Atlantic Jet Stream Research Articles

Persistent Summer North Atlantic Jet Variability: Dynamical Feedbacks and Model‐Observation Discrepancies

AbstractPersistent fluctuations in the latitudinal position of the North Atlantic (NATL) jet stream are closely linked to extreme weather anomalies, particularly over Europe. Accurately representing jet stream persistence in climate models is essential for enhancing the reliability of future regional extreme weather projections. This study investigates the summer persistence of the NATL jet stream latitudinal fluctuations and the Summer North Atlantic Oscillation (SNAO) using data from CMIP6 models and ERA5 reanalysis. Our findings reveal that CMIP6 models generally overestimate persistence compared to ERA5 during the historical period. Utilizing the relative vorticity tendency equation, we assess the strength of the eddy‐mean flow feedback and identify a positive relationship between persistence and feedback strength across models. We also found that a large fraction of the intermodel spread in precipitation persistence is associated with differences in the persistence of jet fluctuations.

Impact of Arctic Stratospheric Polar Vortex on Mediterranean Precipitation

Abstract The Mediterranean precipitation has significant implications for the local ecology and human livelihoods. This study explores the impact of the Arctic stratospheric polar vortex on the Mediterranean precipitation during the wet season (winter) using reanalysis data and large-scale ensemble experiments that are forced by anomalous stratospheric polar vortex states. Our findings reveal that total and extreme precipitation over the Mediterranean region are increased under weak polar vortex (WPV) conditions and decreased under strong polar vortex (SPV) conditions. Moisture budget suggests that the changes in vertical motion over the Mediterranean play the dominant role in precipitation changes. It is further found that the tropospheric quasi-stationary wave train propagating from Greenland to the European continent is enhanced under WPV conditions, accompanied by warming advection over the eastern Mediterranean and vorticity advection in the upper troposphere over the whole region of the Mediterranean. Consequently, the vertical motion and precipitation there are enhanced. The southward shift of the Atlantic jet stream under WPV conditions leads to enhanced moisture transport toward the Mediterranean, providing a favorable moisture source for precipitation. In addition, the background baroclinicity in the lower troposphere around the northern Mediterranean is enhanced under WPV conditions, intensifying frontogenesis and cyclonic activities, which are commonly accompanied by intensified precipitation. These results are consistent with earlier literature and suggest that the changes in the stratospheric polar vortex may be used as a potential predicting factor for Mediterranean precipitation.

Mid-Holocene climate at mid-latitudes: assessing the impact of Saharan greening

Abstract. During the first half of the Holocene (11 000 to 5000 years ago), the Northern Hemisphere experienced a strengthening of the monsoonal regime, with climate reconstructions robustly suggesting a greening of the Sahara region. Palaeoclimate archives also show that this so-called African humid period (AHP) was accompanied by changes in climate conditions at middle to high latitudes. However, inconsistencies still exist in reconstructions of the mid-Holocene (MH) climate at mid-latitudes, and model simulations provide limited support in reducing these discrepancies. In this paper, a set of simulations performed using a climate model are used to investigate the hitherto unexplored impact of Saharan greening on mid-latitude atmospheric circulation during the MH. Numerical simulations show Saharan greening has a year-round impact on the main circulation features in the Northern Hemisphere, especially during boreal summer (when the African monsoon develops). Key findings include a westward shift in the global Walker Circulation, leading to modifications in the North Atlantic jet stream in summer and the North Pacific jet stream in winter. Furthermore, Saharan greening modifies atmospheric synoptic circulation over the North Atlantic, enhancing the effect of orbital forcing on the transition of the North Atlantic Oscillation phase from predominantly positive to negative in winter and summer. Although the prescription of vegetation in the Sahara does not improve the proxy–model agreement, this study provides the first constraint on the influence of Saharan greening on northern mid-latitudes, opening new opportunities for understanding MH climate anomalies in regions such as North America and Eurasia.

