How do winds drive western boundary currents surrounding New Zealand?

Abstract

New Zealand is situated at a cross-roads of different ocean currents. To the north, the East Australian Current, the primary western boundary current (WBC) of the South Pacific gyre flows into the region. To the south, the Antarctic Circumpolar Current (ACC) makes a large northward excursion near the country. The winds are the cause of these ocean currents. The ocean circulation from the wind forcing is the winddriven circulation. This thesis is focused on the wind-driven circulation. The volume transport around New Zealand in SOSE was compared with previous observational studies and it was largely in agreement. Vorticity balances of the flows around New Zealand were examined in the Southern Ocean State Estimate (SOSE). The vorticity terms in the South Pacific were examined and analysed whether the South Pacific winds were the dominant factor. A perturbation experiment was performed by adding a wind stress anomaly in the South Pacific to examine wave generation and propagation toward New Zealand. The results show the bottom pressure torque is the dominant term in the vorticity balance in the WBC; it reveals the importance of the bottom bathymetry in the WBC. In the WBC region, dominant terms are advection and stretching along the flow, while dissipation consistently contributes to the volume transport. The regional integration of terms shows dissipation becomes dominant over the wider region. In the Southland Current region, the local winds are dominant, while in other current regions local winds are negligible. The Island Rule describes the circulation around an island due to wind stress and the Coriolis force, but it is not a perfect tool to describe the circulation east of New Zealand because the bathymetry is too complicated. Integration of vorticity terms across the South Pacific shows the remote wind is the main term north from 48°S, and further south dissipation, advection, and stretching become large and non-negligible, which suggest the strong effect from the ACC. The Sverdrup balance describes the subtropical South Pacific Ocean but fails to describe where the WBC and ACC flow. In the WBC region, dissipation is dominant, and it is the same near the ACC in the South Pacific. The various waves generated by the wind anomaly experiment were identified. Barotropic Rossby waves propagate to the boundary region. Internal gravity waves propagate westwards and also reach to the north of New Zealand. The waves are bringing energy from the winds over to the boundary. This study shows the connection and the relation of winds, waves, and currents around New Zealand to improve the future projections of boundary currents around New Zealand.