Showing papers in "Geophysical Research Letters in 1997"
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TL;DR: In this article, the authors examined the chronology of interdecadal climatic regime shifts using instrumental data over the North Pacific, North America and the tropical oceans, and reconstructed climate records for North America.
Abstract: The chronology of interdecadal climatic regime shifts is examined, using instrumental data over the North Pacific, North America and the tropical oceans, and reconstructed climate records for North America. In the North Pacific and North America, climatic regime shifts around 1890 and in the 1920s with alternating polarities are detected, whose spatial structure is similar to that of the previously-known climatic shifts observed in the 1940s and 1970s. Sea-surface temperatures in the tropical Indian Ocean-maritime continent region exhibit changes corresponding to these four shifts. Spectra obtained by the Multi-Taper-Method suggest that these regime shifts are associated with 50–70 year climate variability over the North Pacific and North America.
The leading mode of the empirical orthogonal functions of the air-temperature reconstructed from tree-rings in North America exhibits a spatial distribution that is reminiscent of instrumentally observed air-temperature differences associated with the regime shifts. The temporal evolution of this mode is characterized by a 50–70 year oscillation in the eighteenth and nineteenth centuries. This result, combined with the results of the analyses of the instrumental data, indicates that the 50–70 year oscillation is prevalent from the eighteenth century to the present in North America.
809 citations
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TL;DR: In this article, a prototype system has been developed to monitor the instantaneous global distribution of ionospheric irregularities, using the worldwide network of Globa Positioning System (GPS) receivers.
Abstract: A prototype system has been developed to monitor the instantaneous global distribution of ionospheric irregularities, using the worldwide network of Globa Positioning System (GPS) receivers. Case studies in this pape indicate that GPS receiver loss of lock of signal tracking may be associated with strong phase fluctuations. It is shown that a network-based GPS monitoring system will enable us to study the generation and evolution of ionospheric irregularities continuously around the globe under various solar and geophysical conditions, which is particularly suitable for studies of ionospheric storms, and for space weather research and applications.
701 citations
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TL;DR: In this paper, the authors studied possible braking mechanisms of high-speed ion flows in the near Earth central plasma sheet for radial distances between 9 and 19 Earth Radii (RE) on the basis of observations made by the AMPTE/IRM satellite.
Abstract: We have studied possible braking mechanisms of high-speed ion flows in the near-Earth central plasma sheet for radial distances between 9 and 19 Earth Radii (RE) on the basis of observations made by the AMPTE/IRM satellite. Flows with velocities in excess of 400 km/s are almost always Earthward for this range, indicating that the source of the flows is beyond 19 RE. Though the occurrence rate of the high-speed flows substantially decreases when the satellite comes closer to the Earth, high-speed flows with velocities higher than 600 km/s are still observed. We suggest that the high-speed flows are stopped at a clear boundary between the regions of dipolar field and tail-like field in the plasma sheet. The boundary corresponds to the inner edge of the neutral sheet. The average jump of the magnetic field at the boundary, which is estimated from the observations by assuming a pressure balance, is 6.7 nT. The inertia current caused by the braking of the flow and the current caused by pileup of the magnetic flux at the stopping point are quantitatively estimated and discussed in relation to the formation of the substorm current wedge.
480 citations
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TL;DR: A comprehensive statistical analysis of how an index of the Southern Oscillation changed from 1882 to 1995 was given by Trenberth and Hoar as discussed by the authors, with a focus on the unusual nature of the 1990-1995 El Nino-Southern Oscillations (ENSO) warm event in the context of an observed trend for more El NINO and fewer La Nina events after the late 1970s.
Abstract: A comprehensive statistical analysis of how an index of the Southern Oscillation changed from 1882 to 1995 was given by Trenberth and Hoar [1996], with a focus on the unusual nature of the 1990–1995 El Nino-Southern Oscillation (ENSO) warm event in the context of an observed trend for more El Nino and fewer La Nina events after the late 1970s. The conclusions of that study have been challenged by two studies which deal with only the part of our results pertaining to the length of runs of anomalies of one sign in the Southern Oscillation Index. They therefore neglect the essence of Trenberth and Hoar, which focussed on the magnitude of anomalies for certain periods and showed that anomalies during both the post-1976 and 1990-mid-1995 periods were highly unlikely given the previous record. With updated data through mid 1997, we have performed additional tests using a regression model with autoregressive-moving average (ARMA) errors that simultaneously estimates the appropriate ARMA model to fit the data and assesses the statistical significance of how unusual the two periods of interest are. The mean SOI for the post-1976 period is statistically different from the overall mean at <0.05% and so is the 1990-mid-1995 period. The recent evolution of ENSO, with a major new El Nino event underway in 1997, reinforces the evidence that the tendency for more El Nino and fewer La Nina events since the late 1970s is highly unusual and very unlikely to be accounted for solely by natural variability.
