Time series analysis of diurnal cycles in small‐scale turbulence

Two new time series techniques are employed to study the diurnal effect at between 30 and 60 metres depth in the upper equatorial ocean. A modified spectral approach is proposed to identify the period of an unequally spaced series. Based on the exceedances of mixing activities, a new criterion is introduced to identify local nights and days. These two techniques together provide a simple way to demonstrate the existence of the diurnal cycle in deep stratified layers in the ocean. Instead of entertaining complicated models, the proposed approach suggests a simple way to identify the periodicity of an unequally spaced data set of this nature. Similar diurnal effects are detected in the analyses of a subsequent data set. © 1998 John Wiley & Sons, Ltd.