# Presentation: 594527

## Climate Stability and Instability: Transition from Flywheel to Driver?Jochem Marotzke School of Ocean and Earth ScienceSouthampton Oceanography Centre Southampton, SO14 3ZHUnited Kingdom

## Slide 2

## NOAA Global SST Analysis, 4 - 9 November 2002

## North Atlantic warmer than North Pacific  NADW formation not a simple forced response to stronger cooling by atmosphere: If it were, NA should be colder than NP.

- Ocean circulation active in setting fundamental properties
- High North Atlantic sea surface salinity (SSS) crucial for NADW formation
- Ocean circulation can, in principle, maintain NA SSS greater than NP SSS without bias in forcing such as Atlantic-to-Pacific atmospheric water vapour transport (Marotzke & Willebrand, 1991).
- True in reality? - “without bias in forcing”? Coupled GCMs give equivocal answers (e.g., Manabe & Stouffer, 1999).

## Is there another circulation mode that the MOC could attain?

## Is there another circulation mode that the MOC could attain?

- Could transitions to another mode be abrupt?

## Flywheel or Driver?

- Conceptual, mostly steady-state; illustrated w/ simple GCMs

- Is there another circulation mode that the MOC could attain?

- Confirmation requires continuous MOC observations
- How can this be done?

- Could transitions to another mode be abrupt?

- Would an MOC transition be a passive response to external forcing, or be self-driven, possibly following a trigger?

## Mixing in Stratified Waters (I):

- Sandström (1908, 1916; see Colin de Verdière 1993): Heating below cooling is required so that fluid can act as a heat engine (buoyancy-driven flow exists)
- Jeffreys (1926): Expansion below contraction is crucial, which is possible in presence of mixing even if heating & cooling occur at the same pressure
- Munk (1966): Mixing heats upwelling deepwater
- Weyl (1968): Mixing converts turbulent kinetic energy into potential energy, which is needed to drive flow
- Munk and Wunsch (1998): Energy for mixing derives significantly both from tides and from wind

## Mixing in Stratified Waters (II):

- GCMs with fixed diffusivity: MOC increases with density gradient (e.g., Scott, thesis 2000)
- With fixed amount of energy available for mixing, MOC might decrease with density gradient (Walin 1990, Lyle 1997, Huang 1998,  Nilsson & Walin 2001, Oliver, thesis in prep.)

- Series of GCM experiments: Nilsson & Walin (submitted):

- Mixing and MOC:
- Flywheel or Driver - Meaningless question?

## Expect mixing to matter mainly over very long timescales 

- Expect mixing to matter mainly over very long timescales
- Time-dependent situations?
- Kevin Oliver (UEA, thesis in prep.): Considers transient behaviour in isopycnic box model with energy-dependent mixing (Nilsson & Walin, 2001)

## Oliver (Thesis, UEA, in prep.)

## Oliver (Thesis, UEA, in prep.)

**F****F**** increased from 0.3 to 0.4 Sv**

**F****F**** decreased from 0.4 to 0.3 Sv**

## Wang et al. 1999, idealised global model: “NADW” collapses under doubling of FW forcing within 1000 years

- NB: Collapse timescale unpredictable within factor 2

**BUT**:
- Steady-state: NADW increases with FW forcing
- NADW consistent with Rooth (1982) box model
- Total nearly constant

## Convective mixing & sinking are different processes:

- Mauritzen (1996): DSOW derives from gradually sinking Atlantic Water, not convection in central Greenland Sea gyre
- Marotzke & Scott (1999): Sinking possible without convective mixing; sinking expected near boundaries
- Spall & Pickart (2001): Convective mixing & sinking co-located near sloping topography

## If convective mixing is unimportant, why do we pay so much attention to its fate in the North Atlantic?

## If high-latitude salinity is so important in the North Atlantic, why is the freshwater part of the surface buoyancy flux so small?

**If high-latitude salinity is so important in the North Atlantic, why is the freshwater part of the surface buoyancy flux so small?**

**Schmitt et al., 1989**

## Large & Nurser, 2001

**Large & Nurser, 2001**

**Blue: Ocean heat loss**

**Red: Ocean water gain**

**Red: Ocean density gain**

## Pole-to-equator (and top-to-bottom) density contrast is dominated by temperature: The pycnocline is a thermocline 

- Pole-to-equator (and top-to-bottom) density contrast is dominated by temperature: The pycnocline is a thermocline

## Pole-to-equator (and top-to-bottom) density contrast is dominated by temperature: The pycnocline is a thermocline 

- Water is dense because it is cold (from high latitudes)
- Which high latitudes ventilate deep ocean depends on SSS
- Density contrasts between high latitudes (competing DW formation sites) much smaller than between pole & equator
- Cross-equatorial coupling between high latitudes crucial

