Observations of the Meridional Overturning Circulation and

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Observations of the Meridional Overturning
Circulation and Implications for Past and Future
Change Carl Wunsch, MIT NCAR July 2008
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Oceanic change has become a public, over-heated
subject.
Sea change why global warming could leave
Britain feeling the cold No new ice age yet,
but Gulf Stream is weakening Atlantic current
came to halt for 10 days in 2004 James
Randerson, science correspondentFriday October
27, 2006The Guardian, London Scientists have
uncovered more evidence for a dramatic weakening
in the vast ocean current that gives Britain its
relatively balmy climate by dragging warm water
northwards from the tropics. The slowdown, which
climate modellers have predicted will follow
global warming, has been confirmed by the most
detailed study yet of ocean flow in the
Atlantic. Most alarmingly, the data reveal that a
part of the current, which is usually 60 times
more powerful than the Amazon river, came to a
temporary halt during November 2004.
a normally respectable newspaper
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The nightmare scenario of a shutdown in the
meridional ocean current which drives the Gulf
stream was dramatically portrayed in The Day
After Tomorrow. The climate disaster film had
Europe and North America plunged into a new ice
age practically overnight. Although no scientist
thinks the switch-off could happen that quickly,
they do agree that even a weakening of the
current over a few decades would have profound
consequences. Warm water brought to Europe's
shores raises the temperature by as much as 10C
in some places and without it the continent would
be much colder and drier. Researchers are not
sure yet what to make of the 10-day hiatus. "We'd
never seen anything like that before and we don't
understand it. We didn't know it could happen,"
said Harry Bryden, at the National Oceanography
Centre, in Southampton, who presented the
findings to a conference in Birmingham on rapid
climate change. Is it the first sign that the
current is stuttering to a halt? "I want to know
more before I say that," Professor Bryden
said. Lloyd Keigwin, a scientist at the Woods
Hole Oceanographic Institution, in Massachusetts,
in the US, described the temporary shutdown as
"the most abrupt change in the whole climate
record". He added "It only lasted 10 days. But
suppose it lasted 30 or 60 days, when do you ring
up the prime minister and say let's start
stockpiling fuel? How can we rule out a longer
one next year?" Prof Bryden's group stunned
climate researchers last year with data
suggesting that the flow rate of the Atlantic
circulation had dropped by about 6m tonnes of
water a second from 1957 to 1998. If the current
remained that weak, he predicted, it would lead
to a 1C drop in the UK in the next decade. A
complete shutdown would lead to a 4C-6C cooling
over 20 years. The study prompted the UK's
Natural Environment Research Council to set up an
array of 16 submerged stations spread across the
Atlantic, from Florida to north Africa, to
measure flow rate and other variables at
different depths. Data from these stations
confirmed the slowdown in 1998 was not a "freak
observation"- although the current does seem to
have picked up slightly since. Special
reports
Lloyd Keigwin, a scientist at the Woods Hole
Oceanographic Institution, in Massachusetts, in
the US, described the temporary shutdown as "the
most abrupt change in the whole climate
record". He added "It only lasted 10 days. But
suppose it lasted 30 or 60 days, when do you ring
up the prime minister and say let's start
stockpiling fuel? How can we rule out a longer
one next year?"
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Temperature Proxies The Greenland ice core
M. Bender, GISP2 core
D-O events
Bolling-Allerod
Last Glacial Maximum
Holocene
instrumental record
15oC
Younger Dryas
-60,000y
-10,000y

• Not temperature, but clearly related to it.
• Dominant features
• Glacial/interglacial
• Apparent Holocene stability
• (3) Glacial period instability (rapid
  fluctuations,
• the Dansgaard-Oeschger, D-O, events)

