In this study, we use a novel pan-Arctic sea ice-ocean coupled model to examine the effects of tides on sea ice and the mixing of water masses. Two 30 year simulations were performed: one with explicitly resolved tides and the other without any tidal dynamics. We find that the tides are responsible for a $15% reduction in the volume of sea ice during the last decade and a redistribution of salinity, with surface salinity in the case with tides being on average $1.0-1.8 practical salinity units (PSU) higher than without tides. The ice volume trend in the two simulations also differs: 22.09 3 10 3 km 3 /decade without tides and 22.49 3 10 3 km 3 /decade with tides, the latter being closer to the trend of 22.58 3 10 3 km 3 /decade in the PIOMAS model, which assimilates SST and ice concentration. The three following mechanisms of tidal interaction appear to be significant: (a) strong shear stresses generated by the baroclinic clockwise rotating component of tidal currents in the interior waters; (b) thicker subsurface ice-ocean and bottom boundary layers; and (c) intensification of quasi-steady vertical motions of isopycnals (by $50%) through enhanced bottom Ekman pumping and stretching of relative vorticity over rough bottom topography. The combination of these effects leads to entrainment of warm Atlantic Waters into the colder and fresher surface waters, supporting the melting of the overlying ice.
Mixing in the ocean and shelf seas is critical for the vertical distribution of dynamically active properties, such as density and biogeochemical tracers. Eight different decadal simulations are used to assess the skill of vertical turbulent mixing schemes (TMS) in a 3‐D regional model of tidally active shelf seas. The TMS differ in the type of stability functions used and in the Ozmidov/Deardorff/Galperin limiter of the turbulence length scales. We review the dependence of the critical Richardson and Prandtl numbers to define the “diffusiveness” of the TMS. The skill in representing bias and variability of stratification profiles is assessed with five different metrics: surface and bottom temperatures and pycnocline depth, thickness, and strength. The assessment is made against hydrography from three data sets (28,000 profiles in total). Bottom and surface temperatures are found to be as sensitive to TMS choice as to horizontal resolution or heat flux formulation, as reported in other studies. All TMS underrepresent the pycnocline depth and benthic temperatures. This suggests physical processes are missing from the model, and these are discussed. Different TMSs show the best results for different metrics, and there is no outright winner. Simulations coupled with an ecosystem model show the choice of TMS strongly affects the ecosystem behavior: shifting the timing of peak chlorophyll by 1 month, showing regional chlorophyll differences of order 100%, and redistributing the production of chorophyll between the pycnocline and mixed layer.
We explore dense water cascading (DWC), a type of bottom-trapped gravity current, on multidecadal time scales using a pan-Arctic regional ocean-ice model. DWC is particularly important in the Arctic Ocean as the main mechanism of ventilation of interior waters when open ocean convection is blocked by strong density stratification. We identify the locations where the most intense DWC events occur and evaluate the associated cross-shelf mass, heat, and salt fluxes. We find that the modeled locations of cascading agree well with the sparse historical observations and that cascading is the dominant process responsible for cross-shelf exchange in the boundary layers. Simulated DWC fluxes of 1.3 Sv (1 Sv = 10 6 m 3 /s) in the Central Arctic are comparable to Bering Strait inflow, with associated surface and benthic Ekman fluxes of 0.85 and 0.58 Sv. With ice decline, both surface Ekman flux and DWC fluxes are increasing at a rate of 0.023 and 0.0175 Sv/year, respectively. A detailed analysis of specific cascading sites around the Beaufort Gyre and adjacent regions shows that autumn upwelling of warm and saltier Atlantic waters on the shelf and subsequent cooling and mixing of uplifted waters trigger the cascading on the West Chukchi Sea shelf break. Lagrangian particle tracking of low salinity Pacific waters originating at the surface in the Bering Strait shows that these waters are modified by brine rejection and cooling, and through subsequent mixing become dense enough to reach depths of 160-200 m. Plain Language Summary In this study we explore dense water cascading, а specific type of bottom-trapped gravity current. This current of very dense waters originates on the shelves, due to winter cooling and sea ice freezing, and slowly propagates downslope to deep waters. It is specifically important in the Arctic Ocean as the main mechanism of deep water mass formation and carbon storage. We use numerical model of the Arctic Ocean to predict preferable locations of the cascading, its intensity mass, and heat fluxes. Our results are in agreement with available very sparse observations. Our model predicts that with sea ice melting, cascading formation will accelerate. One of the crucial processes contributing to the upper ocean to abyssal exchange in the Arctic Ocean is shelf convection, also known as dense water cascading (DWC), a specific type of bottom-trapped gravity current (Shapiro et al., 2003). At high latitudes, the dense waters feeding DWC are formed on shallow shelves in the cold season due to heat loss to the atmosphere, resulting in freezing and subsequent brine injection into the water column. Preconditioning of DWC happens if the convection reaches the bottom, forming a well-mixed water mass denser than the waters downslope. In this case, dense water propagates mainly along-slope (due to Coriolis forcing) and downslope (due to bottom friction) as a bottom boundary current, mixing with the ambient waters en route. Descending DWC waters interact with bottom topographic features such as sills and canyons (Chap...
Abstract.Results of a sensitivity study are presented from various configurations of the NEMO ocean model in the Black Sea. The standard choices of vertical discretization, viz. z levels, s coordinates and enveloped s coordinates, all show their limitations in the areas of complex topography. Two new hybrid vertical coordinate schemes are presented: the "s-on-top-of-z" and its enveloped version. The hybrid grids use s coordinates or enveloped s coordinates in the upper layer, from the sea surface to the depth of the shelf break, and z-coordinates are set below this level. The study is carried out for a number of idealised and real world settings. The hybrid schemes help reduce errors generated by the standard schemes in the areas of steep topography. Results of sensitivity tests with various horizontal diffusion formulations are used to identify the optimum value of Smagorinsky diffusivity coefficient to best represent the mesoscale activity.
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