Pinatubo Eruption Research Articles

Arctic oscillation response to the 1991 Pinatubo eruption in the SKYHI general circulation model with a realistic quasi‐biennial oscillation

Stratospheric aerosol clouds from large tropical volcanic eruptions can be expected to alter the atmospheric radiative balance for a period of up to several years. Observations following several previous major eruptions suggest that one effect of the radiative perturbations is to cause anomalies in the Northern Hemisphere extratropical winter tropospheric circulation that can be broadly characterized as positive excursions of the Arctic Oscillation (AO). We report on a modeling investigation of the radiative and dynamical mechanisms that may account for the observed AO anomalies following eruptions. We focus on the best observed and strongest 20th century eruption, that of Mt. Pinatubo on 15 June 1991. The impact of the Pinatubo eruption on the climate has been the focus of a number of earlier modeling studies, but all of these previous studies used models with no quasi‐biennial oscillation (QBO) in the tropical stratosphere. The QBO is a very prominent feature of interannual variability of tropical stratospheric circulation and could have a profound effect on the global atmospheric response to volcanic radiative forcing. Thus a complete study of the atmospheric variability following volcanic eruptions should include a realistic representation of the tropical QBO. Here we address, for the first time, this important issue. We employed a version of the SKYHI troposphere‐stratosphere‐mesosphere model that effectively assimilates observed zonal mean winds in the tropical stratosphere to simulate a very realistic QBO. We performed an ensemble of 24 simulations for the period 1 June 1991 to 31 May 1993. These simulations included a realistic prescription of the stratospheric aerosol layer based on satellite observations. These integrations are compared to control integrations with no volcanic aerosol. The model produced a reasonably realistic representation of the positive AO response in boreal winter that is usually observed after major eruptions. Detailed analysis shows that the aerosol perturbations to the tropospheric winter circulation are affected significantly by the phase of the QBO, with a westerly QBO phase in the lower stratosphere resulting in an enhancement of the aerosol effect on the AO. Improved quantification of the QBO effect on climate sensitivity helps to better understand mechanisms of the stratospheric contribution to natural and externally forced climate variability.

Measurements of stratospheric volcanic aerosol optical depth from NOAA TIROS Observational Vertical Sounder (TOVS) observations

We show that the infrared optical depth of stratospheric volcanic aerosols produced by the eruption of Mount Pinatubo in June 1991 may be retrieved from the observations of the High‐Resolution Infrared Radiation Sounder (HIRS‐2) on board the polar meteorological satellites of the National Oceanic and Atmospheric Administration (NOAA). Evolution of the concentration in time and in space, in particular the migration of the aerosols from the tropics to the Northern and Southern Hemispheres, is found to be consistent with our knowledge of the consequences of this eruption. The method relies on the analysis of the differences between the satellite observations and simulations from an aerosol‐free radiative transfer model using collocated radiosonde data as the prime input. Thus aerosol optical depths are retrieved directly without making assumptions about the aerosol size distribution or absorption coefficient. The aerosol optical depths reached a maximum in August 1991 in the tropical zone (0.055 at 8.3 μm, 0.03 at 4.0 μm, and 0.02 at 11.1 μm). The peak occurred in November 1991 in the southern midlatitudes and in March/April 1992 in the northern midlatitudes. A reanalysis of the almost 25 year archive of NOAA TIROS‐N Operational Vertical Sounder (TOVS) observations holds considerable promise for improved knowledge of the atmosphere loading in volcanic aerosols.

THE SPATIOTEMPORAL FLUCTUATION AND SPATIAL DISTRIBUTION CHARACTERISTICS IN ATMOSPHERIC CO2 CONCENTRATION ON A GLOBAL SCALE

In recent years, global warming is progressing and increasing CO2 concentration in the atmosphere is generally its major cause. Accordingly many researchers of various fields have been carried out on CO2. But there are many uncertainties on CO2. We considered that it is necessary to make a precise model to clarify these uncertainties. So we thought that it was significant to grasp the variation characteristics of atmospheric CO2 concentration.Then, in this paper we investigated to grasp the fluctuation characteristics of atmospheric CO2 concentration, such as the spatiotemporal fluctuation and spatial distribution characteristics, by using the atmospheric CO2 concentration practical data that has been equipped in recent years. Moreover we confirmed how much influence the Mt. Pinatubo Eruption would have had on the atmospheric CO2 concentration fluctuation because it has not been clarified.

