Abstract
Abstract. The worldwide distribution of the small degree-3 M1 ocean tide is investigated using a quasi-global data set of over 800 tide gauge records and a global tide model. M1 is confirmed to have a geographical variation in the Atlantic consistent with the suggestion of Platzman (1984b) and Cartwright (1975) that M1 is generated in the ocean as a consequence of the spatial and temporal overlap of M1 in the tidal potential and one (or at least a small number of) diurnal ocean normal mode(s). As a consequence, it is particularly strong around the UK and on North Sea coasts (amplitudes ∼10 mm). This analysis shows that their suggestion is also consistent to a great extent with the observed small amplitudes in the Pacific and Indian oceans. However, there are differences at the regional and local level which require much further study via more sophisticated ocean tidal modelling. By contrast, what is called the M1' tide (a combination of several degree-2 lines in the tidal potential with frequencies close to that of M1) is shown to have a geographical distribution consistent with expectations from other degree-2 diurnal tides, apart from locations such as around the UK where tidal interactions introduce complications. As far as I know, this is the first time that these small tidal constituents have been mapped on a global basis and, in particular, the first time that the ocean response to the degree-3 component of the tidal potential has been investigated globally.
Highlights
M1 is a small tidal constituent with a frequency of 1 cycle per lunar day. It arises from the degree-3 component of the tidal potential, unlike larger constituents such as M2, the predominant semidiurnal tide in the ocean with a frequency double that of M1, which originate from the degree-2 component
What analysts referred to as “M1” was more likely to be due to other tidal lines within the “M1 group”, which have frequencies slightly different from M1 itself and which originate from the degree-2 component of the tidal potential
Many aspects of the spatial variation of M1 described by the model and the tide gauges are to be seen in the Platzman synthesis of Fig. 2
Summary
M1 is a small tidal constituent with a frequency of 1 cycle per lunar day. It arises from the degree-3 component of the tidal potential, unlike larger constituents such as M2, the predominant semidiurnal tide in the ocean with a frequency double that of M1, which originate from the degree-2 component. Agnew (2007), Pugh and Woodworth (2014) and other texts can be consulted for explanations of why the tidal potential contains degree-2 and degree-3 (and degree-4, etc.) components.M1 was not identified unambiguously in tide gauge records until as late as 1968 when it was observed in data from Cuxhaven, Germany (Cartwright, 1975). Cartwright (1975, 1976) used very long tide gauge records (several with 18 or more years of data) to demonstrate that the true degree-3 M1 was large (amplitude ∼ 1 cm) around the UK and on North Sea coasts. This was consistent with the suggestion of Platzman (published later in Platzman, 1984b) that M1 is forced by one (or a small number of) normal mode(s), especially one with a period of 25.7 h which is strong in the Atlantic rather than in the other ocean basins. This was consistent with the suggestion of Platzman (published later in Platzman, 1984b) that M1 is forced by one (or a small number of) normal mode(s), especially one with a period of 25.7 h which is strong in the Atlantic rather than in the other ocean basins. Amin (1982) used an additional seven long records from the west coast of Great Britain, confirming the large M1 in this region and showing that amplitudes increased going north
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