Investigating Dansgaard-Oeschger events via a 2-D ocean model Kevin Lewis California Institute of Technology

Climate change cycles Orbital Frequencies – 20-100 kyr Major climate shifts also recorded on 1-10 kyr timescales Dansgaard et al., 1993

Dansgaard-Oeschger Events Characterized by rapid warming in the N. Atlantic, followed by slower cooling Quasi-Periodic, with a timescale of ~1400 years Recorded by diverse climate proxies Evidence for global climatic effects (Data From ftp://ftp.ngdc.noaa.gov/paleo/icecore/greenland/summit/grip/isotopes/gripd18o.txt)

Other Evidence of D-O events Sediment cores from the Santa Barbara Basin (Hendy and Kennet, 1999)

Other Evidence of D-O events Stalagmites from Eastern China (Wang et al., 2001)

Global Map of D-O records http://www2.ocean.washington.edu/oc540/lec01-31/

Global Thermohaline Circulation

Atlantic Circulation Deep water formation Deep Water is formed at the Northern and Southern extents of the Atlantic Ocean This deep circulation has an overturning timescale of ~103 years Surface currents strongly influence climate in many areas, as in the N. Atlantic Plane of 2-D model (Source: Ge101 lecture notes) Deep water formation

Previous Work (Briefly) D-O events have been studied by many others using models of all complexities Many of these models require specific additional assumptions (Ganopolski and Rahmstorf, 2001)

Goals Study changes in the overturning circulation using a model of intermediate complexity (between box models and full 3-D climate models) Investigate the possibility of thermobaric energy release in connection to changes in circulation Look for a self-sustaining oscillatory mode intrinsic to the ocean, with no time-dependent parameters Introduce only the minimum number of parameters/assumptions necessary

Thermobaric Potential Energy The effect of temperature on density increases with pressure Cold, fresh water can be stable above or below warm, salty water Warm, Salty water Cold, Fresh water

Equations Variables: Equations: Streamfunction, ψ Density, ρ Salinity, S Temperature, θ Equations: Model Assumptions: Boussinesq Mixed boundary conditions for S,θ No flux, no-slip sides/bottom (u=0) stress-free top surface (ζ=0), with freshwater and salinity flux

Model Parameters N-S symmetric, time independent fluxes Constant horizontal and vertical diffusivities Linear equation of state

Observed Stable Modes Thermal Stratification Salt Stratification Heating of deep water from surface Rapid sinking of Cold, Fresh surface reservoir Strong surface current, transporting salt to one pole Temperature (oC) Temperature (oC) Method: Find the balance point between these modes

Results Centennial-scale oscillations can be sustained for >10^4 years Transitions between two separate modes occur quasi-periodically Timescale is diffusion controlled Trigger is a thermobaric PE reservoir at the surface (Cold, Fresh water) Solution is symmetric

Results

Stability Diagram for viscosities Below 5000 m^2/s vertical diffusivity, no oscillations have been observed A small transition region is present, where irregular oscillations are observed

Effect of Thermobaricity Without the thermobaric term for density, oscillations are impossible

Effect of Geothermal Heating Deep ocean must be warmed to create a thermobaric instability Geothermal has not been observed to affect outcome Temperature (oC) Heating From Above Heating from below

Future work Explore parameters governing oscillations Horizontal and Vertical diffusivities Heat and Salinity Fluxes Investigate the effects of model assumptions, e.g.: Mixed boundary conditions No Convective Adjustment

Conclusions Centennial-Scale oscillations have been observed for a given range of parameters Presence of oscillations is largely limited by vertical diffusion Warming of deep ocean from surface is able to create a thermobaric instability

Misc. Parameters Relaxation time – 23 days Dt – 2 hours Write Out time – 4 years Vert. Diffusion – 5e-3 m^2/s Horiz. Diffusion – 5e3 m^2/s Alpha1 = 8e-5, Alpha2 = 3e-12, Beta = 9e-4

Possible Improvements Variable diffusivities with depth, ν = ν(z) Convective adjustment Geothermal Heating Sea Ice parameterization In general, additional parameters should only be added when necessary