Atmospheric rivers (ARs) play a major role in transporting heat and moisture into the Arctic, yet their thermodynamic structure and regional impacts remain poorly understood. Here, we adopt a combined Eulerian-Lagrangian framework to investigate two intense ARs that penetrated into the central Arctic within one week in April 2020 during the MOSAiC field campaign. This study provides a comprehensive view of their large-scale dynamics, moisture sources, and thermodynamic evolution.The first AR entered the Arctic via the Siberian sector, driven by a highly anomalous quasi-stationary anticyclone over north-central Siberia. The second followed an Atlantic pathway and was associated with an unusually deep and persistent cyclone over Baffin Bay. Despite their distinct origins and pathways, both events produced extreme surface impacts, including widespread warming across Eurasia exceeding 9 degrees C over a 7 d period and intense precipitation along the Greenland coast and in the central Arctic. The events coincided with a notable decline in sea ice extent along eastern Greenland and in the Barents-Kara Sea, that is highly correlated with the AR-induced warming and rainfall.Backward trajectory analysis of parcels associated with extreme Arctic precipitation reveals distinct pathways and thermodynamic evolution. During both AR events, a subset of air parcels exhibiting classic AR characteristics is identified. These warm, moist, low-pressure airmasses ascend upon arrival and release intense precipitation. Moisture sources, however, differed by pathway: the Atlantic AR drew from the warm Gulf Stream region, while the Eurasian AR was fed by continental Eurasia. These findings highlight the diverse origins and mechanisms of ARs and their capacity to drive rapid Arctic climate and cryospheric changes.

Abstract. Accurately representing polar stratospheric clouds (PSCs) in global Chemistry-Climate Models and Earth System Models is important as they play a key role in springtime ozone depletion in polar regions by activating both chlorine and bromine species through heterogeneous reactions and denitrifying the stratosphere. Here, we present and evaluate an updated PSC parameterisation scheme implemented in the UK Earth System Model (UKESM1.1). The scheme includes the kinetic formation of nitric acid trihydrate (NAT) particles, the formation of supercooled ternary solution (STS) droplets assuming thermodynamic equilibrium, and accounts for ice and sulphate aerosols. To evaluate the new scheme, we compare modelled PSC production with satellite observations from the Cloud-Aerosol and Lidar with Orthogonal Polarization (CALIOP) instrument and model concentrations of gas-phase nitric acid and ozone concentrations with observations from the Aura Microwave Limb Sounder (MLS) instrument for the 2008 Antarctic and 2009/2010 Arctic winters. In comparison with the current, simpler scheme, the updated parameterisation increases both the range of PSC types that form and the seasonal variability, resulting in better agreement with CALIOP observations. It also slows the growth of NAT particles and enables nitric acid partitioning between gas, liquid, and solid phases, leading to improved agreement with MLS observations. However, there is comparatively little impact on stratospheric ozone, with the exception near the edge of the polar vortex where the new scheme improves comparisons with MLS observations.

In recent years a number of record-breaking, even record shattering, extreme weather and climate events have occurred over Antarctica. Such events can drive increased surface melt, thinning and even break-up of Antarctica’s ice shelves. They also pose threats to Antarctic species, ecosystems and the globally important services they provide. However, our knowledge and understanding of how extreme events over Antarctica may respond under climate forcing is lacking. To addresses this gap, the ExtAnt project is an ambitious four-year programme of research that brings together leading UK and international scientists to use new modelling resources and methods to elucidate drivers of extreme events in Antarctica. It aims to provide a comprehensive assessment of present day and future high impact extreme weather events in Antarctica, and associated risks. Key foci for impacts are surface melt on ice shelves and the highly specialised Antarctic biodiversity.Recent science highlights will be presented on characteristics and drivers of extreme events and a new database of Antarctic extremes. An example of current early initial analysis relates to large ensembles, which shows that global climate models exhibit larger biases in mid-tropospheric daily meridional wind extremes at 65°S in summer (too weak) than in winter, in contrast to larger winter biases in the mean climatology. There is a fairly small, but clear, increase in the magnitude of meridional wind extremes in summer in the ozone hole period compared with the pre-ozone period. Wider implications the results so far will be discussed along with future plans for the project in downscaling (using both machine learning and traditional approaches), event attribution and surface melt modelling.