Impact of natural variability in the 11-year mesopause region temperature observation over Fort Collins, CO (41°N, 105°W)

The Colorado State University Sodium Lidar has measured temperatures in the mesopause region (80–105 km) for over 11 years. Based on 7 years' observation, an episodic change with a warming of 11.8 K in 1993 at the mean mesopause altitude of 98 km, attributable to Mt. Pinatubo eruptions, was reported. In this paper, we focus on the solar cycle effects. With 11 years of data, we observed a maximum solar response of 0.06 K/SFU at 99 km, which decreases at lower and higher altitudes to nearly zero and appears to change sign at ∼82 and ∼104 km. The phase changes are consistent with earlier midlatitude observation with incoherent scatter radar above and Rayleigh lidar below the altitudes reported here, providing clear experimental evidence of dynamical influences throughout different layers of Earth's atmosphere. We discuss the altitude dependence of response amplitudes and the effect of volcanic and solar flux variability have on the mesopause region temperatures, as well as long-term temperature trends.

Spatio-temporal variability in volcanic sulphate deposition over the past 2 kyr in snow pits and firn cores from Amundsenisen, Antarctica

AbstractIn the framework of the European Project for Ice Coring in Antarctica (EPICA), a comprehensive glaciological pre-site survey has been carried out on Amundsenisen, Dronning Maud Land, East Antarctica, in the past decade. Within this survey, four intermediate-depth ice cores and 13 snow pits were analyzed for their ionic composition and interpreted with respect to the spatial and temporal variability of volcanic sulphate deposition. The comparison of the non-sea-salt (nss)-sulphate peaks that are related to the well-known eruptions of Pinatubo and Cerro Hudson in AD 1991 revealed sulphate depositions of comparable size (15.8±3.4 kg km–2) in 11 snow pits. There is a tendency to higher annual concentrations for smaller snow-accumulation rates. The combination of seasonal sodium and annually resolved nss-sulphate records allowed the establishment of a time-scale derived by annual-layer counting over the last 2000 years and thus a detailed chronology of annual volcanic sulphate deposition. Using a robust outlier detection algorithm, 49 volcanic eruptions were identified between AD 165 and 1997. The dating uncertainty is ±3 years between AD 1997 and 1601, around ±5 years between AD 1601 and 1257, and increasing to ±24 years at AD 165, improving the accuracy of the volcanic chronology during the penultimate millennium considerably.

Retrieval of the aerosol optical depth in the UV‐B at Uccle from Brewer ozone measurements over a long time period 1984–2002

Since 1984, Brewer spectrophotometer #16 has measured the ozone column at Uccle near Brussels in Belgium (50°48′N, 4°21′E, 100 m) from the direct sun observations at five isolated wavelengths in the UV‐B: 306.3 nm, 310.1 nm, 313.5 nm, 316.7 nm and 320.1 nm. We have used the Langley Plot Method (LPM) to retrieve the aerosol optical depth (AOD) from these observations over a long time period: from 1984 to November 2002. A seasonal variation of the AOD is clearly observed with mean AOD values of about 0.4 and 0.9 at 306.3 nm in winter and in summer respectively. The magnitude of these AODs is comparable to the AODs measured by AERONET sunphotometers at other places. We succeeded to demonstrate the impact of the eruption of Mount Pinatubo on the retrieved AOD in the UV‐B at Uccle in winter. Trend analysis on the mean annual AOD gives significant negative trends at the 2σ level only from 1989 to 2002 at all wavelengths and not over the whole period 1984–2002. The annual trend amounts to −2.46 ± 0.37%/year at 306.3 nm over 1989–2002. Significant negative seasonal trends at the 2σ level are only found in winter at all wavelengths (−2.66 ± 0.65%/year at 306.3 nm for example) and in summer at 306.3 nm (−1.24 ± 0.55%/year). In summer, in autumn and in spring at the other 4 wavelengths the trends are not significant at the 2σ level.

