. DATA61, CSIRO, Clayton, Victoria, Australia

Rising sea levels are placing more coastal assets at risk from inundation. To date, a static inundation approach referred to as the ‘bathtub’ model has been used to investigate the coastal regions and infrastructure at risk from overland flooding due to storm tides (the combination of atmospheric-driven storm surge and astronomical tide) and rising sea levels. The bathtub approach suffers the disadvantage of; (1) not accounting for the time dependence of coastal flooding and hence the duration of flood events and (2) considering only the overland inundation and not potential inundation that can occur due to backwater effects within underground drainage networks. CSIRO’s DATA61 group have developed the CFAST modelling tool to address these shortcomings and in collaboration with CSIRO Oceans and Atmosphere are developing the capability to enable coastal practitioners to more realistically model the impact of sea level rise and storm tides on inundation.

Extreme value analysis of storm tide water levels provide a design height for inundation studies. These extreme water levels can be added to the projection of changes in global mean sea level to investigate future extreme water levels. For dynamic modelling of inundation, a time series (not a single height) of water level is required to force the hydrodynamic model boundary. In this study we demonstrate and evaluate the impacts arising from differently constructed extreme water level boundary condition scenarios of surge and tide and use the CFAST hydrodynamic model to simulate the dynamic inundation. The CFAST model is aimed to assist coastal councils investigate the location and duration of the local impact of sea level rise on the extreme inundation events of coastal assets.

Combined effects of monsoon driven waves and macro-tidal currents on sediment resuspension and distribution

Fernando P. Andutta¹, Ruth G. Patterson², Xiao Hua Wang³ 

1. Griffith Climate Change Response Program, Griffith University, Gold Coast, Qld, Australia
2. Charles Darwin University, Darwin, NT, Australia
3. UNSW, Canberra, ACT, Australia

Sediment transport processes in remote tropical and macro-tidal estuaries are typically understudied due to costly instrumentation requirements, access difficulties and the extreme weather conditions. Dry season sediment transport in Darwin Harbour, northern Australia, is governed by tidal asymmetry. In contrast, few if any studies have focused on the Australian summer monsoon period. We monitored turbidity and suspended sediment concentration (SSC) using sea bed moorings at more than seven locations in Darwin Harbour over one Australian summer monsoon season. We compared our results with tide, wind, river discharge, wave height and period, and ocean currents recorded over the same period. We found that the maximum SSC (330 mgL^-1) correlated with a peak in significant wave height (>1.3 m) and consistently strong (~7.5 ms^-1) long lasting (25-29 days) north to northwesterly winds. Peak SSC was six times the spring tide SSC outside of active monsoon periods. A striking and paradoxical feature of this study was that the maximum SSC coincided with neap tides and low river discharge, due to unusually low rainfall for the first active monsoon in the study period. Based on the literature, we expected the neap tide and low discharge combination to produce less turbid conditions except during cyclones. However, we found that SSC was highest during a neap tide and low freshwater discharge because active monsoon conditions superposed tidal dominance for sediment resuspension. We found that significant wave heights were associated with northwesterly wind driven ocean swell, because fetch distance to the north is not adequate to produce such wave heights. Therefore, active monsoons (occurring 1-4 times per year), drive waves which superpose tidal dominance for sediment resuspension and dispersal. These findings should be taken into account for all future port developments and infrastructure projects by including wave modelling in preliminary hydrodynamic modelling.