Wind, shelf-current and density-driven circulation in Poverty Bay, New Zealand

This thesis describes the three-dimensional hydrodynamics of Poverty Bay in context of its bathymetry, forcing parameters and typical circulation patterns. The thesis provides a model of hydrodynamic processes in an exposed and stratified coastal embayment. Two field experiments were undertaken, with associated data analyses and interpretation, and numerical models were used to support and extend the data analyses. Continental-shelf-currents exhibited a strong non-tidal component (mean 0.09 m s⁻¹, maximum 0.32 m s⁻¹) with relatively small tidal flows (mean 0.04 m s⁻¹, maximum 0.12 m s⁻¹). Wavelet analyses across the 8-512 hour cycle band showed that wind-forcing explained a large proportion of energy contained in the non-tidal shelf-current component. Coastal-trapped-waves with amplitudes of ≤ 0.1 m and velocity ≤ 0.2 m s⁻¹ also contributed to non-tidal shelf-current motion. Correlation and regression analyses showed that CTWs generated by wind-driven water flux through Cook Strait, could account for up to 40% of variance in Gisborne shelf-currents, at timescales of 2-20 day period. Simulated sea-levels showed 88% energy attenuation occurred as CTWs exited Cook Strait, but only 3% attenuation occurred as CTWs travelled up-coast from Riversdale, therefore a barotropic CTW generated by a Cook Strait flux had approximately 9% its original energy on reaching Gisborne. Continental-shelf-currents formed occasional eddies of diameter ∼4 km and velocity ∼0.1 m s⁻¹ in the lee of Tuaheni Point. Continental-shelf-currents contributed ∼0.01 m s⁻¹ to circulation in depths less than 18 m, small in comparison to those typically measured of 0.05-0.10 m s⁻¹. Circulation was dominated by the combined influence of wind stress and river discharge, and since the prevailing wind created circulation patterns similar to the Waipaoa River discharge, both processes contributed to the prevailing anticyclonic horizontal circulation observed in Poverty Bay. River discharge was important for its