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- Strategic Goals: Science, Facilities & Technology
- 1. Exploring atmospheric, Earth system, and solar processes, variability, and change
- 2. Investigating the interaction of the atmosphere, the broader Earth system, and human society
- 3. Improving prediction of weather, climate, and other atmospheric phenomena
- 4. Developing community models
- 5. Enabling innovative field experiments and measurement campaigns
- 6. Developing new instrumentation
- Research Catalog
CGD 2008 Profiles in Science: Stephen Yeager
Summary of achievements
Stephen Yeager
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
Yeager, S.G., and W.G. Large, 2007: Observational Evidence of Winter Spice Injection. J. Phys. Oceanogr., 37, 2895-2919.
Figure 1.
High resolution figure
Abstract: Temperature and salinity (T-S) profiles from the global array of Argo floats support the existence of spice-formation regions in the subtropics of each ocean basin where large, destabilizing vertical salinity gradients coincide with weak stratification in winter. In these characteristic regions, convective boundary layer mixing generates a strongly density-compensated (SDC) layer at the base of the well-mixed layer. The degree of density compensation of the T-S gradients of an upper-ocean water column is quantified using a bulk vertical Turner angle (Tub) between the surface and upper pycnocline. The winter generation of the SDC layer in spice-formation zones is clearly seen in Argo data as a large-amplitude seasonal cycle of Tub in regions of the subtropical oceans characterized by high mean Tub. In formation regions, Argo floats provide ample evidence of large, abrupt spice injection (T-S increase on subducted isopycnals due to vertical mixing) associated with the winter increase in Tub. A simple conceptual model of the spice-injection mechanism is presented that is based on known behavior of convective boundary layers and supported by numerical model results. It suggests that penetrative convective mixing of a partially density-compensated water column will enhance the Turner angle within a transition layer between the mixed layer and the upper pycnocline, generating seasonal T-S increases on density surfaces below the mixed layer. Observations are consistent with this hypothesis. In OGCMs, regions showing high Tub mean and seasonal amplitude are also the sources of significant interannual spice variability in the permanent pycnocline. Decadal changes in the North Pacific of a model hindcast simulation show qualitative resemblance to the observed multiyear time series from the Hawaii Ocean Time series (HOT) station ALOHA. Modeled pycnocline variations near Hawaii can be linked to high Tub seasonality and winter spice injection within a formation region upstream of ALOHA, suggesting that spice injection may explain the origins of observed large, interannual variations on isopycnals in the ocean interior.
Figure caption: Multiyear time series from Argo profiler WMO 5900117 of South Atlantic upper ocean (a) salinity contoured at 0.2, (b) temperature contoured at 1°C, (c) Turner angle, Tu, with density overlaid (contoured at 0.25 kg/m3), (d) bulk vertical Turner angle between the surface and 200m, and (e) isopycnal salinity on six σ0 surfaces: 25.2, 25.4, 25.5, 25.6, and 25.7 kg/m3. In (c), Tu values of 60°, 71.6°, 77°, and 85° correspond approximately to
Rρ = 3.7, 2, 1.6, and 1.2, respectively, and cross-hatch indicates that vertical gradients are too small to compute Tu.
Large, W.G. and S.G. Yeager, 2008: The Global Climatology of an Interannually Varying Air-Sea Flux Data Set. Climate Dynamics, doi:10.1007/s00382-008-0441-3.
Figure 2.
High resolution figure
Abstract: The air-sea fluxes of momentum, heat, freshwater and their components have been computed globally from 1948 at frequencies ranging from 6-hourly to monthly. All fluxes are computed over the 23 years from 1984 to 2006, but radiation prior to 1984 and precipitation before 1979 are given only as climatological mean annual cycles. The input data are based on NCEP reanalysis only for the near surface vector wind, temperature, specific humidity and density, and on a variety of satellite based radiation, sea surface temperature, sea-ice concentration and precipitation products. Some of these data are adjusted to agree in the mean with a variety of more reliable satellite and in sitmeasurements, that themselves are either too short a duration, or too regional in coverage. The major adjustments are a general increase in wind speed, decrease in humidity and reduction in tropical solar radiation. The climatological global mean air-sea heat and freshwater fluxes (1984-2006) then become 2 W/m2 and -0.1 mg/m2 per second, respectively, down from 30 W/m2 and 3.4 mg/m2 per second for the unaltered data. However, decadal means vary from 7.3 W/m2 (1977-1986) to -0.3 W/m2 (1997-2006). The spatial distributions of climatological fluxes display all the expected features. A comparison of zonally averaged wind stress components across ocean sub-basins reveals large differences between available products due both to winds and to the stress calculation. Regional comparisons of the heat and freshwater fluxes reveal an alarming range among alternatives; typically 40 W/m2 and 10 mg/m2 per second, respectively. The implied ocean heat transports are within the uncertainty of estimates from ocean observations in both the Atlantic and Indo-Pacific basins. They show about 2.4 PW of tropical heating, of which 80% is transported to the north, mostly in the Atlantic. There is similar good agreement in freshwater transport at many latitudes in both basins, but neither in the South Atlantic, nor at 35°N.
