Example of marine heatwave (MHW) event at ORS065 in 2003. (Top) Hourly upwelling favorable wind stress, (bottom) temperature anomaly from the 90th percentile climatology (which is the threshold used to identify MHWs). Black crosses indicate the start and end of the event at each depth when detected. Note that the MHW started during downwelling-favorable winds and was not detected at the surface. Graphic: Schaeffer and Roughan, 2017 / GRL

By Peter Hannam
21 May 2017

(The Sydney Morning Herald) – Taking a dip at Sydney's beaches remains an attractive option even this far into the autumn, and the projections of climate change mean you soon won't have to be an ice-berger to swim year round.

"Sydney will have tropical waters by between 2040-60," Adriana Verges, a marine ecologist at the University of NSW, said. "Summers [will be] above 25, winter 19 degrees."

Those celebrating the future demise of the wetsuit, though, might want to take a look beyond the shallows.

A paper published in Geophysical Research Letters this month highlighted the extent of warmth wasn't being captured by the readily available surface temperature measurements.

"Satellites are not getting the full picture," Moninya Roughan, an associate professor at UNSW's Coastal and Regional Oceanography Lab and co-author of the paper said. "They are missing the peak and intensity, and sometimes the duration [of marine heat waves]." […]

"Weeks [of heatwaves] are a long time when you're a marine organism, a small creature, at the bottom of the food chain," Professor Roughan said. […]

An impact of the so-called tropicalisation of temperate waters was that herbivorous species, such as rabbitfish, silver drummers and sea urchins, were moving into rich ecosystems such as kelp forests.

"They are essentially destroying the habitat that is the foundation for the entire ecological community," Dr Verges said.

Bursts of heat can also take their toll. The extreme event off Western Australia in the 2010-11 summer – the worst recorded in 160 years of records – killed about 150 kilometres of the kelp's range, which had not recovered, Dr Verges said. [more]

'Maximum damage': What's going wrong in our deep blue and warming sea

ABSTRACT: Marine heatwaves (MHWs) are becoming more common with record events occurring around the world and unprecedented biological impacts including mass mortality and habitat shifts. However, little is known about the statistical characteristics of MHWs due to the lack of long-term in situ observations. Using two historical data sets spanning from 1953 (and 1992) to 2016, we use a seasonally varying climatology and temperature anomalies to identify and characterize MHW events down to 100 m depth in coastal waters off southeastern Australia. We show that MHWs regularly extend the full depth of the water column, with a maximum intensity below the surface. Extreme temperatures at depth are driven by local downwelling favorable winds that mix the water column and reduce the stratification. These results show the importance of considering subsurface hydrography and that sea surface temperature is insufficient to fully understand MHWs which are having disastrous ecological consequences in coastal regions globally.

Subsurface intensification of marine heatwaves off southeastern Australia: The role of stratification and local winds



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