Benjamin Pohl

The hyperarid Namib Desert relies on fog and low clouds (FLCs) as a critical water source, a role likely to grow in importance under a warmer, drier climate. While the climatological mean conditions favoring FLC formation are well understood, its day-to-day drivers are less clear. FLCs have been linked to transient synoptic-scale disturbances, but previous studies relied on two-state (e.g. fog versus clear-sky) composites, which mixed different synoptic states and could not resolve a disturbance's evolution across the coast. To fill this gap, we extend an existing weather-type (WT) classification of the south-eastern Atlantic atmospheric circulation to the full annual cycle, and combine it with 22 years of satellite FLC observations, ERA5 reanalysis, and more than a decade of station observations. We find that in the Namib, FLC occurrence is strongly tied to the life cycle of eastward-propagating mid-latitude disturbances, that drive long-range moisture transport ahead of mid-latitude troughs, and favor free-tropospheric drying and boundary-layer deepening on their rear flanks. This response persists for around three days and is state-dependent, varying with season and latitude. Along the south coast where radiation and advection-radiation fog rather than stratus-interception fog dominate, FLC occurrence instead responds oppositely to the same weather types, pointing to a systematic link between dominant fog type and synoptic sensitivity. These results establish transient mid-latitude disturbances as a unifying, multi-scale control on Namib fog variability and a physical basis for anticipating its response to future circulation change.

Lower-tropospheric ozone (O3) pollution is a major environmental and societal concern in East and Southeast Asia (ESEA), where concentrations often exceed the thresholds recommended by the World Health Organization, causing hundreds of thousands of premature deaths each year. While lower-tropospheric O3 plays a key role in air quality, its systematic spatial variability and the influence of synoptic and climatological drivers on its distribution over long periods have not yet been characterised. In this study, we describe the most commonly occurring spatial patterns of lower-tropospheric O3 during spring (March and April), the season of its highest concentrations. These regional spatial patterns are called O3 regimes, which are derived from chemical reanalyses during over 21 years using a k-means clustering algorithm. This method allows us to identify four distinct regimes of O3 regional distribution, which are validated with respect to multispectral IASI+GOME2 satellite observations. Our results highlight the occurrence of a specific regime characterised by anticyclonic and high insolation conditions over ESEA, that favour the occurrence of biomass burning in Indochina and lead to positive O3 anomalies extending from Southeast Asia towards the Pacific. Its occurrence is significantly correlated with El Niño conditions. Conversely, a contrasting regime is associated with La Niña, which enhance convection and cloudiness, while producing a negative anomaly of tropospheric O3 over most of the region. These results highlight that lower-tropospheric O3 in spring exhibits different distribution patterns, which can be strongly driven by synoptic conditions and large-scale climate variability.

Atmospheric blocking is a key driver of persistent circulation anomalies and associated extreme events in the Southern Hemisphere, yet its characteristics around Antarctica remain poorly understood due to methodological diversity and the absence of a consolidated, long-term dataset. This study presents a new multi-method Antarctic atmospheric blocking dataset covering the period 1979–2024, derived from ERA5 reanalysis and constructed using multiple blocking detection approaches applied consistently across the Southern Hemisphere (25–90° S). The dataset integrates diagnostics based on 500 hPa geopotential height and vertically integrated potential vorticity within a unified framework for spatial filtering, event definition, and temporal tracking. It provides instantaneous blocking masks, spatiotemporally tracked event catalogs, time series, and aggregated climatologies that are directly comparable across methods. The results reveal only weak large-scale similarities in Antarctic blocking across detection approaches, mainly related to high-latitude occurrence and seasonal modulation. In contrast, pronounced method-dependent diversity is evident in blocking frequency, spatial extent, the number of detected blocking events, and persistence. Geopotential height-based methods identify a broader spectrum of anticyclonic flow regimes, including events extending into the Antarctic interior, whereas potential vorticity-based methods isolate fewer, more spatially confined events that emphasize dynamically coherent upper-level disturbances near the polar vortex. Event-based diagnostics further reveal systematic trade-offs between event frequency and duration, illustrating how different methodological choices preferentially capture either shorter-lived circulation anomalies or more persistent blocking structures. These contrasts arise from the diverse dynamical expressions of blocking at high southern latitudes, indicating that no single diagnostic fully captures Antarctic blocking behavior. Key uncertainties relate to threshold sensitivity, spatial filtering, and diagnostic formulation, which should be considered when interpreting blocking statistics and inter-method differences. By providing a consistent and openly accessible resource, archived on Zenodo at https://doi.org/10.5281/zenodo.18329806 (Bozkurt et al., 2026), this dataset allows direct intercomparison of blocking definitions, supports evaluation of climate models over Antarctica, and provides a foundation for future studies of blocking-related circulation variability and extreme events.