Figure caption: Time series over 57 years (1950 through 2006) of annual mean CORE.2 fluxes: a) air-sea heat flux in W/m2, b) air-sea freshwater flux, excluding runoff, in mg/m2/s, averaged over the global ocean and the Atlantic, Pacific, Indian and Southern, but not the Arctic basins.
Support:NOAA grant no. NA06GP0428 and by the National Science Foundation through its sponsorship of the National Center for Atmospheric Research.
Griffies, S. M., A. Biastoch, C. Boning, F. Bryan, G. Danabasoglu, E. P. Chassignet, M. H. England, R. Gerdes, H. Haak, R. W. Hallberg, W. Hazeleger, J. Jungclaus, W. G. Large, G. Madec, A. Pirani, B. L. Samuels, M. Scheinert, A. S. Gupta, C. A. Severijns, H. L. Simmons, A. M. Treguier, M. Winton, S. Yeager, J. Yin, 2008: Coordinated Ocean-ice Reference Experiments (COREs). Ocean Modelling, in press.
Figure 3.
High resolution figure
Abstract: Coordinated Ocean-ice Reference Experiments (COREs) are presented as a tool to explore the behavior 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 and Yeager (2004) was developed for coupled ocean and sea ice models. We found it to be suitable for our purposes, even though its evaluation originally focused 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, with identification of biases promoting avenues for model and/or forcing dataset development.
Figure caption: Time series of the annual mean Atlantic meridional overturning circulation (AMOC) index (vertical axis) for model years 1-500 (horizontal axis) in units of Sv from the seven participating models in this study. The index is computed as the maximum AMOC streamfunction at 45N in the region beneath the wind driven Ekman layer. Note that the GFDL-MOM simulation was extended to 600 years to verify that it was reaching a steady state for the overturning. An AMOC with realistic transport strength and structure is important for maintaining a realistic ocean climate. It is sometimes quite difficult to realize a stable overturning circulation, especially in ocean-ice models. The behavior of the ocean-ice models in this study indeed reflects on this sensitivity, with some models ``refusing'' to stabilize at a circulation reflecting observations (e.g., for North Atlantic Deep Water (NADW), about 15 Sv), whereas others appear to reach a stable value either with a very weak salinity restoring (NCAR-POP, FSU-HYCOM, and MPI), or stronger restoring (GFDL-MOM and Kiel-ORCA). It is notable that the two isopycnal models appear to have the most difficulty reaching a steady state, with the GFDL-HIM simulation showing large amplitude variations, whereas the KNMI-MICOM simulation settles into a very weak, nearly absent, overturning circulation in the NADW cell. We also note that the FSU-HYCOM simulation exhibits a nontrivial level of interannual variability relative to the simulations from NCAR-POP, GFDL-MOM, Kiel-ORCA, and MPI. Given the steady nature of the repeating annual forcing, the FSU-HYCOM variability is internally generated. The mechanism for variability has not been determined.
Support: NCAR is sponsored by the National Science Foundation.
Doney, S. C., S. Yeager, G. Danabasoglu, W. G. Large, and J. C. McWilliams, 2007: Mechanisms governing interannual variability of upper ocean temperature in a global ocean hindcast simulation. J. Phys. Oceanogr., 37, 1918-1938.
Figure 4.
High resolution figure
Abstract: The interannual variability in upper-ocean (0-400 m) temperature and governing mechanisms for the period 1968-97 are quantified from a global ocean hindcast simulation driven by atmospheric reanalysis and satellite data products. The unconstrained simulation exhibits considerable skill in replicating the observed interannual variability in vertically integrated heat content estimated from hydrographic data and monthly satellite sea surface temperature and sea surface height data. Globally, the most significant interannual variability modes arise from El Niño-Southern Oscillation and the Indian Ocean zonal mode, with substantial extension beyond the Tropics into the midlatitudes. In the well-stratified Tropics and subtropics, net annual heat storage variability is driven predominately by the convergence of the advective heat transport, mostly reflecting velocity anomalies times the mean temperature field. Vertical velocity variability is caused by remote wind forcing, and subsurface temperature anomalies are governed mostly by isopycnal displacements (heave). The dynamics at mid- to high latitudes are qualitatively different and vary regionally. Interannual temperature variability is more coherent with depth because of deep winter mixing and variations in western boundary currents and the Antarctic Circumpolar Current that span the upper thermocline. Net annual heat storage variability is forced by a mixture of local air-sea heat fluxes and the convergence of the advective heat transport, the latter resulting from both velocity and temperature anomalies. Also, density-compensated temperature changes on isopycnal surfaces (spice) are quantitatively significant.
Figure caption: Spatial maps of the (left) first and (right) second EOF (1993-97) for the (top) model and (middle) T/P monthly SSH anomalies after the removal of the average seasonal cycle for 1993-95. The contour interval is 1 cm, and the variances associated with each EOF are given as percentages of the respective total variances. (bottom) The time series of the TOPEX/Poseidon and model first-mode principal components.