463 citations
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TL;DR: In this paper, an alternate emission mechanism is proposed, where the solar wind contains a large number of minor/heavy ion species with a range of charge states, such as O6+, C5+, N5+, and Si10+, producing ions which can be highly excited and consequently emit photons in the extreme ultraviolet and x-ray part of the spectrum.
Abstract: Recently, Lisse et al. (1996) reported on exciting observations by the Rontgen X-ray satellite (ROSAT) of x-ray and extreme ultraviolet emissions from comet C/Hyakutake 1996 B2. The spatial distribution of the emissions was displaced sunward of the nucleus and the spatial extent was about 105 km. Lisse et al. (1996) suggested that the emission could be explained by thermal bremsstrahlung associated with hot electrons, possibly due to solar wind interaction effects. In the present paper, an alternate emission mechanism is proposed. The solar wind contains a large number of minor/heavy ion species with a range of charge states, such as O6+, C5+, N5+, and Si10+. These ions will readily charge transfer with cometary neutrals, producing ions which can be highly excited and consequently emit photons in the extreme ultraviolet and x-ray part of the spectrum. The photon emission rate is proportional to the solar wind heavy ion flux and hence to the solar wind flux and, with some assumptions concerning the solar wind velocity, to the solar wind number density. The emission rate should be greatest downstream of the bow shock along the sun-comet axis in agreement with the observed spatial distribution. The x-ray images are really images of the line of sight integration of the solar wind density convoluted with the cometary neutral density. A total EUV/x-ray luminosity for comet Hyakutake from this charge transfer mechanism agrees with the observed luminosity of 4 × 1015 ergs s−1 within a factor of two.
399 citations
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TL;DR: In this paper, the authors fit Kappa functions to 16,000 velocity distribution functions measured in the solar wind by the electron plasma instrument on board Ulysses, and found that the electron distributions are observed to have important high velocity tails in the fast solar wind but are closer to a Maxwellian in the slow wind.
Abstract: We fit Kappa functions to 16,000 velocity distribution functions measured in the solar wind by the electron plasma instrument on board Ulysses. Statistically, the electron distributions are observed to have important high velocity tails in the fast solar wind but are closer to a Maxwellian in the slow wind. We also discuss how this result could support a recent kinetic model of the solar wind proposed by Maksimovic, Pierrard and Lemaire [1997].
385 citations
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TL;DR: In this article, the number size distribution of ambient submicron and ultrafine aerosol particles have been measured on a continuous basis (every 10 minutes) for three quarters of the year 1996, at a forest site in Southern Finland.
Abstract: Number size distribution of ambient submicron and ultrafine aerosol particles have been measured on a continuous basis (every 10 minutes) for three quarters of the year 1996, at a forest site in Southern Finland. Continuous monitoring offers additional insight over the diurnal dynamics of the submicron size distribution, including existence of clearly separate size modes as well as events of new particle formation. Selected examples of the measured size distributions are presented, including the particle formation events observed at the measurement site. Typical characteristics of days with particle formation events versus days of no events are discussed.
381 citations
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TL;DR: In this paper, high-velocity friction experiments have been performed on a pair of hollow-cylindrical specimens of gabbro initially at room temperature, at slip rates from 7.5 mm/s to 1.8 m/s, with total circumferential displacements of 125 to 174 m, and at normal stresses to 5 MPa, using a rotary-shear high-speed friction testing machine.
Abstract: High-velocity friction experiments have been performed on a pair of hollow-cylindrical specimens of gabbro initially at room temperature, at slip rates from 7.5 mm/s to 1.8 m/s, with total circumferential displacements of 125 to 174 m, and at normal stresses to 5 MPa, using a rotary-shear high-speed friction testing machine. Steady-state friction increases slightly with increasing slip rate at slip rates to about 100 mm/s (velocity strengthening) and it decreases markedly with increasing slip rate at higher velocities (velocity weakening). Steady-state friction in the velocity weakening regime is lower for the non-melting case than the frictional melting case, due perhaps to severe thermal fracturing. A very large peak friction is always recognized upon the initiation of visible frictional melting, presumably owing to the welding of fault surfaces upon the solidification of melt patches. Frictional properties thus change dramatically with increasing displacement at high velocities, and such a non-linear effect must be incorporated into the analysis of earthquake initiation processes.
