climate stability and instability: transition from flywheel to driver? jochem marotzke school of...
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Climate Stability and Instability: Transition from Flywheel to
Driver?
Jochem Marotzke School of Ocean and Earth ScienceSouthampton Oceanography Centre
Southampton, SO14 3ZHUnited Kingdom
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?
• Discuss intricacies using the example of ocean mixing
• Conceptual, mostly steady-state; illustrated w/ simple GCMs
Flywheel or Driver?
• 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
•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.)
FF increased from 0.3 to 0.4 Sv
FF 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?
Schmitt et al., 1989
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
• 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
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.)
• Convective mixing determines dominant high latitudes but not global deepwater formation rate
• Cooling dominates buoyancy flux in DW formation region
• Interhemispheric (& interocean?) dynamics central
Summary Part I:
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
• 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?
• 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
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
Blue:
Covered
Red:
MOC
Blue:
Recon-struction
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)
OCCAM
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)