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Highlights: [•] We investigate an effect of numerical diffusion on water mass transformation. [•] We investigate a dependency of a numerical diffusion effect on explicit diffusivity. [•] Numerical diffusion seems to be inevitable for current eddy-resolving models. [•] We must adequately evaluate an effect of numerical diffusion. [Copyright &y& Elsevier]

Abstract: The formation and outflow of dense shelf water (DSW) from the Adélie and George V Land region and the seasonal variation in the ocean circulation therein are investigated using a global ice-ocean model with fine resolution around East Antarctica. The model reproduces key elements of the overall system described by previous observational studies: modified Circumpolar Deep Water intrusions into the Adélie Depression, enhanced sea-ice production in the coastal polynya regime, and seasonal variation in the export of DSW through the Adélie Sill. We find two distinct seasonal circulation patterns in the Adélie Depression. The first is a vertical circulation formed over the southwestern flank of the Mertz Bank in winter, with a southeastward inflow of light water in the upper layer and a northwestward outflow of dense water in the lower layer. The second is a surface-intensified clockwise circulation trapped in the Adélie Depression during summer. This seasonal variation is attributed to the spinning-up and down of the haline circulation which is driven by the brine-rejection from enhanced sea ice formation in the coastal polynyas. In the model a similar haline circulation also develops in the Mertz Depression. Assuming a critical density of 27.88 (27.80)kgm−3 for outflowing DSW, the annual mean export from the Adélie and Mertz Sills are 0.21 (0.37) Sv and 0.12 (0.32) Sv, respectively. The model clearly shows a dual-system for DSW formation and export from the Adélie and Mertz Depressions, consistent with recent observational analysis over the continental slope and rise. [Copyright &y& Elsevier]

Abstract: Progress of North Pacific modeling over the past decade is reviewed with a focus on studies under minimally idealized setups, which enables us to directly compare modeling results with observed features. Modeling targets are here categorized into three categories: surface currents, subsurface and intermediate waters, and deep circulation. These three are very different in terms of the level of observation-based understanding and what has been achieved by modeling. Progress of modeling is constrained by that of observation, and emergence of high-resolution observed datasets is encouraging higher and more precise modeling in each of these categories. [Copyright &y& Elsevier]

Abstract: Formation processes of the hydrographic structures and meridional overturning circulations in the equatorial Pacific are investigated using an ocean general circulation model where a high accuracy tracer advection scheme, the second-order moment scheme, is implemented. It is also addressed why the equatorial thermocline tends to be diffuse in conventional ocean models. The upper equatorial thermocline water is fed by the subtropical cell of the northern hemisphere, which lies over the lower thermocline water fed by the subtropical cell of the southern hemisphere. Additional experiments reveal that even slight numerical diffusion induced by a third-order tracer advection scheme disturbs the thermodynamic balance of the Pacific equatorial thermocline and makes it significantly diffuse. Magnitude of vertical numerical diffusion is of primary importance in properly reproducing the Pacific equatorial thermocline. Smaller vertical diffusion makes the isopycnals around 25.5σθ shallower and consequently intensifies the equatorward transport by the lower part of the southern hemisphere subtropical cell. This cold water transport from the southern hemisphere is also important in further tightening the equatorial thermocline, and it is significantly reduced by horizontal numerical diffusion under less accurate advection schemes. [ABSTRACT FROM AUTHOR]

Abstract: A non-hydrostatic ocean model using an effective Poisson solver is developed. The Poisson solver is a combination of the multigrid method, the Krylov-subspace method, and the sparse approximate inverse. Its numerical cost only linearly increases with total number of computational cells, and it also has high parallel computing efficiency. The numerical cost of the non-hydrostatic model described in the present paper remains only twice of that of a hydrostatic model, even with non-smooth topography and with a huge number of computational grid cells on massively parallelized computer systems. Therefore, it has a potential to expand the applicability of non-hydrostatic ocean models. We also present the preliminary result of the high-resolution non-hydrostatic experiment on Ice Shelf Water overflow in the southern Weddell Sea, which shows good agreement with observations in terms of the pathway of dense water and velocity field. [Copyright &y& Elsevier]