Disentangling Forced Trends in the North Atlantic Jet From Natural Variability Using Deep Learning

AbstractRegional weather variability and extremes over Europe are strongly linked to variations in the North Atlantic jet stream, especially during the winter season. Projections of the evolution of the North Atlantic jet are essential for estimating the regional impacts of climate change. Therefore, separating forced trends in the North Atlantic jet from its natural variability is an extremely relevant task. Here, a deep learning based method, the Latent Linear Adjustment Autoencoder (LLAE), is used for this purpose on an ensemble of fully‐coupled climate simulations. The LLAE is based on a variational autoencoder and an additional linear component. The model uses detrended temperature and geopotential to predict the component of the zonal wind associated with natural variability. The residual between this prediction and the original wind field is interpreted as the forced component of the jet. The method is first tested for the geostrophic wind for which the forced trend can be obtained analytically from the difference between geostrophic wind computed from detrended and full geopotential. Despite the large variability of the total trends, the LLAE is shown to be effective in extracting the forced component of the trend for each individual ensemble member in both geostrophic and full wind fields. The LLAE‐derived forced trend shows an increase in the upper‐level zonal wind speed along a southwest–northeast oriented band over the ocean and a jet extension toward Europe. These are common characteristics over different periods and show some similarities to the upper‐level zonal wind speed trend obtained from the ERA5 reanalysis.

Investigating the medium‐range predictability of European heatwave onsets in relation to weather regimes using ensemble reforecasts

AbstractIn this study, the medium‐range predictability of heatwave (HW) onsets in four midlatitude European regions is investigated statistically with the help of ensemble reforecasts for the period 2001–2018. The concept of Euro‐Atlantic weather regimes is adopted to characterise HWs (about 50 in each region) and to study whether forecast skill may depend on the large‐scale dynamical setup. HW onsets over the British Isles and Scandinavia are mainly associated with Scandinavian and European blocking regimes, whereas the “no regime” case is observed more frequently for Central Europe. Stratified by weather regime, the predictability of heatwave onsets is then studied by means of a multiple metric‐based analysis of European Centre for Medium‐Range Weather Forecasts (ECMWF) and Global Ensemble Forecast System Version 12 (GEFSv12) ensemble reforecasts. For two of the regions considered, Central Europe and the British Isles, a conclusive picture is obtained: medium‐range predictive skill is significantly higher for HW onsets associated with Scandinavian or European blocking compared with cases with no pronounced regime. This skill advantage mostly concerns the large‐scale flow and, to some extent, 850‐hPa temperatures, but is generally not reflected in the correct prediction of near‐surface temperatures. Finally, we investigate for two regions how exceptionally good or poor forecasts are related to the atmospheric state during or shortly after forecast initialisation. At 10 days lead time, poor large‐scale flow predictive skill for Central European HW onsets is linked to anomalously high baroclinicity further upstream and an intensified North Atlantic jet stream, whereas good forecasts on average feature an initial state close to climatology. Forecast skill for near‐surface temperatures is not affected by such dynamical precursors, but rather by pre‐existing soil‐moisture anomalies. For the British region, exceptionally good forecasts of both large‐scale flow and near‐surface temperatures are associated with an already established continental blocking. In contrast to Central Europe, pre‐existing soil‐moisture anomalies play less of a role there.

The seasonal characteristics of English Channel storminess have changed since the 19th Century

Information from a variety of sources has suggested that increased storminess was experienced across the British Isles in the late eighteenth/early nineteenth century. However, it is not clear how stormy that period was relative to current conditions. Using newly recovered barometric pressure data that extend back to 1748 we have constructed a measure of geostrophic wind speed for the English Channel region using a pressure-triangle approach. We show that the 1790−1820s was a period of increased storminess across the region. This storminess extended throughout the year, which is different to comparable increases observed since the 1990s, which were confined to the winter season. While a strengthened North Atlantic jet stream is implicated in both periods, in the earlier period it is likely that the storm track shifted slightly to a more southerly location. We discuss the potential forcing mechanisms responsible for the changes in storminess over this multi-century timeframe.