Risk modeling, assessment, and management of lahar flow threat.

The 1991 eruption of Mount Pinatubo in the Philippines is considered one of the most violent and destructive volcanic activities in the 20th century. Lahar is the Indonesian term for volcanic ash, and lahar flows resulting from the massive amount of volcanic materials deposited on the mountain's slope posed continued post-eruption threats to the surrounding areas, destroying lives, homes, agricultural products, and infrastructures. Risks of lahar flows were identified immediately after the eruption, with scientific data provided by the Philippine Institute of Volcanology, the U.S. Geological Survey, and other research institutions. However, competing political, economic, and social agendas subordinated the importance of scientific information to policy making. Using systemic risk analysis and management, this article addresses the issues of multiple objectives and the effective integration of scientific techniques into the decision-making process. It provides a modeling framework for identifying, prioritizing, and evaluating policies for managing risk. The major considerations are: (1) applying a holistic approach to risk analysis through hierarchical holographic modeling, (2) applying statistical methods to gain insight into the problem of uncertainty in risk assessment, (3) using multiobjective trade-off analysis to address the issue of multiple decisionmakers and stakeholders in the decision-making process, (4) using the conditional expected value of extreme events to complement and supplement the expected value in quantifying risk, and (5) assessing the impacts of multistage decisions. Numerical examples based on ex post data are formulated to illustrate applications to various problems. The resulting framework from this study can serve as a general baseline model for assessing and managing risks of natural disasters, which the Philippines' lead agency-the National Disaster Coordinating Council (NDCC)-and other related organizations can use for their decision-making processes.

Oxidized sulfur-rich mafic magma at Mount Pinatubo, Philippines

Basaltic fragments enclosed in andesitic dome lavas and pyroclastic flows erupted during the early stages of the 1991 eruption of Mount Pinatubo, Philippines, contain amphiboles that crystallized during the injection of mafic magma into a dacitic magma body. The amphiboles contain abundant melt inclusions, which recorded the mixing of andesitic melt in the mafic magma and rhyolitic melt in the dacitic magma. The least evolved melt inclusions have high sulfur contents (up to 1,700 ppm) mostly as SO4 2–, which suggests an oxidized state of the magma (NNO+1.4). The intrinsically oxidized nature of the mafic magma is confirmed by spinel–olivine oxygen barometry. The value is comparable to that of the dacitic magma (NNO+1.6). Hence, models invoking mixing as a means of releasing sulfur from the melt are not applicable to Pinatubo. Instead, the oxidized state of the dacitic magma likely reflects that of parental mafic magma and the source region in the sub-arc mantle. Our results fit a model in which long-lived SO2 discharge from underplated mafic magma accumulated in the overlying dacitic magma and immiscible aqueous fluids. The fluids were the most likely source of sulfur that was released into the atmosphere during the cataclysmic eruption. The concurrence of highly oxidized basaltic magma and disproportionate sulfur output during the 1991 Mt. Pinatubo eruption suggests that oxidized mafic melt is an efficient medium for transferring sulfur from the mantle to shallow crustal levels and the atmosphere. As it can carry large amounts of sulfur, effectively scavenge sulfides from the source mantle and discharge SO2 during ascent, oxidized mafic magma forms arc volcanoes with high sulfur fluxes, and potentially contributes to the formation of metallic sulfide deposits.

A GCM Study of Volcanic Eruptions as a Cause of Increased Stratospheric Water Vapor

Abstract Recent general circulation model (GCM) experiments have shown that idealized climatic perturbations that increase the temperature of the tropical tropopause region can cause larger than expected surface temperature increases. This is because the extra water vapor that is transported into the stratosphere acts as a positive radiative forcing agent. Since major volcanic eruptions in the Tropics also perturb temperatures in the region of the tropical tropopause, evidence is sought for the existence of such a mechanism by comparing model simulations and observations of the period following the eruption of Mount Pinatubo in 1991. Lower-stratosphere temperature perturbations are simulated using a GCM of intermediate complexity, and increases in water vapor concentration are found in the lower stratosphere that decay slowly in the years following the peak temperature increase. Such increases are consistent with the limited observations that exist over this period, as well as earlier work done with simpl...

