1. Jake Langmead-Jones 10/10/2016 @Jlangmeadjones
The Role of Ocean Circulation in Climate Simulations, Freshwater Hosing and Hysteresis
Jake Langmead-Jones
10/10/2016
@Jlangmeadjones

2. Key questions What are coupled models?
How are ocean circulation models set up?
How are ocean circulation models refined?
Why are ocean circulation models useful?
What are the uses of coupled models?
An example of a coupled model in use

3. Coupled models simulate both atmospheric and oceanic circulations, allowing fluxes between the two cells
Models typically simulate atmosphere, oceans, land surfaces and sea ice
Models typically have resolutions of ~1-4° by ~1-4° grid spacing
Oceanic models have vertical layers
Models allow fluxes of heat or tracers to pass between reservoirs
Models allow physical processes in each of the cells to influence each other

4. Ocean models are set up through a combination of equations of motion, parameterizations and physical forcing
Processes that are larger than model resolution can be physically simulated:
Wind
Heat
Large scale circulation
Salinity
Turbulent Mixing
Distribution of vegetation
Processes that are smaller than model resolution have to be parameterized:
Clouds
Eddies
Deep overflows
Evaporation
Sublimation
Rain
Rivers
Melting of ice
Processes external to the system have to be added as forcing:
CO2 concentration
Solar forcing
Aerosol concentration
Volcanic input of aerosols

5. Ocean models are refined using control runs and comparing the results to observed data
Control runs are initiated at 20th century conditions and run until they reach equilibrium
The data produced can be compared to observations of the past to determine how well the model imitates the past
It also allows model “drift” to be identified which can be rectified in two ways:
Adding fluxes to the model
Reassessing the physical set up of the model

6. Ocean models use flux adjustments or changes to the physics to prevent “drift”
Models that use flux adjustments
Used in models that “drift” to an unrealistic equilibrium
Additional fluxes of heat and fresh water added to models
This allows models to accurately recreate modern day observations
Models that do not use flux adjustments
Model equilibrium is close to modern day conditions
Produce results further from observed data but are thought to better model physical processes

7. Oceans are included in models because they have a profound effect on atmospheric temperature
On timescales greater than decades, oceans give the best prediction of climate:
SSTs have a crucial impact on atmospheric circulation
Ocean circulation patterns are thought to be key drivers of heat distribution as seen in paleoclimates
Oceanic cells influence atmospheric temperature in a variety of ways such as:
ENSO
Interactions between ocean and atmosphere dominate Pacific SSTs on inter-annual timescales
MOC
Overturning of the North Atlantic results in elevated temperatures in the poles and reduced temperatures in the tropics
Subduction of water masses
Subduction of water masses drives surface heat into the ocean, retarding anthropogenic heating of the atmosphere

8. Models can be used to test our understanding of circulation in the past as well as predict how circulation might change in the future
Assuming models are sound, they can be compared to paleo-data to elucidate circulation patterns
The aim for modelers is to build a model that accurately simulates different circulation patterns (e.g modern day and LGM) without requiring differential circulation patterns
Models can test how the future climate will respond to forcing
Typically this involves either hosing the model with fresh water or increasing the CO2
Effective models would allow temperature, sea level and nutrients to be forecast

9. Manabe & Stouffer (1999) used a climate model to explore how fresh water effects MOC and hence global climate
Younger Dryas was an unexpected glaciation in a period of otherwise global warming
There was rapid onset and escape
The current hypothesis is that global warming caused melting of NA glaciers, resulting in large scale addition of fresh water to the high latitude North Atlantic
This addition slowed (Manabe & Stouffer, 1999) or stopped (Manabe & Stouffer, 1988) MOC by decreasing the density of high altitude surface waters
In turn, this decreased the poleward transport of heat resulting in glaciations
Manabe & Stouffer (1999) used a coupled ocean model to add freshwater to the high latitudes and simulate the Young Dryas Glaciation

10. Looking into the setup of the model, it is easy to see why Andy is so skeptical of modelling
The model does has twelve vertical layers in ocean with a horizontal resolution of 4°x4°
Model assumes present day climate conditions, not the condition at 15kyr
Water is fluxed evenly into the area ‘A’ at a rate of 0.1 Sv
To avoid drift, the model adds a constant flux of heat and water to the oceans every year

11. The model causes changes to MOC, freshening the high latitudes as well

12. The sub-polar gyre also strengthens as a result coupled with weakening of the tropical gyre

13. The model clearly shows that SST has an effect on global surface air temperature and matched with Broeker (1995) paleo-data

14. The model also gives insight into the dynamics of MOC perturbations
The model also correctly predicts the recovery of MOC once freshwater has been switched off, albeit slower than in the geological record
The freshwater input is switched off after 500 years
Atmospheric cooling dies off by 750th year
MOC recovers by the 900th year
The model predicts decadal oscillations in the strength of MOC, which is seen in the geological record
Manabe & Stouffer (2000) show that latitude of freshwater input is important
When seawater is added at the low latitude the response is 4-5 times smaller

15. Manabe and Stouffer (1999) also show the effect that anthropogenic heating will have on MOC
Models were run simulating an increase in CO2 to 2 – 4 times preindustrial levels
Both models showed a weakening and shallowing of MOC
This is due to increased precipitation at the poles
However both models also showed that once CO2 addition ceased MOC recovered to full strength over the next millennium

16. Summary
Oceanic circulation models are important as water masses influence surface air temperature
The oceans are more predictable than the atmosphere on long time scales
Oceanic circulation models are useful (depending on your opinion) for simulating past climate and forecasting future climates