369 citations
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TL;DR: In this paper, the authors measured iron in the water column and conducted iron-enrichment bottle-incubation experiments at a station in the central Ross Sea (76°30′S, 170°40′W), first, in the presence of melting sea ice, and 17 days later, in ice-free conditions.
Abstract: During summer 1995–96, we measured iron in the water column and conducted iron-enrichment bottle-incubation experiments at a station in the central Ross Sea (76°30′S, 170°40′W), first, in the presence of melting sea ice, and 17 days later, in ice-free conditions. We observed a striking temporal change in mixed-layer dissolved iron concentrations at this station, from 0.72–2.3 nM with sea ice present, to 0.16–0.17 nM in ice-free conditions. These changes were accompanied by a significant drawdown in macronutrients and an approximate doubling of algal (diatom) biomass. Our incubation experiments suggest that conditions were iron-replete in the presence of sea ice, and iron-deficient in the absence of sea ice. We surmise that bioavailable iron was released into seawater from the melting sea ice, stimulating phytoplankton production and the biological removal of dissolved iron from the mixed layer, until iron-limited conditions developed. These observations suggest that the episodic release of bioavailable iron from melting sea ice is an important factor regulating phytoplankton production, particularly ice-edge blooms, in seasonally ice-covered Antarctic waters.
342 citations
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TL;DR: In this paper, a data-based hypothesis on the mechanism of the El Nino-Southern Oscillation (ENSO) is presented, which emphasizes the importance of off-equator sea surface temperature and sea level pressure variations west of the dateline for initiating equatorial easterly winds over the far western Pacific.
Abstract: A data-based hypothesis is presented on the mechanism of the El Nino-Southern Oscillation (ENSO), a major determinant of interannual global climate variability. The hypothesis emphasizes the importance of off-equator sea surface temperature and sea level pressure variations west of the dateline for initiating equatorial easterly winds over the far western Pacific. These winds compete with westerly winds over the equatorial central Pacific enabling the coupled ocean-atmosphere system to oscillate. Consistent with this hypothesis, an analogical oscillator model is constructed that produces ENSO-like oscillations. The proposed mechanism differs from the delayed oscillator paradigm in that wave reflection at the western boundary is not a necessary condition for the coupled ocean-atmosphere system to oscillate.
339 citations
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TL;DR: In this article, a statistical survey of GEOTAIL observations reveals the following properties of the near Earth plasma sheet (−15 < XGSM′ < −50 Re): during the periods when the northward IMF dominates, the plasma sheet becomes significantly cold and dense, the best correlations between the near-Earth plasma sheet and the IMF parameters occur when the latter quantities are averaged over 9−4+3 hours prior to the observations, and temperatures diminish and densities increase near the dawn and dusk flanks of the plasmas.
Abstract: A statistical survey of GEOTAIL observations reveals the following properties of the near-Earth plasma sheet (−15 < XGSM′ < −50 Re): During the periods when the northward IMF dominates, (1) the plasma sheet becomes significantly cold and dense, (2) the best correlations between the plasma sheet and the IMF parameters occur when the latter quantities are averaged over 9−4+3 hours prior to the plasma sheet observations, and (3) temperatures diminish and densities increase near the dawn and dusk flanks of the plasma sheet. We suggest that during prolonged northward IMF periods (∼ several hours) there is a slow diffusive transport of the plasma from the solar wind into the plasma sheet through the the magnetotail flanks.
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TL;DR: In this article, a detailed ocean/sediment carbon cycle model was used to simulate the response of the carbonate cycle in the ocean to a range of anthropogenic CO2 release scenarios.
