Coherent signals in the temperature and height of the sea surface off North Cape, New Zealand

Abstract

Dynamic heights of the ocean surface to the north‐east of North Cape, New Zealand are determined from 18 months of Topex‐Poseidon altimeter measurements along a repeat track crossing the East Auckland current system. The anomalies in the heights and in the cross‐track geostrophic currents derived from these heights both portray strong cyclic signals with period of c. 140 days travelling towards New Zealand, and a signal with period of at least 18 months in the opposite sense. Sea‐surface temperature (SST) anomalies and anomalies in the along‐track SST gradients exhibit similar periodicity. The coherence between anomalies in velocity and temperature gradient and that between anomalies in height and temperature both reach c. 0.74 at the period of 140 days and 0.88 for the long‐period signal. A correspondence is also shown between the positions of maximum flow in the East Auckland current and positions of high sea‐surface temperature gradient. Winds are shown to affect the strength of the 140‐day signal in the temperature gradient data, and to impair its coherence with the matching signal in the velocity anomaly data. In particular, if temperature gradient data are smoothed through times and places where the recent history of the curl of the wind‐stress is significantly positive (Ekman pumping occurring) the coherence in the 140‐day signal between the velocity and temperature gradient is increased from 0.74 to 0.92. Similar impacts on the longer period signal are not found, nor are they evident for either of the two prominent signals in the coherence between SST and height anomalies.