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Onno Bokhove and Vijaya Ambati

forcing frequencies were then used to drive the harmonic “wind” forcing provided via the Ekman pumping and suction due to an oscillating rigid lid. Various forcing strengths have been imposed in which a match with the linear calculations is best suited by weak forcing, whereas better visualization requires a larger signal-to-noise ratio and, consequently, stronger forcing. The latter promotes, however, the emergence of nonlinear effects. We, therefore, also compare the experimental streamfunction

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R. M. Samelson

). Lying between the wind-driven ( Luyten et al. 1983 ) upper thermocline gyres above, and the diffusion-driven ( Stommel and Arons 1960 ; Stommel and Webster 1962 ) abyssal circulation below, this cell is evidently driven by a poorly understood combination of these two mechanisms. Early theoretical efforts to explore the dynamics of this cell focused on the diffusive mechanism, in which diabatic forcing in the form of interior turbulent mixing drives heat downward, resulting in midlatitude upwelling

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Baylor Fox-Kemper and Raffaele Ferrari

diffusivities or viscosities: instead the TRM Parsons model compellingly illustrates the roles of eddies in a gyre circulation. In the TRM formulation, eddy effects appear as a force in the momentum equations—a fact extensively exploited here and in many atmospheric eddy–mean flow interaction studies. It is shown that eddies transfer momentum downward in the ocean interior and buoyancy poleward in the ocean mixed layer. The TRM Sverdrup relation is simply modified in the presence of eddies. The next section

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Yafang Zhong and Zhengyu Liu

-like structure ( Mantua et al. 1997 ) for all the cases (not shown). This suggests that the pattern of the PMV is set up predominantly by local atmospheric variability forcing. We further examine the PMV by studying the power spectrum for unfiltered SST anomalies averaged over the center of PMV SST variability in the broad KOE region (35°–45°N, 140°E–140°W; denoted by the rectangular box in Fig. 1a ). The distinguished PMV peaks at quasi-50-yr in CTRL ( Fig. 2a ) and PC-ET ( Fig. 2b ; Zhong et al. 2008

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Michael A. Spall

significantly downstream of sills. There is clearly a distinct and separate process resulting from surface buoyancy forcing that is responsible for the sinking of the dense waters within the marginal sea that produced the dense overflow waters. There is also downwelling in less constrained regions of buoyancy loss, such as the Labrador Sea ( Pickart and Spall 2007 ). Several recent studies have considered the formation of dense waters on shelves by local surface forcing and its subsequent spreading by

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Carl Wunsch and Patrick Heimbach

bound (e.g., Wunsch 2008 ). The specific solution is denoted v3.35 and differs quantitatively in numerous ways from the unoptimized control run (v3.0) that represented the starting estimate. A brief description of the v3.35 ECCO–GODAE estimate is that it is the result of a least squares fit of a GCM to a global, weighted dataset, 1992–2006. Comparisons (not shown) of the equivalent results in the “control” solution obtained by forcing the Massachusetts Institute of Technology GCM (MITgcm) with the

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J. A. Whitehead

that although over geological time the solar forcing is modified by orbital changes, the ocean surface temperature has evidently not always responded linearly. Climate records reveal occasional changes of atmospheric temperature of many degrees within “fast” times (order of 50 yr). Some of these are attributed to abrupt transitions of the thermohaline circulation regime ( Broecker et al. 1985 ; Boyle 1990 ; Keigwin and Jones 1994 ; Keigwin et al. 1994 ; Bard et al. 1996 ; Broecker 1997

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Paola Cessi and Christopher L. Wolfe

conservation, it is possible to obtain an approximate estimate of the zonally integrated transport streamfunction by knowing the wind stress forcing and the buoyancy difference across the basin (i.e., the buoyancy on the eastern and western boundaries; Hirschi and Marotzke 2007 ; Marotzke 1997 ). Defining the meridional transport streamfunction, Ψ, such that it is possible to relate Ψ to the wind stress, τ , distributed over the Ekman layer as a body force, and the difference in buoyancy on the eastern

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K. Shafer Smith and John Marshall

from a balance between large-scale thermal and wind forcing, which drive the mean state to be baroclinically unstable, and local eddy fluxes that act to remove mean PV gradients. This is sensible if large-scale forcing restores the mean faster than, or comparable to the rate at which eddies erode it. However, when the mean state is unstable at very small scales, this may not be a reasonable assumption. The geostrophic mean shear computed from hydrography (the thin line in Fig. 6a ) is an

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