- Cooling dominates buoyancy flux in DW formation region
- Interhemispheric (& interocean?) dynamics central

## Tziperman 1997

**Tziperman 1997**

**Wang et al. 1999**

**Klinger & Marotzke 1999**

## Convective mixing determines dominant high latitudes but not global deepwater formation rate

- Interhemispheric (& interocean?) dynamics central

- Diapycnal mixing works on overall density contrast
- Controls global rate of upwelling deepwater
- Efficiency of convective mixing unimportant for global rate
- Distribution over competing high latitudes depends on surface density, hence SSS
- High latitudes with deepest convective mixing dominate  (Needs to be qualified: Topography, overflows etc.)

## Summary Part I:

- Cooling dominates buoyancy flux in DW formation region
- Interhemispheric (& interocean?) dynamics central

- Mixing and MOC:
- Flywheel or Driver - Meaningless question?

- Timescales critical in dependence on mixing and FW forcing

- Oceanic and atmospheric processes linked inextricably

## Confirmation (of hypotheses of what controls MOC and its variability) requires continuous MOC observations as a starting point

- How can this be done?

## 26.5°N MOC Monitoring Proposal

**26.5°N MOC Monitoring Proposal**
- PIs: Jochem Marotzke, Stuart Cunningham, Harry Bryden (SOC)
- Submitted to **NERC RAPID **Programme (which is funded with £20M over 6 years)
- Requested: £4.7M over 5 years
- Would support 2 Post-docs, 1 Research Assistant, 1 Ph.D. Student
- Funding decision expected 25/26 November

## Why 26.5°N?

**Why 26.5°N?**
- Near Atlantic heat transport maximum - captures total heat transport convergence into North Atlantic
- South of area of intense heat loss ocean  atmosphere over Gulf Stream extension
- MOC dominates heat transport at 26.5°N
- Heat transport variability dominated by velocity fluctuations (Jayne & Marotzke, 2001)
- Florida Strait transport monitored for >20 years   (now: Johns, Baringer & Beal, Miami, collaborators)
- 4 modern hydrographic occupations

## Approach: Integrated thermal wind(geostrophy)

- Ekman contribution to MOC included
- Surface layer Ekman transport assumed to return independent of depth

## Slide 27

## Slide 28

## Model-based experiment design:

**Model-based experiment design:**
- Funded through NERC prior to conception of RAPID
- Joël Hirschi (post-doc), Johanna Baehr (M.Sc. student)
- “Deploy” antenna in high-resolution models, OCCAM (1/4°; SOC, Webb et al.; Hirschi),  FLAME (1/3°; IfM Kiel, Böning et al.; Baehr )
- See Hirschi et al. poster

## Slide 30

## Blue:

- Covered

- Red:
- MOC
- Recon-struction

## Red: MOC 	Blue: Reconstruction

- Red: MOC 	Blue: Reconstruction

- Black: OCCAM Heat Transport 	Green: Reconstruction

- OCCAM

- FLAME

## Red: MOC 	Blue: Reconstruction 	      Cyan: 300 realisations with random error (1 Sv Florida Strait; 0.01 kgm-3)

- Red: MOC 	Blue: Reconstruction 	      Cyan: 300 realisations with random error (1 Sv Florida Strait; 0.01 kgm-3)

- OCCAM

## Blue: Reconstruction      Cyan: Thermal Wind 	Green: Ekman

- Blue: Reconstruction      Cyan: Thermal Wind 	Green: Ekman

- FLAME

- OCCAM

## Transition from Flywheel to Driver:

- Importance of mixing in MOC dynamics

- Nature and location of mixing matter but are unknown (interior & boundary mixing; base of SO mixed layer; energetics)

**1. What have we learned during the WOCE period?**

- MOC could reorganise

- Dynamics of convection

## Transition from Flywheel to Driver:

- DBE visualised inhomogeneity of mixing

- Deep Indian Ocean MOC: Well studied in WOCE projects (despite lack of WOCE 32S section); considerable deep mixing required to balance inflow.

**2. What specifically was the WOCE contribution?**

- Hydrographic sections gave accurate global estimate of MOC

## Transition from Flywheel to Driver:

- Continuous observations of MOC drivers (heat & FW budgets of convection areas)

- Estimates of global distribution of mixing

**3. What is required in the future (I)?**

- Continuous observations of the MOC at selected latitudes

## Transition from Flywheel to Driver:

**3. What is required in the future (II)?**

- Model-based experiment design for climate time series: Rational resource allocation

- Ocean (and coupled) models that represent coupled nature of mixing

- Improved (or development of) conceptual understanding of interaction between high latitudes (within and across oceans)