Strength of the winter North Atlantic jet stream has deviated from its natural trend under anthropogenic warming

Abstract The North Atlantic jet stream (NAJ) has a profound impact on the climate of the North Atlantic–European sector, especially in winter. Observations show that the winter NAJ (NAJw) has strengthened over the past ~140 yr. However, it remains unclear whether this long-term trend has deviated from the natural variability. Here, we present a 2500-yr-long reconstruction of NAJw strength using high-quality stalagmite δ18O records from southeastern Europe. Our results show that the NAJw weakened during both the Roman Warm Period (300 B.C.–A.D. 200) and the Medieval Warm Period (A.D. 900–1250) but that it has strengthened under anthropogenic warming (since A.D. 1850). This indicates that its current trend has already deviated from the natural variability. The best explanation for this present anomalous trend of NAJw strength is that it was triggered by the appearance of the North Atlantic warming hole under anthropogenic forcing. This anomalous trend suggests that continued global warming may further strengthen the NAJw in the future.

Critical role of biomass burning aerosols in enhanced historical Indian Ocean warming

The tropical Indian Ocean (TIO) has experienced enhanced surface warming relative to the tropical mean during the past century, but the underlying mechanisms remain unclear. Here we use single-forcing, large-ensemble coupled model simulations to demonstrate that changes of biomass burning (BMB) aerosols have played a critical role in this TIO relative warming. Although the BMB aerosol changes have little effect on global mean temperatures due to regional cancellation, they significantly influence the pattern of warming over the tropical oceans. The reduction of BMB aerosols over the Indian subcontinent induces a TIO warming, while the increase of BMB aerosols over South America and Africa causes a cooling of the tropical Pacific and Atlantic, respectively. The resultant TIO relative warming leads to prominent global climate changes, including a westward expanded Indo-Pacific warm pool, a fresher TIO due to enhanced rainfall, and an intensified North Atlantic jet stream affecting European hydroclimate.

Subseasonal Prediction of Central European Summer Heatwaves with Linear and Random Forest Machine Learning Models

Abstract Heatwaves are extreme near-surface temperature events that can have substantial impacts on ecosystems and society. Early warning systems help to reduce these impacts by helping communities prepare for hazardous climate-related events. However, state-of-the-art prediction systems can often not make accurate forecasts of heatwaves more than two weeks in advance, which are required for advance warnings. We therefore investigate the potential of statistical and machine learning methods to understand and predict central European summer heatwaves on time scales of several weeks. As a first step, we identify the most important regional atmospheric and surface predictors based on previous studies and supported by a correlation analysis: 2-m air temperature, 500-hPa geopotential, precipitation, and soil moisture in central Europe, as well as Mediterranean and North Atlantic sea surface temperatures, and the North Atlantic jet stream. Based on these predictors, we apply machine learning methods to forecast two targets: summer temperature anomalies and the probability of heatwaves for 1–6 weeks lead time at weekly resolution. For each of these two target variables, we use both a linear and a random forest model. The performance of these statistical models decays with lead time, as expected, but outperforms persistence and climatology at all lead times. For lead times longer than two weeks, our machine learning models compete with the ensemble mean of the European Centre for Medium-Range Weather Forecast’s hindcast system. We thus show that machine learning can help improve subseasonal forecasts of summer temperature anomalies and heatwaves. Significance Statement Heatwaves (prolonged extremely warm temperatures) cause thousands of fatalities worldwide each year. These damaging events are becoming even more severe with climate change. This study aims to improve advance predictions of summer heatwaves in central Europe by using statistical and machine learning methods. Machine learning models are shown to compete with conventional physics-based models for forecasting heatwaves more than two weeks in advance. These early warnings can be used to activate effective and timely response plans targeting vulnerable communities and regions, thereby reducing the damage caused by heatwaves.

