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) Stewart Turner (Uni Cambridge, ANU) Andreas Klocker (Uni Tasmania) Paul Linden (Uni Cambridge) Jan Zika (NOC, Southampton) Peter McIntosh (CSIRO, Hobart) Felicity Graham (Uni Tasmania) Steve Griffies (NOAA GFDL, Princeton) Sjoerd Groeskamp (Uni Tasmania) Nathan Bindoff (Uni Tasmania, Hobart) Stefan Riha (UNSW Sydney) Bill Dewar (Florida State Uni) Yuehua Li (UNSW Sydney) Chris Garrett (Uni Victoria, Canada) David Woolsey (UNSW Sydney) Juergen Willebrand (GEOMAR, Kiel) Peter Gent (NCAR, Boulder) Rainer Feistel (IOW, Warnemunde) Rich Pawlowicz (UBC, Vancouver) Dan Wright + (BIO, Canada) Richard Greatbatch (GEOMAR, Kiel) John Church (CSIRO, Hobart) Bernadette Sloyan (CSIRO, Hobart) Paul Barker (UNSW Sydney) David Jackett + (CSIRO, Hobart)
The ocean’s role in climate
The northward flux of heat
The horizontal ocean circulation
The vertical ocean circulation
The vertical ocean circulation
The layered nature of the ocean
Vertical mixing by internal wave breaking (the Standard Model)
The ocean’s role in climate Diapycnal spread of a tracer at 300m K v =0.17 x 10 -4 m 2 s -1 Ledwell et al. 1998
Climate Change
The ocean’s role in climate
The ocean’s role in climate
Emissions versus concentrations
The ocean’s role in climate
Global emissions pathways to give us a 67% chance of not exceeding 2 o C warming
Historical versus present carbon emissions
The ocean’s role in climate
Our future climate is up to us:- will we be rational or stupid?
The ocean’s role in climate
The ocean’s role in climate
Share of energy from renewable sources in 2012 (%) In October 2014 the European Union agreed to cuts its greenhouse gas emission by 40% by 2030 (compared to the emissions of 1990).
Back to the Ocean
Energy content changes in different components of the Earth system for two periods (1961–2003 and 1993–2003).
The ocean’s role in climate
Ocean Heat Content, integrated from the surface 2000 metres depth in the global ocean. X Chen, and K Tung Science 2014;345:897-903
Argo
The ocean’s role in climate
The ocean’s role in climate
The ocean’s role in climate: some of my research
The ocean’s role in climate: some of my research
My first publication; a photograph of nothing.
When an under-sea oil well “blows out”
When mixing causes un-mixing
What is an appropriate average velocity? McDougall and McIntosh (1996, 2001)
What is an appropriate average velocity:- Transport of water of given density classes McDougall and McIntosh (1996, 2001)
The vertical ocean circulation
Vertical mixing by internal wave breaking (the Standard Model)
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.
Cabbeling and Thermobaricity on the diagram
Thermobaricity Thermobaric vertical advection through an surface (with average pressure of 1400 dbar)
The ill-defined nature of “neutral surfaces” The local direction of neutral mixing is the plane that is normal to
The helical nature of neutral trajectories For a neutral surface to be well-defined, Neutral Helicity, must be zero everywhere on the surface.
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.
The ill-defined nature of “neutral surfaces” (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 diagram for the Atlantic, 250 dbar to the bottom While this plot of all the data from both the North & South Atlantic looks “solid” or “full” on the diagram, … Colour is latitude; blue in the south, green at the equator, red in the north.
The global ocean is quite “thin” in space. Atlantic Ocean, 500dbar to 3300dbar
another view of the Atlantic Atlantic Ocean, 500dbar to 3300dbar
The ill-defined nature of “neutral surfaces” (c) Zero helicity requires that the contours of constant and be parallel in an isobaric surface, that is, Figure from David
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.
Diapycnal flow caused by Neutral Helicity
The spatial variation of the vertical motion caused by four strange mixing processes Sum of vertical transports caused by the nonlinear equation of state on an surface (average pressure = 1400 dbar)
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” of breaking internal waves in the Southern Ocean.
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.
What are these “density” surfaces?
An Atlantic cross-section showing various types of density surface
Improvements in the fictitious diapycnal diffusion of density (Veronis error)
What is “heat” in the ocean? Specific heat capacity at constant pressure, c p (J kg -1 K -1 ) at p = 0 dbar
What is “heat” in the ocean?
A new internationally adopted definition of seawater in terms of Absolute Salinity and Conservative Temperature, TEOS-10
The ocean’s role in climate
Inverse model estimates of ocean mixing and circulation
Revision: the key messages of this talk
Energy content changes in different components of the Earth system for two periods (1961–2003 and 1993–2003).
Ocean Heat Content, integrated from the surface 2000 metres depth in the global ocean. X Chen, and K Tung Science 2014;345:897-903
The ocean’s role in climate
The ocean’s role in climate
Global emissions pathways to give us a 67% chance of not exceeding 2 o C warming
The layered nature of the ocean
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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