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Trevor J McDougall
Royal Society of Tasmania, 4th November 2014
What has the Ocean got to do with Climate?
Ocean Physics, School of Mathematics and Statistics
What has the Ocean got to do with Climate? Trevor J McDougall Royal - - PowerPoint PPT Presentation
What has the Ocean got to do with Climate? Trevor J McDougall Royal - - PowerPoint PPT Presentation
What has the Ocean got to do with Climate? Trevor J McDougall Royal Society of Tasmania, 4 th November 2014 Ocean Physics, School of Mathematics and Statistics Acknowledgements Gary Brown (Uni Adelaide, Princeton) Yuzhu You (Uni Sydney)
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The ocean’s role in climate
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The northward flux of heat
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The horizontal ocean circulation
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The vertical ocean circulation
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The vertical ocean circulation
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The layered nature of the ocean
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Vertical mixing by internal wave breaking
(the Standard Model)
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The ocean’s role in climate
Diapycnal spread of a tracer at 300m Kv=0.17 x 10-4 m2s-1 Ledwell et al. 1998
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Climate Change
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The ocean’s role in climate
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The ocean’s role in climate
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Emissions versus concentrations
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The ocean’s role in climate
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Global emissions pathways to give us a 67% chance of not exceeding 2oC warming
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Historical versus present carbon emissions
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The ocean’s role in climate
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Our future climate is up to us:- will we be rational or stupid?
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The ocean’s role in climate
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The ocean’s role in climate
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In October 2014 the European Union agreed to cuts its greenhouse gas emission by 40% by 2030 (compared to the emissions of 1990).
Share of energy from renewable sources in 2012 (%)
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Back to the Ocean
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Energy content changes in different components of the Earth system for two periods (1961–2003 and 1993–2003).
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The ocean’s role in climate
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Ocean Heat Content, integrated from the surface 2000 metres depth in the global ocean.
X Chen, and K Tung Science 2014;345:897-903
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Argo
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The ocean’s role in climate
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The ocean’s role in climate
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The ocean’s role in climate: some of my research
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The ocean’s role in climate: some of my research
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My first publication; a photograph of nothing.
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When an under-sea oil well “blows out”
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When mixing causes un-mixing
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What is an appropriate average velocity?
McDougall and McIntosh (1996, 2001)
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What is an appropriate average velocity:- Transport of water of given density classes
McDougall and McIntosh (1996, 2001)
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The vertical ocean circulation
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Vertical mixing by internal wave breaking
(the Standard Model)
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Cabbeling and Thermobaricity
This is a “parcel view” of lateral thermobaricity and cabbeling Cabbeling occurs when the water parcels intimately mix at the molecular level. The diapycnal thermobaric advection occurs as the parcels move together, before mixing. This advection is made irreversible by the molecular mixing.
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Cabbeling and Thermobaricity
- n the diagram
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Thermobaricity
Thermobaric vertical advection through an surface
(with average pressure of 1400 dbar)
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The local direction of neutral mixing is the plane that is normal to
The ill-defined nature of “neutral surfaces”
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The helical nature of neutral trajectories
For a neutral surface to be well-defined, Neutral Helicity, must be zero everywhere on the surface.
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The ill-defined nature of “neutral surfaces”
Neutral helicity can be written as
where the thermobaric coefficient is
H being zero implies
(a) that the line lies in an isobaric surface, and (b) that contours of p and in a neutral tangent plane are parallel, and (c) that and data in an isobaric surface describe a line (rather than an area) on the diagram.
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(a) Zero helicity requires that lie in the p surface since has to be zero.
Scanned map of p and theta on an approximately neutral surface.
The ill-defined nature of “neutral surfaces”
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The diagram for the Atlantic, 250 dbar to the bottom
Colour is latitude; blue in the south, green at the equator, red in the north.
While this plot of all the data from both the North & South Atlantic looks “solid” or “full” on the diagram, …
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Atlantic Ocean, 500dbar to 3300dbar
The global ocean is quite “thin” in space.
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another view of the Atlantic
Atlantic Ocean, 500dbar to 3300dbar
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Figure from David
(c) Zero helicity requires that the contours of constant and be parallel in an isobaric surface, that is,
The ill-defined nature of “neutral surfaces”
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Vertical motion due to the ocean not being 100% “skinny”
Because the ocean is not totally “empty” in space, fluid can migrate vertically through any “density” surface simply by cork- screwing its way along helical neutral trajectories without the need for any dissipation of mechanical energy. If the global ocean volume in space were not so tiny, it would make no sense to study diapycnal mixing, tidal mixing, and diapycnal tracer diffusion.
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Diapycnal flow caused by Neutral Helicity
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The spatial variation of the vertical motion caused by four strange mixing processes
Sum of vertical transports caused by the nonlinear equation
- f state on an surface (average pressure = 1400 dbar)
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The zonally-averaged dianeutral velocity
Zonal mean of the sum of four different vertical transports, each caused by the nonlinear nature of the equation of state . => These nonlinear processes are larger than the “standard model”
- f breaking internal waves in the Southern Ocean.
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The globally-integrated dianeutral transports
Vertical profile of globally integrated transports caused by neutral helicity (its spatial form, green), cabbeling (blue), thermobaricity (red), and their sum (black). => The nonlinear equation of state causes downwelling of about 6 Sv, mostly in the Southern Ocean.
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What are these “density” surfaces?
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An Atlantic cross-section showing various types of density surface
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Improvements in the fictitious diapycnal diffusion of density (Veronis error)
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Specific heat capacity at constant pressure, cp (J kg-1 K-1) at p = 0 dbar
What is “heat” in the ocean?
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What is “heat” in the ocean?
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A new internationally adopted definition of seawater in terms
- f Absolute Salinity and
Conservative Temperature, TEOS-10
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The ocean’s role in climate
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Inverse model estimates of ocean mixing and circulation
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Revision: the key messages of this talk
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Energy content changes in different components of the Earth system for two periods (1961–2003 and 1993–2003).
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Ocean Heat Content, integrated from the surface 2000 metres depth in the global ocean.
X Chen, and K Tung Science 2014;345:897-903
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The ocean’s role in climate
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The ocean’s role in climate
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Global emissions pathways to give us a 67% chance of not exceeding 2oC warming
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The layered nature of the ocean
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Vertical motion due to the ocean not being 100% “skinny”
Because the ocean is not totally “empty” in space, fluid can migrate vertically through any “density” surface simply by cork- screwing its way along helical neutral trajectories without the need for any dissipation of mechanical energy. If the global ocean volume in space were not so tiny, it would make no sense to study diapycnal mixing, tidal mixing, and diapycnal tracer diffusion.
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