# Presentation: 909582

## The Northwest Corner of the Atlantic and Rapid Climate Change

- Matthew Hecht
- with
- Frank Bryan,_x0004_ Rick Smith, Mathew Maltrud
- and contributions from others

**Notes:**
From others: global work involves various intersecting collaborations.
Background: _x0005_ER Ocns are great for providing beautiful plots, we’d like to think we’re going beyond pretty pictures. This is the more ambitious range of what we’d like to explore.

## ...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)

- ...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)

**Notes:**
(would also like to better understand paleoclimate transitions)

## 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

**Notes:**
This statement for Imperial modelling group.

## 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

- Brief review of modern, and
- Likely glacial configuration
- We’re particularly concerned here with transition between modes
- Preconditioning
- Feedbacks which stabilize the transition

**Notes:**
So that was a vertical perspective on the circulation. How about a horizontal perspective on these modes?

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

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

**Notes:**
Pretty good indication of modern GS/NAC system from 1936.

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

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

**Notes:**
Cross-hatched regions are where waters of Sub-Polar Front and subtropical gyre are found.
Note the branching off of the Azores Current.

## 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

- 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

**Notes:**
Will have to 

## Surface currents and iceberg dispersal at Last Glacial Max, from Robinson, Maslin and _x0005_McCave, Paleoceanography, 1995

- Surface currents and iceberg dispersal at **Last Glacial Max**, from Robinson, Maslin and _x0005_McCave, *Paleoceanography*, 1995

**Notes:**
Here’s what Robinson et al surmise from ocean sediments.
Note containment of subtrop gyre.

## 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

- 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. 

- Conceptual mechanism for rapid THC response to onset of northern deep water formation through **T**opographic and **P**lanetary 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

**Notes:**
Ties into the question...

## 21rst century climate

- What concern do we have for ocean circulation?

**Notes:**
Okay, for now let’s leave LGM-to-Modern transition.
How about century 21?

## Response of North Atlantic Overturning Circulation

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

**Notes:**
Brown curve -- the stable one -- is with “isopycnal” vertical coordinate.
HYCOM hybrid of z-coord w/ isopyc gets a slowdown too (though RB says THC is a “loose cannon”!)

## Stouffer et al, J Clim. 2006

- CMIP “water hosing” simulations, 100 yrs at 0.1Sv (0.1x106m3/s), then 100 years of recovery.

**Notes:**
Did Ron give 30% as average reduction after 100 years?
Could 0.1Sv today only come from rapid melting of Greenland Icesheet?
Note that dashed means EMIC.

## Stouffer et al, J Clim. 2006

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

**Notes:**
Dashed mean EMIC.

## 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

- 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

**Notes:**
NW Corner: there just isn’t any penetration of subtrop gyre water around Grand Banks, no penetration into southern Lab Sea.

## CCSM.3

- control

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

## Slide 19

## 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

**Notes:**
Well, it is possible to get this...

## 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

- 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

**Notes:**
Sea surface height variability (cm) from 1/10 model (above) for 1991-1993 and TOPEX-ERS1 (below) for 1995-1997.
Weak NAC. If you look carefully you’ll see separation occurs late.

## 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

## Slide 28

## Gulf Stream paths

- from 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

**Notes:**
Model tracks at 10 day intervals.
Obs are from Watts 1995.

## 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

**Notes:**
Emphasize that strong DWBC allows for isolation from topography.

## Obs

- (AVISO)

- Sea Surface Height
- Variability (1998-2000)

- C=1

- C=1/2

- C=1/4

**Notes:**
Whereas GS sep was good in more viscous case, NW corner better with lower viscosity.
Not much room to satisfy both. We recommend middle value.

## 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)

- 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

**Notes:**
We’ve slipped one in with higher dissipation yet, the level we’ve used at 1/2 the resolution.
Whereas we don’t’ see support for competing explanation of Ozgokmen et al at Hatteras (competing with Thompson and Schmitz), 
this mechanism may explain reattachment at SENR (which is more our concern here!).
(_x0005_Deep EKE, barotropization related through BC/BT energy conversion.)

## 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.

- 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

**Notes:**
Point out Newfoundland and Flemish Cap.
Temperature, with velocity vectors.
Barotropic structure qualitatively similar, as seen in velocity vectors at 729m (here) and other depths. Reflects importance of topography in this region.
Baroclinic structure is very different.

## Peak velocities in NAC match pretty well
but see how 10 cm/s isotach is at 1600 m in model, 3500 m in obs

- but see how 10 cm/s isotach is at 1600 m in model, 3500 m in obs

- C=1/4

- +32.5

- -20.0

**Notes:**
Transport numbers are across denser classes: LSW, GFZW, DSOW.
Comment on thinning of “LSW” layer.
Although” DWBC” transport is too large, core of DSOW is weak.
Model numbers from 1998--2000 (would be -19.0 and +19.3 for 1990-1992).

## 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

- 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

**Notes:**
For brevity, not showing results here.

- 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

**Notes:**
Top two panels differ only in full vs pbc topography.
Bottom two only in degree of smoothing (very moderate vs moderate)
Obs on the right.

## Factors particular to global configuration
Lateral boundary conditions (lack thereof)
Horizontal grid

- Lateral boundary conditions (lack thereof)
- Horizontal grid

**Notes:**
We understand sensitvities much better now, owing to work in North Atlantic context.
Still strong influences of these factors which are particular to global configuration.

## Sea Surface Heights

- Global

- 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)

**Notes:**
At this point we had already eliminated surface forcing as “the difference” between sector NA and fully global.
Grid issues include aspect ratio, orientation, grid spacing (topography?).

## 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

- Horizontal grid discretization

**Notes:**
We understand sensitvities much better now, owing to work in North Atlantic context.
Still strong influences of these factors which are particular to global configuration.

## Displaced Pole Grid

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

**Notes:**
(in CCSM pole is in Greenland; in CRIEPI x3, it’s in Siberia)
Aspect ratio? Max/min area?

## Tripole Grid

- Lat/lon grid in
- southern hemisphere,
- w/ pole in Antarctica

- Displace pole grid
- W/ poles in
- North America,
- Siberia

- Transition
- at something
- like 30oN
- so that
- singularities
- remain well
- within land

**Notes:**
Note: this has been an option in POP for some time.
A number of other groups doing same.
CCSM has preferred to stick with dipole grid for now.

## 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

**Notes:**
Using 

## 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

- ...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

**Notes:**
Eddy resolving models get much more realistic circulation.
Can we apply this to long integrations to study paleoclimate, results requiring ensembles, etc?