Alexander B. Rabinovich

Spectral analysis of tsunami waves: Separation of source and topography effects

A new approach is proposed to separate the influence of source and topography in observed tsunami spectra and to reconstruct the initial source spectrum. The method assumes a linear tide gauge response to external forcing and is based on comparative analysis of tsunami and background spectra. Evaluation of the ratio of tsunami to background spectra gives functional characteristics that are invariant of station location and dependent only on the source parameters. Three events, the Urup tsunami of December 22, 1991 (northwest Pacific), the Hokkaido Southwest (Okushiri) tsunami of July 12, 1993 (Sea of Japan), and the Shikotan tsunami of October 4, 1994 (northwest Pacific), are used as examples of the proposed approach. The source spectra reconstructed from the analysis of different tide gauges were found to be in good agreement with each other and with the seismological, geodetic, and field survey data.

Constrained circulation at Endeavour ridge facilitates colonization by vent larvae

Understanding how larvae from extant hydrothermal vent fields colonize neighbouring regions of the mid-ocean ridge system remains a major challenge in oceanic research. Among the factors considered important in the recruitment of deep-sea larvae are metabolic lifespan, the connectivity of the seafloor topography, and the characteristics of the currents. Here we use current velocity measurements from Endeavour ridge to examine the role of topographically constrained circulation on larval transport along-ridge. We show that the dominant tidal and wind-generated currents in the region are strongly attenuated within the rift valley that splits the ridge crest, and that hydrothermal plumes rising from vent fields in the valley drive a steady near-bottom inflow within the valley. Extrapolation of these findings suggests that the suppression of oscillatory currents within rift valleys of mid-ocean ridges shields larvae from cross-axis dispersal into the inhospitable deep ocean. This effect, augmented by plume-driven circulation within rift valleys having active hydrothermal venting, helps retain larvae near their source. Larvae are then exported preferentially down-ridge during regional flow events that intermittently over-ride the currents within the valley.

The landslide tsunami of November 3, 1994, Skagway Harbor, Alaska

We show that the tsunami of November 3, 1994 in Skagway, Alaska was generated by an underwater landslide formed during the collapse of a cruise ship wharf undergoing construction at the head of Taiya Inlet. This event occurred at a time of extreme low tide and was not associated with a regional seismic event or incoming oceanic tsunami. Persistent wave motions with an amplitude of 1 m and a period of 3 min recorded by a tide gauge in Skagway Harbor following the landslide are linked to the formation of a cross-inlet seiche and quarter-wave resonance within the harbor. The high Q factor for the harbor (Q ≈ 20) indicates weak dissipation and strong resonance within the harbor.

Generation of meteorological tsunamis (large amplitude seiches) near the Balearic and Kuril Islands

Extreme atmosphere-induced seiche oscillations occasionally occur in specific inlets and bays of the world ocean causing severe damage to coastal areas, ships and port constructions. Ciutadella inlet (Menorca Island, Western Mediterranean) can be singled out as a place where such large seiches, locally known as rissaga, are quite common. Similar (although weaker) oscillations are also regularly observed in bays of Shikotan Island (South Kuril Islands, northwestern Pacific). Several spectacular events in these regions, identified in the first part of this study (Rabinovich and Monserrat, 1996), are analysed to determine the atmospheric parameters responsible for the generation of large-amplitude seiches. Their generation mechanism was shown to be quite different from that causing ordinary background oscillations. Coincidence of some external factors and certain resonance effects seem to be necessary to produce the destructive waves. In particular, rissaga waves in Ciutadella inlet were found to be related to significant atmospheric disturbances propagating from the southwest, coinciding with the orientation of the inlet, and having a phase speed of about 3 m/s, which is close to the phase speed of long waves offshore from Menorca. Pronounced resonant properties of the inner basin strongly amplify incoming waves in Ciutadella inlet. In contrast, the bays of the northwestern coast of Shikotan Island are protected from normally incident atmosphere-induced waves by the elongated Kunashir Island, hence the whole situation there is not so favorable for the excitation of large seiches.

Evidence for nonlinear interaction between internal waves of inertial and semidiurnal frequency

Observational evidence is presented for pervasive nonlinear interaction between inertial and M 2 frequency oscillations in the northeast Pacific. Enhanced currents at the sum frequency (termed the fM 2 frequency) have the rotary spectral properties of freely propagating waves suggesting permanent energy exchange from the inertial and semidiurnal bands to the fM 2 band. The interaction may be responsible for the greater than predicted decay of inertial energy observed in the ocean and may contribute significantly to oceanic mixing.

On the reconstruction of the transfer function for atmospherically generated seiches

A method recently proposed to separate source and topographic effects in observed tsunami spectra is revised and applied to atmospherically generated seiches. The method is used to investigate the origin of abnormal seiche events (‘rissaga waves’) recorded in the region of Ciutadella, Balearic Islands. The reconstructed open-sea source spectra and the observed atmospheric pressure spectra are combined to estimate the ‘transfer function’ between the atmosphere and the sea surface. This function is used to predict sea level spectra for given locations near to the coast. Theoretically computed spectra for two ‘rissaga’ events are in good agreement with observations.

