Determining the internal structure of the ocean off north-east New Zealand from surface measurements

The assumption that variability in the circulation around the east coast of the North Island of New Zealand can be described using satellite-derived sea level and temperature and two empirical modes is tested. Potential errors are quantified if such an assumption is used to derive the internal fields from surface observations. Data from 303 CTD (conductivity-temperature-depth) casts made in the region between 1994 and 1999 are first tested against a much larger XBT (expendable bathy-thermograph) data set to show that they are representative of the expected variability, and then used to compute the empirical modes. The empirical modes are not baroclinic modes, but probably represent the lateral meanderings and variations in strength of the East Auckland Current, together with the presence of Rossby waves in this region. If surface temperature and height can be measured precisely, 60-70% of the variance in the internal temperature and velocity perturbations can be explained with the model. The standard deviation of reconstructed internal temperature errors is about 0.6 degrees near the surface and less with depth. The standard deviation in velocity errors is 5 cm s(-1).