Global mapping of nonseismic sea level oscillations at tsunami timescales

Abstract

Present investigations of sea level extremes are based on hourly data measured at coastal tide gauges. The use of hourly data restricts existing global and regional analyses to periods larger than 2 h. However, a number of processes occur at minute timescales, of which the most ruinous are tsunamis. Meteotsunamis, hazardous nonseismic waves that occur at tsunami timescales over limited regions, may also locally dominate sea level extremes. Here, we show that nonseismic sea level oscillations at tsunami timescales (<2 h) may substantially contribute to global sea level extremes, up to 50% in low-tidal basins. The intensity of these oscillations is zonally correlated with mid-tropospheric winds at the 99% significance level, with the variance doubling from the tropics and subtropics to the mid-latitudes. Specific atmospheric patterns are found during strong events at selected locations in the World Ocean, indicating a globally predominant generation mechanism. Our analysis suggests that these oscillations should be considered in sea level hazard assessment studies. Establishing a strong correlation between nonseismic sea level oscillations at tsunami timescales and atmospheric synoptic patterns would allow for forecasting of nonseismic sea level oscillations for operational use, as well as hindcasting and projection of their effects under past, present and future climates.

Seiches in the Eastern Caribbean

“…Finally, even though the seiches of the eastern Caribbean are smaller than those reported in some other parts of the world (and the currents associated with them will also be small), it has been shown that they can be as large as 10s cm in places. Therefore, as Vilibić and Šepić (2017) have remarked in the context of high frequency sea level variability in general, they should be taken into account when extreme levels are calculated for coastal engineers. Even if there is a 'continuous seiche', for which the forcing may not be understood perfectly, then it should be possible to combine the statistics of seiche variability with the extreme levels determined by conventional analysis of hourly data into better estimates of sea level return probabilities.…”

“…Its data archive was not intended to be a resource for scientific research, although some investigators of sea level variability have taken advantage of it (e.g. Vilibić and Šepić 2017).…”

“…Figure SM2.1c shows the variation in total wind speed at the same locations using ERA-Interim information, and the dominant zonal character of the trade winds results in a similar plot being obtained for zonal wind speed. Vilibić and Šepić (2017) suggested a dependence between the magnitude of high-frequency (< 2 hour) sea level oscillations and the strength of mid-troposphere zonal winds. In turn, that suggests that high-frequency air pressure (or wind) oscillations, which tend to be more intense during high wind conditions, might be a generator of seiches, transferring energy to the sea though resonant or near-resonant forcing.…”

“…In turn, that suggests that high-frequency air pressure (or wind) oscillations, which tend to be more intense during high wind conditions, might be a generator of seiches, transferring energy to the sea though resonant or near-resonant forcing. The main focus of Vilibić and Šepić (2017) was on the latitudinal dependence of such processes, instead of variability through the year. However, Figure SM2.1(c) suggests that such a link does not apply in the present case.…”

“…The relationship suggested by Vilibić and Šepić (2017) can also be investigated using 6-minute wind and air pressure records from NDBC meteorological stations. Two records from our area (Charlotte Amalie and Barbuda) are almost complete during 2015-16.…”