Estimates of the coherence of ocean currents over vertical distances

Abstract

Pairs of moored current meters at Site D have been used to collect simultaneous series of ocean current measurements over vertical separations of from 1 to 1500 m. Within the range of frequencies that can be resolved with the limited time series available, currents are not coherent across the seasonal thermocline. Otherwise currents are generally coherent over some range of lower frequencies, and incoherent at higher frequencies. It was found that the frequency at which the coherence drops below 0·5 varies inversely as the separation, δZ, between sensors according to the relation: f 0·5 δZ = 13 m/hr. At small separations, the maximum coherent frequency is apparently related to the Brunt-Väisälä frequency. More extensive measurements are needed to obtain a good description of the relation between coherence and vertical separation, but the results presented here may be adequate for defining vertical sampling intervals in the design of the next stage of array experiments.