Modelled coastal circulation and Lagrangian statistics from a large coastal embayment: The case of Bay of Plenty, Aotearoa New Zealand

In this study, a high-resolution one-way nested, hindcast ROMS model was developed to analyse the coastal circulation and Lagrangian statistics within the Bay of Plenty (BoP) region in Aotearoa, New Zealand. The Bay of Plenty Model (BoPM) was statistically evaluated against a set of multiple remote sensing and in situ observations from 2003-2004, forming the analysis period for this study. Overall, the BoPM possesses good skill reproducing ocean water temperature, salinity, sea level and water column velocity over tidal and non -tidal timescales (Willmott skill >0.8 for most variables). Root-mean-squared errors of <1 C for ocean water temperature, approximate to 0.15 for salinity (an exception present during winter), <8 cm s-1 for water column velocity and 0.09 m for non-tidal sea surface height are achieved. Over the 2-year period, nearshore modelled sea surface currents are correlated to local wind forcing on the western and central region of the BoP consistent with coastal wind-driven upwelling dynamics. Up to 30% of the cross-shelf current variability is explained by along-shore wind stress, consistent with previous observational studies. Meanwhile, the eastern region has no significant correlation to the along-shore winds, suggesting other forcings must be considered. Circulation patterns and Lagrangian statistics under two distinct atmospheric conditions over the two years were analysed: January, with predominantly moderate upwelling-favourable winds and July, with highly variable and stronger winds. January conditions show an eastward flowing large-scale boundary current, the East Auckland Current (EAUC), and a quasi-stationary eddy, the East Cape Eddy (ECE), close to the shelf break, while July conditions show the EAUC and ECE located further from shore. A series of particle release experiments from inner-, mid-shelf, and shelf break locations are used to identify trajectory and dispersion variability between western, central, and eastern sections of the BoP under January and July conditions. Particles released under January conditions tend to flow eastward following the EAUC. Under July conditions, inner-and mid-shelf releases converge towards the central BoP, where bathymetric changes and islands create pathways for particles to exit the continental shelf. Relative dispersion (R2) under January conditions shows a ballistic dispersion regime (R2 approximate to t2) over the first 10 days followed by a diffusive regime (R2 approximate to t1) after 15 days. Under July conditions, a ballistic-like regime is maintained throughout a 30-day period, likely due to strong mixing in the BoP associated with submesoscale processes. Particles released in proximity to East Cape headland that remain in the water column for > 5 days show higher dispersion relative to releases from the central and western regions of the BoP. However, 70%-90% of particles released on the eastern inner-shelf strand to the coastline in <5 days. This suggests that the eastern region can act as a retention zone, constraining the particles towards the shore. Drift duration increases for particles released further from the shore, where offshore mesoscale currents influence them more, and islands become important receptor locations.