The Arctic is warming rapidly, with atmospheric rivers (ARs) amplifying ice melt, extreme precipitation, and abrupt temperature shifts. Detecting ARs in the Arctic remains challenging, because AR detection algorithms designed for mid-latitudes perform poorly in polar regions. This study introduces a regional deep learning (DL) image segmentation model for Arctic AR detection, leveraging large-ensemble (LE) climate simulations. We analyse historical simulations from the Climate Change in the Arctic and North Atlantic Region and Impacts on the UK (CANARI) project, which provides a large, internally consistent sample of AR events at 6-hourly resolution and enables a close comparison of AR climatology across model and reanalysis data. A polar-specific, rule-based AR detection algorithm was adapted to label ARs in simulated data using multiple thresholds, providing training data for the segmentation model and supporting sensitivity analyses. U-Net-based models are trained on integrated water vapour transport, total column water vapour, and 850 hPa wind speed fields. We quantify how AR identification depends on threshold choices in the rule-based algorithm and show how these propagate to the U-Net-based models. This study represents the first use of the CANARI-LE for Arctic AR detection and introduces a unified framework combining rule-based and DL methods to evaluate model sensitivity and detection robustness. Our results demonstrate that DL segmentation achieves robust skill and eliminates the need for threshold tuning, providing a consistent and transferable framework for detecting Arctic ARs. This unified approach advances high-latitude moisture transport assessment and supports improved evaluation of Arctic extremes under climate change.

Synoptic weather systems play a crucial role in transporting moisture to Antarctica. Climate models project significant changes in these systems, including a wintertime intensification and a summertime poleward shift, with implications for Antarctic ice mass balance. Our analysis of CMIP6 model output shows synoptic moisture fluxes across the Antarctic Circle increasing by 2-6% per decade under high-emission scenarios, accounting for 24% of winter and 93% of summer total moisture transport trends. This increase is mainly associated with enhanced eddy moisture anomalies rather than stronger eddy wind anomalies that are often used to gauge storm track activity. Eddy-driven moisture variability also accounts for a substantial fraction of inter-model uncertainty in future projections. Furthermore, using a large-ensemble approach, we show that differences between reanalysis and multi-model mean externally forced trends could possibly be due to natural climatic variability, while potential model biases cannot be excluded.

In recent decades, the Arctic has warmed nearly four times faster than the global average, undergoing profound changes as a result. A key factor in this accelerated warming is the meridional transport of atmospheric water vapour. Particularly, intense intrusions of moisture and heat, so-called atmospheric rivers (ARs), are rare phenomena to reach the high latitudes, but can have severe impacts on the Arctic environment.In this study, we examine an AR pair in April 2020 using a combination of Eulerian and Lagrangian methods alongside observational data from Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition. The event consisted of two distinct ARs that followed separate pathways - one across Siberia and the other across the Atlantic - before converging in the central Arctic within the span of one week. Large-scale atmospheric circulation patterns associated with these ARs show a combination of low and high pressure systems on the flanks of the ARs, channelling moisture and heat northward. Notably, our results show that the Siberian AR was linked to extreme heat anomalies, whereas the Atlantic AR primarily transported abundant moisture.Backward air parcel trajectories calculated using LAGRANTO provide new insights into the complex dynamics of Arctic ARs, revealing details of their distinct pathways and moisture source regions. Analysis of these trajectories also uncovers a strong connection between the observed sea ice melt in the Barents-Kara Sea and the interaction of an AR with the ice edge, underscoring the significant influence of ARs on the Arctic climate system.