Abstract: The long term abiological sinks for anthropogenic CO2 will be dissolution in the oceans and chemical neutralization by reaction with carbonates and basic igneous rocks. We use a detailed ocean/sediment carbon cycle model to simulate the response of the carbonate cycle in the ocean to a range of anthropogenic CO2 release scenarios. CaCO3 will play only a secondary role in buffering the CO2 concentration of the atmosphere because CaCO3 reaction uptake capacity and kinetics are limited by the dynamics of the ocean carbon cycle. Dissolution into ocean water sequesters 70–80% of the CO2 release on a time scale of several hundred years. Chemical neutralization of CO2 by reaction with CaCO3 on the sea floor accounts for another 9–15% decrease in the atmospheric concentration on a time scale of 5.5–6.8 kyr. Reaction with CaCO3 on land accounts for another 3–8%, with a time scale of 8.2 kyr. The final equilibrium with CaCO3 leaves 7.5–8% of the CO2 release remaining in the atmosphere. The carbonate chemistry of the oceans in contact with CaCO3 will act to buffer atmospheric CO2 at this higher concentration until the entire fossil fuel CO2 release is consumed by weathering of basic igneous rocks on a time scale of 200 kyr.
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TL;DR: A cruise to the Ross Sea polynya in November-December, 1994 to measure the contribution of phytoplankton during the austral spring to the annual productivity of the region was conducted by.
Abstract: Although satellite data of surface layer pigments have suggested that the daily productivity in the Ross Sea is among the largest found in any marine system, no modern oceanographic cruise has entered the Ross Sea polynya to quantitatively assess the austral spring productivity over time scales of days to weeks. We conducted a cruise to the Ross Sea polynya in November–December, 1994 to measure the contribution of phytoplankton during the austral spring to the annual productivity of the region and found markedly enhanced levels of phytoplankton biomass. Chlorophyll concentrations were greater than 3 µg l−1 in mid-November, a time when the polynya was covered by a thin (ca. 20–30 cm) layer of ice. Particulate matter concentrations increased through time, and by early December chlorophyll and particulate carbon concentrations exceeded 10 µg l−1 and 53 µmol l−1, respectively. Primary productivity also increased through time: the mean productivity in early December equaled 3.53 g C m−2 d−1, and maximum measured rates exceeded 6 g C m−2 d−1. Productivity based on nitrate disappearance averaged 1.52 g C m−2 d−1 (with a maximum rate of 2.49 g C m−2 d−1), suggesting that the bloom's new production was also substantial. The Ross Sea polynya is the most southerly location in the Antarctic where phytoplankton growth is initiated this early and which supports such high standing stocks by early December. Inclusion of this production in a carbon budget for the region suggests that this area supports an annual production of 200 g C m−2, the largest of any region in the Southern Ocean, and confirms its hyperproductive nature.
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TL;DR: Partial melt compositions of natural carbonated peridotite (KLB-1 + 2.5 wt% CO2) have been determined at 3 GPa using the diamond aggregate method as mentioned in this paper.
Abstract: Partial melt compositions of natural carbonated peridotite (KLB-1 + 2.5 wt% CO2) have been determined at 3 GPa using the diamond aggregate method. Melt obtained at 1350 °C is carbonatitic, and with increasing temperature melt composition becomes silicate-rich. These melts are lower in SiO2 and Al2O3 and higher in MgO and CaO compared to dry partial melts of KLB-1 generated at the same pressure. The 1400 and 1450 °C liquids have melilititic compositions, which are in good agreement with those of natural alkali-basalts from continental regions. At higher temperatures, partial melts are richer in SiO2, and the compositions of liquids at 1475 and 1525 °C approximate nephelinite and basanite respectively. However, they deviate from natural alkaline rocks in that the CaO/(CaO+MgO) ratios are smaller, suggesting that the natural nephelinite and basanite magmas were formed at lower pressures. The compositional trend of natural alkaline rocks, melilitites-nephelinites-basanites, may reflect the effect of decreasing pressure.
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TL;DR: In this paper, the authors used predicted GPS orbits for automated computation of vertical column water vapor within 30 minutes of GPS data collection, based on a 4 month comparison, near real-time GPS column Water vapor agrees with radiosondes and radiometers within 2 mm rms.
Abstract: We describe sensing of atmospheric column water vapor in near real-time using the Global Positioning System (GPS). We use predicted GPS orbits for automated computation of vertical column water vapor within 30 minutes of GPS data collection. Based on a 4 month comparison, near real-time GPS column water vapor agrees with radiosondes and radiometers within 2 mm rms. Our near real-time column water vapor data are posted hourly at www.unavco.ucar.edu. They are available for assimilation in numerical weather models and for other applications.
