ocean circulation and coupling with the atmosphere arnaud czaja 1. ocean heat storage &...
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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
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Part I
Ocean heat storage and transport
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Ha Ho
+
Poleward energy transport
=
Net energy loss at top-of-the atmosphere
Imbalance between and = energy (heat) storage
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Poleward heat transport and storage are small…
oaoP HHPWRS ,120)1( 2
Energy exchanged at top-of-atmosphere :
Planetary albedo Solar constant
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SeasonalHeat storage
)(10 A
o
SPW
dzdycTdxt
S
Q4
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Trenberth & Caron, 2001
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Ganachaud & Wunsch, 2003
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Sometimes effects of heat storage and transport are hard to
disentangle
• Is the Gulf Stream responsible for “mild” European winters?
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“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 windsof 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 evergreenrobes; while in the same latitude, on this side, the coasts of Labrador are fastbound in fetters of ice.”
Maury, 1855.
Eddy surface airtemperature from NCAR reanalysis(January, CI=3K)
WARM!
COLD!
Lieutenant Maury “The Pathfinder of the Seas”
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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
FseaairS
OOO QQt
ThC
Atmosphere
seaairQ
FQ
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Seager et al. (2002)
Q3
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Part II
Some key oceanic observations
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World Ocean Atlas surface temperature
ºC
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Thermocline
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World Ocean Atlas Salinity (0-500m)
psu
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The “great oceanic conveyor belt”
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Matsumoto, JGR 2007
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“Circulation” scheme
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Broecker, 2005NB: 1 Amazon River ≈ 0.2 Million m3/s
Q5
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In – situ velocity measurements
Location of “long”(~2yr) currentmeters
Dep
th
Amplitude oftime variability
From Wunsch (1997, 1999)NB: Energy at period < 1 day
was removed
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1 yr
NB: Same velocity vectors but rotated
Moorings in the North Atlantic interior (28N, 70W = MODE)
Schmitz (1989)
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Direct ship observations
NB: 1m/s = 3.6kmh = 2.2mph = 1.9 knot
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Surface currents measured from Space
y
Pfu
o
1
Time mean sea surface height Standard deviation of sea surface height
“Geostrophic balance”
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10-yr average sea surface height deviation from geoid
Subtropical gyres
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10-yr average sea surface height deviation from geoid
Antarctic Circumpolar Current
Subpolar gyres
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ARGO floats (since yr 2000)
Coverage by depths
Coverage by lifetime
T/S/P profiles every 10 days
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Sv2010max
All in-situ observations can be interpolated dynamically using numerical ocean models
136101 smSv
From Wunsch (2000)
Overturning Streamfunction(Atlantic only)
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RAPID – WATCH array at 26N
Q2
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RAPID – WATCH array at 26N
14 m
illion
s £
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Part III
Ocean heat uptake and anthropogenic forcing of climate
change
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Heat storage and Climate change
The surface warming due to +4Wm-2 (anthropogenicforcing) is not limited to the mixed layer.
Heat exchanges between the mixed layer and deeper layers control the timescale of the surface warming.
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Anthropogenic forcing
Net surface ocean heating
Upper ocean cooling via diabatic processes
Upper ocean cooling via massexchange with deep ocean
Weak vertical ocean heat transport
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Anthropogenic forcing
Net surface ocean heating
Upper ocean cooling via diabatic processes
Upper ocean cooling via massexchange with deep ocean
Large vertical ocean heat transport
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The Environmental Physics Climate Model
Tropics
ExtraTropicsOcean
OH2AT
1ST 2ST
1OT 2OT
AH
OH
AtmosphereTA1
Hea
t co
nten
t (J
)
http://www.sp.ph.ic.ac.uk/~aczaja/EP_ClimateModel.html
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Upper (0-750m) ocean heat content vs TOA imbalance: observations
Wong et al (2006)
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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
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Ocean heat uptake in wind driven gyres
• Global downward ocean heat transport driven by winds.
• Strength:
1233 430
10.410 KWmyr
mwc Ekpo
Levitus (1988)
Williams & Follows (2012)
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Buoyancy driven circulations and 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)
Cooling
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Interior mixing & ocean heat uptake
Osborne (1998)Upward heat flux
Downward heat flux
Verticalheat flux(Wm-2)
+100
-100
Equator North PoleSouth Pole
deep
er
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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.
SolidEarth
g
02
1 22 hNW ref z
gN ref
refref
2where
Z=0Z=h
Reference density
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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 energeticdisplacements
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The movie…