Jessica A. Benthuysen

Coral reef metabolism and carbon chemistry dynamics of a coral reef flat

Global carbon emissions continue to acidify the oceans, motivating growing concern for the ability of coral reefs to maintain net positive calcification rates. Efforts to develop robust relationships between coral reef calcification and carbonate parameters such as aragonite saturation state (Ωarag) aim to facilitate meaningful predictions of how reef calcification will change in the face of ocean acidification. Here we investigate natural trends in carbonate chemistry of a coral reef flat over diel cycles and relate these trends to benthic carbon fluxes by quantifying net community calcification and net community production. We find that, despite an apparent dependence of calcification on Ωarag seen in a simple pairwise relationship, if the dependence of net calcification on net photosynthesis is accounted for, knowing Ωarag does not add substantial explanatory value. This suggests that, over short time scales, the control of Ωarag on net calcification is weak relative to factors governing net photosynthesis.

The unprecedented 2015/16 Tasman Sea marine heatwave

The Tasman Sea off southeast Australia exhibited its longest and most intense marine heatwave ever recorded in 2015/16. Here we report on several inter-related aspects of this event: observed characteristics, physical drivers, ecological impacts and the role of climate change. This marine heatwave lasted for 251 days reaching a maximum intensity of 2.9 °C above climatology. The anomalous warming is dominated by anomalous convergence of heat linked to the southward flowing East Australian Current. Ecosystem impacts range from new disease outbreaks in farmed shellfish, mortality of wild molluscs and out-of-range species observations. Global climate models indicate it is very likely to be that the occurrence of an extreme warming event of this duration or intensity in this region is respectively ≥330 times and ≥6.8 times as likely to be due to the influence of anthropogenic climate change. Climate projections indicate that event likelihoods will increase in the future, due to increasing anthropogenic influences.

Freshening anomalies in the Indonesian throughflow and impacts on the Leeuwin Current during 2010–2011

During the 2010–2011 La Nina and Ningaloo Nino, excessive precipitations in the Maritime Continent and Indonesian-Australian Basin caused surface waters to freshen by 0.3 practical salinity unit in the southeast Indian Ocean. The low-salinity anomalies are observed to be carried westward by the Indonesian throughflow and the South Equatorial Current and transmitted into the poleward flowing eastern boundary current, the Leeuwin Current, along the Western Australian coast. Low-salinity anomalies contribute to about 30% of the anomalous increase of the southward Leeuwin Current transport during the evolution of the 2010–2011 Ningaloo Nino, resulting in unprecedented warming off the coast of Western Australia. Episodical freshening of the Leeuwin Current has been observed at the Rottnest coastal reference station of Western Australia during extended La Nina conditions over the past several decades; low-salinity anomalies at the station during the 2010–2011 Ningaloo Nino are comparable with strong historical events.

Intrusive upwelling in the Central Great Barrier Reef

In the Central Great Barrier Reef, the outer continental shelf has an open reef matrix that facilitates the exchange of waters with the Coral Sea. During austral summer, cool water intrudes onto the shelf along the seafloor. Temperature observations reveal cool, bottom intrusions during a 6 year period from the Queensland Integrated Marine Observing Systems Palm Passage mooring. A metric is used to identify 64 intrusion events. These intrusions predominantly occur from October to March including the wet season. During an event, the outer-shelfs near-bottom temperature decreases by 1–3°C typically over 1 week. The near-bottom salinity tends to increase, while near-surface changes do not reflect these tendencies. Intrusion events occur predominantly with either weakening equatorward winds or poleward wind bursts. A regional hydrodynamic model for the Great Barrier Reef captures the timing and amplitude of these intrusions. During intrusion events, isotherms tend to uplift over the continental slope and onto the shelf and the East Australian Current intensifies poleward. Over the shelf, a bottom-intensified onshore current coincides with bottom cooling. For numerous events, the model diagnostics reveal that the cross-shelf flow is dominated by the geostrophic contribution. A vertical circulation tilts the isopycnals upward on the southern side of the passage, causing an along-shelf density gradient and geostrophic onshore flow with depth. While wind fluctuations play a major role in controlling the along-shelf currents, model results indicate that a concurrent topographically induced circulation can assist the onshore spread of cool water.

