Welcome to CGD

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Summary of achievements

The research interest that underlies my research is the physics of the upper ocean and feedbacks with the atmosphere. Of particular interest are low frequency, large scale phenomena, and the role of the upper ocean as a conduit between the interior ocean and the atmosphere. Therefore, upper ocean models for global climate modeling studies are a particular focus. I am involved in small scale experiments, both observational and numerical (LES), that are designed to determine important upper ocean processes, and the signatures of these processes, so that their large scale significance can be assessed and modeled. The atmosphere-ocean forcing functions are a very important aspect of the problem, and I continue to try to improve our knowledge of them.

Ocean and Sea-Ice Modeling for Coupled Climate Models: Major products are state of the art ocean and sea-ice model components for the NCAR Community Climate System Model (CCSM). These components are being tested extensively and exercised in studies of the global ocean and of the impacts of sea-ice and atmospheric boundary conditions. There is currently a concerted effort to force a coupled ocean, sea-ice system in as similar a manner as practical to when coupled to an atmospheric model. Growing activities are the assessment of component performance within a coupled CCSM integration, as well as analysis of the coupled model output. Objectives are to investigate the interaction of the atmosphere and ocean in decadal climate fluctuations and to identify possible sources of climate drift in coupled models.

Forcing Numerical Models: The goal here is to specify the frequency and wavenumber content of forcing functions required to force numerical ocean models. One task is to quantify the ocean model response to high wavenumber (25-500 km) winds. Since important responses have been found, a means of obtaining the needed high wavenumber information at the required frequency from satellite scatterometer measurements was developed. The product is an annual cycle (August 1996 through July 1997) of global scatterometer based winds. The resolution is one-half degree in both the zonal and meridional directions and every six hours in time.

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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Donner, L.J. and W.G. Large. 2008: Climate Modeling. Annual Rev. of Env. and Resources, 33, 1-17, doi:10.1146/annurev.eg.33.101308.100001.

Figure 2: High resolution figure

Abstract: Climate models simulate the atmosphere, given atmospheric composition and energy from the sun, and include explicit modeling of, and exchanges with, the underlying oceans, sea ice, and land. The models are based on physical principles governing momentum, thermodynamics, cloud microphysics, radiative transfer, and turbulence. Climate models are evolving into Earth-system models, which also include chemical and biological processes and afford the prospect of links to studies of human dimensions of climate change. Although the fundamental principles on which climate models are based are robust, computational limits preclude their numerical solution on scales that include many processes important in the climate system. Despite this limitation, which is often dealt with by parameterization, many aspects of past and present climate have been successfully simulated using climate models, and climate models are used extensively to predict future climate change resulting from human activity.

Figure caption: Spatial distributions of correlations of SST anomalies with anomalies averaged over the white box that defines the Niño 3 region from (a) the Hadley Centre Sea Ice and SST data set (HadISST) and (b) simulations (61) of the Communtity Climate System Model version 3.5 (CCSM3.5).

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Legg, S., B. Briegleb, Y. Chang, E.P. Chassignet, G. Danabasoglu, T. Ezer, A.L. Gordon, S. Griffies, R. Hallberg, L. Jackson, W. Large, T.M. Özgökmen, H. Peters, J. Price, U. Riemenschneider, W. Wu, X. Xu and J. Yang. 2009: Improving Oceanic Overflow Representation in Climate Models: The Gravity Current Entrainment Climate Process Team. Bulletin of the American Meteorological Society, 90, 657-670, doi:doi:10.1175/2008BAMS2667.1.

Figure 3: High resolution figure

Abstract: Oceanic overflows are bottom-trapped density currents originating in semienclosed basins, such as the Nordic seas, or on continental shelves, such as the Antarctic shelf. Overflows are the source of most of the abyssal waters, and therefore play an important role in the large-scale ocean circulation, forming a component of the sinking branch of the thermohaline circulation. As they descend the continental slope, overflows mix vigorously with the surrounding oceanic waters, changing their density and transport significantly. These mixing processes occur on spatial scales well below the resolution of ocean climate models, with the result that deep waters and deep western boundary currents are simulated poorly. The Gravity Current Entrainment Climate Process Team was established by the U.S. Climate Variability and Prediction (CLIVAR) Program to accelerate the development and implementation of improved representations of overflows within large-scale climate models, bringing together climate model developers with those conducting observational, numerical, and laboratory process studies of overflows. Here, the organization of the Climate Process Team is described, and a few of the successes and lessons learned during this collaboration are highlighted, with some emphasis on the well-observed Mediterranean overflow. The Climate Process Team has developed several different overflow parameterizations, which are examined in a hierarchy of ocean models, from comparatively well-resolved regional models to the largest-scale global climate models.

Figure Caption: Salinity in the Mediterranean outflow plume shown as a function of latitude and depth along a section at 8.5°W from (a) the World Ocean Circulation Experiment (WOCE) observations and (b) HY COM regional simulation at 0.08° horizontal resolution with 28 layers in the vertical. The regional model was integrated for six months, and salinity is shown for the end of the integration.