Global evidence for co-variability of tidal constituents

Abstract Understanding how tides respond to short-term climate variability and long-term climatic trends is central to projecting future coastal hazards. While past research has focused on tide-surge or tide-river interactions, the relationships among tidal constituents themselves remain unexplored. Here we present the first global analysis of tidal constituent co-variability using tide gauge records available since mid–20 th century. For each gauge, we identify the dominant constituent and evaluate its rank correlation with other major constituents (M2, N2, S2, O1, K1, MK3, MS4) using a moving window approach that ensures representing nonlinear interactions in high temporal resolution. Correlations were first quantified over the full observational record to characterize the multi-decadal co-variability across the observation network. Subsequently, comparisons between an earlier period (1950–1980) and a more recent period (1981–2019) were conducted to evaluate the presence of nonstationarity in these relationships. Finally, correlation structures were examined under distinct El Niño–Southern Oscillation (ENSO) phases to assess the extent to which large-scale climate variability modulates these dependencies. Results show that tidal constituents exhibit significant co-variability at roughly one-fifth of stations analyzed, with significant changes over time. Significantly larger number of locations (almost twice) exhibit co-variability in the recent era when compared to the earlier period (especially in the North Pacific). ENSO further modulates these relationships, with distinct patterns found during El Niño and La Niña years. These findings reveal that tidal constituents do not vary independently, but co-evolve under both interannual climate variability and long-term change. Recognizing this overlooked dimension of tides provides new insight into tidal dynamics and has important implications for assessing future compound coastal flooding in a warming climate.