Presentation on theme: "Ocean circulation and coupling with the atmosphere Arnaud Czaja 1. Ocean heat storage & transport 2. Key observations 3. Ocean heat uptake and global warming."— Presentation transcript:
Ocean circulation and coupling with the atmosphere Arnaud Czaja 1. Ocean heat storage & transport 2. Key observations 3. Ocean heat uptake and global warming 4. Mechanisms of ocean-atmosphere coupling
Sometimes effects of heat storage and transport are hard to disentangle Is the Gulf Stream responsible for “mild” European winters?
“Every West wind that blows crosses the Gulf Stream on its way to Europe, and carries with it a portion of this heat to temper there the Northern winds of winter. It is the influence of this stream upon climate that makes Erin the “Emerald Isle of the Sea”, and that clothes the shores of Albion in evergreen robes; while in the same latitude, on this side, the coasts of Labrador are fast bound in fetters of ice.” Maury, 1855. Eddy surface air temperature from NCAR reanalysis (January, CI=3K) WARM! COLD! Lieutenant Maury “The Pathfinder of the Seas”
Model set-up (Seager et al., 2002) Full Atmospheric model Ocean only represented as a motionless “slab” of 50m thickness, with a specified “q- flux” to represent the transport of energy by ocean currents Atmosphere
Part III Ocean heat uptake and anthropogenic forcing of climate change
Heat storage and Climate change The surface warming due to +4Wm-2 (anthropogenic forcing) is not limited to the mixed layer. Heat exchanges between the mixed layer and deeper layers control the timescale of the surface warming.
Anthropogenic forcing Net surface ocean heating Upper ocean cooling via diabatic processes Upper ocean cooling via mass exchange with deep ocean Weak vertical ocean heat transport
Anthropogenic forcing Net surface ocean heating Upper ocean cooling via diabatic processes Upper ocean cooling via mass exchange with deep ocean Large vertical ocean heat transport
The Environmental Physics Climate Model Tropics Extra Tropics Ocean Atmosphere T A1 Heat content (J) http://www.sp.ph.ic.ac.uk/~aczaja/EP_ClimateModel.html
Upper (0-750m) ocean heat content vs TOA imbalance: observations Wong et al (2006)
Mechanisms of heat exchange between upper and deep layers Wind driven circulation pumping down of warm subtropical waters; upwelling of cold, high latitude waters. Buoyancy driven circulations sinking of dense water and upwelling of light water (= overturning circulations + eddy driven + convection). Mixing isopycnal diffusion and breaking internal gravity waves. Q1
Ocean heat uptake in wind driven gyres Global downward ocean heat transport driven by winds. Strength: Levitus (1988) Williams & Follows (2012)
Buoyancy driven circulations and ocean heat uptake : Total temperature change in the 10 th decade after 2XCO2 (idealised ocean basin) Temperature change due to change in ocean currents Temperature change in absence of change in ocean currents. Xie and Vallis (2011) Cooling
Interior mixing & ocean heat uptake Osborne (1998) Upward heat flux Downward heat flux Vertical heat flux (Wm-2) +100 -100 Equator North Pole South Pole deeper
Motions in the ocean are not isotropic: “neutral” surfaces In the simplest case of a waterworld at rest, a fluid parcel does work against the buoyancy force when displaced upward or downward. Motions along z=cst are energetically neutral. Solid Earth where Z=0 Z=h Reference density
Motions in the ocean are not isotropic: “neutral” surfaces In the real ocean, neutral surfaces take the shape of a bowl due to the distortion of spheres by the seafloor topography, surface heating, cooling and winds. Neutral surfaces in the Atlantic WOCE A16 NB: These surfaces can be approximated as surfaces of constant density (“isopycnals”). Neutrally energetic displacements
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