Significant changes have occurred during the last few decades across the North Atlantic climate system, including in the atmosphere, ocean, and cryosphere. These large-scale changes play a vital role in shaping regional climate and extreme weather events across the UK and Western Europe. This review synthesizes the characteristics of observed large-scale changes in North Atlantic atmospheric and oceanic circulations during past decades, identifies the drivers and physical processes responsible for these changes, outlines projected changes due to anthropogenic warming, and discusses the predictability of these circulations. On multi-decadal time scales, internal variability, anthropogenic forcings (especially greenhouse gases), and natural forcings (such as solar variability and volcanic eruptions) are identified as key contributors to large-scale variability in North Atlantic atmospheric and oceanic circulations. However, there remain many uncertainties regarding the detailed characteristics of these various influences, and in some cases their relative importance. We therefore conclude that a better understanding of these drivers, and more accurate quantification of their relative roles, are crucial for more reliable decadal predictions and projections of regional climate for the North Atlantic and Europe.

Near‐surface marine winds in coastal Antarctica have global importance, as they affect ocean circulation and sea‐ice variability. We test the sensitivities of simulated near‐surface winds and wind stress in coastal Antarctica to uncertain aspects of regional atmospheric model configuration. The UK Met Office Unified Model (MetUM) is run in a limited‐area setup over 10 months, evenly split between austral summer and winter. Tests include varying horizontal grid spacing, stable boundary‐layer representation, surface exchange of momentum, and subgrid orographic drag. We focus especially on winds over the Cape Darnley polynya, which is important for Antarctic Bottom Water formation. Output from the MetUM correlates well with low‐level winds from sonde, station, and satellite observations, especially in the summer months. However, the shape of the vertical profile of wind speed depends strongly on model configuration. Enabling the subgrid orography scheme and enhancing the scale of subgrid mountains leads to major reductions in near‐surface wind speed over the steep coastal slopes and over the Cape Darnley polynya, which we attribute to an upslope shift in the extent of katabatic flow. Near‐surface winds and wind stress over near‐shore ocean regions are also highly sensitive to the configuration of sea‐ice roughness, which disproportionately affects strong winds, again leading to especially large impacts over Cape Darnley. Stable boundary‐layer representation has a moderate impact on boundary‐layer winds but a relatively small effect at the surface. Varying horizontal grid spacing between 4, 12, and 40 km has a relatively minor impact on winds, even though the steepness of the coastal terrain is greatly affected. The results underscore the sensitivity of Antarctic marine coastal winds to some highly model‐dependent aspects of atmospheric model physics, with implications for simulation of atmosphere–ocean coupling, sea‐ice dynamics, and Antarctic Bottom Water production.

Polar stratospheric clouds (PSCs) play a fundamental role in depleting stratospheric ozone. Heterogeneous reactions on their surfaces increase the concentration of active chlorine, which can catalytically destroy ozone and prolong ozone depletion by denitrifying and dehydrating the stratosphere. However, parametrisations of PSC formation is poorly included in global chemistry-climate models due to the complexity of the microphysical processes involved in PSC particle formation. This limits our ability to project the future recovery of the stratospheric ozone and the resulting climate impacts.In this work, the representation of PSCs in the UK Earth System Model (UKESM) has been improved by refining the particle formation schemes to 1) kinetically determine the growth of nitric acid trihydrate particles rather than using a thermodynamic assumption, and 2) include the growth of supercooled ternary solution particles through the uptake of nitric acid rather than using a sulphate aerosol climatology. To validate these changes, the simulated PSCs are converted into optical properties and evaluated against satellite data. Here, a comparison of the results from the new PSC scheme in the UKESM with the observations from the satellite-borne Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) is presented. Whether the changes to the formation parameterisation improve the model’s ability to accurately simulate PSCs both temporally and spatially in the Northern and Southern hemisphere is assessed and the effect on stratospheric denitrification in the model is examined.

