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Local Grid Refinement in New Zealand’s Earth System Model: Tasman Sea Ocean Circulation Improvements and Super-Gyre Circulation Implications
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  • Erik Behrens,
  • Jonny Williams,
  • Olaf Morgenstern,
  • Phil Sutton,
  • Graham Rickard,
  • Mike Williams
Erik Behrens

Corresponding Author:[email protected]

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Jonny Williams
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Olaf Morgenstern
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Phil Sutton
National Institute of Water and Atmosphe
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Graham Rickard
National Institute of Water and Atmospheric Research (NIWA)
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Mike Williams
National Institute of Water and Atmospheric Research
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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.
Jul 2020Published in Journal of Advances in Modeling Earth Systems volume 12 issue 7. 10.1029/2019MS001996