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TL;DR: In this article, a new formulation of an approximate conservation relation is proposed for stationary Rossby waves on a zonally varying basic flow, utilizing that A and e are both related to the wave activity pseudomomentum.
Abstract: A new formulation of an approximate conservation relation is proposed for stationary Rossby waves on a zonally varying basic flow, utilizing that A (proportional to wave enstrophy) and e (proportional to wave energy) are both related to the wave activity pseudomomentum. For stationary Rossby waves, it is shown in the limit of a small-amplitude, plane wave on a slowly varying, unforced non-zonal flow that a particular linear combination of A and e, namely, M ≡ (A + e)/2, is independent of the wave phase, even if unaveraged, and is conserved under steady, unforced and nondissipative conditions. It is also shown that the three-dimensional flux of M is parallel to the local group velocity in the WKB limit. The flux could be a useful diagnostic tool, as shown in an example that presents a “snapshot” of the three-dimensional propagation of a stationary Rossby wavetrain in the real atmosphere. Our conservation relation is a generalization of that for stationary Rossby waves on a zonally-uniform basic flow derived by Plumb.
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TL;DR: In this article, the authors confirm the lead of the Antarctic Cold Reversal on the Younger Dryas cold event by at least 1.8 kyr, which suggests that the northern and southern hemispheres were in anti-phase during this cold event.
Abstract: The transition from the Last Glacial to the Holocene is a key period for understanding the mechanisms of global climate change. Ice cores from the large polar ice sheets provide a wealth of information with good time resolution for this period. However, interactions between the two hemispheres can only be investigated if ice core records from Greenland and Antarctica can be synchronised accurately and reliably. The atmospheric methane concentration shows large and very fast changes during this period. These variations are well suited for a synchronisation of the age scales of ice cores from Greenland and Antarctica. Here we confirm the proposed lead of the Antarctic Cold Reversal on the Younger Dryas cold event. The Antarctic cooling precedes the Younger Dryas by at least 1.8 kyr. This suggests that northern and southern hemispheres were in anti-phase during the Younger Dryas cold event. A further result of the synchronisation is that the long-term glacial-interglacial increase of atmospheric CO2 was not interrupted during the Younger Dryas event and that atmospheric CO2 changes are not necessarily dominated by changes in the North Atlantic circulation.
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TL;DR: Combining SO2 plume emissions from Mt Etna in 1975-1995 with the magma S content suggests that between 35 and 59 km 3 of basalt were degassed for sulfur over two decades, only 10-20% of which actually extruded except during extensive lava outbreak (1992), the SO2 flux was mostly supplied by endogenous degassing of non-erupted basalt, replaced at a timeaveraged rate of 45-8 m3/s as discussed by the authors.
Abstract: Combining SO2 plume emissions from Mt Etna in 1975-1995 with the magma S content suggests that between 35 and 59 km 3 of basalt were degassed for sulfur over two decades, only 10-20% of which actually extruded Except during extensive lava outbreak (1992), the SO2 flux was mostly supplied by endogenous degassing of non-erupted basalt, replaced at a time-averaged rate of 45-8 m3/s The unerupted degassed magma cannot be accomodated by the upper plumbing system (whose maximum capacity is estimated at 06 km3), nor by simple intrusive growth of the volcanic pile Being denser than undegassed melt, most of it was probably removed by gravitational convection in subvolcanic feeders Ultimately, part of this degassed magma may solidify within the crust, contributing to the accretion of a wide 'plutonic' complex, >_3 times larger than the volcano itself, that has grown within its sedimentary basement the largest dataset for a volcano worldwide The measurements were intermittent from 1975 to 1987 [Allard et al, 1991, and references therein] but routineous since then [Caltabiano et al, 1994; Allard et al, 1994a; Bruno et al, 1994, 1996] The SO2 flux was found to vary within a main range from _<1000 tons/day (t/d) during low passive fuming to 10000-12000 t/d during high Strombolian activity (95% of data) A few peaks at over 20000 t/d occurred during lava-fountaining paroxysms in 1990-1991 (03 % of data) However, high fluxes without Table Sulfur Output and Magma Degassing Budget of Etna, 1975-1995 Year SO2 output Degassed magma (tons) (106 m 3)
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TL;DR: In this article, the authors detected five solar impulsive ∼1−10µ keV electron events while the WIND spacecraft was inside the magnetic cloud observed upstream of the Earth on October 18−20, 1995.