The D-O events appear to represent major warmings
occurring sometimes in less than 10 years! How
could such things happen?
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The favored explanation (see Al Gore, The Day
After Tomorrow, The Guardian (London), The
Independent (London), The Economist, Nature,
Science, .) is that the ocean circulation shuts
off in the North Atlantic.
Is this credible?
A GREAT GRAPHIC! BUT THE NOTION THAT OCEANIC
FLOW IS A ONE-DIMENSIONAL RIBBON IS A LOVELY
FANTASY.
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Claims a 30 reduction in overturning
circulation, 50 reduction in lower NADW
transport since 1957. Huge publicity.
Nature, December 2005
Data used are 5 snapshots since 1957.
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• Some basic, background issues.
• The expression thermohaline circulation should
  be suppressed. on the basis that it has become
  essentially meaningless through misuse.
• There is a circulation of heat (temperature) and
  of salt. They are very different (they have
  entirely different boundary conditions). The
  expression has been used in at least several
  mutually contradictory ways
• The circulation of heat and salt (which are
  different).
• The circulation driven by heat and salt fluxes at
  the sea surface.
• The circulation driven by density anomalies
• The circulation driven by pressure anomalies
• The abyssal circulation.
• The abyssal circulation driven by abyssal density
  anomalies

Example "The thermohaline circulation (THC) is a
global pattern of currents that arises from
gradients in density, and hence hydrostatic
pressure, between different regions in the
world's oceans." E. Hawkins and R. Sutton, 2007,
Clim. Dyn., 29, 745-762. Or, Bulletin of the
American Meteorological Society, June 2006, P.
803, "...the thermohaline circulation, as
measured in the Florida Straits, was near the
long-term mean. (What does this mean?)
Note, at least, that the ocean circulation is
almost indistinguishable from geostrophic
balance. Does flow drive pressure gradients or
vice-versa?
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Contrary to hundreds of papers about box models,
conveyor belts, zonally integrated models,
etc., the ocean circulation is three-dimensional
and highly time-dependent. What does one see?
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The ocean is a noisy place and so its easy to
be fooled by short-term variations
Surface elevation anomaly
Bottom pressure anomaly
cm
From Estimating the Circulation and Climate of
the Ocean (ECCO) consortium (MIT, JPL, GFDL, U.
of Hamburg) results. A very dynamic, basically
turbulent system, whose sampling is a major
challenge.
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Red segment denotes western end of RAPID array
Dudley Chelton, 2008. Gridded altimetric data
alone. 1 cm produces about 7Sv transport at
mid-latitudes
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Some people seem to be able to sit at their desks
and determine how this was different in the past
and how it will be in the future. An extremely
impressive intellectual feat!
Macdonald and Wunsch, 1996, Nature. Oversimplifi
ed versions of the actual circulation
Lumpkin Speer, JPO, 2008
The complicated pathways are an essential
ingredient in understanding how the system works,
how it might change in the future, and how it
might have been in the past.
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How fast can the ocean gross baroclinic structure
change? Recall Veronis and Stommel (1956)
L/vgroup
This is the initial signal velocity, not the
adjustment time.
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The energetics of the system largely determine
how fast things can change.
Ferrari Wunsch, 2009, Ann. Rev. Fluid Mechs.
Rates of exchange of energy between ocean and
atmosphere, and between the components of oceanic
energy involved in the general circulation are
all O(1012W). Changing the abyssal N. Atlantic
temperature by 1 degree C leads to a PE change of
order 1022J for a time scale of 1010s or about
300 years---unless energy transfer rates are
greatly changed in the process.
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T. Stocker, Science, 1998
(referring to Dansgaard-Oeschger events)
Numerous modeling studies (13) have shown that
changes in the meridional heat transport in the
Atlantic Ocean, caused by sudden changes of the
Atlantic's thermohaline circulation, are
resulting in antiphase behavior of north and
south. A sudden increase of the northward
meridional heat flux draws more heat from the
south and leads to a warming in the north that is
synchronous with a cooling in the south (14). It
has been shown that the stability of this
circulation is limited (5) and that changes in
the surface salinity can trigger major
reorganizations of this circulation. More
importantly, simulations demonstrate that the
amplitudes and rapidity of events compare well
with the paleoclimatic record (15).
15
Total oceanatmosphere heat transport from
(mainly) observations. Total from ERBE. Ocean
from hydrography (Ganachaud), atmosphere as
residual.
heat in from sun
heat radiated to space
heat radiated to space
northward
Wunsch, 2005, J. Climate
southward
ocean
At high northern latitudes, the poleward
transport of heat is dominated by the atmosphere.
Suppose one interrupted the North Atlantic part.
Can the North Atlantic Ocean tail wag the
atmospheric dog? Recall, too, Bjerknes
compensation.
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What is observed to be going on?
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Among the Data Types
WOCE
Argo
T/P, Jason
GRACE
How to synthesize? Estimation/optimal control
problem Use a model (MITgcm) and its adjoint
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An independent estimate, in which we attempt to
use all the data, no matter what type it is, from
1992 onward. How to put those together to create
an understanding of what the three-dimensional
ocean is doing over days to decades? About 1
billion data constraints used.
Some withheld data (TOGA/TAO, drifter velocities,
tomography,)
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The ECCO-GODAE setup, v2