Antarctic coastal surface winds affect ice-sheet stability, sea ice, and local ecosystems. The strongest coastal winds are especially important due to the nonlinear relationship between wind speed and wind stress. We investigate the dynamics of extreme coastal winds using a simplified momentum budget calculated across the period 2010-2020 from the ERA5 reanalysis. The pressure-gradient forcing term in the budget is decomposed into a large-scale component and one associated with the temperature deficit layer. The role of budget terms across the coastal sector is compared for weak and strong winds. We then calculate composites of the top 100 easterly wind events across six east Antarctic coastal sectors, identifying terms responsible for the evolution of coastal extremes. A simple balance of terms exists offshore, dominated by large-scale forcing, contrasting with the complex balance in the onshore sector where katabatic forcing is large. Large-scale forcing explains 57% of offshore coastal wind-speed variance overall, improving to 81% when budget terms associated with the temperature deficit layer and horizontal advection are included, with significant regional variation. The residual term plays an increasingly active role as wind speed increases. Extremes in all coastal sectors are associated with a synoptic-scale transient dipole of pressure anomalies driving warm-air advection. Although katabatic forcing is a very large term in magnitude, it is found to play a passive role, declining as wind speeds increase during extreme conditions. In some regions, an anomalous southerly component develops during extremes, which we attribute to an ageostrophic barrier wind. This research underscores the major role for large-scale forcing in Antarctica's coastal winds, but also reveals a significant regional locally driven component. The results have implications for improving numerical model simulations of coastal easterlies and for studying their impacts on ocean circulation, sea ice, and ice-shelf basal melt. The strongest winds in coastal Antarctica are important for ocean circulation and sea ice, but the role of local processes such as katabatic winds remains uncertain. We decompose the ERA5 momentum budget to show that composite-mean extreme wind events in coastal Antarctica are predominantly associated with large-scale forcing, indicated by the dark orange line. To explain short-time-scale variability more generally, other terms accounting for pooling of cold air and katabatic forcing are critical, especially for more moderate winds. image

In this study, available large ensemble datasets in the Coupled Model Intercomparison Phase 6 (CMIP6) archive were used to provide the first multi-variate overview of the evolution of extreme seasons over Antarctica and the Southern Ocean during the 20th and 21st centuries following medium-to-high radiative forcing scenarios. The results show significant differences between simulated changes in background mean climate and changes in low (10th percentile) and high (90th percentile) extreme seasons. Regional winter warming is most pronounced for cold extremes. In summer, there are more pronounced increases in high extremes in precipitation and westerly wind during the ozone hole formation period (late 20th century), affecting coastal regions and, in particular, the Antarctic Peninsula. At midlatitudes, there is a reduction in the range of summer season wind extremes. Suggested mechanisms for these differences are provided relating to sea ice retreat and westerly jet position.

AbstractImprovement of subseasonal to seasonal North Atlantic winter forecasting requires better prediction of the North Atlantic Oscillation (NAO), the dominant mode of variability in the Northern Hemisphere. Despite recent research demonstrating the importance of stratosphere‐troposphere coupling for NAO predictability, the driving mechanisms and implications are not fully understood. This study reveals that the October upper stratosphere is highly relevant to polar vortex development and predictability of winter NAO. We derive a simple index based on the strength of meridional wind in the upper stratospheric surf zone and find that anomalously poleward motion is associated with a significantly stronger polar vortex, which predicts the subsequent winter surface NAO with a correlation coefficient of r = 0.40.

Meteorological records at Signy Station in the South Orkney Islands (SOIs) have recently been digitized to cover the period of 1947–1995. This study compares the newly available near‐surface air temperatures at Signy with those from a nearby station, Orcadas and with reanalysis datasets to provide a more comprehensive picture of the weather and climate variability in the SOIs. Temperatures from both stations show a higher degree of variability in winter than summer, but the variability differs in terms of its relationship to the dominant wind directions and sea ice influences. The two stations differ markedly in terms of their respective warm temperature events, largely due to orography‐induced föhn winds at Signy Station as northwesterlies flow over Coronation Island. ERA5 reproduces the monthly to annual averages exceedingly well but underestimates both cold and warm tails of station temperatures. Temperature trends in the SOIs are also considered in terms of changes in large‐scale circulation and the sea surface temperature over the Brazil‐Falkland Confluence. However, caution is required in interpreting the long‐term temperature trends estimated from reanalysis data as most of the reanalyses show a cold bias before 1979, which is most likely caused by misrepresentation of the sea ice.

