East Auckland Current ocean model configuration and analysis files
Creators: name: Santana, Rafael
Publisher: name: Zenodo
Year: 2022

description: This model is based on the Regional Ocean Modeling System (ROMS), a primitive-equation, hydrostatic, and free-surface ocean model that solves the Reynolds-averaged form of the Navier–Stokes equations. ROMS is a fully nonlinear, finite-difference model that uses terrain-following (sigma) vertical coordinates and horizontal orthogonal or curvilinear Arakawa C grid (Shchepetkin and McWilliams, 2003, 2005; Haidvogel et al., 2008). The model domain (290 x 150) is rotated 52.14° clockwise to better resolve the NZNES and spans 332 km offshore at the widest point (near North Cape). The domain has horizontal resolution of approximately 2 km, which roughly captures coastline variability and still resolves the continental shelf and slope without large computational cost. The model has 30 vertical sigma layers and model bathymetry was interpolated from the 250 m resolution bathymetric data set built by the National Institute of Water and Atmospheric Research (NIWA - https://niwa.co.nz/our-science/oceans/bathymetry). We use a vertical discretisation scheme that increases the resolution near the surface and bottom by applying stretching function type 4 and transformation equation option 2 (Shchepetkin and McWilliams, 2005, 2009). The vertical resolution is higher at the upper 200 m (from 4 to 30 layers). On the slope (depth&lt;1000 m), the vertical resolution is higher than 66 m and in the open ocean the thickest level is 233 m (3838.6 m depth). Baroclinic modes are resolved using a time step of 180 s, while the barotropic time step is 6 s. Model surface forcing is from the Japanese atmospheric 55-year reanalysis for driving ocean models (JRA55-do, Tsujino et al., 2018). A previous study demonstrated that JRA55-do had the highest correlation with observed winds in comparison to other atmospheric forcing datasets in the Southwest Pacific (Taboada et al., 2019). Atmospheric forcing fields of wind speed, net shortwave radiation, downward longwave radiation, relative humidity, temperature, rain, and pressure are specified every 3 h and used to compute the surface fluxes of stress, heat and freshwater using the bulk flux parameterisation of Fairall et al., (2003). The model uses initial and boundary conditions of SSH, temperature, salinity, and velocities from HYCOM-NCODA (Chassignet et al., 2009) versions 91.1 and 91.2 which cover period of simulations generated here. de Souza et al., (2021) analysed the performance of four global reanalysis that assimilate SSH, SST and Argo data on the New Zealand waters. The authors found that HYCOM-NCODA (8 km resolution) had higher SSH and SST variability than low-resolution (25 km) satellite surface observations and other reanalyses with similar grid-spacing. HYCOM-NCODA had velocity standard deviation similar to that observed at M4 and M5, even though GLORYS (8 km global ocean reanalysis, Lellouche et al., 2018) better represented temperature and salinity profiles in the region. Annual average discharge from several rivers are included as lateral forcing in the model. The boundary forcing is applied daily using \citet{chapman1985numerical} condition for free surface, Shchepetkin condition (Mason et al., 2010) for barotropic velocities and mixed radiation-nudging (Marchesiello et al., 2001) for baroclinic velocities, temperature and salinity. A 5-day nudging coefficient is applied towards the lateral boundaries. The model aims to simulate continental shelf, slope and rise regions, including the offshore extent of the EAuC and its eddy variability. This and the upcoming studies using the simulations analysed here focus on intra-annual variability and tides are not included as forcing.