Measurement of tsunamis in the open ocean

Abstract

The spectrum of long waves, which are a background to tsunamis, is analyzed on the basis of records of near‐bottom pressure sensors obtained in the Northwest Pacific during the first and second USA‐USSR expeditions on the investigation of tsunamis in the open ocean (1975 and 1978). Instrumental trends, tidal oscillations, and quasistationary longwave noise were contained in the records. Special filters were used to pick out small waves generated by the seismicity of the ocean bottom. A decrease of noise level from 10² cm (including tides) to 10°¹ cm could be reached. The level of long‐wave noise is found to depend on the instrument's location. Minimal disturbances in the records were observed at stations situated on the edge of the continental slope. The influence of cyclones passing over the instrument's site is deduced. It shows an increase in noise level on Nyquist frequency (0.5 min°¹), which possibly is caused by the action of swell generated by the cyclone. Seismicity of the region under investigation for the second expedition (August‐October 1978) is described, and the recurrence of tsunamis is estimated, including microtsunamis. Taking into account this analysis, records were filtered and sections corresponding to probable arrivals of tsunamis from most strong earthquakes were selected. The anomalous disturbance of ocean level with a height of about 0.5 cm was found. Presumably, it was generated by an earthquake with magnitude M = 6.     

Variability of baroclinic tidal currents on the Mackenzie Shelf,the Southeastern Beaufort Sea

Semidiurnal tidal currents on the outer shelf of the Mackenzie Shelf in the Beaufort Sea were found to be strongly influenced by the locally generated baroclinic tide. Two primary factors are involved in this process: (1) the sharp shelf break along the northeastern Mackenzie Shelf, promoting the generation of vigorous internal tidal waves; and (2) the proximity to critical latitudes for M₂ and N₂ motions locking these waves and preventing them from leaving the source region. As a result, internal tides are resonantly trapped between the shelf and critical latitudes. The physical properties and temporal variations of tidal motions were examined using current meter measurements obtained from 1987–1988 at four sites (SS1, SS2, SS3, and SS4) offshore of the shelf break at depths of ∼200 m. Each mooring had Aanderaa RCM4s positioned at ∼35 m below the surface and ∼50 m above the bottom. Complex demodulation was used to compute the envelopes (amplitude modulation) of these components. A striking difference in the variability of clockwise (CW) and counterclockwise (CCW) tidal currents was found. The CW tides are highly variable, have greater amplitude, exhibit a burst-like character associated with wind events and contain about 80% of the total energy of the semidiurnal tidal currents. In contrast, the CCW components have a more regular temporal regime with distinct monthly, fortnightly and 10-day modulation at astronomical periodicities associated with frequency differences M₂–N₂ (0.03629 cpd), S₂–M₂ (0.06773 cpd), and S₂–N₂ (0.10402 cpd). Significant horizontal correlation of the CW current envelopes was found only between stations near the northeast Mackenzie Shelf, indicating this to be the main area of baroclinic internal wave generation.     

The resonant generation of shelf waves by a moving cyclone

The resonant generation of shelf waves by the local atmospheric disturbance displacing longshore is considered. The model, which is more precise compared with that in refs 3 and 4 owing to the consideration of dispersion of the trapped waves, was applied to the analysis. It is shown that comparatively low-frequency waves are generated most effectively when the cyclone displacement rate is close to the maximum phase velocity of a single shelf mode.Translated by Mikhail M. Trufanov.     

Exoplanet status report: Observation, characterization and evolution of exoplanets and their host stars

After the discovery of more than 400 planets beyond our Solar System, the characterization of exoplanets as well as their host stars can be considered as one of the fastest growing fields in space science during the past decade. The characterization of exoplanets can only be carried out in a well coordinated interdisciplinary way which connects planetary science, solar/stellar physics and astrophysics. We present a status report on the characterization of exoplanets and their host stars by reviewing the relevant space- and ground-based projects. One finds that the previous strategy changed from space mission concepts which were designed to search, find and characterize Earth-like rocky exoplanets to: A statistical study of planetary objects in order to get information about their abundance, an identification of potential target and finally its analysis. Spectral analysis of exoplanets is mandatory, particularly to identify bio-signatures on Earth-like planets. Direct characterization of exoplanets should be done by spectroscopy, both in the visible and in the infrared spectral range. The way leading to the direct detection and characterization of exoplanets is then paved by several questions, either concerning the pre-required science or the associated observational strategy.     