Extreme warm events in the South Orkney Islands (SOIs) are investigated using synoptic observations from Signy and Orcadas stations for 1947–1994 and 1956–2019 respectively. Defining the extremes as temperatures exceeding the 95th percentile of the temperature distribution, we reveal the characteristics and associated drivers of the warm events, especially the top 10 events in both summer and winter. At both stations, extreme warm events often involve a combined effect of atmospheric rivers (ARs) and localised föhn warming, with distinct characteristics due to the station locations relative to Coronation Island, the largest and highest island of the SOIs. For example, warm events at Signy are warmer (by an average of around 3°C) than the corresponding concurrent temperatures at Orcadas. The number of warm events per year has significantly increased over the record periods at both stations, which could potentially impact ecosystems by increasing melting of snow and ice. Extreme warm events at Signy are dominated by föhn warming in combination with ARs originating from the Southern Atlantic Ocean, where warm, moisture‐rich air is rapidly advected towards the islands by enhanced northerly winds. By contrast, the Orcadas warm extremes involve both warm‐air advection and föhn warming associated with enhanced northwesterlies/westerlies with ARs originating in the Pacific Ocean that travel across the Drake Passage. Simulation of one of the top 10 warm events for Signy station using a 1‐km grid spacing configuration of the atmosphere‐only UK Met Office Unified Model is used to disentangle the role of local versus large‐scale forcing. We find that the majority of the warming can be attributed to föhn effects for the case study. These results demonstrate the complexity of Antarctic temperature extremes.

Climate model biases in the North Atlantic (NA) low-level tropospheric westerly jet are a major impediment to reliably representing variability of the NA climate system and its wider influence, in particular over western Europe. A major aspect of the biases is the occurrence of a prominent early-winter equatorward jet bias in Coupled Model Inter-comparison Project Phase 5 (CMIP5) models that has implications for NA atmosphere-ocean coupling. Here we assess whether this bias is reduced in the new CMIP6 models and assess implications for model representation of NA atmosphere-ocean linkages, in particular over the sub-polar gyre (SPG) region. Historical simulations from the CMIP5 and CMIP6 model datasets were compared against reanalysis data over the period 1861–2005. The results show that the early-winter equatorward bias remains present in CMIP6 models, although with an approximately one-fifth reduction compared to CMIP5. The equatorward bias is mainly associated with a weaker-than-observed frequency of poleward excursions of the jet to its northern position. A potential explanation is provided through the identification of a strong link between NA jet latitude bias and systematically too-weak model-simulated low-level baroclinicity over eastern North America in early-winter. CMIP models with larger equatorward jet biases exhibit weaker correlations between temporal variability in speed of the jet and sea surface conditions (sea surface temperatures and turbulent heat fluxes) over the SPG. The results imply that the early-winter equatorward bias in jet latitude in CMIP models could partially explain other known biases, such as the weaker-than-observed seasonal-decadal predictability of the NA climate system.

The influence of the Semi‐Annual Oscillation (SAO) on the timing and evolution of major sudden stratospheric warmings (SSWs) is examined using the 2008/2009 SSW as the primary case‐study. When the zonal winds in both the troposphere and the SAO region of the equatorial upper stratosphere/lower mesosphere are relaxed towards reanalysis fields in the UK Met Office Unified Model, a remarkably accurate representation of the January 2009 SSW is achieved. The accurate timing of the SSW is determined by the SAO zonal wind relaxation. The westerly‐to‐easterly phase transition of the SAO in the lower mesosphere (0.1–0.5 hPa) is found to be a key factor for this influence. It defines an initial conical‐shaped vortex that determines the upward propagation of wave activity and subsequent evolution of wave mean‐flow interaction. Internal transient wave reflection in the subtropics and associated wave‐induced acceleration of the mean‐flow is found to be an important component, strengthening the vortex and thus delaying the onset of the SSW. The sensitivity of SSW timing to the equatorial westerly winds in the lower mesosphere is further explored in the context of all major SSWs during the 1979–2018 period. The timing of SSWs is found to be significantly correlated with the timing of the equinoctial westerly‐to‐easterly phase transition at 0.3 hPa in early winter (r = 0.79). This relationship is discussed in the context of the more widely recognised influence of the quasi‐biennial oscillation (QBO). These results suggest that accurate simulation of the timing of SAO phase transitions, as well as knowledge of the QBO phase, is likely to provide additional and extended Northern Hemisphere wintertime seasonal forecast skill.