Subseasonal reversals of winter surface air temperature in mid-latitude Asia and the roles of westward-shift NAO

Reversals of surface air temperature anomalies over mid-latitude Asia (SATMA) have become more frequent. Six winters have been reversed over the last decade, which had serious negative consequences for agricultural production, ecological systems, and human health. This subseasonal reversal can be observed in the second mode of season-reliant empirical orthogonal function analysis, showing a persistent opposite anomaly between early and late winter. The difference in SATMA between early and late winter exceeds 3 °C when the reversal occurs. At the time of the reversal, the North Atlantic jet stream extends eastward in early winter and recedes westward in late winter. The main influencing system changes from the Scandinavia teleconnection to the Ural blocking high, accompanied by a change from strong southerly to northerly winds. These factors jointly lead to the reversal of SATMA between early and late winter. More importantly, the North Atlantic Oscillation (NAO) has been shown to affect SATMA variability in a variety of different ways. Changes in SATMA are influenced not only by the intensity of the NAO, but also by its spatial distribution. When the southern center of the NAO shifts westward and is located over the North Atlantic, atmospheric circulation and SATMA in early and late winter show similar changes to those seen in SATMA reversal winters. In contrast, when the NAO southern center shifts eastward and is located over Western Europe, persistent anomalies are observed in atmospheric circulation and SATMA throughout the winter. Therefore, when studying the relationship between SATMA and the NAO, attention should be paid to the different responses of SATMA to the spatial distribution of the NAO at subseasonal timescales.

Persistent anomalies of the North Atlantic jet stream and associated surface extremes over Europe

Unusual, persistent configurations of the North Atlantic jet stream affect the weather and climate over Europe. We focus on winter and on intraseasonal and seasonal time scales, and study persistent jet anomalies through the lens of large deviation theory using Coupled Model Intercomparison Project (CMIP6) simulations of the MPI-ESM-LR model and ERA5 reanalysis data. The configurations of interest are defined as long-lasting anomalies of a few months in jet latitude, speed or zonality. Our results show that persistent temperature and precipitation extremes over large European regions are anomalously frequent during the unusual, persistent jet configurations we identify. Furthermore, the relative increase in frequency of surface extremes is larger for more intense surface extremes and/or more extreme jet anomalies. This is relevant in the context of the predictability of these extremes. The highest extreme event frequencies at the surface are observed in case of precipitation over the Mediterranean and Western Europe during anomalously zonal and/or fast jet events, pointing to these jet anomalies matching rather homogeneous large scale atmospheric configurations with a clear surface footprint. Additionally, our results emphasise the usefulness of large deviation rate functions to estimate the frequency of occurrence of persistent jet anomalies. They therefore provide a tool to statistically describe long-lasting anomalies, much like extreme value theory may be used to investigate shorter-lived extreme events.

Case study on the influence of synoptic-scale processes on the paired H2O–O3 distribution in the UTLS across a North Atlantic jet stream

