The Northwest Corner of the Atlantic and Rapid Climate Change

Matthew Hechtwith

Frank Bryan, Rick Smith, Mathew Maltrud and contributions from others

…or, how our work in eddy-resolving ocean modeling applies to the question

How to better characterize thermohaline circulation (THC) stability in terms of response to– greenhouse gas forcing– possible increases in fresh water flux (Greenland)

and, perhaps

• Estimation of stability of the THC may present an opportunity for more advanced approaches to ocean modeling

we first review• What is known about modes of North Atlantic circulation– Switching between modes that may have happened in the past

Modes of Atlantic THC

Modern (also Dansgaard-Oeschger?)

Glacial (Atlantic Intermediate but no

Atlantic Deep Water)

Heinrich event shutdown

Rahmstorf, Nature 2002

• How about the Gulf Stream/North Atlantic Current system in these modes?– Brief review of modern, and– Likely glacial configuration

• We’re particularly concerned here with transition between modes– Preconditioning– Feedbacks which stabilize the transition

10ºC isotherm, from Rossby, Reviews of Geophys 1996, taken in turn from Iselin, 1936.

“Major currents of the northern North Atlantic”, from Rossby, Reviews of Geophys 1996, taken in turn from Krauss, 1986.

• Modern circulation with – Gulf Stream – North Atlantic Current – penetration into the “Northwest Corner”

was much different during the last glacial period– Instead, the paleo-Gulf Stream fed a more limited subtropical gyre•Think of the Pacific, no high latitude penetration of the subtropical gyre

Surface currents and

iceberg dispersal at Last Glacial Max, from Robinson, Maslin and McCave,

Paleoceanography, 1995

• Rossby and Nilsson have discussed a mechanism for rapid switching from this paleo-Atlantic circulation to modern circulation– Given preconditioning of Atlantic and then resumption of deep water formation

– topographic and planetary waves communicate this shift to Grand Banks region, and

– North Atlantic Current forms, turning north at Grand Banks and into NW Corner

– stronger (and now interhemispheric) THC makes for stronger Florida Current

Conceptual mechanism for rapid THC response to onset of

northern deep water formation

through

Topographic and Planetary waves, from Rossby and Nilsson, JGR

2005.

Development of NAC:Was transition truly

“rapid”?• Learn more about this in Birmingham?– Answer consequential to paleoclimate research– Answer, though simple, may take time to settle• Question of Atlantic Circulation driving the atmosphere, or atmosphere (winds) driving the ocean (Wunsch 2006)?

• Regardless, – Circulation which feeds North Atlantic deep water formation modeled poorly in climate models

– Stability of modern Atlantic circulation to “anthropogenic forcing” may depend on realism of this circulation• mesoscale variability required for realistic circulation in this region

21rst century climate

• What concern do we have for ocean circulation?

Response of North Atlantic Overturning

Circulation

From the Intergovernmental Panel on Climate Change (IPCC)Third Assessment Report, 2001

Stouffer et al, J Clim. 2006

CMIP “water hosing” simulations, 100 yrs at 0.1Sv (0.1x106m3

/s), then 100 years of recovery.

Stouffer et al, J Clim. 2006

Extreme water hosing (10x) leads to shutdown in all models:

• Rossby and Nilsson discuss role of NAC in– Transport of saltier waters into deep water formation region

– Maintenance of newly resumed deep convection• (Maintenance of modern deep convection)

• So what happens in an ocean climate model?– Ocean climate models don’t form a Northwest Corner

Back to the real world

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

CCSM.3

control

Sea Surface Height from ocn climate model (CCSM.3 control

simulation)

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Large sea surface temperature errors in the southern Lab Sea are apparent.

Associated biases in salinity may be a larger issue for modeling of THC stability.

Collins et al, J Clim 2006

Eddy-resolving ocean models for climate

science?• Eddies essential to mean circulation of Northwest Atlantic

• THC stability would be climate science application where high res has a role– opportunity to support “IPCC-class” climate simulation

(again, the multi-scale problem)

Possible to get good Gulf Stream/North

Atlantic Current in “eddy-resolving”

model

0.1 model of SMBH

TOPEX/Pos. obs (Le Traon et al 1998

Sea Surface Height Variability

• Resolution was part of the story– We’ve since learned that a good Gulf Stream system is far from assured, even at 0.1º

SSH Variability from Maltrud and McClean 2004

1/10º POP, dipole grid, full cells:

TOPEX-ERS1

How to improve global GS/NAC?