El Chichón Volcano (Chiapas Volcanic Belt, Mexico) Transitional Calc-Alkaline to Adakitic-Like Magmatism: Petrologic and Tectonic Implications

The rocks of the 1982 eruption of El Chichón volcano (Chiapas, Mexico) display a series of geochemical and mineralogical features that make them a special case within the NW-trending Chiapas volcanic belt. The rocks are transitional between normal arc and adakitic-like trends. They are anhydrite-rich, and were derived from a water-rich, highly oxidized sulfur-rich magma, thus very much resembling adakitic magmas (e.g., the 1991 Pinatubo eruption). We propose that these rocks were generated within a complex plate tectonic scenario involving a torn Cocos plate (Tehuantepec fracture zone) and the ascent of hot asthenospheric mantle. The latter is supported by an outstanding negative S-wave anomaly widely extending beneath the zone, from 70 to 200 km in depth. The adakitic-like trend would be derived from the direct melting of subducting Cocos plate, whereas the transitional rocks would have resulted from the mixing of two poles, one reflecting a mantle source, and the other, the already mentioned adakitic melts. The basaltic source would also account for the high sulfur content and δ34S values of the El Chichón system (about +5.8), as result of a contribution of SO2 in fluids released from an underlying mafic magma.

Spatial and temporal variability of the stratospheric aerosol cloud produced by the 1991 Mount Pinatubo eruption

As a critical quality control step toward producing a stratospheric data assimilation system for volcanic aerosols, we conducted a comparison between Stratosphere Aerosol and Gas Experiment (SAGE) II aerosol extinction profiles and aerosol backscatter measured by five lidars, both in the tropics and midlatitudes, for the two‐year period following the 1991 Mt. Pinatubo eruption. The period we studied is the most challenging for the SAGE II retrieval because the aerosol cloud caused so much extinction of the solar signal that in the tropics few retrievals were possible in the core of the cloud. We compared extinction at two wavelengths at the same time that we tested two sets of conversions coefficients. We used both Thomason and Jäger's extinction‐to‐backscatter conversion coefficients for converting lidar backscatter profiles at 0.532 μm or 0.694 μm wavelengths to the SAGE II extinction wavelengths of 0.525 μm and 1.020 μm or the nearby ones of 0.532 μm and 1.064 μm respectively. The lidars were located at Mauna Loa, Hawaii (19.5°N, 155.6°W), Camagüey, Cuba (21.4°N, 77.9°W), Hefei, China (31.9°N, 117.2°W), Hampton Virginia (37.1°N, 76.3°W), and Haute Provence, France (43.9°N, 5.7°W). For the six months following the eruption the aerosol cloud was much more heterogeneous than later. Using two alternative approaches, we evaluated the aerosol extinction variability of the tropical core of the Pinatubo stratospheric aerosol cloud at the timescale of 1–2 days, and found it was quite large. Aerosol variability played the major role in producing the observed differences between SAGE II and the lidars. There was in general a good agreement between SAGE II extinction measurements and lidar derived extinction, and we conclude that all five lidar sets we compared can be used in a future data assimilation of stratospheric aerosols. This is the most comprehensive comparison yet of lidar data with satellite data for the Pinatubo period.

Volcanic eruptions recorded in the Illimani ice core (Bolivia): 1918–1998 and Tambora periods

Abstract. Acid layers of volcanic origin detected in polar snow and ice layers are commonly used to document past volcanic activity on a global scale or, conversely, to date polar ice cores. Although most cataclysmic eruptions of the last two centuries (Pinatubo, El Chichon, Agung, Krakatoa, Cosiguina, Tambora, etc.) occurred in the tropics, cold tropical glaciers have not been used for the reconstruction of past volcanism. The glaciochemical study of a 137 m ice core drilled in 1999 close to the summit of Nevado Illimani (Eastern Bolivian Andes, 16°37' S, 67°46' W, 6350 m asl) demonstrates, for the first time, that such eruptions are recorded by both their tropospheric and stratospheric deposits. An 80-year ice sequence (1918-1998) and the Tambora years have been analyzed in detail. In several cases, ash, chloride and fluoride were also detected. The ice records of the Pinatubo (1991), Agung (1963) and Tambora (1815) eruptions are discussed in detail. The potential impact of less important regional eruptions is discussed.

