Why do high tides vary month to month?

The Moon's slow orbital wobble and the seasons set the monthly stage

The biggest astronomical reason month-to-month high tides differ is a slow change in the Moon's orbital path, called lunar precession. Over about 18.6 years, the Moon's orbit tilts in a way that slightly raises or lowers the highest astronomical tides. A projection framework that included this effect found that lunar precession influenced high tide levels widely, and was especially noticeable in Bangkok, Cardiff, and Manila [1]. Separately, a U.S. study showed that combining sea-level rise with this nodal cycle of tidal amplitude can produce sharp jumps in how often high-tide flooding occurs, with the mid-2030s possibly bringing rapid increases in multiple U.S. coastal regions [2].

On top of the Moon's cycle, the ocean itself has a seasonal rhythm. Sea level is not constant through the year: it can rise when ice melts earlier or freezes later, and when ocean circulation shifts [1]. In the United States, annual cycles and short-term sea-level anomalies cause extreme seasons or months when many high-tide flooding days cluster together [2]. A prediction model for U.S. tide gauges relies on astronomical tide predictions, sea-level trends, and the tendency of monthly sea-level anomalies to persist—meaning a high-sea-level month tends to be followed by another high one—to forecast flooding months in advance [3].

Local geography shifts which months are worst

Because the Moon's cycle and seasonal sea level combine differently at each coast, the months of greatest high-tide flooding are not the same everywhere. In projections for popular travel destinations, Bangkok was expected to see 89% of its high-tide flooding days between October and February, while Manila's frequency was projected to be nearly four times higher between June and August than in any other month. Jacksonville's flooding clustered between October and December, and New York City's was most common in October and December [1]. These differences show that a monthly high-tide pattern is a local signature, not a global rule.

In estuaries, the seabed itself can change month to month and alter tides. In China's Qiantang Estuary, strong seasonal variations in water level appeared beyond the usual spring–neap cycle. A river flood could erode the bed and double the tidal range in the upper reach, after which sediment accumulation gradually reduced it again under normal river flow. Over a month or a single spring–neap cycle, the tidal range in the upper estuary tracked the tidal range at the mouth well, but that relationship blurred when a whole year of data was considered together because the channel shape kept evolving [4].

Why a few months can matter more than the yearly average

Monthly clustering has a practical consequence: a coastal community can experience a critical number of high-tide flooding days in one month or season one to two decades before that frequency becomes the yearly norm [2]. In other words, the annual average can understate the risk in the worst month. This is why monthly-granularity projections are useful for planners and travelers—they identify not just which years but which months are most likely to bring repeated flooding [1].

The same monthly signal appears in coastal waters beyond flooding. In Japan's Bungo Channel, the intensity of a tidal front—a boundary between well-mixed and stratified water—varied on a fortnight cycle, and that fortnightly pattern depended on the month because background stratification and residual currents change seasonally [5]. That is a different kind of measurement from tide height, but it reinforces the same idea: tidal behavior has a monthly rhythm shaped by both the Moon and the season.

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