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This study explores the seasonal variations in snow grain size on the East Antarctic Plateau, where dry metamorphism occurs, by using microwave radiometer observations from 2000 to 2022. Local meteorological conditions and large-scale atmospheric phenomena have been considered in order to explain some peculiar changes in the snow grains. We find that the highest ice divide is the region with the largest grain size in the summer, mainly because the wind speed is low. Moreover, some extreme grain size values with respect to the average (over +3σ) were identified. In these cases, the ERA5 reanalysis revealed a high-pressure blocking close to the onsets of the summer increase in the grain size. It channels moisture intrusions from the mid-latitudes, through atmospheric rivers that cause major snowfall events over the plateau. If conditions of weak wind and low temperature occur during the following weeks, dry snow metamorphism is facilitated, leading to grain growth. This determines anomalous high maximums of the snow grain size at the end of summer. These phenomena confirm the importance of moisture intrusion events in East Antarctica and their impact on the physical properties of the ice sheet surface, with a co-occurrence of atmospheric rivers and seasonal changes in the grain size with a significance of over 95 %. [ABSTRACT FROM AUTHOR]

This study explores the grain size seasonal variations on the East Antarctic Plateau, where dry metamorphism occurs, by using microwave radiometer observations from 2000 to 2022. Local meteorological conditions and large scale atmospheric phenomena have been considered in order to explain some peculiar changes of the snow grains. We find that the highest ice divide is the region with the largest grain size in the summer, mainly because the wind speed is low. Moreover, some extreme grain size values with respect to the average (over +3σ) were identified. In these cases, the ERA5 reanalysis revealed a high pressure blocking/ridge situation in the proximity of the onsets of the summer increase of the grain size, conveying the relatively warm and moist air coming from the mid latitudes, often associated with atmospheric rivers. If weak wind and low temperature conditions occur during the following weeks, dry snow metamorphism is facilitated, leading to grain growth. This determines anomalous high maximums of the snow grain size at the end of summer. These phenomena confirm the importance of moisture intrusion events in the East Antarctica and their impact on the physical properties of the ice sheet surface, with a co-occurrence of atmospheric rivers and seasonal changes of the grain size significant over 95%. [ABSTRACT FROM AUTHOR]

During austral winter, a compact low cloud deck over the South Atlantic contrasts with clear sky over southern Africa, where forest fires triggered by dry conditions emit large amounts of biomass burning aerosols (BBAs) in the free troposphere. Most of the BBA burden crosses the South Atlantic embedded in the tropical easterly flow. However, midlatitude synoptic disturbances can deflect part of the aerosol from the main transport path towards southern extratropics. In this study, the first objective classification of the synoptic variability controlling the spatial distribution of BBA in southern Africa and the South Atlantic during austral winter (August to October) is presented. By analysing atmospheric circulation data from reanalysis products, a six-class weather regime (WR) classification of the region is constructed. The classification reveals that the synoptic variability is composed of four WRs, representing disturbances travelling at midlatitudes, and two WRs accounting for pressure anomalies in the South Atlantic. The WR classification is then successfully used to characterise the aerosol spatial distribution in the region in the period 2003–2017, in both reanalysis products and station data. Results show that the BBA transport towards southern extratropics is controlled by weather regimes associated with midlatitude synoptic disturbances. In particular, depending on the relative position of the pressure anomalies along the midlatitude westerly flow, the BBA transport is deflected from the main tropical route towards southern Africa or the South Atlantic. Moreover, the WRs accounting for midlatitude disturbances show organised transition sequences, which allow one to illustrate the evolution of the BBA northerly transport across the region in the context of a wave pattern. The skill in characterising the BBA transport shown by the WR classification indicates the potential for using it as a diagnostic/predictive tool for the aerosol dynamics, which is a key component for the full understanding and modelling of the complex radiation–aerosol–cloud interactions controlling the atmospheric radiative budget in the region. [ABSTRACT FROM AUTHOR]

High alpine rock wall permafrost is extremely sensitive to climate change. Its degradation has a strong impact on landscape evolution and can trigger rockfalls constituting an increasing threat to socio-economical activities of highly frequented areas; quantitative understanding of permafrost evolution is crucial for such communities. This study investigates the long-term evolution of permafrost in three vertical cross sections of rock wall sites between 3160 and 4300m above sea level in the Mont Blanc massif, from the Little Ice Age (LIA) steady-state conditions to 2100. Simulations are forced with air temperature time series, including two contrasted air temperature scenarios for the 21st century representing possible lower and upper boundaries of future climate change according to the most recent models and climate change scenarios. The 2-D finite element model accounts for heat conduction and latent heat transfers, and the outputs for the current period (2010-2015) are evaluated against borehole temperature measurements and an electrical resistivity transect: permafrost conditions are remarkably well represented. Over the past two decades, permafrost has disappeared on faces with a southerly aspect up to 3300ma.s.l. and possibly higher. Warm permafrost (i.e. > -2 °C) has extended up to 3300 and 3850ma.s.l. in N and S-exposed faces respectively. During the 21st century, warm permafrost is likely to extend at least up to 4300ma.s.l. on S-exposed rock walls and up to 3850ma.s.l. depth on the N-exposed faces. In the most pessimistic case, permafrost will disappear on the S-exposed rock walls at a depth of up to 4300ma.s.l., whereas warm permafrost will extend at a depth of the N faces up to 3850ma.s.l., but possibly disappearing at such elevation under the influence of a close S face. The results are site specific and extrapolation to other sites is limited by the imbrication of local topographical and transient effects. [ABSTRACT FROM AUTHOR]

El Nino-Southern Oscillation (ENSO) is a naturally occurring mode of tropical Pacific variability, with global impacts on society and natural ecosystems. While it has long been known that El Nino events display a diverse range of amplitudes, temporal evolution and spatial patterns, the realization that ENSO impact can be highly sensitive to this event-to-event diversity is driving a renewed interest in the subject. However, very little has been done in understanding the potential impact of decadal modes of Pacific sea-surface temperature (SST) on this ENSO diversity, as well as on its impacts on rainfall worldwide. Using several ENSO indices, describing both central and eastern Pacific events, and indices portraying the Pacific Decadal Oscillation (PDO) and the Interdecadal Pacific Oscillation (IPO) derived from three different observed SST data sets, this study aims at assessing the impact of decadal variability on ENSO diversity and its impacts on rainfall at the global scale. The results are then compared with those obtained in GFDL-CM3 pi-control run, which shows good performance in reproducing impact of Pacific SST variability at interannual and decadal timescales. [ABSTRACT FROM AUTHOR]