CGD's Dr. Gokhan Danabasoglu

Abstract

The effects of a prescribed surface intensification of the thickness (and isopycnal) diffusivity on the solutions of an ocean general circulation model are documented. The model is the coarse resolution version of the ocean component of the National Center for Atmospheric Research (NCAR) Community Climate System Model version 3 (CCSM3). Guided by the results of Ferreira, Marshall, and Heimbach (2005), we employ a vertical dependence of the diffusivity which varies with the stratification, N², and is thus large in the upper ocean and small in the abyss. We experiment with vertical variations of diffusivity which are as large as 4000 m² s^-1 within the surface diabatic layer, diminishing to 400 m² s^-1 or so by a depth of 2 km. The new solutions compare more favorably with the available observations than those of the control which uses a constant value of 800 m² s^-1 for both thickness and isopycnal diffusivities. These include an improved representation of the vertical structure and transport of the eddy-induced velocity in the upper-ocean North Pacific, a reduced warm bias in the upper ocean, including the equatorial Pacific, and improved southward heat transport in the low- to mid-latitude Southern Hemisphere. There is also a modest enhancement of abyssal stratification in the Southern Ocean.

Figure caption: Time-mean, eddy-induced normal transport in potential temperature bins (1°C bin interval) in the upper ocean integrated along the repeated hydrographic ship track in the tropical North Pacific from a) CONTROL, b) ITN2 in which both the isopycnal and thickness diffusivities vary similarly in the vertical, c) TN2 in which only the thickness diffusivity varies in the vertical and the isopycnal diffusivity is constant at 800 m² s^-1, and d) measured by Roemmich and Gilson (2001, J. Phys. Oceanogr., 31, 675-687). The numbers denoted by S and N indicate the cumulative southward and northward transports in the upper ocean, respectively. The integrals extend to 945 and 800 m in the model and observations, respectively.

Support: partially supported by the NSF grant OCE-0336827 for the Climate Process Team on Eddy Mixed-Layer Interactions (CPT-EMILIE). John Marshall acknowledges support from the NASA-funded ECCO2 project and NSF's Polar Program. The computational resources were provided by the Scientific Computing Division of the National Center for Atmospheric Research (NCAR). NCAR is sponsored by the National Science Foundation.

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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.

Reference Stephen Yeager's Research Catalog for this publication.

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Wu, W., G. Danabasoglu, and W. G. Large, 2007: On the effects of parameterized Mediterranean overflow on North Atlantic ocean circulation and climate. Ocean Modelling, 19, 31-52, doi:10.1016/j.ocemod.2007.06.003.

Abstract

A parameterized Mediterranean overflow, based on the marginal sea boundary condition of Price and Yang (1998), has been implemented in the ocean component of the Community Climate System Model to represent exchanges through the Strait of Gibraltar, associated entrainment and intrusion of overflow product water into the Atlantic. Previously, in coarse resolution model versions with a closed Strait, this physics has been either missing in uncoupled configurations or both only partially and unphysically treated as a surface salt exchange when fully coupled. Parameter choices are evaluated by comparing climatologically forced solutions to observations and process model results. The two major criteria satisfied by the implementation in a fully coupled climate model and a global ocean model are stable solutions and projection of the overflow signal across the Atlantic basin at about 1000 m depth. Both of these configurations are low resolution, and in both the transports of inflow, source and entrainment water are all within the range of observed estimates, but there is too little product water. This bias is attributed to inadequate modeling of water masses in the Mediterranean source region. Nevertheless, the properties of the product water differ little from observed estimates and both the uncoupled and coupled models develop a Mediterranean salt tongue that spreads west and south from the Strait with a signature reminiscent of the observed hydrography. The improvements relative to either blocking the Strait, or excavating a too wide channel are presented. In the coupled solution, the impact of the improved overflow physics on the global climate is minimal, with North Atlantic sea surface temperatures and heat fluxes changing generally by less than 1°C and 15 W m^-2, respectively. However, there is interesting spatial variability in the coupling strength, which ranges between ±20 W m^-2 °C^-1 in the coupled case.

Figure caption: North Atlantic time-mean salinity (left panels) and temperature (right panels) distributions at a depth of 1100 m (a-b) without the parameterized Mediterranean overflow (PMO) (x3ocn) and (c-d) with PMO (x3pmo).

Support: supported by NSF Grant OCE-0336834 for the Climate Process Team on Gravity Current Entrainment. The computational resources were partially provided by the Scientific Computing Division of the National Center for Atmospheric Research (NCAR). NCAR is sponsored by the National Science Foundation.

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Danabasoglu, G., R. Ferrari, and J. C. McWilliams, 2007: Sensitivity of an ocean general circulation model to a parameterization of near-surface eddy fluxes. J. Climate (in press).

Abstract

A simplified version of the near-boundary eddy flux parameterization of Ferrari and McWilliams (2007) has been implemented in the NCAR Community Climate System Model (CCSM3) ocean component for the surface boundary only. This scheme includes the effects of diabatic mesoscale fluxes within the surface layer. The experiments with the new parameterization show significant improvements compared to a control integration that tapers the effects of the eddies as the surface is approached. Such surface tapering is typical of present implementations of eddy transport in some current ocean models. The comparison is also promising versus available observations and results from an eddy-resolving model. These improvements include the elimination of strong, near-surface, eddy-induced circulations and a better heat transport profile in the upper-ocean. The experiments with the new scheme also show reduced abyssal cooling and diminished trends in the potential temperature drifts. Furthermore the need for any ad-hoc, near-surface taper functions is eliminated. The impact of the new parameterization is mostly associated with the modified eddy-induced velocity treatment near the surface. The new parameterization acts in the depth range exposed to enhanced turbulent mixing at the ocean surface. This depth range includes the actively turbulent boundary layer and a transition layer underneath, composed of waters intermittently exposed to mixing. The mixed layer, i.e. the regions of weak stratification at the ocean surface, is found to be a good proxy for the sum of the boundary layer depth and transition layer thickness.

Figure caption: Upper-ocean vertical profiles of zonally-integrated, time-mean total advective (Eulerian mean + eddy-induced) heat transport at 49.4°S from CONTROL, NSEF which uses the new surface eddy flux parameterization, and an eddy-resolving model (ER) as a measure of "truth."

Support: partially supported by the NSF grant OCE-0336827 for the Climate Process Team on Eddy Mixed-Layer Interactions (CPT-EMILIE). The computational resources were provided by the Scientific Computing Division of the National Center for Atmospheric Research (NCAR). NCAR is sponsored by the National Science Foundation.