Abstract: Five solar impulsive ∼1–10² keV electron events were detected while the WIND spacecraft was inside the magnetic cloud observed upstream of the Earth on October 18–20, 1995. The solar type III radio bursts produced by these electrons can be directly traced from ∼1 AU back to X-ray flares in solar active region AR 7912, implying that at least one leg of the cloud was magnetically connected to that region. Analysis of the electron arrival times shows that the lengths of magnetic field lines in that leg vary from ∼3 AU near the cloud exterior to ∼1.2 AU near the cloud center, consistent with a model force-free helical flux rope. Although the cloud magnetic field exhibits the smooth, continuous rotation signature of a helical flux rope, the ∼0.1-1 keV heat flux electrons and ∼1–10² keV energetic electrons show numerous simultaneous abrupt changes from bidirectional streaming to unidirectional streaming to complete flux dropouts. We interpret these as evidence for patchy disconnection of one end or both ends of cloud magnetic field lines from the Sun.
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TL;DR: In this paper, the seasonally averaged flux of isoprene to the atmosphere was estimated to be 1.7 × 107 molecules cm−2 s−1, which may be significant for atmospheric chemistry in locations remote from land as it is the only known source of atmospheric isoperesidual in these regions.
Abstract: In this paper we show evidence that isoprene emission from the oceans is strongly seasonally dependent and is correlated with the chlorophyll content of the water from measurements in the North Sea and Southern Ocean. We estimate the seasonally averaged flux of isoprene to the atmosphere to be 1.7 × 107 molecules cm−2 s−1, which may be significant for atmospheric chemistry in locations remote from land as it is the only known source of atmospheric isoprene in these regions. We observe a strong seasonal cycle of several other NMHCs in seawater at high latitudes, with a maximum in summer. This will distort current estimates of the annual marine flux of NMHCs to the atmosphere which may need to be reduced by up to an order of magnitude to account for lower emissions in winter.
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TL;DR: In this paper, the total ozone observations from the Total Ozone Mapping Spectrometer (TOMS) instruments during March 1997 reveal an extensive region of low column densities in the Arctic region centered near the north pole.
Abstract: Total ozone observations from the Total Ozone Mapping Spectrometer (TOMS) instruments during March 1997 reveal an extensive region of low column densities in the Arctic region centered near the north pole. Values were below 250 Dobson units for nearly a two week period during this period, and were correlated with the position of the northern lower stratospheric polar vortex. The March 1997 average total ozone column densities were more than 30% lower than the average of column densities observed during the 1979–1982 March period.
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TL;DR: In this paper, the photodissociation quantum yields for acetone in the 290-320 nm wavelength region for pressures and temperatures characteristic of the upper troposphere were compared with those obtained during the NASA and NOAA sponsored Stratospheric Tracers of Atmospheric Transport (STRAT) field campaign.
Abstract: This paper summarizes measured photodissociation quantum yields for acetone in the 290-320 nm wavelength region for pressures and temperatures characteristic of the upper troposphere. Calculations combine this laboratory data with trace gas concentrations obtained during the NASA and NOAA sponsored Stratospheric Tracers of Atmospheric Transport (STRAT) field campaign, in which measurements of OH, HO_(2), odd-nitrogen, and other compounds were collected over Hawaii, and west of California during fall and winter of 1995/1996. OH and HO_(2) concentrations within 2 to 5 km layers just below the tropopause are ∼50% larger than expected from O_(3), CH_(4), and H_(2)O chemistry alone. Although not measured during STRAT, acetone is inferred from CO measurements and acetone-CO correlations from a previous field study. These inferred acetone levels are a significant source of odd-hydrogen radicals that can explain a large part of the discrepancy in the upper troposphere. For lower altitudes, the inferred acetone makes a negligible contribution to HO_(x) (HO+HO_(2)), but influences NO_(y) partitioning. A major fraction of HO_(x) production by acetone is through CH_(2)O formation, and the HO_(x) discrepancy can also be explained by CH_(2)O levels in the 20 to 50 pptv range, regardless of the source.
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TL;DR: In this article, a solution of the kinetic-MHD wave equations for this process using a realistic equilibrium profile including full ion Larmor radius effects and waveparticle resonance interactions for electrons and ions to model the dissipation is presented.