Atmospheric rivers (ARs) represent the main intrusions of moisture and heat into Antarctica, exerting a major influence on the continent's surface mass balance. Yet, due to geometric and directional constraints, existing detection algorithms often fail to track their evolution inland after landfall or in regions where abrupt directional changes occur. We introduce DARK (Detecting ARs using their Kurvature), a new Antarctic AR detection framework designed to overcome these limitations. DARK applies a strict 98th-percentile threshold to total integrated vapor transport and computes AR length along the curved axis to evaluate the 2000 km AR criterion. This enables the continuous detection of ARs with complex geometries, including those that curve, overturn, or extend across the South Pole. An additional AR-children module identifies smaller but still intense moisture remnants that detach from parent ARs after landfall yet continue to transport vapor and heat inland. The resulting climatology shows that DARK ARs account for about 18 % of total Antarctic precipitation and are linked to roughly half of top 1 % daily precipitation anomalies, 60 % of top 1 % daily maximum temperature anomalies, and 80 % of compound warm-and-wet events. DARK provides a more detailed assessment of AR-related precipitation and temperature impacts in the South Pole region. Despite slightly higher occurrence, risk ratio analysis shows that DARK ARs more effectively capture the most intense events than earlier Antarctic schemes. Including AR-children further strengthens these associations, especially over Victoria Land, where they contribute to about one third of AR-related precipitation.

Regional Climate Models (RCMs) are widely used to provide higher resolution climate information by improving the representation of surface heterogeneity, mesoscale processes and orographic effects. However, their performance remains sensitive to domain configuration, the choice of driving GCM, and structural differences among RCMs. This study analyzes five GCM–RCM combinations common to both the Africa (AFR) and Middle East and North Africa (MENA) CORDEX domains using simulations from RegCM4 and RCA4 to quantify the relative contributions of domain, GCMs and RCMs, to precipitation uncertainty (during the historical period 1981–2005 and by 2070–2100 under the RCP8.5 scenario). Both RCMs and GCMs consistently reproduce the observed large-scale seasonal rainfall patterns, with a summertime (JJAS) maximum over Sudan and western-central Ethiopia, and bimodal rainfall regimes (MAM and OND rains) across the Horn of Africa and parts of the Arabian Peninsula. RCMs generally reduce major seasonal biases present in the GCMs but unevenly improve the representation of topographically driven precipitation. Orographic effects strongly influence RCM biases, with RCA4 showing a pronounced increase of overestimation with elevation, while RegCM4 exhibits a weaker altitude dependence. Uncertainty analysis shows that GCMs dominate precipitation variability (48.1%), followed by RCMs (26.4%) and domain contributions (25.5%), with strong regional contrasts linked to topography, coastal influences, and lateral boundary conditions. Future projections confirm the persistence of this behaviour, with GCM forcing governing the large-scale response, while RCM and domain effects introduce secondary but spatially structured modifications consistent with the historical period. This finding suggests that adaptation decisions will need to be made in the context of high uncertainty concerning regional changes in precipitation.

The Southern Annular Mode (SAM) represents the dominant pattern of climate variability in the extratropical Southern Hemisphere. However, its signature in coastal East Antarctic ice core records is unclear. A daily synoptic typing data set for the southern Indian Ocean constructed using self‐organizing maps on 500 hPa daily geopotential height anomalies was used to investigate the relationship between the SAM and ice core records from Law Dome and Mount Brown South. Results indicate that synoptic weather regimes can either enhance or suppress the signature of the SAM on surface weather and ice core records. Weather regimes that are more likely during negative SAM and are associated with positive temperature anomalies over East Antarctica support the observed relationship between the SAM and the Law Dome δ ¹⁸ O record since 1979. In contrast, weather regimes that describe meridional moisture flux toward Antarctica explain more snowfall accumulation variability at the ice core sites compared to the SAM, which contributes to suppressing the SAM signal in snowfall accumulation records. Results highlight the importance of considering the synoptic‐scale dynamics on the relationship and stationarity of the SAM influence on surface weather and interpretation of ice core records. Given the limited robust relationships between the SAM and the ice core records investigated, caution should be taken before using these records to reconstruct past SAM variability. Instead, the coastal East Antarctic ice core records provide insight into the occurrence of meridional weather regimes, which are often associated with extreme temperature and precipitation conditions.

