Choice of reference surface for geostrophic currents around New Zealand

Abstract

The depths of reference surfaces for determining geostrophic currents in the Tasman and South‐western Pacific Basins and in the Hikurangi Trench are examined using Defant's method for the determination of the “layer of no motion”. These depths are then checked by comparing the directions of the resulting currents with the direction of flow deduced from hydrological evidence and, in the Hikurangi Trench, from an examination of the correlation between surface currents and the mass field. On this basis the reference surface was found to be at 2300 dbars in the Tasman Basin, 2000–2500 dbars in the Southwestern Pacific Basin and >1500 dbars in the head of the Hikurangi Trench.