Abstract: Large amplitude compressional type ULF waves can propagate from the magnetosheath to the magnetopause where there are large gradients in density, pressure and magnetic field. These gradients efficiently couple compressional waves with shear/kinetic Alfven waves near the Alfven field-line resonance location (ω=k∥υA). We present a solution of the kinetic-MHD wave equations for this process using a realistic equilibrium profile including full ion Larmor radius effects and wave-particle resonance interactions for electrons and ions to model the dissipation. For northward IMF a KAW propagates backward to the magnetosheath. For southward IMF the wave remains in the magnetopause but can propagate through the k∥=0 location. The quasilinear theory predicts that transport due to KAWs at the magnetopause primarily results from the perpendicular electric field coupling with magnetic drift effects with diffusion coefficient D⟂ ∼ 109 m²/s. For southward IMF additional transport can occur because magnetic islands form at the k∥=0 location. Due to the broadband nature of the observed waves these islands can overlap leading to stochastic transport which is larger than that due to quasilinear effects.
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TL;DR: In this article, the authors derived an early 1996 emission rate of 5.9±0.2 Gg SF6 and an interhemispheric exchange time of 1.3± 0.1 years.
Abstract: Sulfur hexafluoride (SF6), an anthropogentically produced compound that is a potent greenhouse gas, has been measured in a number of NOAA GMDL air sampling programs. These include high resolution latitudinal profiles over the Atlantic and Pacific oceans, weekly flask samples from seven remote, globally distributed sites, hourly in situ measurements in rural North Carolina, and a series of archived air samples from Niwot Ridge, Colorado. The observed increase in atmospheric mixing ratio is consistent with an overall quadratic growth rate, at 6.9±0.2% yr−1 (0.24±0.01 ppt yr−1) for early 1996. From these data we derive an early 1996 emission rate of 5.9±0.2 Gg SF6 yr−1 and an interhemispheric exchange time of 1.3±0.1 years.
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TL;DR: This article showed that the slowdown of the West Antarctic ice stream is restricted to its lower part, and occurred because of loss of lubrication on localized "sticky spots" at the bed of the ice stream.
Abstract: The dynamic behavior of the West Antarctic ice sheet is of interest because of the possibility that it may change and cause rapid sea-level rise. Attention is focused on the fast-moving and rapidly responding ice streams that drain the ice sheet. One of these, ice stream C, largely stopped about a century ago, and some models for this shutdown postulate negative feedbacks that would tend to stabilize the ice-sheet. Here, new data are presented indicating that the slowdown of the ice stream is restricted to its lower part, and occurred because of loss of lubrication on localized “sticky spots” at the bed of the ice stream. The increased friction probably arises from a topographic accident of the glacier bed that has directed lubricating water to the neighboring ice stream B, together with slow drawdown of the ice sheet, rather than from any general stabilizing feedbacks.
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TL;DR: In this article, the frequency-magnitude distribution of earthquakes, characterized using the b-value, is examined as a function of space beneath Mount St. Helens and Mt. Spurr.
Abstract: The frequency-magnitude distribution of earthquakes, characterized using the b-value, is examined as a function of space beneath Mount St. Helens (1988–1996), and Mt. Spurr (1991–1995). At Mount St. Helens, two volumes of anomalously high b (b > 1.3) can be observed at depths of 2.6–3.6 km below the crater floor and below 6.4 km. These anomalies coincide with (1) the depth of vesiculation of ascending magma, and (2) the suggested location of a magma chamber at Mount St. Helens. Study of Mt. Spurr reveals an area of high b-value (b ≥ 1.3) at a depth of about 2.3–4.5 km below the crater floor of the active vent Crater Peak. We propose that the higher material heterogeneity in the vicinity of a magma chamber or conduit due to vesiculation of the ascending magma is the main cause of the increased b-value at shallow depths. Alternatively, interaction of magma with groundwater may have increased pore pressure and lowered the effective stress. The deeper anomaly at Mount St. Helens is likely caused by high thermal stress gradients in the vicinity of the magma chamber. Our results indicate that detailed mapping of the frequency-magnitude distribution can be used as a tool to trace vesiculation and locate active magma chambers.
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TL;DR: In this article, the authors measured the exchange flow between the Red Sea and the Gulf of Aden-Indian Ocean through the Bab el Mandab Strait for 10 months, June 1995-March 1996.
