Abyssal circulation around New Zealand—a comparison between observations and a global circulation model

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Observations of abyssal currents off eastern New Zealand are compared to results from the Los Alamos National Laboratory (LANL) global ocean circulation model. Physical oceanographic measurements are few along the 6000-km long path of the abyssal flow so they are supplemented by geological data including bottom photographs, nephelometer profiles, sediment analyses, and high resolution seismic profiles. While greatly increasing the spatial coverage of the observations, the geological data have limitations concerning temporal aspects of the circulation, e.g., the resolution of seismic records restricts identification of bottom current action to periods of ∼12,000 years or more. Despite these limitations, the model compares well with observations for the region south of Chatham Rise (43°S). There, the model shows a highly energetic, topographically steered flow that coincides with zones of seafloor erosion, active bedload transport, and prominent benthic nepheloid layers. In contrast, correlation is less clear to the north of Chatham Rise. Model output and observations agree on flow directions in regions of marked topography, but in areas of subdued relief, model current directions depart from reality. Such departures are because the model bathymetry has (1) a grid too coarse to resolve small but key current pathways, and (2) step-like contours that artificially guide the flow even though in nature the seabed may have low relief. The model also underestimates current intensity as measured by eddy kinetic energy (EKE) and volume transport. However, these deficiencies can be reduced by improving the model bathymetry and better representating the oceanic processes such as the interaction of Rossby waves with the bathymetry.