Observations of the Meridional Overturning Circulation andImplications for Past and Future Change

Carl Wunsch, MIT

NCAR July 2008

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, 2006

The 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

Oceanic change has become a public, “over-heated” subject….

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?"

M. Bender, GISP2 coreTemperature Proxies: The Greenland ice core

Not temperature, but clearly related to it.

Dominant features:(1) Glacial/interglacial(2) Apparent Holocene stability(3) Glacial period instability (rapid fluctuations, the `Dansgaard-Oeschger,’ D-O, events)

Holocene

Younger Dryas

Last Glacial Maximum

Bolling-Allerod

15oC

-60,000y -10,000y

D-O events

The D-O events appear to represent major warmings occurring sometimes in less than 10 years! How could such things happen?

instrumental record

is supposed to shut down occasionally---when fresh waterfloods the North Atlantic---preventing sinking.

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.

A GREAT GRAPHIC! BUT THE NOTION THAT OCEANIC FLOW IS A ONE-DIMENSIONAL RIBBON IS A LOVELY FANTASY.

Is this credible?

Nature, December 2005 Claims a 30% reduction in overturning circulation, 50% reduction in lower NADW transport since 1957. Huge publicity.

Data used are 5“snapshots” since 1957.

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:

(1) The circulation of heat and salt (which are different).(2) The circulation driven by heat and salt fluxes at the sea surface. (3) The circulation driven by density anomalies(4) The circulation driven by pressure anomalies(5) The abyssal circulation.(6) The abyssal circulation driven by abyssal density anomalies

Note, at least, that the ocean circulation is almost indistinguishable from geostrophic balance. Does flow drive pressure gradients or vice-versa?

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?)

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?

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.

Surface elevationanomaly:

Bottom pressure anomaly

cm

The ocean is a “noisy” place and so it’s easy to be fooled by short-term variations:

Red segment denotes western end of RAPID array

Dudley Chelton, 2008.Gridded altimetric data alone. 1 cm produces about 7Sv transport at mid-latitudes

Macdonald and Wunsch, 1996, Nature.

Oversimplified versions of the actual circulation

The complicated pathways are an essential ingredient in understandinghow the system works, how it might change in the future, and how it mighthave been in the past.

Lumpkin & Speer, JPO, 2008

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!

How fast can the ocean gross baroclinic structure change?

Recall Veronis and Stommel (1956):

This is the initial signal velocity, not the adjustment time.

L/v g

roup

The energetics of the system largely determine how fast things can change.

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.

Ferrari & Wunsch, 2009,

Ann. Rev. Fluid Mechs.

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).

T. Stocker, Science, 1998

(referring to Dansgaard-Oeschger events)

Total ocean+atmosphere heat transport from (mainly) observations. Total from ERBE. Ocean from hydrography (Ganachaud), atmosphere as residual.

ocean

heat in from sunheat radiated tospace

heat radiated tospace

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.

northward

southward

Wunsch, 2005, J. Climate

What is observed to be going on?

Among the Data Types

How to synthesize? Estimation/optimal control problem:Use a model (MITgcm) and its adjoint:

Argo T/P, Jason

GRACE

WOCE

Some withheld data (TOGA/TAO, drifter velocities, tomography,…)

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.

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

and seek the stationary point.

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:

‘

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.

vectors of Lagrange multipliers, AKA, the adjoint or dual solution

misfit to Initial conditions

misfit to the observations

adjustable parameters (controls)

the model

Two major difficulties: the size of the problem, and the need to understand errors in everything.

After adjustment, the model is run forward in time, in ordinary free mode, using the adjusted parameters.

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.

Thompson et al.,2008, Nature

Zonal sum, 15-year time-meanmeridional vol. transport/meter

upper 300m only

Zonal integrals are not particle pathways.

Zonal integral, vertical velocity, 15-year time mean.

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.

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 45°N. 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).

W. Broecker, Science, 2003:

Stammer, JGR, 2008Consider the basic time scale of change.Do we see anything inconsistent with the elementary theoretical ideas?

fresh water anomalies created at Greenland/Antarctic margins

Stammer, 2008, JGR

upper (0-1165m, 1165-4450m, 4450-bottom), intermediate, andabyssal ocean mass flux

heat (enthalpy) fluxes

3- monthly average mass flux

26N North Atlantic

Expanded scale, variability month to month of midwater mass flux,1165-4450m. Aliasing is a serious issue in the use of synoptic sectionsto estimate time-average values.

26N in Pacific

3-monthly avg. zonal integrals

12-yr. means

Antarctica to Australia

mass flux

temperature flux

salt flux

First EOF, zonal volume monthly data

First EOF zonal volume,annual mean data ~40% of the low frequency variance

~44% of the variance.

2nd EOF meridional volume flux based on monthly means. ~15% of low frequency variance

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

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.)

A Summary StatementThere 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. (Didn’t 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.

If you are going to forecast, bet on ocean persistence….

Thank you