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Discussion papers | Copyright
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 29 Jun 2018

Research article | 29 Jun 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Ocean Science (OS).

A mechanistic classification of double tides

J. A. Mattias Green1, David G. Bowers1, and Hannah A. M. Byrne2 J. A. Mattias Green et al.
  • 1School of Ocean Sciences, Bangor University, Menai Bridge, UK
  • 2Department of Evolutionary Biology, Uppsala University, Uppsala, Sweden

Abstract. Double high or low tides are usually explained by adding a higher harmonic to the dominating tide. In its simplest form, the criterion for a double tide is that the amplitude ratio between the higher harmonic and the dominating constituent is larger than 1/n2 where n is the ratio of their periods. However, it is not always clear how the higher harmonic becomes large enough to generate the double tide. This is rectified here by identifying three possible ways to enhance the higher harmonic enough to produce a double tide. Using TPXO9, the latest version of the altimetry constrained global tide database, potential locations for all three classes are identified and the existence of double tides are then evaluated using historic long-term tide gauge data from nearby locations. Thirteen locations with double tides were identified this way across the classes, of which seven are discussed further and shown to fit the classification scheme. The search criterion for classes 1 and 2, based on the amplitudes of M2, S2, and M4, work well with TPXO9 and suggests over 400 locations with double tides. The main reason we cannot identify more double tide locations is a lack of TG data, especially in the polar areas. Class 3, which requires an embayment resonant for the higher harmonic initially provided over 8000 potential locations, but only a few of these were in embayments. This class thus requires more manual work to identify the locations. It is concluded that the mechanism behind double tides in most textbooks needs to be revised because they are far more frequent in both space and time than previously thought.

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Short summary
In a double tide the ocean reaches high or low tide, starts to fall or rise, only to go back to a new high or low. Here, we describe three ways this can happen by dividing locations with observed double tides into three classes. This showed that double tides are more common than we thought, and more complicated than most textbooks claim because they only describe one class of double tides. This matters to shipping, coastal flood management, and other disciplines interested in sea-level change.
In a double tide the ocean reaches high or low tide, starts to fall or rise, only to go back to...