Climate model biases in the North Atlantic (NA) low-level tropospheric westerly jet are a major impediment to reliably representing variability of the NA climate system and its wider influence, in particular over western Europe. We highlight an early-winter equatorward jet bias in Coupled Model Inter-comparison Project (CMIP) models and assess whether this bias is reduced in the CMIP6 models in comparison to the CMIP5 models. Historical simulations from the CMIP5 and CMIP6 are further compared against reanalysis data over the period 1862-2005. The results show that an equatorward bias remains significant in CMIP6 models in early winter. Almost all CMIP5 and CMIP6 model realizations exhibit equatorward climatological jet latitude biases with ensemble mean biases of 3.0° (November) and 3.0° (December) for CMIP5 and 2.5° and 2.2° for CMIP6. This represents an approximately one-fifth reduction for CMIP6 compared to CMIP5. The equatorward jet latitude bias is mainly associated with a weaker-than-observed frequency of poleward daily-weekly excursions of the jet to its northern position. A potential explanation is provided. Our results indicate a strong link between NA jet latitude bias and systematically too-weak model-simulated low-level baroclinicity over eastern North America in early-winter. Implications for model representation of NA atmosphere-ocean linkages will be presented. In particular CMIP models with larger equatorward jet biases tend to exhibit weaker correlations between temporal variability in jet speed and sea surface conditions over the NA sub-polar gyre (SPG). This has implications for the ability of climate models to represent key aspects of atmospheric variability and predictability that are associated with atmosphere-ocean interactions in the SPG region.

Not only the climate in the troposphere is changing, also at higher altitudes long-term trends have been observed. The global mean middle and upper atmosphere have been cooling, resulting in atmospheric contraction and a decline in thermosphere density at fixed height, mainly driven by the increase in atmospheric CO2 concentration. The secular variation of the Earth’s magnetic field is an additional driver of long-term change in the upper atmosphere, causing trends that are strongly location-dependent. While magnetic field changes are most important for the ionosphere, they also affect the temperature and wind structure throughout the thermosphere, mainly via changes in the strength and geographic distribution of Joule heating. Simulations with the Whole Atmosphere Community Climate Model eXtension (WACCM-X) suggest that perturbations induced by magnetic field changes in the lower thermosphere climate can further propagate downward via vertical dynamical coupling. Our results show a significant response in the zonal mean temperature and zonal wind in the Southern Hemisphere (SH) middle- to high-latitude troposphere, stratosphere, and mesosphere of up to ±2 K and ±2 m/s, as well as regionally significant changes in Northern Hemisphere (NH) polar surface temperatures of up to ±1.3 K, in December-January-February. In the SH, changes in gravity wave filtering in the thermosphere induce a change in the residual circulation that extends down into the upper mesosphere, where further changes in the zonal mean wind climatology are generated, together with changes in local planetary wave generation and/or amplification and gravity wave filtering. This induces an anomalous residual circulation that extends down into the troposphere. The NH middle atmosphere response is zonally asymmetric, consisting of a significant change in the positioning and shape of the upper stratospheric polar vortex, which is dynamically consistent with the surface temperature response. While the details of the lower and middle atmosphere responses may not be entirely accurate due to model limitations, the results from our simulations do indicate that dynamical coupling within the atmosphere can conceivably result in upper atmosphere processes having a significant effect on surface climate.

High surface temperatures are important in Antarctica because of their role in ice melt and sea level rise. We investigate a high temperature event in December 1989 that gave record temperatures in coastal East Antarctica between 60° and 100°E. The high temperatures were associated with a pool of warm lower tropospheric air with December temperature anomalies of >14°C that developed in two stages over the Amery Ice Shelf. First, there was near‐record poleward warm advection within an atmospheric river. Second, synoptically driven downslope flow from the interior reached unprecedented December strength over a large area, leading to strong descent and further warming in the coastal region. The coastal easterly winds were unusually deep and strong, and the warm pool was advected westwards, giving a short period of high temperatures at coastal locations, including a surface temperature of 9.3°C at Mawson, the second highest in its 66‐year record.