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Circulation driven by horizontal differential heating is studied, using a doublewalled Plexiglas tank (20 × 15 × 2.5 cm 3 ) filled with salt water. For instances of heating/cooling from above and below, results indicate that there is always quasi-equilibrium circulation. In contrast to most previous results from experimental/numerical studies, circulation in our experiments appears in the form of a shallow cell adjacent to the boundary of thermal forcing. The non-dimensional streamfunction maximum confirms the 1/5-power law of Rossby, Ψ ∼ RaL . Dissipation rate measured in the experiments appears to be consistent with theory.For cases of heating/cooling from a sloping bottom, circulation is similar to cases with a flat bottom; circulation is strong if heating is below cooling, but it is rather weak if heating is above cooling. Nevertheless, circulation in all cases is visible to the naked eye.

[1] The analysis of an updated monthly climatology of observed temperature and salinity from the U.S. Navy Generalized Digital Environment Model reveals a basinscale cyclonic circulation over the deep South China Sea (SCS). The cyclonic circulation lies from about 2400 m to the bottom. The boundary current transport of the cyclonic circulation is around 3.0 Sv. Our results suggest that the cyclonic circulation is mainly forced by the Luzon overflow, with bottom topography playing an important role. The structures of potential temperature, salinity, and potential density in the deep SCS are consistent with the existence of the cyclonic circulation. Specifically, low salinity water is found in the interior region west of Luzon Island, and surrounded by saline Pacific water in boundary current regions to the north, west and southwest. Our results show the potential density distribution and the corresponding cyclonic circulation in deep SCS are primarily controlled by salinity variations in the deep basin.

[1] Mesoscale eddies dominate oceanic kinetic energy at sub-initial frequencies. Their three-dimensional structure has, however, remained obscure, hindering better understanding of eddy dynamics. Here by applying the composite analysis of satellite altimetry and Argo float data to the globe, we show that despite remarkable regional differences in amplitude, extent and polarity, etc., mesoscale eddies have a universal structure in normalized stretched coordinates. Horizontally, the associated pressure anomaly is well described by a function of the normalized radial distance from the eddy center R(r n ) = (1-r n 2 /2) exp(-r n 2 /2), whereas vertically it is sinusoidal in a stretched coordinate z s = R z 0 (N/f )dz, where N and f are the buoyancy frequency and the Coriolis parameter.

The effect of double-diffusive mixing on the general circulation is explored using the GFDL MOM2 model. The motivation for this comes from the known sensitivity of the thermohaline circulation to the vertical diffusivity and the earlier work of Gargett and Holloway, who studied the effects of a simple nonunity ratio between heat and salt diffusivities in a GCM. In this work, a more realistic, yet conservative, parameterization of the doublediffusive mixing is applied, with the intensity depending on the local density ratio R ϭ ␣T z /␤S z. A background diffusivity is used to represent non-double-diffusive turbulent mixing in the stably stratified environment. The numerical model is forced by relaxation boundary conditions on both temperature and salinity at the sea surface. Three control experiments have been carried out: one with the double-diffusive parameterization (DDP) determined by the local density ratio, one with constant but different diffusivities for heat and salt as previously considered by Gargett and Holloway (GHD), and the other with the same constant diapycnal eddy diffusivity for both heat and salt (CDD). The meridional overturning in run DDP is 22% less than in run CDD, and the maximum poleward heat transport is about 8% less. In comparison, the overturning rate and poleward heat transport in run GHD display reductions that are about half as large. The interior temperature and salinity in run DDP and GHD are higher than in run CDD, with the change in run DDP more than twice that in run GHD. In addition, in DDP and GHD, the density ratio distribution becomes closer to unity than in run CDD, with the change in run DDP being larger than in GHD. Interestingly, the double diffusion is stronger in the western boundary current region than the interior, implying a close relation between vertical shear and the intensity of double diffusion. These results indicate a greater sensitivity of the thermohaline circulation to double diffusion than had previously been suspected due to the tendency of the double-diffusive mixing to generate self-reinforcing flows. This effect appears to be more significant when the double-diffusive mixing is applied only when the stratification is favorable rather than uniformly applied. In addition, parameter sensitivity experiments suggest that double diffusion could have stronger effects on the meridional overturning and poleward heat transport than modeled here since the parameterizations chosen are rather conservative. * WHOI Contribution Number 9239.

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