Abstract
The Bay of Plenty (BoP), in the northeast of Aotearoa New Zealand, is a large focus of marine economic activity. The BoP possesses the largest natural harbour in Aotearoa, which is responsible for 41% of the country’s exports. Additionally, the Whakatōhea iwi (tribe) established the world’s first large open ocean green-lipped mussel farm in the eastern region of the BoP. Understanding coastal hydrodynamics such as coastal circulation and material dispersion is crucial. This dissertation aims to describe aspects of the Eulerian and Lagrangian hydrodynamics of the coastal region of the BoP.
We developed the Bay of Plenty Model (BoPM), a realistic coastal-resolving long-term BoP simulation, to achieve our objective. This 25+ year simulation was statistically evaluated against a set of multiple remote sensing and in situ observations. The BoPM had a good skill in reproducing ocean water temperature, salinity, sea level, and water column velocity over tidal and non-tidal timescales (Willmot skill >0.8 for most variables). The BoPM outperforms the nation-wide coarser resolution mode (Moana Ocean Hindcast, 5 km horizontal resolution), demonstrating the benefits of coastal-resolving simulations for coastal hydrodynamics research.
We explored BoP circulation and dispersion drivers using the BoPM. Up to 30% of the cross-shelf current variability in the central and western BoP comes from along-shore wind stress. The eastern region had no significant correlation to the along-shore winds. Lagrangian statistics suggest that the eastern region can act as a retention zone, constraining particles to the nearshore. Moreover, particles released further from the shore have a greater influence from the offshore mesoscale features and less influence from the local winds, resulting in the loss of particles from the coastal region.
The combination of climatological Eulerian and Lagrangian approaches identified persistent climatological flow structures throughout the Bay of Plenty. (1) A transport barrier north of the retention zone. (2) A seasonally-varying hook-like or barrier feature that disrupts the continuous eastward flow in the eastern region of the BoP. (3) Shadowed regions associated with island features incorporated into the study.
Three different virtual particle behavioural parametrizations were examined to assess their influence on coastal connectivity. Our study reveals that pelagic larvae duration (PLD) is the most important factor among the simulated behaviours in the quantification of coastal connectivity. Moreover, the eastern region displays higher self-recruitment, reinforcing the presence of a retention zone in the region.
Our findings suggest that the BoP is subject to high interannual variability. Therefore, we investigated the correlation between the Southern Oscillation Index (SOI) and the detrended 27-year time series of sea surface and atmospheric variables. Our findings imply that the SOI has a weak influence in the region, and composites of long-term trends must be considered to understand the BoP's interannual variability mechanisms.
The current thesis added to our understanding of oceanographic connectivity, average circulation, and the mechanism of interannual and event-scale variability of the coastal circulation of the BoP. In addition, we indicate the potential areas for future research in which the BoPM may prove to be a valuable tool.