Flow through the Mernoo Saddle, New Zealand

Abstract

Data were analysed from two current meters moored 100 and 300 m off the seabed in the Mernoo Saddle from June to November 1988. The flow was dominated by the lunar semidiurnal constituent M 2 with an amplitude of 15.1 cm s −1 at the upper level and 15.8 cm s −1 at the lower level. The major axes of the tidal ellipses for the semidiurnal constituents were aligned more or less alongshore whereas those of the two largest diurnal constituents, Oj and Ki, were aligned across‐shore. Peak tidal period flows of 28 cm s −1 (upper) and 33 cm s −1 (lower) were recorded. Over the entire record, net flow was to the north at an average speed of 3.1 cm s −1 but there were periods of up to 5 weeks during which net flow was to the south. Strong southward flow events lasting 10–12 days were recorded on three occasions and the maximum current speed recorded was 67 cm s −1 to the south. Satellite imagery was used to show that these southward flow events coincide with the influx of warm water at the surface within 5 km of the coast near Kaikoura, and we suggest that the southward flows are caused by this influx of warm water which is carried by eddies from the south of the Wairarapa eddy to the area immediately north of the Mernoo Saddle. There is some indication that internal tides may be significant in this area but inertial currents were found to be weak.