Isaac V. Fine

Estimating Mixed Layer Depth from Oceanic Profile Data

Abstract Estimates of mixed layer depth are important to a wide variety of oceanic investigations including upper-ocean productivity, air–sea exchange processes, and long-term climate change. In the absence of direct turbulent dissipation measurements, mixed layer depth is commonly derived from oceanic profile data using threshold, integral, least squares regression, or other proxy variables. The different methodologies often yield different values for mixed layer depth. In this paper, a new method—the split-and-merge (SM) method—is introduced for determining the depth of the surface mixed layer and associated upper-ocean structure from digital conductivity–temperature–depth (CTD) profiles. Two decades of CTD observations for the continental margin of British Columbia are used to validate the SM method and to examine differences in mixed layer depth estimates for the various computational techniques. On a profile-by-profile basis, close agreement is found between the SM and de facto standard threshold met...

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.

The landslide‐generated tsunami of November 3, 1994 in Skagway Harbor, Alaska: A case study

We examine the origin and behavior of the catastrophic tsunami that impacted Skagway Harbor at the head of Taiya Inlet, Alaska, on November 3, 1994. Geomorphologic and tide gauge data, combined with numerical simulation of the event, reveal that the tsunami was generated by an underwater landslide formed during the collapse of a cruise-ship dock undergoing construction. Use of a fine-grid model for Skagway Harbor and a coarse-grid model for Taiya Inlet enables us to explain many of the eyewitness accounts and to reproduce the dominant oscillations in the tide gauge record, including the persistent (∼1 h) 3-min oscillation in Skagway Harbor. The occurrence of the landslide is linked to critical overloading of the slope materials at a time of extreme low tide.

Japan's 2011 Tsunami: Characteristics of Wave Propagation from Observations and Numerical Modelling

We use a numerical tsunami model to describe wave energy decay and transformation in the Pacific Ocean during the 2011 Tohoku tsunami. The numerical model was initialised with the results from a seismological finite fault model and validated using deep-ocean bottom pressure records from DARTs, from the NEPTUNE-Canada cabled observatory, as well as data from four satellite altimetry passes. We used statistical analysis of the available observations collected during the Japan 2011 tsunami and of the corresponding numerical model to demonstrate that the temporal evolution of tsunami wave energy in the Pacific Ocean leads to the wave energy equipartition law. Similar equipartition laws are well known for wave multi-scattering processes in seismology, electromagnetism and acoustics. We also show that the long-term near-equilibrium state is governed by this law: after the passage of the tsunami front, the tsunami wave energy density tends to be inversely proportional to the water depth. This fact leads to a definition of tsunami wave intensity that is simply energy density times the depth. This wave intensity fills the Pacific Ocean basin uniformly, except for the areas of energy sinks in the Southern Ocean and Bering Sea.

On Numerical Simulation of the Landslide-Generated Tsunami of November 3, 1994 in Skagway Harbor, Alaska

A three-dimensional, shallow-water numerical model for a viscous landslide with full slide-wave interaction (Kulikov et al.; 1996; Fine et al., 1998) has been modified to include the subaerial component of the landslide. The model is used to simulate the November 3, 1994 tsunami in Skagway, Alaska generated by collapse of the PARN Dock. Results show that the dock slide moved down the steep (30–35°) slope of Taiya Inlet and was guided along the trough at the base of the slope, consistent with geomorphological findings. The leading tsunami wave, propagating in front of the advancing slide, impacted the Alaska State Ferry Terminal and the NOAA tide gauge site as a positive wave (crest), consistent with the tide gauge record and with the results of laboratory modelling by Raichlen et al. (1996). Computed wave heights for the PARN Dock failure (13 m at the Ferry Terminal, 7.7 m at the tide gauge site, and 1.3 in the Small Boat Harbor) agree closely with the tide gauge record and eyewitness accounts. The computed 3.0 min period for the fundamental long-wave mode for Skagway Harbor is nearly identical to the observed period. Estimates of the Q-factor (Q≈24) are comparable to observed values (Q≈21), suggesting significant tsunami energy retention in the harbour. Energy loss appears to be through radiation damping rather than from frictional effects. A detailed examination of the slide motion and associated tsunami waves in the vicinity of the PARN Dock reveals that, in the first few seconds, a “wall of water” would have formed opposite the dock and that the floating Ferry Terminal would have been impacted 15 to 20 s after onset of the event, consistent with eyewitness accounts. The floating debris observed at the still-standing northern portion of the dock was apparently carried alongshore by a secondary wave crest originating near the collapsed southern part of the dock.

Northern Adriatic meteorological tsunamis: Assessment of their potential through ocean modeling experiments

Potential for generation of meteotsunami waves via open ocean resonance has been documented for the shallow northern Adriatic, based on a set of barotropic numerical modeling experiments. Model simulations were forced by a bell-shaped traveling atmospheric (air pressure, wind) disturbance, with shape and propagation parameters chosen in accordance with measurements done during several observed northern Adriatic meteotsunamis. Air pressure disturbances were found to generate much larger meteotsunami waves than wind disturbances, with wind disturbances having a limited influence in the very coastal and shallow areas only. Numerical simulations reveal that the most important factor for generation of large meteotsunami waves is matching between the speed of the atmospheric disturbance and the speed of long-ocean waves. Already a small (∼10%) deviation from resonant conditions stops the wave growth and dramatically decreases height of predicted waves. A train of atmospheric disturbances can significantly increase maximum wave heights at selected locations at which multiple reflections and superimpositions of meteotsunami waves occur. Sensitivity of model simulations to resonant conditions and limited cross-propagation width of atmospheric disturbance explain the localization of destructive meteotsunami waves in a limited area during destructive historic events. Mapping of maximum predicted wave heights indicates places with large meteotsunami hazard potential, matching the locations where real events were observed, and may be a useful tool for assessing vulnerability and risks in coastal areas during extreme sea level events.