Abstract. During a research flight of the Wave-driven ISentropic Exchange (WISE) campaign, which was conducted over the eastern North Atlantic on 1 October 2017, the composition of the upper troposphere and lower stratosphere (UTLS) across the North Atlantic jet stream was observed by airborne, range-resolved differential absorption lidar (DIAL) profiles. We investigate how the high variability in the paired H2O and O3 distribution along the two-dimensional lidar cross section is affected by synoptic-scale weather systems, as revealed by the Lagrangian history of the observed air masses. To this aim, the lidar observations are combined with 10 d backward trajectories along which meteorological parameters and derived turbulence diagnostics are traced. The transport and mixing characteristics are then projected to the vertical cross sections of the lidar measurements and to the H2O–O3 phase space to explore linkages with the evolution of synoptic-scale weather systems and their interaction. Tropical, midlatitude, and arctic weather systems in the region of the jet stream and the related transport and mixing explain the complex H2O and O3 distribution to a large extent: O3-rich stratospheric air from the high Arctic interacts with midlatitude air from the North Pacific in a northward-deflected jet stream associated with an anticyclone over the US and forms a filament extending into the tropopause fold beneath the jet stream. In the troposphere, lifting related to convection in the intertropical convergence zone (ITCZ) and two tropical cyclones that continuously injected H2O into dry descending air from the tropical Atlantic and Pacific form filamentary H2O structures. One tropical cyclone that transitioned into a midlatitude cyclone lifted moist boundary layer air, explaining the highest tropospheric H2O values. During the two days before the observations, the air with mixed tropospheric and stratospheric characteristics experienced frequent turbulence along the North Atlantic jet stream, indicating a strong influence of turbulence on the formation of the extratropical transition layer (ExTL). This investigation highlights the complexity of stirring and mixing processes and their close connection to interacting tropospheric weather systems from the tropics to the polar regions, which strongly influenced the observed fine-scale H2O and O3 distributions. The identified non-local character of mixing should be kept in mind when interpreting mixing lines in tracer–tracer phase space diagrams.

Decadal variation of the summer extreme high temperature days in northern Eurasia during 1960–2018

Unprecedented heat waves have been demonstrated sweeping across much of the Northern Hemisphere in recent summers. However, this study reveals that for northern Eurasia (30°–70°N, 10°–130°E), significant increases in summer extreme high temperature days (EHTDs) have already commenced since the mid-1990s, with the peaks centered on the surrounding areas of the Caspian Sea and Lake Baikal. Results indicate that compared with the period of 1960–1994 (P1), during 1995–2018 (P2) high-pressure and anticyclonic anomalies occupy the areas around the Caspian Sea and Lake Baikal, thus suppressing local cloud cover and precipitation, enhancing the solar radiation and high-temperature anomalies. The anticyclonic anomaly over Lake Baikal shows a close relationship with the increasing trend of global temperature, and the anomalous anticyclone over the Caspian Sea is under the influence of the warm North Atlantic and anomalous upper-troposphere jet stream during P2. The warmer than normal North Atlantic leads to the high-pressure anomaly over the Caspian Sea by modulating the Rossby wave activity. In addition, during P2, the temperatures in the middle North Atlantic and Western Europe show greater increments than those over higher latitudes, and thus the temperature gradient enhances the westerly thermal wind, which therefore leads to an anomalous upper-level jet stream to the North of the Caspian Sea. Under this circumstance, the Caspian Sea is located to the right side of the anomalous jet exit, inducing the suppressed upward motion over there and favoring the occurrence of more EHTDs after the mid-1990s.

The British–Okhotsk Corridor Pattern and Its Linkage to the Silk Road Pattern

Abstract Based on observation and reanalysis datasets, numerical experiments with a simple dynamical model, and climate model outputs, this study investigates the second leading waveguide teleconnection along the summer polar front jet (PFJ) over Eurasia on the interannual time scale, the British–Okhotsk Corridor (BOC) pattern. The BOC pattern explains 20.8% of the total variance over northern Eurasia, which is only slightly lower than the first leading teleconnection, the British–Baikal Corridor pattern. It consists of four zonally oriented action centers over the British Isles, western Russia, northern Siberia, and the Sea of Okhotsk. It is primarily confined to northern Eurasia and leads to wavelike temperature and precipitation anomalies along its routine. Besides, it is occasionally coupled to the dominant waveguide teleconnection along the subtropical jet (STJ), the Silk Road pattern (SRP). A bifurcated wavelike pattern appears over Eurasia when the coupling is strong, with two branches of waves over the PFJ and the STJ, respectively. The fluctuations of the BOC–SRP linkage play a profound role in shaping the dominant climate variability modes over Eurasia. Numerical experiments with a simple dynamical model suggest that the basic flow cannot directly influence the BOC–SRP linkage by affecting the propagation condition of Rossby waves. Nevertheless, the basic flow can indirectly influence the linkage by changing the exciting locations of the BOC pattern through modulating the wave–mean flow interaction at the exit of the Atlantic jet stream. The climate model INMCM4.0 can reproduce the observed BOC–SRP linkage and its time-varying characteristics, supporting the observation and the proposed mechanism. Significance Statement Over Eurasia, extreme summer weather events are often related to long-lasting stagnant atmospheric circulation anomalies in the upper troposphere, which can be well determined by a few recurring modes called atmospheric teleconnections. Atmospheric teleconnections over Eurasia usually propagate along the subtropical jet (STJ) and the polar front jet (PFJ). Although the teleconnections along the STJ have been well understood, the teleconnections along the PFJ are currently not fully understood. This paper investigates the British–Okhotsk Corridor (BOC) pattern, a newly defined major summer teleconnection along the PFJ. The BOC pattern can be occasionally coupled to a dominant teleconnection along the STJ. Fluctuations of this linkage play a profound role in shaping the dominant surface climate variability modes over Eurasia. Observation and reanalysis datasets, numerical experiments with a simple dynamical model, and climate model output are used to understand this linkage in the study.