Abstract: Observations of deep ocean temperature and salinity in the Labrador and Greenland Seas indicate that there is negative correlation between the activities of deep convection in these two sites. A previous study suggests that this negative correlation is controlled by the North Atlantic Oscillation (NAO). In this study, we discuss this deep convection seesaw by using a coupled atmosphere and ocean general circulation model. In this simulation, the deep convection is realistically simulated in both the Labrador and Greenland Seas and their negative correlation is also recognized. Regression of sea level pressure to wintertime mixed layer depth in the Labrador Sea reveals strong correlation between the convection and the NAO as previous studies suggest, but a significant portion of their variability is not correlated. On the other hand, the convection in the Greenland Sea is not directly related to the NAO, and its variability is in phase with changes in the freshwater budget in the GIN Seas. The deep convection seesaw found in the model is controlled by freshwater transport through the Denmark Strait. When this transport is larger, more freshwater flows to the Labrador Sea and less to the Greenland Sea. This leads to lower upper-ocean surface salinity in the Labrador Sea and higher salinity in the Greenland Sea, which produces negative correlation between these two deep convective activities. The deep convection seesaw observed in the recent decades could be interpreted as induced by the changes in the freshwater transport through the Denmark Strait, whose role has not been discussed so far. [Copyright &y& Elsevier]

Abstract: We investigate the formation of dense water in the Greenland, Iceland, and Norwegian (GIN) Seas by focusing on salt balance between oceanic salt transport and sea surface flux. We conduct ocean general circulation model (OGCM) experiments under a sea surface salinity (SSS) restoring boundary condition with a horizontal resolution of 2° and those where the resolution is increased for northern high latitudes. A comparison of the salt transport at each passage and passive tracer distribution between the experiments reveals the following features: (1) in higher-resolution experiments, a stronger current at the Iceland–Scotland passage can carry saline water from the Atlantic Ocean to the GIN Seas and (2) higher resolution can represent the stronger West Spitsbergen Current that confines fresher water from the Arctic Ocean to the east coast of Greenland, thereby allowing saline water from the Atlantic Ocean to flow toward the Greenland Sea more efficiently. As the resolution increases, they contribute to a more realistic simulation of salinity distribution in the GIN Seas. These improvements are critically important for a realistic simulation of the Atlantic meridional overturning circulation (AMOC) when the model is forced without the SSS restoring condition. In OGCM experiments forced by climatological freshwater flux, the modeled AMOC in the experiment with the resolution of 2° is unrealistically weak because the inability to simulate the salt transport into the GIN Seas dramatically reduces the salinity in the GIN Seas and weakens deep water formation there. As the resolution increases, the AMOC becomes stronger due to a more realistic representation of ocean salt transport into the GIN Seas. These results demonstrate that the adoption of high resolution at least for the northern high latitudes facilitates realistic simulation of deep water formation in the GIN Seas and AMOC. [Copyright &y& Elsevier]

To clarify the characteristics and mechanism of mesoscale variability in the deep ocean, we analyzed historical mooring observations conducted at Site R (30°N, 147°E) south of the Kuroshio Extension during 1978–85 and the 10-year output of an eddy-resolving ocean general circulation model. The power spectral density (PSD) of current velocity at a depth of 5000 m had a peak at periods of 45–75 days at two mooring stations that were zonally 98 km apart and at 54 days at Site R in the model. In the model, the PSD at 54 days at 5000 m had a horizontal maximum 100 km north of Site R as well as in the Kuroshio Extension region 500 km north of Site R. Within the high-PSD region near Site R, variability in the velocity was coherent, and its phase propagation was characterized by zonal and meridional wavenumbers of −2.9×10 −5 and 0.1×10 −5 rad m −1 , respectively, the former of which was comparable to the value of −2.6×10 −5 rad m −1 obtained from the mooring observations. Such wavenumbers matched well with the dispersion relation of barotropic topographic Rossby waves (TRWs). Backward ray tracing and PSD distribution suggested that the barotropic TRWs at Site R were generated in the Kuroshio Extension region, and their energy propagated southward. [ABSTRACT FROM AUTHOR]