Global distribution of total ozone and lower stratospheric temperature variations

Abstract. This study gives an overview of interannual variations of total ozone and 50 hPa temperature. It is based on newer and longer records from the 1979 to 2001 Total Ozone Monitoring Spectrometer (TOMS) and Solar Backscatter Ultraviolet (SBUV) instruments, and on US National Center for Environmental Prediction (NCEP) reanalyses. Multiple linear least squares regression is used to attribute variations to various natural and anthropogenic explanatory variables. Usually, maps of total ozone and 50 hPa temperature variations look very similar, reflecting a very close coupling between the two. As a rule of thumb, a 10 Dobson Unit (DU) change in total ozone corresponds to a 1 K change of 50 hPa temperature. Large variations come from the linear trend term, up to -30 DU or -1.5 K/decade, from terms related to polar vortex strength, up to 50 DU or 5 K (typical, minimum to maximum), from tropospheric meteorology, up to 30 DU or 3 K, or from the Quasi-Biennial Oscillation (QBO), up to 25 DU or 2.5 K. The 11-year solar cycle, up to 25 DU or 2.5 K, or El Niño/Southern Oscillation (ENSO), up to 10 DU or 1 K, are contributing smaller variations. Stratospheric aerosol after the 1991 Pinatubo eruption lead to warming up to 3 K at low latitudes and to ozone depletion up to 40 DU at high latitudes. Variations attributed to QBO, polar vortex strength, and to a lesser degree to ENSO, exhibit an inverse correlation between low latitudes and higher latitudes. Variations related to the solar cycle or 400 hPa temperature, however, have the same sign over most of the globe. Variations are usually zonally symmetric at low and mid-latitudes, but asymmetric at high latitudes. There, position and strength of the stratospheric anti-cyclones over the Aleutians and south of Australia appear to vary with the phases of solar cycle, QBO or ENSO.

Impact of the Mount Pinatubo eruption on cirrus clouds formed by homogeneous freezing in the ECHAM4 GCM

Volcanic emissions may have the potential to alter cirrus cloud properties. Here we conduct different sensitivity studies with the ECHAM4 general circulation model for 2.5 years after the Mount Pinatubo eruption (July 1991 to December 1993) and compare homogeneous cirrus formation caused by sedimenting sulfate particles produced in the eruption plume with homogeneous cirrus formation in the undisturbed atmosphere. In the first scenario, the sulfate aerosol mass from Pinatubo is added to the background aerosol concentration assuming a monomodal aerosol. Here the aerosol concentration increases by up to 3000 cm−3 in 1992, which can be regarded as an upper limit more representative for the first months after the eruption. The ice crystal number concentration increases by up to 1 cm−3 near the tropical tropopause more than doubling the pre‐existing concentration in this region one year after the eruption. In the second, more realistic, scenario the Pinatubo aerosol is added to a bimodal background size distribution as a separate large particle mode. Here the aerosol number concentration increases by 10–25 cm−3, which can be regarded as a lower bound more representative for what has been observed in 1992. Then the ice crystal number increases at most 50% in the tropics in 1992. Satellite observations show an increase in ice water path starting in 1992 that could be related to either the Mount Pinatubo eruption or the El Niño event or both but a decrease in total cloud cover. While there is no trend on cloud microphysical or optical properties in our second scenario, the first scenario shows a pronounced increase in ice water path and a noticeable impact on cloud radiative forcing.

The crater lake and hydrothermal system of Mount Pinatubo, Philippines: evolution in the decade after eruption

