Comparison of model and observational ocean circulation climatologies for the New Zealand region

Primitive equation three‐dimensional regional models driven by global models can be used to generate climatologies of the ocean circulation. With 1/12° resolution, these model climatologies begin to resolve the coastal circulation and it is appropriate to ask whether they are adequate to replace observational climatologies. We compare circulation climatologies derived from a z coordinate 36 vertical layer model, run with three different diffusivities with an observational climatology (CSIRO Atlas of Regional Seas, CARS), and with surface velocities derived from Lagrangian drifters deployed under the Global Drifter Program (GDP). All the climatologies capture the major features in the mean circulation. South of the Chatham Rise, and east of New Zealand, the model climatologies agree better with GDP than does CARS. North of the Chatham Rise, the models overestimate the strength of the East Cape Eddy. Wave number spectra from the models suggest that instability processes which are important in the mean are captured with diffusion less than 25 m2 s−1. The model with lowest diffusion may be providing the best estimate of the mean circulation, but at the expense of numerical instability.