In Paris late last month the General Assembly of UNESCO's
Intergovernmental Oceanographic Commission (IOC) accepted the case for the
introduction of a new international thermodynamic description of seawater,
cast in terms of a new salinity variable called Absolute Salinity.
Hobart-based CSIRO Wealth from Oceans Flagship scientist, Dr Trevor
McDougall, made the case during his presentation of the Bruun Memorial
Lecture to the Paris meeting.
"Scientists will now have an accurate measure of the heat content of
seawater for inclusion in ocean models and climate projections," Dr
McDougall says.
"Variations in salinity and heat influence ocean currents and measuring
those variations are central to quantifying the ocean's role in climate
change. The new values for salinity, density and heat content should be in
widespread use within 18 months."
Marine scientists have been searching for the 'magic formula' for
measuring salinity – which varies from ocean to ocean and between
tropical, temperate and polar regions – for more than 150 years.
"These variations in salinity and temperature are responsible for
driving deep ocean currents and the major vertical overturning
circulations of the world's oceans, which transfer ocean heat towards the
Arctic and Antarctic regions," Dr McDougall says.
Unchanged since the last assessment 30 years ago, the case to review
ocean thermodynamic measurements began in 2005 when the Scientific
Committee on Oceanic Research (SCOR) established a working group, chaired
by Dr McDougall. Supporting him were Dr Rainer Feistel from the
Leibniz-Institut für Ostseeforschung in Warnemünde (Germany), Dr Frank
Millero, from the Rosenstiel School of Marine and Atmospheric Science at
the University of Miami in Florida, Dr Dan Wright of the Bedford Institute
of Oceanography, Canada and Dr David Jackett of CSIRO.
Salinity, comprising the salts washed from rocks, is measured using the
conductivity of seawater – a technique which assumes that the composition
of salt in seawater is the same in all the world's oceans.
"The new approach, involving Absolute Salinity, takes into account the
changes in the composition of seasalt between different ocean basins
which, while small, are a factor of about 10 larger than the accuracy with
which scientists can measure salinity at sea," Dr McDougall says.
Until the new description of seawater is widely adopted, ocean models
will continue to assume that the heat content of seawater is proportional
to a particular temperature variable called "potential temperature".
"The new description allows scientists to calculate the errors involved
by using this approximation while also presenting a much more accurate
measure of the heat content of seawater, namely Conservative Temperature,"
Dr McDougall says.
"The difference is mostly less than 1ºC at the sea surface, but it is
important to correct for these biases in ocean models."
The Centre for Australian Weather and Climate Research is a partnership
between CSIRO and the Bureau of Meteorology.
CSIRO initiated the National Research Flagships to provide
science-based solutions in response to Australia's major research
challenges and opportunities. The 10 Flagships form multidisciplinary
teams with industry and the research community to deliver impact and
benefits for Australia.
SOURCE: CSIRO