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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
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 ) SoR  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
Qairsea
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…