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

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

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 30-60 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

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

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

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

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

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

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

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

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

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

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

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

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

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