Orographic resolution driving the improvements associated with horizontal resolution increase in the Northern Hemisphere winter mid-latitudes

Abstract. In recent years much attention has been devoted to the investigation of the impact of increasing the horizontal resolution of global climate models. In the present work, a set of atmosphere-only idealized sensitivity simulations with EC-Earth3 has been designed to disentangle the relative roles of increasing the resolution of the resolved orography and of the atmospheric grid. Focusing on the Northern Hemisphere winter, it is shown that if the grid is refined while keeping the resolved orography unchanged, model biases are reduced only on some specific occasions. Conversely, increasing the resolved (or mean) orography is found to clearly reduce several important systematic model errors, including synoptic transient eddies, the North Atlantic jet stream variability, and atmospheric blocking frequency and duration. From an analysis of the radiation budget it is concluded that the large changes in radiative fluxes caused by the resolution increase – something commonly observed in climate models – have a relevant impact on the atmospheric circulation, partially offsetting the benefits obtained from the increase in orographic resolution. These findings point to the necessity of always tuning climate models to fully exploit the benefits of high horizontal resolution.

Interactions between the stratospheric polar vortex and Atlantic circulation on seasonal to multi-decadal timescales

Abstract. Variations in the strength of the Northern Hemisphere winter polar stratospheric vortex can influence surface variability in the Atlantic sector. Disruptions of the vortex, known as sudden stratospheric warmings (SSWs), are associated with an equatorward shift and deceleration of the North Atlantic jet stream, negative phases of the North Atlantic Oscillation, and cold snaps over Eurasia and North America. Despite clear influences at the surface on sub-seasonal timescales, how stratospheric vortex variability interacts with ocean circulation on decadal to multi-decadal timescales is less well understood. In this study, we use a 1000 year preindustrial control simulation of the UK Earth System Model to study such interactions, using a wavelet analysis technique to examine non-stationary periodic signals in the vortex and ocean. We find that intervals which exhibit persistent anomalous vortex behaviour lead to oscillatory responses in the Atlantic Meridional Overturning Circulation (AMOC). The origin of these responses appears to be highly non-stationary, with spectral power in vortex variability at periods of 30 and 50 years. In contrast, AMOC variations on longer timescales (near 90-year periods) are found to lead to a vortex response through a pathway involving the equatorial Pacific and quasi-biennial oscillation. Using the relationship between persistent vortex behaviour and the AMOC response established in the model, we use regression analysis to estimate the potential contribution of the 8-year SSW hiatus interval in the 1990s to the recent negative trend in AMOC observations. The result suggests that approximately 30 % of the trend may have been caused by the SSW hiatus.