Atmospheric rivers (ARs) represent the main intrusions of moisture and heat into Antarctica, exerting a major influence on the continent’s surface mass balance. Yet, due to geometric and directional constraints, existing detection algorithms often fail to track their evolution inland after landfall or in regions where abrupt directional changes occur. We introduce DARK (Detecting ARs using their Kurvature), a new Antarctic AR detection framework designed to overcome these limitations. DARK applies a strict 98th-percentile threshold to total integrated vapor transport and computes AR length along the curved axis to evaluate the 2000-km AR criterion. This enables the continuous detection of ARs with complex geometries, including those that curve, overturn, or extend across the South Pole. An additional AR-children module identifies smaller but still intense moisture remnants that detach from parent ARs after landfall yet continue to transport vapor and heat inland. The resulting climatology shows that DARK ARs account for about 18 % of total Antarctic precipitation and are linked to roughly half of top 1 % daily precipitation anomalies, 60 % of top 1 % daily maximum temperature anomalies, and 80 % of compound warm-and-wet events. DARK provides a more detailed assessment of AR-related precipitation and temperature impacts in the South Pole region. Despite slightly higher occurrence, risk ratio analysis shows that DARK ARs more effectively capture the most intense events than earlier Antarctic schemes. Including AR-children further strengthens these associations, especially over Victoria Land, where they contribute to about one third of AR-related precipitation.

Karst aquifers play a crucial role in global water resources. Characterizing their thermal dynamics is essential for improving our understanding of their functioning. However, monitoring karst systems is challenging due to their strong heterogeneity, anisotropy, and limited accessibility. To address these challenges, we deployed an 800‐m‐long fiber‐optic cable within the unsaturated zone of a karst system in the Jura Mountains (eastern France). The objective was to investigate the spatial and temporal variability of cavity air and underground river water temperatures over a 6‐month monitoring period. The results highlight a gradual increase in cavity air temperature (CAT) and cavity river temperature (CRT) over the study period, consistent with the diffusion of surface air temperature (SAT). Superimposed on this long‐term influence, short‐term temperature fluctuations linked to precipitation events reveal the role of advective heat transfer associated with rapid infiltration and mixing in the main conduit. Temperature and electrical conductivity contrasts between the underground river and a lateral tributary indicate that multiple reservoirs contribute to aquifer recharge.

Atmospheric blocking is a key driver of persistent circulation anomalies and associated extreme events in the Southern Hemisphere, yet its characteristics around Antarctica remain poorly understood due to methodological diversity and the absence of a consolidated, long-term dataset. This contribution investigates how methodological choices in blocking detection influence the inferred characteristics of Antarctic blocking and discusses the implications for large-scale circulation variability and climate extremes. Using ERA5 reanalysis for the period 1979 to 2024, we apply several established blocking diagnostics based on geopotential height and potential vorticity within a unified spatiotemporal framework. By standardising filtering, event identification, tracking, and aggregation procedures, we isolate differences that arise specifically from the diagnostic formulation rather than from implementation details. The comparison reveals substantial method dependent variability in blocking frequency, spatial extent, persistence, and intensity, particularly at high southern latitudes where circulation regimes differ from classical midlatitude blocking. Geopotential height based diagnostics identify a broader range of quasi stationary anticyclonic anomalies, including events extending toward the Antarctic continent, while potential vorticity based diagnostics isolate fewer and more spatially confined events associated with dynamically coherent upper level disturbances near the polar vortex. These methodological contrasts have direct implications for how blocking related climate extremes are interpreted, including links to temperature anomalies, moisture intrusions, and surface melt episodes. Differences in diagnosed event duration and location can substantially alter the attribution of extreme conditions to blocking regimes. Ongoing work examines how blocking characteristics identified by different diagnostics relate to variability in large scale circulation modes such as the Southern Annular Mode and ENSO, highlighting the importance of methodological awareness when assessing teleconnections and long term variability. Overall, the results demonstrate that Antarctic atmospheric blocking cannot be fully characterised by a single diagnostic perspective and that method dependence must be explicitly considered in studies of polar circulation variability, climate extremes, and future change.