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

Abstract

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 role in stratifying the water column, typically producing an ∼2m mixed surface layer with salinity < 34.5 psu compared with the marine background of 34.8-35.0 psu. This reduced vertical mixing, enhanced shear, and facilitated faster horizontal velocities, thus creating higher sensitivity to wind-driven processes. Under most conditions, the wind was the most important contributor to circulation with a response time of 1 hour for the upper and lower layers and ∼5 hours in the mid-water column. Simulations suggested that when Waipaoa River discharge exceeds 60 m³s⁻¹ it begins to dominate wind-induced circulation, but this occurs only -11% of the time. The long-term wind-record suggested that offshore winds prevail 77% of the time, and these drive a time-averaged upwelling vertical circulation of 0.03-0.10 m s⁻¹ and an anticyclonic mid-water gyre of -0.04 m s⁻¹. The time-averaged vertical circulation is characterised by the offshore flow of buoyant surface water and onshore flow of dense marine water, while a time-averaged horizontal anticyclonic gyre exists in the mid-water column as water flows in past Tuaheni Point, circles the bay and flows out past Young Nicks Head. The time-averaged anticyclonic gyre primarily results from the interaction between the prevailing north-northwest winds and the bathymetry, since the prevailing wind pushes more surface water toward the southern side and out past Young Nicks Head (at ∼0.10 m s⁻¹) than past Tuaheni Point (∼0.03 m s⁻¹). Additionally, the bay is deeper in the northern entrance and this is the preferred channel for bottom-return-flows, having time-averaged shoreward flow of ∼0.05 m s⁻¹. Variation from typical time-averaged circulation occurred approximately 23% of the time, when downwelling was induced during times of onshore wind stress.

Barotropic forcing by the Waipaoa River began to exert dominance over forcing by wind-stress when the discharges reached 0.5% of the bay volume per day, the plume growing to ∼30% of total bay volume through entrainment and diffusion. Embayments with similar freshwater discharge to total volume ratios are expected to also show strong river-induced circulation.

Numerical simulations showed that the bathymetric features having largest influence on circulation were the cross-shore seabed gradient and the shape of the headlands. Correct depth representation was more important to circulation modelling in the inner bay, where headland effects were minor or non-existent, whereas the headlands influenced currents closer to the bay entrance, particularly in the lower water column. Abrupt headlands favoured eddy formation inside embayments. The cross-shore seabed gradient guided up- and downwelling currents, and thus provided an important control on the response of the mid and lower water column to surface-driven flow. The enclosed geometry of an embayment creates recirculation-favourable pressure gradients in comparison to islands, reefs and headlands and consequently recirculation occurs earlier. An embayment eddy parameter E was developed to predict the presence of shelf-current-driven embayment eddies.