Separation of Short Time Series of Currents into “Fluctuations,” “Tides,” and “Mean” Flow

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Abstract
With standard low-frequency velocity data from current meter moorings, pressure gradient–driven mean flow is determined by low-pass filtering, while tides are estimated by fitting tidal constituents, with accuracy and numbers of constituents determined by record length. With the advent of higher-frequency measurements from cabled coastal ocean observatories, current data also include supertidal variability (fluctuation) associated with a variety of turbulent and internal wave processes. To examine the relationships of such fluctuations to variability of the tides and/or potentially time-variable mean flows within which they are embedded, it is highly desirable to find a method whereby these flow components can be separated over relatively short periods of intensified event-scale forcing. A method is presented that first isolates “fluctuations” and then separates the remaining longer time-scale variability into “tides” and a remaining “mean,” without recourse to extraction of a fitted tide with the error inherent in such a fit over short data records.

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