Numerical Modeling of Tsunami Generation by Submarine and Subaerial Landslides

Recent catastrophic tsunamis at Flores Island, Indonesia (1992), Skagway, Alaska (1994), Papua New Guinea (1998), andIzmit, Turkey (1999) have significantly increased scientific interest in landslides, and slide-generated tsunamis. Theoretical investigations and laboratory modeling further indicate that purely submarine landslides are ineffective at tsunami generation compared with subaerial slides. In the present study, we undertook several numerical experiments to examine the influence of the subaerial component of slides on surface wave generation, and to compare the tsunami generation efficiency of viscous, and rigid-body slide models. We found that a rigid-body slide produces much higher tsunami waves than a viscous (liquid) slide. The maximum wave height, and energy of generated surface waves were found to depend on various slide parameters, and factors, including slide volume, density, position, and slope angle. For a rigid-body slide, the higher the initial slide above sea level, the higher the generated waves. For a viscous slide, there is an optimal slide position (elevation) which produces the largest waves. An increase in slide volume, density, and slope angle always increases the energy of the generated waves. The added volume associated with a subaerial slide entering the water is one of the reasons that subaerial slides are much more effective tsunami generators than submarine slides. The critical parameter determining the generation of surface waves is the Froude number, Fr (the ratio between slide, and wave speeds). The most efficient generation occurs near resonance when Fr = 1.0. For purely submarine slides with p 2 ≤0.2 g-cm-3, the Froude number is always less than unity, and resonance coupling of slides, and surface waves is physically impossible. For subaerial slides there is always a resonant point (in time and space) where Fr = 1.0 for which there is a significant transfer of energy from a slide into surface waves. This resonant effect is the second reason that subaerial slides are much more important for tsunami generation than submarine slides.

Observations of seamount‐attached eddies in the North Pacific

Satellite-tracked drifters were used to examine eddy activity in the vicinity of the Emperor Seamount Chain (ESC) in the North Pacific during 1991–1993. The trajectories of two drifters drogued at a depth of 120 m revealed a pair of counterrotating mesoscale eddies attached to the leeside of Ojin/Jingu Seamount in the summer of 1992. The eddies had diameters of 75–100 km and rotational speeds of 20–40 cm/s at 120 m. Sea surface height anomalies derived from blended TOPEX/ERS-1 satellite altimetry revealed that the eddies had a surface manifestation as well. One of the drifters made five loops within the cyclonic eddy over a period of 62 days, during which time the eddy translated westward, toward the seamount, at 2.9 cm/s. This is one of the first observations demonstrating an extended attachment of a topographically generated eddy to a seamount. Drifters drogued at a depth of 15 m which crossed the ESC in the summer of 1991 and winter of 1993 revealed no eddy activity, most likely because of a decoupling of the topographic influence to the 15 m flow at their crossing latitude over the Nintoku Seamount (summit depth at 1000 m). The implication is that eddy formation within the mixed layer near the ESC is confined to the region around the taller Ojin/Jingu and Kinmei Seamounts (summit depths at 800 m and 100 m, respectively). It is suggested that long-lived eddies attached to the leeside of the ESC can profoundly influence local biological production and water exchange between the western and eastern basins of the North Pacific.

Oceanic Odyssey of a satellite-tracked drifter: North Pacific variability delineated by a single drifter trajectory

A near-surface satellite-tracked drifter launched off the east coast of the Kuril Islands on September 4,1993 began a 2.5-year Odyssey across the North Pacific Ocean. During its travels, the drifter encountered numerous energetic oceanographic regimes as it moved from the region of the Kuril-Kamchatka Trench to the continental margin of the Kuril Islands, through Friza Strait into the Sea of Okhotsk, seaward again through Bussol’ Strait, and then eastward across the North Pacific. Oceanic features detected along the basin-wide trajectory include a quasi-permanent anticyclonic eddy over the Kuril-Kamchatka Trench, open-ocean wind-driven inertial oscillations, coastal-trapped diurnal shelf waves, semidiurnal tidal currents, transient cyclonic and anticyclonic eddies, through-strait flows, and wave-like mesoscale meanders. The single drifter track delineates the dynamically-rich variability of upper ocean currents, emphasizes the marked difference in flow dynamics between boundary and open ocean regions, and provides a time-scale for the movement of surface waters across the entire North Pacific.

Evidence of Diurnal Shelf Waves in Satellite-Tracked Drifter Trajectories off the Kuril Islands

Abstract Satellite-tracked surface drifters deployed in September 1993 in the vicinity of the Kuril–Kamchatka Trench were advected onto the Pacific continental shelf of the Kuril Islands where they encountered strong (40–50 cm s−1) diurnal tidal currents. One of the drifters subsequently passed through Friz Strait into the Sea of Okhotsk, experiencing intense (>100 cm s−1) diurnal currents in the strait and strong (35–40 cm s−1) diurnal currents over the Okhotsk shelf of the Kuril Islands. The across-shelf structure of the diurnal tidal currents is shown to be consistent with that of free, topographically trapped subinertial waves propagating along the continental margin of the islands. Of the three continental shelf wave models considered (a barotropic model with zero mean flow, a barotropic model with nonzero alongshore mean flow, and a baroclinic model based on the observed density structure), only the baroclinic model accurately explains the main features of the diurnal currents for the Pacific and Ok...