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Summary of achievements

As the CCSM ocean model liason, I support the development, execution, and analysis of the ocean component of the Community Climate System Model, and provide assistance to community users of POP code and data. Some specific examples of my work in this role over the past year include: development of revision-controlled, publically-released ocean diagnostic routines which automate simulation analysis; development of POP modifications to enable vertical grid flexibility and multiyear interior restoring; and involvement in a project aimed at utilizing CCSM as a tool for decadal climate projections. An ongoing line of research with Bill Large has been the development of improved ocean surface boundary condition datasets--an effort which has facilitated the Common Ocean Reference Experiment (CORE) protocol of the CLIVAR Working Group for Ocean Model Development. In FY08, a second, extended version of this global, multiyear atmospheric state dataset was released to the ocean modelling community, along with a corresponding air-sea flux product which has been used to quantify the scales of interannual changes in ocean forcing components. Use of these multiyear datasets to generate high-fidelity ocean hindcast simulations has enabled studies of ocean multidecadal variability which would not be possible using observations alone. An example of this research is the identification of a generation mechanism for ocean spice variability, with a manuscript published this year, and ongoing exploration of the role that surface salinity variations play in driving change in the ocean interior. A new focus in the context of forced ocean experiments is understanding the sensitivities to high latitude thermohaline forcing, which is poorly constrained by observations. I'm also involved in research examining the possible coupling between ocean biology and ENSO variability.

Publications

Griffies, S. M., A. Biastoch, C. Böning, F. Bryan, G. Danabasoglu, E. Chassignet, M. England, R. Gerdes, H. Haak, R. Hallberg, W. Hazeleger, J. Jungclaus, W. Large, G. Madec, A. Pirani, B. Samuels, M. Scheinert, A. Gupta, C. Severijns, H. Simmons, A. Treguier, M. Winton, S. Yeager, and J. Yin. 2009: Coordinated Ocean-ice Reference Experiments (COREs). Ocean. Modell., 26, 1-46, doi:10.1016/j.ocemod.2008.08.007.

Figure 1: High resolution figure

Abstract: Coordinated Ocean-ice Reference Experiments (COREs) are presented as a tool to explore the behaviour of global ocean-ice models under forcing from a common atmospheric dataset. We highlight issues arising when designing coupled global ocean and sea ice experiments, such as difficulties formulating a consistent forcing methodology and experimental protocol. Particular focus is given to the hydrological forcing, the details of which are key to realizing simulations with stable meridional overturning circulations.

The atmospheric forcing from [Large, W., Yeager, S., 2004. Diurnal to decadal global forcing for ocean and sea-ice models: the data sets and flux climatologies. NCAR Technical Note: NCAR/TN-460+STR. CGD Division of the National Center for Atmospheric Research] was developed for coupled-ocean and sea ice models. We found it to be suitable for our purposes, even though its evaluation originally focussed more on the ocean than on the sea-ice. Simulations with this atmospheric forcing are presented from seven global ocean-ice models using the CORE-I design (repeating annual cycle of atmospheric forcing for 500 years). These simulations test the hypothesis that global ocean-ice models run under the same atmospheric state produce qualitatively similar simulations. The validity of this hypothesis is shown to depend on the chosen diagnostic. The CORE simulations provide feedback to the fidelity of the atmospheric forcing and model configuration, with identification of biases promoting avenues for forcing dataset and/or model development.

Figure caption: Northward heat transport for the global ocean as determined by the ocean models. Results are decadal means from model years 491–500, and the values include both resolved advective and SGS transport contributions. Units are PW=10¹⁵Watts.

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Gent, P.R., S.G. Yeager, R.B. Neale, S. Levis and D.A. Bailey. 2009: Improvements in a half degree atmosphere/land version of the CCSM. Climate Dynamics, 79, 25-58, doi:10.1007/s00382-009-0614-8.

Figure 2: High resolution figure

Abstract: A decadal climate projection between 1980 and 2030 using a nominal 0.5° resolution in the atmosphere and land components has been performed using the Community Climate System Model, version 3.5. The mean climate is compared to a companion simulation using a nominal 2° resolution in the atmosphere and land components. The increased atmosphere resolution has several benefits, and produces a significantly better mean climate. The maximum sea surface temperature biases in the major upwelling regions, including the West Coast of the USA, are reduced by more than 60%. Precipitation patterns are improved in the summer Asian monsoon, mostly due to the better resolved orography, and in the eastern tropical Pacific Ocean south of the equator. The improved precipitation patterns lead to better river flows in many rivers worldwide. The atmospheric circulation in the Arctic also improves, which leads to a better regional sea ice thickness distribution in the Arctic Ocean.

Figure caption: Annual mean Arctic sea ice thickness in meter from (a) 0.5° run, and (b) 2° run.

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Jochum, M. and S. Yeager. 2009: The connection between Labrador Sea buoyancy loss, deep western boundary current strength, and Gulf Stream path in an ocean circulation model. Ocean Modelling, 30, 207-224. [article]

Figure 3: High resolution figure

Abstract: The sensitivity of the North Atlantic gyre circulation to high latitude buoyancy forcing is explored in a global, non-eddy resolving ocean general circulation model. Increased buoyancy forcing strengthens the deep western boundary current, the northern recirculation gyre, and the North Atlantic Current, which leads to a more realistic Gulf Stream path. High latitude density fluxes and surface water mass transformation are strongly dependent on the choice of sea ice and salinity restoring boundary conditions. Coupling the ocean model to a prognostic sea ice model results in much greater buoyancy loss in the Labrador Sea compared to simulations in which the ocean is forced by prescribed sea ice boundary conditions. A comparison of bulk flux forced hindcast simulations which differ only in their sea ice and salinity restoring forcings reveals the effects of a mixed thermohaline boundary condition transport feedback whereby small, positive temperature and salinity anomalies in subpolar regions are amplified when the gyre spins up as a result of increased buoyancy loss and convection. The primary buoyancy flux effects of the sea ice which cause the simulations to diverge are ice melt, which is less physical in the diagnostic sea ice model, and insulation of the ocean, which is less physical with the prognostic sea ice model. Increased salinity restoring ensures a more realistic net winter buoyancy loss in the Labrador Sea, but it is found that improvements in the Gulf Stream simulation can only be achieved with the excessive buoyancy loss associated with weak salinity restoring.

Figure caption: Ocean bathymetry (km) from the POP 1° model with geographical features mentioned in the text.