Ocean circulation
Arnaud Czaja
1. Ocean and Climate
2. Key observations
3. Mechanisms of ocean-atmosphere coupling
Part I
Ocean and Climate
(heat transport and storage)
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
 10 PW ( S A )
Q5
Heat transport: a long history of measurements…
Northward heat transport
Ha+Ho
Ha
Ho
Equator
Pole
Bjerknes’ (1964) monograph. Data from
Sverdrup (1957) & Houghton (1954)
19
1 unit  10 cal / day  0.5PW
Northward heat transport
Ha+Ho
Ha
Ho
10N
30N
50N
70N
Vonder Haar & Oort, JPO 1973.
1 unit  10 cal / yr  1.3PW
22
GERBE
approved!
NB: 1PW = 10^15 W
Pacific
Poleward heat
transport at 24ºN
0.76 +/- 0.3 PW
Atlantic
1.2 +/- 0.3 PW
Atlantic+Pacific
2 +/- 0.4 PW
“Across the same latitude, Ha is 1.7PW. The ocean
therefore can be considered to be more important
than the atmosphere at this latitude in maintaining
the Earth’s budget”.
Hall & Bryden, 1982; Bryden et al., 1991.
GERBE
approved!
(ask more to Chris D.!)
Trenberth & Caron, 2001
GERBE
approved!
Ha+Ho
Ho
Ha
Wunsch, JCl. 2005.
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)
Heat storage and Climate change
The surface warming due to
+4Wm-2 (anthropogenic
forcing) is not limited to the
mixed layer…
How thick is the layer is a
key question to answer to
predict accurately the
timescale of the warming.
Ho = 50m
Ho = 150m
Ho = 500m
NB: You are welcome to
download and run the model :
http://sp.ph.ic.ac.uk/~arnaud
Ensemble mean model results Q1
from the IPCC-AR4 report
Strength of ocean overturning at 30N
(A1B Scenario + constant after yr2100)
Q4
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”
The ocean is conservative below
the surface (≈100m) layer
• Temperature No heat exchange, only
pressure effects.
• Salinity. No phase change in the range of
observed concentration.
Salinity on 1027.6 kg/m3 surface
Conservative nature
of the ocean
Spatial variations of
temperature and salinity
are similar on scales from
several hundreds of kms
to a few kms.
50km
Ferrari & Polzin (2005)
10km
2km
Matsumoto, JGR 2007
“Circulation” scheme
Q6
NB: 1 Amazon River ≈ 0.2 Million m3/s
Broecker, 2005
“Circulation” scheme
Two “sources” of deep water:
NADW: North Atlantic
Deep Water
AABW: Antarctic Bottom
Water
Williams & Follows (2009)
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)
(ask more to Ute and Chris. O.!)
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
Momentum balance
Rotation
rate f/2
East to west
acceleration
fV
East to west
deceleration
NB: f = 2 Ω sinθ
up
North
East
Geostrophic balance!
Rotation
rate f/2
High
Pressure
fV
Low
Pressure
East to west
acceleration
East to west
deceleration
up
North
East
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
The movie…
Friday’s session