Frank Bryan & Gokhan Danabasoglu NCAR Ocean Initialization Issues for Transient Climate Change Ensemble Experiments Frank Bryan & Gokhan Danabasoglu NCAR

Assumptions & Biases Our interest in these experiments include the simulation of the climate through the full 20th century as well as future projections. By initialization we mean designing a sampling strategy to extract initial states from a pre-industrial control integration of the coupled system. My comments reflect a personal research interest in the long-term behavior of the thermohaline circulation.

Recommendations for Transient Experiment Initialization Independent of Ensemble Size Plan and budget for very long (1000+ year) pre-industrial control integrations sufficient to establish a period of stationary statistics that completely overlaps all planned transient integrations Complete control integration(s) and characterize the statistics of their variability prior to starting climate change experiments.

Long-Term Behavior of Control MOC Nakashiki et al (Ocean Modeling, submitted)

Characterize the Variability of the MOC 1990 Controls CCSM3-T85x1 CCSM3-T42x1 CCSM2.2-T42x1 CCSM2-T42x1 CCSM3-T31x3

1990 T85x1 Control MOC EOFs 17-25 years G. Danabasoglu

Initializing Transient Experiments 1990 Control 1% CO2

20th Century Ensemble Members and P-I Control

Ensemble Spread in Heat Storage Gent et al (J. Clim., 2006)

Further Recommendations Cross-WG research program over next 1.5 years to better characterize the global manifestations and physics of the MOC decadal mode. What are the dynamical mechanisms of the decadal oscillations of the MOC? Why does it appear to depend on model resolution? Does the amplitude of the oscillation depend on the mean state? What are the regional and global impacts of the variability? What are the effects on predictability?

A Resource Conservative Initialization Strategy Assuming dominant decadal mode exists: Sample within a single oscillation period (20 to 50 years) of pre-industrial control. Minimum 4-5 members for 20th century experiments Consider dropping members for larger amplitude forcing and/or stabilization scenarios. Samples phase space Minimizes length of control run required