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[1] The unstructured grid finite volume coastal ocean model (FVCOM) system has been expanded to include nonhydrostatic dynamics. This addition uses the factional step method with both split mode explicit and semi-implicit schemes. The unstructured grid finite volume method, combined with a correction of the final free surface from its intermediate value with inclusion of nonhydrostatic effects, efficiently reduces numerical damping and thus ensures second-order accuracy of the solutions with local/global volume conservation. Numerical experiments have been made to fully validate the nonhydrostatic FVCOM, including surface standing and solitary waves in idealized flat-and slopingbottomed channels in homogeneous conditions, the density adjustment problem for lock exchange flow in a flat-bottomed channel, and two-layer internal solitary wave breaking on a sloping shelf. The model results agree well with the relevant analytical solutions and laboratory data. These validation experiments demonstrate that the nonhydrostatic FVCOM is capable of resolving complex nonhydrostatic dynamics in coastal and estuarine regions.

[1] The high-resolution, unstructured grid Finite-Volume Community Ocean Model (FVCOM) was used to examine the physical mechanisms that cause current separation and upwelling over the southeast shelf of Vietnam in the South China Sea (SCS). Process-oriented experiments suggest that the southwesterly monsoon wind is a key physical mechanism for upwelling in that area but not a prerequisite to cause current separation. With no wind forcing, current separation in summer can occur as a result of the encounter of a southward along-shelf coastal current from the north and northeastward buoyancy-driven and stratified tidal-rectified currents from the southwest. The southward current can be traced upstream to the Hong River in the Gulf of Tonkin. This current is dominated by semigeostrophic dynamics and is mostly confined to the narrow shelf along the northern Vietnamese coast. The northeastward currents are generated by tidal rectification and are intensified by the Mekong River discharge and southwesterly monsoon wind forcing. The dynamics controlling this current are fully nonlinear, with significant contributions from advection and vertical turbulent mixing. Upwelling in the current separation zone can be produced by a spatially uniform constant wind field and can be explained using simple wind-induced Ekman transport theory. This finding differs from previous theory in which the regional dipole wind stress curl is claimed as a key mechanism for current separation and upwelling in this coastal region. Our SCS FVCOM, driven by the wind stress, river discharge, and tides, is capable of reproducing the location and tongue-like offshore distribution of temperature as those seen in satellite-derived sea surface temperature imagery.

47 48 A high-resolution unstructured-grid global-regional nested ice-current coupled 49 FVCOM system was configured for the Arctic Ocean and used to examine the impact of 50 model resolution and geometrical fitting on the basin-coastal scale circulation and 51 transport in the pan-Arctic. With resolving steep bottom slope and irregular coastal 52 geometry, the model was capable of simulating the multi-scale circulation and its spatial 53 variability in the Arctic Basin and flow through the Bering Strait, Fram Strait and 54 Canadian Archipelago. The model-simulated annual-mean velocities were in good 55 agreement with observations within the measurement uncertainty and variability due to 56 insufficient sampling. The errors in the flow direction varied with the flow speed, larger 57 in the weak velocity zone and smaller as the velocity increased. In the upper 50-m layer, 58 the annual-mean circulation pattern was dominated by the wind-and ice-drifting-induced 59 anticyclonic circulation in the Arctic Basin and a relatively strong cyclonic slope current 60 along the edge of the continental shelf. In the deep 200-600-m layer, a relatively 61 permanent cyclonic circulation occurred along the steep bottom slope. These annual-62 mean circulations accounted for ~85% of the total kinetic energy variance. De-trending 63 the mean flow, an empirical orthogonal function (EOF) analysis showed that the semi-64 annual and seasonal variability of the sub-tidal flow was dominated by the first and 65 second modes that accounted for ~46% and ~30% of the total variance in the upper 50-m 66 layer and ~58% and 20% in the deep 200-600-m layer. Consistent with observations, the 67 AO-FVCOM-simulated cyclonic slope flow was characterized by a large positive 68 topostrophy. Sensitivity experiment results with various grid configurations suggested 69 that the currents over slopes, narrow straits and water passages featured topographic and 70 3 baroclinic frontal dynamical scales associated with bathymetric slope and internal Rossby 71 deformation radius. Over the Arctic slope, since these two scales are in the same order, 72 the along-slope current could be captured, as the cross-isobath model resolution was 73 refined to resolve the steep bottom topography. Under this condition, there is no need to 74 add Neptune forcing into the momentum equations. The accuracy of the estimation of the 75 transport through the strait and narrow water passage was affected by the model 76 resolution. In Fram Strait where the flow is characterized by strong lateral current shear 77 resulting from the Atlantic inflow and Arctic outflow, the transport estimation could have 78 a significant uncertainty due to both horizontal and vertical sampling resolutions.

[1] The generation, propagation, and dissipation processes of large-amplitude nonlinear internal waves in Massachusetts Bay during the stratified season were examined using the nonhydrostatic Finite-Volume Coastal Ocean Model (FVCOM-NH). The model reproduced well the characteristics of the high-frequency internal waves observed in Massachusetts Bay in August 1998. The model experiments suggested that internal waves over Stellwagen Bank are generated by the interaction of tidal currents with steep bottom topography through a process of forming a large-density front on the western slope of the bank by the release of an initial density perturbation near ebb-flood transition, nonlinear steepening of the density front into a deep density depression, and disintegrating of the density depression into a wave train. Earth's rotation tends to transfer the cross-bank tidal kinetic energy into the along-bank direction and thus reduces the intensity of the density perturbation at ebb-flood transition and density depression in the flood period. The internal wave packet propagates as a leading edge feature of the internal tidal wave, and the faster propagation speed of the highfrequency internal waves in Massachusetts Bay is caused by Earth's rotation. The model experiments suggested that bottom friction can significantly influence the crossbank scale of the density perturbation and thus the density depression during wave generation and the dissipation during the wave's shoaling. Inclusion of vertical mixing using the Mellor-Yamada level 2.5 turbulence closure model had only a marginal effect on wave evolution. The model results support the internal wave theory proposed by Lee and Beardsley (1974) but are in disagreement with the lee-wave mechanism proposed by Maxworthy (1979).