Search

The authors of Lim et al. (2021) have found an error in the code that processed the data displayed in Fig. 2c in the article. The bar graph is supposed [...]

High-resolution transmission electron microscopy (HRTEM) observations of unaltered volcanic air-fall deposits from the ongoing lava dome explosive eruption at Chaiten Volcano, Chilean Patagonia, revealed the presence of highly crystalline silica nanofibers in the respirable fraction of the volcanic ash (240[degrees]C), beta form of cristobalite, with average lengths of hundreds of nanometers and widths on the order of tens of nanometers. We propose that the beta-cristobalite nanofibers are formed during explosive eruptions by the reduction of amorphous silica by carbon monoxide to its reactive suboxide SiO, which is later oxidized to form one-dimensional crystalline silica nanostructures. Nucleation and growth of the nanofibers are enhanced by the high surface area of the micrometer- to nanometer-sized fragments of silica glass in the volcanic column. The formation of nanocrystalline cristobalite fibers during explosive lava dome eruptions poses new challenges for the assessment of the short- and long-term health hazards associated with the respirable nanofibrous components of volcanic ash.

Transient incursions of midlatitude air to the east of the Andes Mountains into subtropical and tropical latitudes are a distinctive feature of the synoptic climatology over South America. The mean synoptic-scale structure of these incursions is documented in this paper on the basis of a compositing analysis of NCEP--NCAR reanalyzed meteorological fields and satellite-based outgoing longwave radiation measurements. Although these incursions are a year-round phenomenon, with relatively modest seasonal changes in their structure, this analysis is focused on the austral summer, when they have their largest impact in the precipitation field. The summertime incursions move equatorward at a mean speed of 10 m s 21 and retain their identities over intervals of about 5 days. The upper-level circulation is characterized by a midlatitude trough--ridge couplet that provides the quasigeostrophic forcing of the system. Before the onset of the incursions of midlatitude air, the approaching upper-level trough tends to deepen the semipermanent area of low pressure over the central plains of the continent (25°--10°S). The strong southward advection of warm, humid air creates favorable conditions for the development of deep convection over central and southern Argentina. During the next two days, a migratory cold anticyclone, steered by the ridge aloft, moves onto the southern plains of South America (40°-- 35°S), establishing a large-scale, southward-directed pressure gradient. Due to the blocking effect of the Andes, the induced low-level circulation assumes the form of an agesotrophic southerly flow that produces the equa-torward advection of the cold air. In this stage, enhanced convection occurs in a well-defined band of the intensified low-level convergence at the leading edge of the cold surge. This synoptic-scale banded structure in the convective cloudiness also emerges as the dominant mode of the day-to-day variability of the deep... [ABSTRACT FROM AUTHOR]

Based on nine years (1983--91) of infrared data from geostationary satellites (the B3 ISCCP product), several features of the diurnal march of the frequency of convective cloudiness over the tropical and subtropical Americas are documented with 3-h temporal resolution and 0.58 30.58 latitude--longitude spatial resolution. The frequency of convective cloudiness in each grid box is defined in terms of the fraction of temporal samples that exhibit cloud-top temperatures colder than 235 K. The effect of varying the threshold is explored and selected results are compared with rainfall estimates based on microwave (SSM/I) imagery. Convective cloudiness over most land areas exhibits a coherent diurnal march with relatively clear mornings, a rapid afternoon buildup, and a more gradual nighttime decay. The highest, coldest convective clouds peak a few hours earlier than those with lower tops. Morning to noontime maxima tend to be prevalent over offshore waters that experience significant convection such as the Gulf of Panama, with the highest convective clouds leading the lower ones by 6 h ormore. During the austral summer, the strongest diurnal march is observed over the Andes Mountains, in a band just inland of the northeast coast of South America and in two intermediate, parallel bands over Amazonia. Between these bands, weak night and morning maxima are observed in some areas. During the boreal summer the strongest diurnal march is observed over Central America and extending northwest along the continental divide as far as the southwestern United States. In comparison to the microwave imagery, the features in convective cloudiness tend to be somewhat larger in spatial scale, and cloudiness over land tends to be biased toward the late afternoon and evening hours. Regions with strong late afternoon/early evening maxima in convective cloudiness tend to experience more diurnal-mean convective cloudiness than those with weak (or morning) maxima... [ABSTRACT FROM AUTHOR]

Central Chile, along the subtropical west coast of South America, has experienced an uninterrupted sequence of dry years) since 2010. The so called Mega Drought (MD) is the longest event on record with detrimental effects on water availability, vegetation and forest fires. Local records and reanalysis data reveal that the exceptional length of the MD results from the reiteration of a circulation dipole characterized by troposheric-deep positive pressure anomalies over the southeast subtropical Pacific and negative pressure anomalies over the Amundsen-Bellinhausen Sea. Historically, ENSO is a major modulator of such dipole, but the ongoing dry period has mostly occurred under ENSO-neutral conditions. Climate model simulations forced with observed global SST and present-day radiative forcing replicate the south Pacific dipole and capture part of the rainfall anomalies during the MD. Further numerical experiments suggest that most of the ocean forcing emanates from the subtropical southwest Pacific, a region that has experienced a marked warming over the last decade. On the other hand, the positive trend of the Southern Annular Mode also contributes to strength of the south Pacific dipole, showing that anthropogenic forcing has also contributed to the maintenance of the central Chile MD. Given the concomitance of natural and anthropogenic forcing, we anticipate only a partial recovery of central Chile precipitation in the decades to come. [ABSTRACT FROM AUTHOR]