Extended Spectral Analysis of Tidal Variability in the North Atlantic Ocean

Abstract

Ocean tides have experienced large-scale changes over the past century, in concert with regionally variable global mean sea level (MSL) rise. Additionally, there can be coherencies between MSL and tidal fluctuations that are active at shorter time scales. This combination of water level variabilities may enhance the probabilities of exceeding flood levels under high-tide events, leading to increased frequency of short-term coastal inundation and nuisance flooding. Previous studies have established the tidal anomaly correlation (TAC) method to analyze covariability of individual tidal components and MSL in the Pacific Ocean, as well as their combination as a proxy for the change in the highest astronomical tide (δ-HAT). Here we extend this methodology to new regions and perform analyses of 170 tide gauges in the North Atlantic Ocean, considering the eight largest gravitational tides and seven overtides. Results indicate that nearly all gauges (95%) exhibit strong individual TACs in one or more tidal constituents, and over half show significant δ-HATs, with a near-equal occurrence of positive and negative tendencies. The most coherent connections of tides and MSL were found on the U.S. East Coast and in the marginal waters of Europe. At some locations, all tidal variabilities act in the same direction, which may amplify possible flood level. At other locations major parts of the tidal spectrum may counteract the MSL rise and partially mitigate extreme water levels due to tidal evolution. In either case, an understanding of regional tidal changes correlated to MSL changes can be instructive in guiding future coastal development efforts.Plain Language Summary Sea level rise is an increasingly important hazard for the health of coastal infrastructure and populations. This risk will increase as rising air temperatures increase ocean temperatures and melt ice sheets. As sea level changes, the ocean tides, long thought to be constant, are also changing, sometimes decreasing and sometimes increasing, due to changes in frictional and resonant properties of local waterways under sea level rise. The tidal component of water levels can be a significant contribution that may further amplify flood-related dangers, with or without the influence of storms. This study extends methodology previously demonstrated for Pacific Ocean to analyze 170 tide gauge locations in the North Atlantic Ocean and its marginal waterways. Our results show that fluctuations in tides are often correlated to sea level modulations, and our approach demonstrated that when all forms of tidal variability are considered, there can be significant impacts to water level, which has implications for future coastal flooding. Overall, more than half of locations examined show a significant increase of tidal ranges under scenarios of increasing mean sea levels.

Sea-level rise exponentially increases coastal flood frequency

“…Non-stationary methods 43,44 , on the other hand, allow for extreme distributions (or their parameters) to vary with time, but typically require longer records of data. Devlin et al (2017a,b, 2019) [45][46][47] and Haigh et al (2019) 48 investigated non-stationary, non-astronomical changes in tidal amplitudes due to changes in mean sea level, which can alter the probabilities of exceeding flood levels during high-tide events. Vousdoukas et al (2018) 18 investigated non-stationary changes in extreme water-level events at the 100-year return period due to climate model projections of different emissions scenarios.…”

Tidal Evolution Related to Changing Sea Level; Worldwide and Regional Surveys, and the Impact to Estuaries and Other Coastal Zones

“…Recent studies have developed a reliable methodology to analyze tidal variability related to MSL variability [7][8][9][10][11]. These methods have been applied to many tide gauge locations worldwide, with a twofold approach.…”

“…A t ðÞ¼abs| Z obs t ðÞ Z pot t ðÞ | (9) P t ðÞ¼θ obs t ðÞÀθ pot t ðÞ (10) The harmonic analysis that generates the As and Ps also provides an MSL timeseries. For each resultant dataset (MSL, A and P), the mean and trend are removed from the time series, to allow direct comparison of their co-variability around the trend.…”

“…A detailed description of the step-by-step method, with additional figures showing the intermediate steps in the process, are provided in the supplementary materials of Devlin et al [8]. The δ-HAT analyses performed for the Atlantic tide gauge stations [10] employed an eight-tide combination, which adds the N 2 ,K 2 ,P 1 and Q 1 tidal constituents to the δ-HAT sum ( Table 2).…”

“…A background will first be given about past research that has identified secular (longterm) non-astronomical changes in ocean tides as well as a summary of past studies of MSL rise. Next will be a description of the methodology of newer efforts that have analyzed the correlated changes in sea levels and ocean tides in the Pacific and Atlantic Oceans, based primarily on the work of Devlin et al [7][8][9][10][11]. Following this will be a summary of significant results in the Pacific and Atlantic basins, as well as results from a focused study of the Hong Kong region, where some of the strongest magnitude changes have been observed.…”