Propagation of a Meteotsunami from the Yellow Sea to the Korea Strait in April 2019

A meteotsunami with a wave height of 0.1–0.9 m and a period of 60 min was observed at tide gauges along the Korea Strait on 7 April 2019, while a train of two to four atmospheric pressure disturbances with disturbance heights of 1.5–3.9 hPa moved eastward from the Yellow Sea to the Korea Strait. Analysis of observational data indicated that isobar lines of the atmospheric pressure disturbances had angles of 75–83° counterclockwise due east and propagated with a velocity of 26.5–31.0 m/s. The generation and propagation process of the meteotsunami was investigated using the Regional Ocean Modeling System. The long ocean waves were amplified due to Proudman resonance in the southwestern Yellow Sea, where the water is deeper than 75 m; here, the long ocean waves were refracted toward the coast on the shallow coastal region of the northern Korea Strait. Refraction and reflection by offshore islands significantly affect the wave heights at the coast. To investigate the effects of an eastward-moving velocity and angle of atmospheric pressure disturbance on the height of a long ocean wave, sensitivity simulations were performed. This result will be useful for the real-time prediction system of meteotsunamis in the Korea Strait.

On the Greenspan Resonance of Meteotsunamis in the Yellow Sea - Insights from the Newly Discovered 2009 Event

The Yellow Sea is recognized as a meteotsunami “hot-spot”, with a relatively high rate of events’ occurrence. The March 2007 and May 2008 meteotsunami events attracted large attention due to their deadly and high impact on the west coast of the Korean Peninsula. However, other small size meteotsunamis remain less known because of their insignificant coastal effect. Yet, a better understanding of meteotsunami hazard in the Yellow Sea requires thorough investigation of both large and small events. This paper reveals the occurrence of a meteotsunami on 11–12 June 2009 in the eastern Yellow Sea. It addresses the analyses of both the recorded sea-level and air-pressure data, the correlation between the atmospheric forcing and the meteotsunami formation, the numerical modeling of meteotsunami propagation, and the resonance effects on the recorded waves. Analysis results evidence a moving air-pressure jump of about 3 hPa that disturbed the sea surface and caused a meteotsunami with wave height up to 0.45 m (crest-to-trough). Both meteorological observations and numerical modeling suggest a speed of 11 to 13 m/s for the atmospheric disturbance propagation, which is much smaller than the optimal condition for Proudman resonance of meteotsunamis in the eastern Yellow Sea. Here, we demonstrate that the Greenspan resonance was responsible for amplifying the incident waves. Despite the insignificant coastal impact of the 11 June 2009 event, its investigation unravels new insights into the formation, amplification, and hazard extent of small size meteotsunamis in the Yellow Sea.

Examination of Generation Mechanisms for an English Channel Meteotsunami: Combining Observations and Modeling

On the morning of 23 June 2016, a 0.70-m meteotsunami was observed in the English Channel between the United Kingdom and France. This wave was measured by several tide gauges and coincided with a heavily precipitating convective system producing 10 m s−1 wind speeds at the 10-m level and 1–2.5-hPa surface pressure anomalies. A combination of precipitation rate cross correlations and NCEP–NCAR Reanalysis 1 data showed that the convective system moved northeastward at 19 ± 2 m s−1. To model the meteotsunami, the finite element model Telemac was forced with an ensemble of prescribed pressure forcings, covering observational uncertainty. Ensembles simulated the observed wave period and arrival times within minutes and wave heights within tens of centimeters. A directly forced wave and a secondary coastal wave were simulated, and these amplified as they propagated. Proudman resonance was responsible for the wave amplification, and the coastal wave resulted from strong refraction of the primary wave. The main generating mechanism was the atmospheric pressure anomaly with wind stress playing a secondary role, increasing the first wave peak by 16% on average. Certain tidal conditions reduced modeled wave heights by up to 56%, by shifting the location where Proudman resonance occurred. This shift was mainly from tidal currents rather than tidal elevation directly affecting shallow-water wave speed. An improved understanding of meteotsunami return periods and generation mechanisms would be aided by tide gauge measurements sampled at less than 15-min intervals.

STUDY ON THE PROUDMAN RESONANCE OF WAVES INDUCED BY A MOVING ATMOSPHERIC PRESSURE DISTURBANCE

A moving atmospheric pressure disturbance can induce a system of forced water waves. As predicted by the linear theory, an infinite wave height will be induced when the Froude number Fr=1, which is known as the Proudman resonance. Fr is defined as the ratio between the moving speed of an atmospheric pressure disturbance and the phase velocity of shallow water wave. The Proudman resonance is thought to be one of main mechanisms for the destructive meteotsunami (Monserrat et al., 2006). In this study, the nonlinear shallow water equations are used to describe the waves induced by a moving pressure disturbance, and the impact factors to the maximum water elevation in the case of Fr=1 are discussed.

Resonance of long waves generated by storms obliquely crossing shelf topography in a rotating ocean

The oceanic forced wave beneath a moving atmospheric disturbance is amplified by Proudman resonance. When modified by the Earth's rotation this classical resonance only occurs if the disturbance time scale is smaller than the inertial period. With or without Coriolis effects, free transients generated by storm forced waves obliquely crossing step changes in water depth at particular angles are shown to resonate by exciting a range of long barotropic free waves. Rotationally influenced slow atmospherically forced waves crossing a vertical coast at a critical angle lead to a form of subcritical resonance, which occurs only when the component of the disturbances' phase velocities along the coast matches that of a free Kelvin wave (KW). In a rotating ocean, transients generated by disturbances crossing a step at a particular angle are shown to excite a free double Kelvin wave (DKW). This new type of resonance only occurs for sufficiently large steps and disturbances with time scale greater than the inertial period. A storm crossing a step shelf can result in the excitation of an infinite set of edge waves, a single KW, a unique DKW and a first-mode continental shelf wave, depending on the topography and the disturbance time scale, translation speed and incident angle. The study of resonances and wave mode excitations generated by storms crossing a coast or a continental shelf may contribute to understanding how a particular combination of the storm characteristics can result in destructive coastal events with time scales encompassing the typical meteotsunami period band (tens of minutes) and storm surges with periods of several hours or days.

Resonance and trapping of topographic transient ocean waves generated by a moving atmospheric disturbance

Proudman resonance amplifies the oceanic forced wave beneath moving atmospheric pressure disturbances. The amplification varies with water depth; consequently, the forced wave beneath a disturbance crossing topography radiates transient free waves. Transients are shown to magnify the effects of Proudman resonance for disturbances crossing the coast or shelf at particular angles. A Snell like reflection law gives rise to a type of resonance for relatively slow moving disturbances crossing a coast in an otherwise flat-bottomed ocean. This occurs for translation speeds less than the shallow water wave speed for disturbances approaching the coast at a critical angle given by the inverse sine of the Froude number of the disturbance. A disturbance crossing the shelf at particular angles can also excite seiche modes of the shelf via generation of a transient at the continental slope. Beyond a typically small angle of incidence, transients generated by a disturbance crossing the continental slope and coast will be trapped on the shelf by internal reflection. The refraction law for a fast-moving forced wave crossing an ocean ridge at greater than a small angle of incidence also results in trapped free-wave transients with tsunami-like periods propagating along the ridge. The subcritical resonance, excitation of shelf modes and trapping of the transients may have implications for storm surges and the generation of destructive meteotsunami.