• 1 degree horizontal resolution
• covering 80N to 80S
• 23 vertical levels
• GM/Redi eddy parameterization
• KPP vertical mixing scheme
• covers 1992 to 2006 (2007 imminent)
• forcing 6-hourly NCEP air-sea fluxes

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ECCO-GODAE estimates are from ordinary
least-squares solutions obtained by adjoining
the model to a model-data misfit function using
an ancient mathematical trick Lagrange
multipliers
misfit to Initial conditions

misfit to the observations
adjustable parameters (controls)
the model
vectors of Lagrange multipliers, AKA, the adjoint
or dual solution
and seek the stationary point.
In control engineering, called the Pontryagin
Minimum Principle, in meteorology 4DVAR, in
oceanography the adjoint method, . Solved by
iteration relying upon knowledge of the partial
derivatives of J with respect to x(t), u(t),
using automatic/algorithmic differentiation
(AD) software tools. Will skip all that here.
After adjustment, the model is run forward in
time, in ordinary free mode, using the adjusted
parameters.
Two major difficulties the size of the problem,
and the need to understand errors in everything.
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Errors and uncertainties

• The case for remaining vigilant
• current instruments at the edge of technology,
  error estimates still fragile
• Recent corrections published for almost all
  observations
• Argo (depth-errors, , Lyman et al., 2007)
• XBT (fall rates, , Gouretski Koltermann, 2006)
• Altimetry (geocenter motion, , e.g. Lavallee et
  al., 2006)
• SST (Thompson et al., 2008)
• Error/uncertainty estimates and updates remain
  crucial, for new observations and model
  representation
• G. Forget and C. Wunsch, 2006 Global
  hydrographic variability and the data weights in
  oceanic state estimates. J. Phys. Oceanogr.,
  2007.
• Ponte, R. M., C. Wunsch and D.Stammer, 2007
  Spatial mapping of time-variable errors in
  Jason-1 and TOPEX/POSEIDON sea surface height
  measurements. JAOT, 2007.
• D. Stammer and A. Koehl and C. Wunsch, 2006
  Impact of the GRACE geoid on ocean circulation
  estimates. JAOT, in press.