Longer and more frequent marine heatwaves over the past century

Heatwaves are important climatic extremes in atmospheric and oceanic systems that can have devastating and long-term impacts on ecosystems, with subsequent socioeconomic consequences. Recent prominent marine heatwaves have attracted considerable scientific and public interest. Despite this, a comprehensive assessment of how these ocean temperature extremes have been changing globally is missing. Using a range of ocean temperature data including global records of daily satellite observations, daily in situ measurements and gridded monthly in situ-based data sets, we identify significant increases in marine heatwaves over the past century. We find that from 1925 to 2016, global average marine heatwave frequency and duration increased by 34% and 17%, respectively, resulting in a 54% increase in annual marine heatwave days globally. Importantly, these trends can largely be explained by increases in mean ocean temperatures, suggesting that we can expect further increases in marine heatwave days under continued global warming.Marine heatwaves are climatic extremes with devastating and long-term impacts on marine ecosystems, fisheries and aquaculture. Here the authors use a range of ocean temperature observations to identify significant increases in marine heatwaves over the past century.

Topographic Beta Spiral and Onshore Intrusion of the Kuroshio Current

This study was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA11020104 and XDA110203052), the National Natural Science Foundation of China (NSFC) (41576023, 41376030, and 41476019), the Foundation for Innovative Research Groups of NSFC (41421005), NSFC-Shandong Joint Fund for Marine Science Research Centers (U1406401), Aoshan Sci-Tec Innovative Project of Qingdao National Laboratory for Marine Science and Technology (2016ASKJ02), the National Key Research and Development Program of China (2017YFC1404000 and 2016YFC1401601), and National Key research and development Plan Sino-Australian Center for Healthy Coasts(2016YFE0101500). It was also supported by the High Performance Computing Center at the IOCAS. We thank Fei Yus group at IOCAS for providing the CTD data. The satellites reanalyzed data (SST, SSH, heat flux, water flux, and wind stress) are publicly available, and their websites have been given where they are used. IOCAS maintains a public website (http://159.226.158.89/owncloud/index.php/s/62RnQxI6HHPjOgp) that includes the cruise data and model results. The intermediate data files and computing codes used in this study are available on request to the authors. The authors declare that they have no conflicts of interest. The altimeter products were produced by Ssalto/Duacs and distributed by Aviso, with support from CNES (http://www.aviso.altimetry.fr/(http:/www.aviso.altimetry.fr/duacs/)%22).

Extreme Marine Warming Across Tropical Australia During Austral Summer 2015–2016

During austral summer 2015–2016, prolonged extreme ocean warming events, known as marine heatwaves (MHWs), occurred in the waters around tropical Australia. MHWs arose first in the southeast tropical Indian Ocean in November 2015, emerging progressively east until March 2016, when all waters from the North West Shelf to the Coral Sea were affected. The MHW maximum intensity tended to occur in March, coinciding with the timing of the maximum sea surface temperature (SST). Large areas were in a MHW state for 3–4 months continuously with maximum intensities over 2°C. In 2016, the Indonesian‐Australian Basin and areas including the Timor Sea and Kimberley shelf experienced the longest and most intense MHW from remotely sensed SST dating back to 1982. In situ temperature data from temperature loggers at coastal sites revealed a consistent picture, with MHWs appearing from west to east and peaking in March 2016. Temperature data from moorings, an Argo float, and Slocum gliders showed the extent of warming with depth. The events occurred during a strong El Nino and weakened monsoon activity, enhanced by the extended suppressed phase of the Madden‐Julian Oscillation. Reduced cloud cover in January and February 2016 led to positive air‐sea heat flux anomalies into the ocean, predominantly due to the shortwave radiation contribution with a smaller additional contribution from the latent heat flux anomalies. A data‐assimilating ocean model showed regional changes in the upper ocean circulation and a change in summer surface mixed layer depths and barrier layer thicknesses consistent with past El Nino events.