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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 Part I Ocean heat storage and transport Net energy loss at top-of-the atmosphere = Poleward energy transport + Ha Imbalance between and = energy (heat) storage Ho Poleward heat transport and storage are small… Energy exchanged at top-of-atmosphere : (1 P ) SoR 120 PW H a , H o 2 Planetary albedo Solar constant Seasonal Heat storage So cTdx dy dz t 10PW ( S A ) Q4 Trenberth & Caron, 2001 Ganachaud & Wunsch, 2003 Sometimes effects of heat storage and transport are hard to disentangle • Is the Gulf Stream responsible for “mild” European winters? WARM! COLD! Eddy surface air temperature from NCAR reanalysis (January, CI=3K) “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. 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 “qflux” to represent the transport of energy by ocean currents Atmosphere TS OCO hO Qair sea QF t Qairsea QF Q3 Seager et al. (2002) Part II Some key oceanic observations World Ocean Atlas surface temperature ºC Thermocline World Ocean Atlas Salinity (0-500m) psu The “great oceanic conveyor belt” Matsumoto, JGR 2007 “Circulation” scheme Q5 NB: 1 Amazon River ≈ 0.2 Million m3/s Broecker, 2005 In – situ velocity measurements Amplitude of time variability Depth Location of “long” (~2yr) currentmeters From Wunsch (1997, 1999) NB: Energy at period < 1 day was removed Moorings in the North Atlantic interior (28N, 70W = MODE) 1 yr Schmitz (1989) NB: Same velocity vectors but rotated Direct ship observations NB: 1m/s = 3.6kmh = 2.2mph = 1.9 knot Surface currents measured from Space 1 P fu o y “Geostrophic balance” Time mean sea surface height Standard deviation of sea surface height 10-yr average sea surface height deviation from geoid Subtropical gyres 10-yr average sea surface height deviation from geoid Subpolar gyres Antarctic Circumpolar Current ARGO floats (since yr 2000) T/S/P profiles every 10 days Coverage by lifetime Coverage by depths All in-situ observations can be interpolated dynamically using numerical ocean models Overturning Streamfunction (Atlantic only) max 10 20Sv 3 1 1Sv 10 m s 6 From Wunsch (2000) RAPID – WATCH array at 26N Q2 RAPID – WATCH array at 26N 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. Weak vertical ocean heat transport Anthropogenic forcing Net surface ocean heating Upper ocean cooling via mass exchange with deep ocean Upper ocean cooling via diabatic processes Large 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 The Environmental Physics Climate Model HA TS 1 HO TS 2 TO 2 TO1 Tropics TA2 Ocean Extra Tropics Heat content (J) H O TA1 Atmosphere 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 Williams & Follows (2012) • Global downward ocean heat transport driven by winds. • Strength: Levitus (1988) 30m o c p wEk 10 4.10 4Wm 2 K 1 yr 3 3 Buoyancy driven circulations and Cooling ocean heat uptake : • Total temperature change in the 10th 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) Interior mixing & ocean heat uptake Upward heat flux Osborne (1998) deeper +100 Downward heat flux Vertical heat flux (Wm-2) -100 South Pole Equator North Pole 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. 1 2 2 W N ref h 0 2 Z=h Z=0 Solid Earth g where N 2 ref g ref ref z 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 NB: These surfaces can be approximated as surfaces of constant density (“isopycnals”). Neutrally energetic displacements WOCE A16 The movie…