This paper describes the development of New Zealand’s Earth System Model (NZESM) and evaluates its performance against its parent model (United Kingdom Earth System Model, UKESM) and observations. The main difference between the two earth system models is an embedded high-resolution (1/5°) nested region over the oceans around New Zealand in the NZESM. Due to this finer ocean model mesh, boundary currents such as the East Australian Current, East Australian Current Extension, Tasman Front and Tasman Leakage and their transports are better simulated in NZESM. The improved oceanic transports have led to a reduction in upper ocean temperature and salinity biases over the nested region. In addition, net transports through the Tasman Sea of volume, heat and salt in the NZESM agree better with previously reported estimates. A consequence of the increased cross-Tasman transports in the NZESM is increased temperatures and salinity west of Australia and in the Southern Ocean reducing the meridional sea surface temperature gradient between subtropics and sub-Antarctic. This also leads to a weakening of the westerly winds between 60S and 45S over large parts of the Southern Ocean, which reduces the northward Ekman transport, reduces the formation of Antarctic Intermediate Water and allows for a southward expansion of the Super-Gyre in all ocean basins. Connecting an improved oceanic circulation around New Zealand to a basin-wide Super-Gyre response is an important step forward in our current understanding of how local scales can influence global scales in a fully coupled earth system model.

Knowledge about the early life history of Antarctic toothfish (Dissostichus mawsoni) is still incomplete, particularly on the spatial and temporal extent of spawning and the subsequent transport of eggs and juveniles from the offshore spawning areas to the continental shelf. This study used a high-resolution hydrodynamic model to investigate the impact of ocean circulation and sea-ice drift on the dispersal of eggs and juvenile Antarctic toothfish. The virtual eggs were released on seamounts of the Pacific-Antarctic ridge in the northern Ross Sea and advected using hydrodynamical model data. Particles were seeded annually over a 14-year period (2002 to 2016) and tracked for three years after release. Spawning success was evaluated based on the number of juveniles that reached known coastal recruitment areas, in the eastern Ross and Amundsen Sea, within three years. Observations show that juveniles (50-100 cm size class) are abundant on the shelf and slope of the Ross and Amundsen Seas. Sensitivities to certain juvenile behaviours were explored and showed that spawning success was reduced by around 70% if juveniles drifted with sea-ice during the second winter season as this carried them into the open ocean away from the shelf region. Spawning success increased during the second winter season if juveniles were entrained in the Ross Gyre circulation or if they actively swam towards the shelf. These modelling results suggest that the ecological advantage of sea-ice association in the early life cycle of toothfish diminishes as they grow, promoting a behaviour change during their second winter.