Plot of temperature at depth over time as measured by the Port Hacking National Reference Station. Note the increase in data from IMOS sensors installed in 2009.


Marine heatwaves – Surface temperature doesn’t tell the whole story

Marine heatwaves (MHWs) defined as ‘discrete prolonged anomalously warm water events’ and associated mass mortality and habitat shifts are becoming more common with record events occurring around the world. However, little is known about the statistical characteristics of MHWs due to the lack of long term in situ observations.

Professor Moninya Roughan, of the Sydney Institute of Marine Science and leader of the IMOS New South Wales Moorings sub-facility, and University of New South Wales colleague Amandine Schaeffer used two historical datasets to investigate temperature changes along the full depth of the water column dating back to 1953.

Around Australia, long- term sampling of the ocean temperature has been conducted at three sites since the 1940-50s, including Port Hacking off Sydney. Known as PH100, this mooring, instrumented to a depth of ∼110 m, was chosen as an IMOS national reference station (NRS), together with nine other sites around the country.

The Port Hacking NRS and a second coastal mooring, ORS065, located 26 km to the north and maintained by Sydney Water Corporation since November 1990 in 65 m of water provided the data for this study.

Using a seasonally-varying climatology and temperature anomalies to identify and characterise MHW events down to 100 m depth, results showed that MHWs regularly extend the full depth of the water column, with a maximum intensity below the surface. 

These data have created one of the first long-term assessments of temperatures from the surface to the seabed as deep as 100 metres. The observations have highlighted that the extent of warmth wasn't being captured by the readily available surface temperature measurements.

‘Satellites are not getting the full picture,’ said Professor Roughan.

‘They are missing the peak and intensity, and sometimes the duration (of marine heat waves),’ she said.

Extreme temperatures at depth are driven by local downwelling favorable winds that mix the water column and reduce the stratification. These new results show the importance of considering sub-surface hydrography, and that sea surface temperature is insufficient to fully understand MHWs which are having disastrous ecological consequences in coastal regions globally.

Marine heatwaves off Sydney – based on at least five consecutive days when temperatures were in the top 10 per cent of readings – were found to last as long as a month. The average duration was between eight to twelve days.

The biggest average anomalies were at 50 m depths and the most extreme temperatures were as much as six degrees above the norm, based on two data sets covering seven and 25 years.

The Great Barrier Reef has attracted much-publicised concern, where two warm summers in a row had triggered unprecedented coral bleaching, affecting about two-thirds of the reef.

Perhaps less widely known, the East Australian Current skirting the eastern seaboard including Victoria and Tasmania is also changing, extending southwards about 350 kilometres in 60 years.

Tropical fish such as rabbitfish help keep seaweed off coral reefs but shift the ecological balance when they move into kelp forests such as those off the east coast of Tasmania.

‘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. 

While the data periods were too short to identify longer term climate trends, an abundance of research suggested marine hot spots were likely to get hotter and there would be worsening impacts.

Key points:

  • Despite the devastating ecological impact s of MHWs, few studies have investigated temperature extremes below the surface.
  • This is the first sub-surface analysis of coastal MHWs using a 60+ year data set
  • MHWs regularly extend to the sea floor, mostly during weak stratification driven by downwelling winds
  • MHWs are most intense at depth, indicating a pressing need for more sub-surface temperature records

The paper presenting this research:

Schaeffer A., and M. Roughan (2017), Sub-surface intensification of marine heatwaves off southeastern Australia: the role of stratification and local winds, Geophys. Res. Lett., 44, doi:10.1002/2017GL073714.

Parts of this article were originally published by the Sydney Morning Herald as 'Maximum damage': What's going wrong in our deep blue and warming sea, by Peter Hannam 

Categories:  Home Slider, NSW-IMOS, news, Facility, National Mooring Network, Node

The buoy at the surface of the Port Hacking National Reference Station.

Example of marine heatwave (MHW) event at ORS065. (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.