The June 1991 eruption of Mount Pinatubo, Philippines breached a significant, pre-eruptive magmatic-hydrothermal system consisting of a hot (>300 °C) core at two-phase conditions and surrounding, cooler (<260 °C) liquid outflows to the N and S. The eruption created a large, closed crater that accumulated hydrothermal upwellings, near-surface aquifer and meteoric inflows. A shallow lake formed by early September 1991, and showed a long-term increase in level of ~1 m/month until an artificial drainage was created in September 2001. Comparison of the temporal trends in lake chemistry to pre- and post-eruptive springs distinguishes processes important in lake evolution. The lake was initially near-neutral pH and dominated by meteoric influx and Cl–SO4 and Cl–HCO3 hydrothermal waters, with peaks in SO4 and Ca concentrations resulting from leaching of anhydrite and aerosol-laden tephra. Magmatic discharge, acidity (pH~2) and rock dissolution peaked in late 1992, during and immediately after eruption of a lava dome on the crater floor. Since cessation of dome growth, trends in lake pH (increase from 3 to 5.5), temperature (decline from 40 to 26 °C), and chemical and isotopic composition indicate that magmatic degassing and rock dissolution have declined significantly relative to the input of meteoric water and immature hydrothermal brine. Higher concentrations of Cl, Na, K, Li and B, and lower concentrations of Mg, Ca, Fe, SO4 and F up to 1999 highlight the importance of a dilute hydrothermal contribution, as do stable-isotope and tritium compositions of the various fluids. However, samples taken since that time indicate further dilution and steeper trends of increasing pH and declining temperature. Present gas and brine compositions from crater fumaroles and hot springs indicate boiling of an immature Cl–SO4 geothermal fluid of near-neutral pH at approximately 200 °C, rather than direct discharge from magma. It appears that remnants of the pre-eruptive hydrothermal system invaded the magma conduit shortly after the end of dome emplacement, blocking the direct degassing path. This, along with the large catchment area (~5 km2) and the high precipitation rate of the area, led to a rapid transition from a small and hot acid lake to a large lake with near-ambient temperature and pH. This behavior contrasts with that of peak-activity lakes that have more sustained volcanic gas influx (e.g., Kawah Ijen, Indonesia; Poas and Rincon de la Vieja, Costa Rica).

Post‐Mount Pinatubo eruption ground‐based infrared stratospheric column measurements of HNO3, NO, and NO2 and their comparison with model calculations

Infrared solar spectra recorded between July 1991 to March 1992 and November 2002 with the Fourier transform spectrometer on Kitt Peak (31.9°N latitude, 111.6°W longitude, 2.09 km altitude) have been analyzed to retrieve stratospheric columns of HNO3, NO, and NO2. The measurements cover a decade time span following the June 1991 Mount Pinatubo volcanic eruption and were recorded typically at 0.01 cm−1 spectral resolution. The measured HNO3 stratospheric column shows a 20% decline from 9.16 × 1015 molecules cm−2 from the first observation in March 1992 to 7.40 × 1015 molecules cm−2 at the start of 1996 reaching a broad minimum of 6.95 × 1015 molecules cm−2 thereafter. Normalized daytime NO and NO2 stratospheric column trends for the full post‐Pinatubo eruption time period equal (+1.56 ± 0.45)% yr−1, 1 sigma, and (+0.52 ± 0.32)% yr−1, 1 sigma, respectively. The long‐term trends are superimposed on seasonal cycles with ∼10% relative amplitudes with respect to mean values, winter maxima for HNO3 and summer maxima for NO and NO2. The measurements have been compared with two‐dimensional model calculations utilizing version 6.1 Stratospheric Aerosol and Gas Experiment (SAGE) II sulfate aerosol surface area density measurements through 1999 and extended to the end of the time series by repeating the 1999 values. The model‐calculated HNO3, NO, and NO2 stratospheric column time series agree with the measurements to within ∼8% after taking into account the vertical sensitivity of the ground‐based measurements. The consistency between the measured and model‐calculated stratospheric time series confirms the decreased impact on stratospheric reactive nitrogen chemistry of the key heterogeneous reaction that converts reactive nitrogen to its less active reservoir form as the lower‐stratospheric aerosol surface area density declined by a factor of ∼20 after the eruption maximum.