Extreme Storm Surges in the Gulf of Finland: Frequency-Spectral Properties and the Influence of Low-Frequency Sea Level Oscillations

Oceanology - An analysis of hourly tide gauge observation data yields time-frequency estimates of storm surges in the Gulf of Finland. Spectral analysis of long-term series made it possible to...     

A VES-IP survey near the geophysical station of Moscow State University, in the Kaluga region

Due to the progress in instrumentation and methods in the most recent decade apparent resistance curves have been supplemented by accurate apparent chargeability curves at some frequencies up to an electrode spacing of 500 m. The comparison of the induced polarization frequency characteristics makes it possible to arrive at additional conclusions on the nature of anomalies. The results of vertical electrical and induced polarization sounding survey (VES-IP) that were carried out under the geophysical field studies of the Moscow State University are described.     

Aeronomical evidence for higher CO2 levels during Earth’s Hadean epoch

According to recent simulations of the Earth’s thermosphere, the exospheric temperature is not expected to rise above 7000-8000 K even under extreme solar EUV conditions anticipated for the early Earth. Rather, when the solar EUV flux exceeds some critical value, the escaping flow of the bulk upper thermosphere starts cooling it due to adiabatic expansion, which results in a decrease of the exobase temperature. Under these extreme conditions, the exobase might have expanded above the magnetopause and the magnetosphere had not been able to protect the upper atmosphere against strong non-thermal erosion by the solar wind.This study shows that a nitrogen-rich terrestrial atmosphere with a present-day composition would have been removed within a few million years during the extreme EUV and solar wind conditions that are expected to have prevailed before the late heavy bombardment period ∼3.8 Ga ago. Our results suggest that a CO₂ amount in the early nitrogen-rich terrestrial atmosphere of at least two orders of magnitude higher than the present-time level was needed to confine the upper atmosphere after the onset of the geodynamo within the shielding magnetosphere and thus might have protected it from complete destruction.     

Non-thermal water loss of the early Mars: 3D multi-ion hybrid simulations

In this study we analyze the non-thermal loss rates of O⁺, O₂⁺ and CO₂⁺ ions over the last 4.5 billion years (Gyr) in the Martian history by using a 3D hybrid model. For this reason we derived the past solar wind conditions in detail. We take into account the intensified particle flux of the early Sun as well as an Martian atmosphere, which was exposed to a sun's extreme ultraviolet (EUV) radiation flux 4.5 Gyr ago that was 100 times stronger than today. Furthermore, we model the evolution of the interplanetary magnetic field by a Weber & Davis solar wind model. The ‘external’ influences of the Sun's radiation flux and solar wind flux lead to the formation of an ionospheric obstacle by photoionization, charge exchange and electron impact. For the early Martian conditions we could show that charge exchange was the dominant ionization mechanism. Several hybrid simulations for different stages in the evolution of the Martian atmosphere, at 1, 2, 5, 10, 30 and 100 EUV, were performed to analyze the non-thermal escape processes by ion pick-up, momentum transfer from the solar wind to the ionosphere and detached ionospheric plasma clouds. Our results show a non-linear evolution of the loss rates. Using mean solar wind parameters the simulations result in an oxygen loss equivalent to the depth of a global Martian ocean of about 2.6 m over the last 4.5 Gyr. The induced magnetic field strength could be increased up to about 2000 nT. A simulation run with high solar wind density results in an oxygen loss of a Martian ocean up to 205 m depth during 150 million years after the sun reached the zero age mean sequence (ZAMS).     

Stratospheric ozone distribution features from the results of simultaneous ground-based microwave measurements in Nizhni Novgorod and Kyrgyzstan

Presented are the results of studying the ozone layer over Nizhni Novgorod and Central Asia using the ground-based millimeter wavelength range equipment. Carried out is the comparison of results of ground-based measurements with the EOS Aura MLS data. The difference in the ozone layer structure at stratospheric heights is revealed during the joint measurements at the mentioned points in November 2010. The analysis demonstrates that this difference is caused by the influence of the polar stratospheric vortex. Proceeding from the experience of similar researches, the conclusion is made on the necessity of organizing the permanent domestic network of the ground-based microwave monitoring of the ozone layer.