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Thompson et al., 2008, Nature
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Zonal sum, 15-year time-mean meridional vol.
transport/meter
upper 300m only
Zonal integrals are not particle pathways.
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Zonal integral, vertical velocity, 15-year time
mean.
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Within error bars, there is consistency of the
meridional heat transport of the model and that
from independent calculations using only thermal
wind balance and Ekman flows.
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W. Broecker, Science, 2003
The Younger Dryas The prevailing view of this
cold snap is that it was triggered by a
catastrophic release of fresh water stored in
proglacial Lake Agassiz (6). This release was
initiated when the retreating margin of the
Laurentide ice sheet opened a lower outlet,
allowing much of the lake's stored water to flood
across the region now occupied by the northern
Great Lakes into the St. Lawrence valley and from
there into the northern Atlantic (Fig. 1). On the
basis of reconstructions of the pre- and
post-diversion shorelines of Lake Agassiz, it has
been estimated that     9500 km3 of water was
released (6). If released over the course of a
single year, this flood would match today's net
annual input of fresh water to the Atlantic Ocean
region north of 45N. In most ocean models, an
input of this magnitude cripples formation of
deep water in the northern Atlantic (i.e., it
greatly weakens or even shuts down the model's
conveyor circulation).
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fresh water anomalies created at
Greenland/Antarctic margins
Stammer, JGR, 2008
Consider the basic time scale of change. Do we
see anything inconsistent with the elementary
theoretical ideas?
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Stammer, 2008, JGR
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Must be very careful about the time scales. A
very large annual cycle in the ocean. Physics
tells us that baroclinic adjustment requires
decades. Conflating paleo-time scales with modern
instrumental time scales makes no sense. Global
ocean observations really only become available
in the 1990s---a very short time! Gross ocean
has a baroclinic stability (from Challenger era
data). Huge issues over sampling that are widely
ignored. Can one really accurately compute
oceanic mean temperatures and salinities 50 or
100 years ago? The data base suggests
otherwise. Much fantasy in this business.
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upper (0-1165m, 1165-4450m, 4450-bottom),
intermediate, and abyssal ocean mass flux
heat (enthalpy) fluxes
26N North Atlantic
3- monthly average mass flux
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Expanded scale, variability month to month of
midwater mass flux, 1165-4450m. Aliasing is a
serious issue in the use of synoptic sections to
estimate time-average values.
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26N in Pacific
3-monthly avg. zonal integrals
12-yr. means
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mass flux
temperature flux
salt flux
Antarctica to Australia
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44 of the variance.
First EOF, zonal volume monthly data
First EOF zonal volume, annual mean data 40 of
the low frequency variance
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2nd EOF meridional volume flux based on monthly
means. 15 of low frequency variance
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First EOF, meridional enthalpy (heat) transport.
Ocean is taking up heat, 3.2W/m2 (in the
NCEP/NCAR reanalysis---ocean data inadequate to
force a smaller value).
First EOF, atm. forcing
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Why is there an obsession with the North
Atlantic? About 10 of the area of the world
ocean. The atmosphere is in contact with it for a
comparatively brief time. Signals of oceanic
influence on the atmosphere are marginal at best.
(Wikipedia article asserts that the MOC exists
only in the Atlantic Ocean. One wonders what the
author knows of the rest of the oceans.)
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A Summary Statement
There is no observational evidence of any major
change taking place in the overturning
circulation of the ocean over the past 15 years
when global ocean observations became
available. All of our theory suggests that in
the perturbation regime, mid and high latitude
baroclinic shifts require multiple decades for
adjustment. The influence of the ocean on the
atmosphere is difficult to detect on decadal time
scales. (Didnt talk about this. Weak
signals.) It appears extremely unlikely that D-O
events are ocean generated. How could the ocean
undergo major baroclinic adjustment in under 10
years? What physics would operate? How would the
energy pathways be accomplished? (Sea level
change timing is not consistent.) The
circulation is very noisy. Whatever true trends
exist will almost surely require many decades to
detect. Five and 10 year observational programs
will tell us something about interannual
variability but little else. Climate is an
intergenerational problem. Coarse resolution
models exhibiting violent respone to massive
fresh water injections need to be understood in
terms of energy transfer rates and numerical
artifacts.
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If you are going to forecast, bet on ocean
persistence.
Thank you