Abstract: The exchange flow between the Red Sea and the Gulf of Aden-Indian Ocean through the Bab el Mandab Strait was measured continuously for 10 months, June 1995–March 1996. ADCP and temperature-salinity chain moorings allow an unprecedented look at the magnitude and seasonal evolution of the inflow layer from the Gulf of Aden, and the high salinity outflow layer from the Red Sea. The timing, structure, and evolution of the summer season mid-depth intrusion of cold, low salinity water into the Red Sea from the Gulf of Aden is measured for the complete intrusion cycle of 1995. We unexpectedly find the deep outflow still strong in June 1995, with speeds of 0.6 m/sec and transport of 0.4 Sv (1 Sv = 106 m³/sec). From July to mid–September, the deep outflow persists but is attenuated to speeds of 0.2 m/sec and transport of 0.05 Sv. The dominant summer feature, the cold low salinity intermediate layer intrusion, persists for 3 months, occupies 70% of the water column in the Strait and carries approximately 1.7 × 1012 m³ of cold nutrient-rich water into the Red Sea. The winter regime begins in mid-September, is fully developed by early November, and continues to the end of our first observation interval in March 1996. Speeds in the lower layer are 0.8–1.0 m/sec and 0.4–0.6 m/sec in the upper layer. At maximum exchange in mid-February, outflow transport reaches 0.7 Sv. Ubiquitous oscillations in current and salinity at synoptic and intraseasonal periods appear closely related to fluctuations in the along-channel wind forcing and perhaps to coastally-trapped waves.
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TL;DR: The most consistent means of investigating the global sea ice cover is by satellite passive microwave sensors, as these are independent of illumination and cloud cover as discussed by the authors, and the overall trends established here serve to better define and strengthen earlier assertions of a reduced ice cover, based on analysis of SMMR and SSMI data taken separately.
Abstract: The most consistent means of investigating the global sea ice cover is by satellite passive microwave sensors, as these are independent of illumination and cloud cover. The Nimbus 7 Scanning Multichannel Microwave Radiometer (SMMR) and the Defense Meteorological Satellite Program (DMSP) Special Sensor Microwave Imager (SSMI) provide information on the global sea ice cover from 1978 to present. The two instruments flew simultaneously during a 6-week overlap period in July and August 1987, thus enabling intercomparison of the two sensors. Brightness temperatures are corrected for instrument drift and calibration differences in order to produce continuous time series of monthly averaged Arctic and Antarctic sea ice extent and sea ice area through the use of the NOrwegian Remote Sensing EXperiment (NORSEX) algorithm, which relates brightness temperatures to ice concentration. Statistical analysis on the time series estimates the decreases in Arctic ice extent and ice area to be 4.5% and 5.7%, respectively, during the 16.8-year observation period. The overall trends established here serve to better define and strengthen earlier assertions of a reduced ice cover, based on analysis of SMMR and SSMI data taken separately. These results are consistent with GCM simulations that suggest retreat of the sea ice cover under global warming scenarios.
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TL;DR: In this paper, the authors identify convection of boundary-layer CH3OOH as a primary source of H ox to the upper troposphere, leading to higher NO/HNO3 ratios than predicted for local photochemical steady-state.
Abstract: Convection in the tropics turns over the upper troposphere at rates (0.08 d−1) comparable to photochemical processes controlling the absolute abundance of HOx (OH + HO2) and the abundance of NOx (NO + NO2) relative to HNO3. Here we identify convection of boundary-layer CH3OOH as a primary source of HOx to the upper troposphere. Turnover of NOx leads to NO/HNO3 ratios much higher than predicted for local photochemical steady-state. Through convective transport the upper troposphere is more photochemically active in producing O3, an important greenhouse gas.
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TL;DR: The 1996-1997 northern hemisphere spring polar vortex was very strong, cold, and symmetric, somewhat similar to those found in the Antarctic spring vortex as discussed by the authors, and was very symmetric from February into late April.
Abstract: The 1996–1997 northern hemisphere spring polar vortex was very strong, cold, and symmetric, somewhat similar to those found in the Antarctic spring vortex The vortex did not form until late in December and was very symmetric from February into late April The spring vortex was characterized by record low temperatures, record low ozone amounts as measured from the Total Ozone Mapping Spectrometer (TOMS) instruments, and a wide band of strong winds in the lower stratosphere Spring wave activity was greatly reduced, with 100 hPa February–March eddy heat fluxes that were lower by a factor of 2 from any previously observed values over the last 18 years