• Factors particular to global configuration– Lateral boundary conditions (lack thereof)– Horizontal grid discretization

• Factors addressable in context of regional North Atlantic model– Here we discuss

• Horizontal dissipation (parameterization, coefficients)

• Preparation of topography (smoothness)

– Also have considered• Vertical mixing• Forcing• Advection (simple centered vs quasi-monotonic)

• How much sensitivity to horizontal dissipation?– coefficients

Gulf Stream pathsfrom intersection of 12°C isotherm and 400m, 1998-2000

• By this point in time (years 13-15) the more viscous (C=1) case has a more realistic separation (obs in green, model mean, 1 and extreme envelope in blue)

C=1, 0.1º

C=1/2, 0.1º

C=1/4, 0.1º

greater dissipation

less dissipation

Stream-coordinate velocities

at “crossover

” (Hatteras)

C=1

C=1/2

C=1/4

Support for Thompson and Schmitz (JPO 1989): DWBC exerts control over separation of Stream

greater dissipation

less dissipation

Obs(AVISO)

Sea Surface HeightVariability (1998-2000)

C=1

C=1/2

C=1/4

greater dissipation

less dissipation

• Deeper penetration of NA Current in less viscous case

• Ozgokmen (97) found jet needed to be highly inertial with low eddy activity to separate and cross f/H, in process study.– We don’t see relation between

low eddy activity and separation at Hatteras

– but maybe high eddy activity for reattachment at Grand Banks

• and Ozgokmen may be correct, even at Hatteras, in terms of isolation from from topography allowing for separation (but perhaps with wrong mechanism for Hatteras, right mechanism for Grand Banks)

Eddy Kinetic Energies, 55º W

C=1/4

C=1

C=8

• How different the density (thermal) fronts are! Yet:

• Many of the features of the flow are similar here, and deeper -- the E/W portions of the flow as it winds northward.

• More viscous case tends to lose much of the NAC out eastern boundary relatively early.

C=1

C=1/4

greater dissipation

less dissipation

• Peak velocities in NAC match pretty well– but see how 10 cm/s isotach is at 1600 m in model,

3500 m in obs

C=1/4

+32.5-20.0

• How much sensitivity to horizontal dissipation?– Parameterization

• Anisotropic Gent-McWilliams Parameterization for Ocean Models– As suggested by Roberts and Marshall (JPO, 1998), adiabatic closure beneficial, even at 0.1º resolution

– Anisotropic GM shown to allow energy levels to remain high, along with use of anisotropic horizontal viscosity• Smith and Gent, JPO 2004

• Preparation of topography• Use of “partial bottom cells”• Additional smoothing (still only moderate)

SSH Variability

BR_full BR_pbc

AK_pbc AK_S3

AVISO

Add pbc’s

Light smoothing of topography

• Factors particular to global configuration– Lateral boundary conditions (lack thereof)

– Horizontal grid

Sea Surface HeightsGlobal

Sector

North Atlantic SSH from fully global 0.1º dipole grid, 3600x2400 points

From sector version of same grid, 1130x1500 pts (restoring boundaries at

N/S, no throughflow)

SSH Variability

SSH Var from fully global 0.1º dipole grid,

3600x2400 points

From sector version of same grid, 1130x1500 pts (restoring boundaries at

N/S, no throughflow)

Global

Sector

• Factors particular to global configuration– Horizontal grid discretization

Displaced Pole

Grid

A sensitivity we’re exploring in global rather than regional configuration: horizontal grid discretization

Tripole Grid

Lat/lon grid insouthern hemisphere,w/ pole in Antarctica

Displace pole gridW/ poles in North America,Siberia

Transitionat somethinglike 30ºNso thatsingularitiesremain wellwithin land

New global eddy-resolving simulations

• Ocean-only study with “transit time distributions” and Lagrangian tracers– Using tripole grid, partial bottom cells, anisotropic GM and viscosity, nearly monotonic advection scheme

• Fully-coupled climate simulation– Using the Community Climate System Model– (Short) control and CO2 increase runs

• Both efforts involve many people at various institutions

Further off: Questions to address

• Smarter ways to bring in effects of mesoscale eddies?– Variable resolution?

– LANS- model?– Higher-order methods?

• Impact of vertical representation (beyond roughness in z-coord)?– Pressure vs density as vertical coordinate (or hybrid)

– Parameterization of mixing in overflows

…Questions to address

• Combine techniques for– Simulation of long-time evolution

• IPCC-class ocean climate model?• Paleo-climate resolution version of ordinary ocean climate model?

• Implicit model?

• With accurate simulation of– Mean transport and preconditioning,– deep water formation, and– thermohaline circulation transition