Climatic, environmental and human consequences of the largest known historic eruption: Tambora volcano (Indonesia) 1815

The 1815 eruption of Tambora volcano (Sumbawa island, Indonesia) expelled around 140 gt of magma (equivalent to ≈50 km3 of dense rock), making it the largest known historic eruption. More than 95% by mass of the ejecta was erupted as pyroclastic flows, but 40% by mass of the material in these flows ended up as ash fallout from the ‘phoenix’ clouds that lofted above the flows during their emplacement. Although they made only a minor contribution to the total magnitude of the eruption, the short-lived plinian explosions that preceded the climactic eruption and caldera collapse were powerful, propelling plumes up to 43 km altitude. Over 71 000 people died during, or in the aftermath of, the eruption, on Sumbawa and the neigh-bouring island of Lombok. The eruption injected ≈60 mt of sulfur into the stratosphere, six times more than was released by the 1991 Pinatubo eruption. This formed a global sulfate aerosol veil in the stratosphere, which resulted in pronounced climate perturbations. Anomalously cold weather hit the northeastern USA, maritime provinces of Canada, and Europe the following year. 1816 came to be known as the ‘Year without a summer’ in these regions. Crop failures were widespread and the eruption has been implicated in accelerated emigration from New England, and widespread outbreaks of epidemic typhus. These events provide important insights into the volcanic forcing of climate, and the global risk of future eruptions on this scale.

Multiscale fracturing as a key to forecasting volcanic eruptions

Among volcanoes reawakening after long repose intervals, the final approach to eruption (∼1–10 days) is usually characterised by accelerating rates of seismicity. The observed patterns are consistent with the slow extension of faults, which continue to grow until they connect a pre-existing array of subvertical fractures and so open a new pathway for magma to reach the surface. Rates of slow extension are here investigated assuming that gravitational loading and magma overpressure create a fluctuating stress field in the country rock. Fluctuations are due to the intermittent growth of small cracks that cannot be detected by monitoring instruments. The rate of detected events is then determined by the frequency with which the concentration of strain energy around the tips of macroscopic faults becomes large enough to permit fault extension. The model anticipates oscillations in seismic event rate about a mean trend that accelerates with time, and identifies the rate of increase in peak event rate as a key indicator of the approach to eruption. The results are consistent with earlier empirical analyses of seismic precursors and, applied to data before the andesitic eruptions of Mt Pinatubo, Philippines, in 1991, and of Soufriere Hills, Montserrat, in 1995, they suggest that, for such volcanoes, reliable forecasts of magmatic activity might eventually be feasible on the order of days before eruption.

Halogen Occultation Experiment and Stratospheric Aerosol and Gas Experiment II observations of tropopause cirrus and aerosol during the 1990s

Averages of Halogen Occultation Experiment (HALOE) aerosol extinction at 121 hPa for 1993–1999 and Stratospheric Aerosol and Gas Experiment (SAGE II) aerosol extinction between 100 and 140 hPa for 1987–1999 are analyzed in the tropics (20°S–20°N). Multiple wavelength techniques for HALOE and SAGE II data are used to distinguish cirrus from aerosol observations following the eruption of Mount Pinatubo in 1991. SAGE II and HALOE cirrus extinction values are 34 and 28% less, respectively, in 1993 than in 1995–1999, while aerosol extinction decreases over the same time period. SAGE II and HALOE decreases in the frequency of occurrence of cirrus in 1993 are qualitatively similar to the SAGE II decreases in the frequency of occurrence of cirrus, discussed by Wang et al. [1995], after the eruption of El Chichon. Tropopause temperature anomalies in 1993 most likely do not account for the decrease in cirrus observed in 1993 by both the HALOE and SAGE II experiments.

Correction to “Lidar backscatter to extinction, mass and area conversions for stratospheric aerosols based on midlatitude balloonborne size distribution measurements”

[1] Size distributions of the stratospheric sulfuric acid aerosol derived from balloonborne particle counter measurements from Laramie, Wyoming, are used to calculate ratios of particle extinction, mass, and surface area to particle backscatter, and the wavelength dependences of particle backscatter and extinction. These ratios may then be used to infer particle extinction, mass, and area from midlatitude lidar data in the spectral range 355 nm to 1064 nm for the time period 1991 to 1999. The conversions are defined in four height intervals from the tropopause to 30 km with a time resolution of four months. Results of conversions from earlier studies are included to span the time period 1979 to 1999. The eruptions of El Chichon (1982) and Pinatubo (1991) strongly influence these conversions. Extended background levels are observed prior to El Chichon and since about 1997.