Self-Organizing Maps Identify Windows of Opportunity for Seasonal European Summer Predictions

We combine a machine learning method and ensemble climate predictions to investigate windows of opportunity for seasonal predictability of European summer climate associated with the North Atlantic jet stream. We particularly focus on the impact of North Atlantic spring sea surface temperatures (SST) on the four dominant atmospheric teleconnections associated with the jet stream: the summer North Atlantic Oscillation (NAO) in positive and negative phases, the Atlantic Ridge (At. Ridge), and Atlantic Low (At. Low). We go beyond standard forecast practices by not only identifying these atmospheric teleconnections and their SST precursors but by making use of these identified precursors in the analysis of a dynamical forecast ensemble. Specifically, we train the neural network-based classifier Self-Organizing Maps (SOM) with ERA-20C reanalysis and combine it with model simulations from the Max Planck Institute Earth System Model in mixed resolution (MPI-ESM-MR). We use two different sets of 30-member hindcast ensembles initialized every May, one for training and evaluation between 1902 and 2008, and one for verification between 1980–2016, respectively. Among the four summer atmospheric teleconnections analyzed here, we find that At. Ridge simulated by MPI-ESM-MR shows the best agreement with ERA-20C, thereby representing with its occurrence windows of opportunity for skillful summer predictions. Conversely, At. Low shows the lowest agreement, which might limit the model skill for early warning of warmer than average summers. In summary, we find that spring SST patterns identified with a SOM analysis can be used to guess the dominant summer atmospheric teleconnections at initialization and guide a sub-selection of potential skillful ensemble members. This holds especially true for At. Ridge and At. Low and is unclear for summer NAO. We show that predictive skill in the selected ensemble exceeds that of the full ensemble over regions in the Euro-Atlantic domain where spring SST significantly correlates with summer sea level pressure (SLP). In particular, we find a significant improvement in predictive skill for SLP, geopotential height at 500 hPa, and 2 m temperature at 3–4 months lead time over Scandinavia, which is robust among the two sets of hindcast ensembles.

Estimating unprecedented extremes in UK summer daily rainfall

The UNSEEN (unprecedented simulated extremes using ensemble) method involves using a large ensemble of climate model simulations to increase the sample size of rare events. Here we extend UNSEEN to focus on intense summertime daily rainfall, estimating plausible rainfall extremes in the current climate. To address modelling limitations simulations from two climate models were used; an initialised 25 km global model that uses parameterised convection, and a dynamically downscaled 2.2 km model that uses explicit convection. In terms of the statistical characteristics that govern very rare return periods, the models are not significantly different from the observations across much of the UK. Our analysis provides more precise estimates of 1000 year return levels for extreme daily rainfall, reducing sampling uncertainty by 70%–90% compared to using observations alone. This framework enables observed daily storm profiles to be adjusted to more statistically robust estimates of extreme rainfall. For a damaging storm in July 2007 which led to surface water flooding, we estimate physically plausible increases in the total daily rainfall of 50%–100%. For much of the UK the annual chance of record-breaking daily summertime rainfall is estimated to be around 1% per year in the present-day climate. Analysis of the dynamical states in our UNSEEN events indicates that heavy daily rainfall is associated with a southward displaced and meandering North Atlantic jet stream, increasing the advection of warm moist air from across Southern Europe and the Mediterranean, and intensifying extratropical storms. This work represents an advancement in the use of climate modelling for estimating present-day climate hazards and outlines a framework for applying UNSEEN at higher spatial and temporal resolutions.

North Atlantic jet stream projections in the context of the past 1,250 years

Reconstruction of the North Atlantic jet stream (NAJ) presents a critical, albeit largely unconstrained, paleoclimatic target. Models suggest northward migration and changing variance of the NAJ under 21st-century warming scenarios, but assessing the significance of such projections is hindered by a lack of long-term observations. Here, we incorporate insights from an ensemble of last-millennium water isotope-enabled climate model simulations and a wide array of mean annual water isotope ([Formula: see text]O) and annually accumulated snowfall records from Greenland ice cores to reconstruct North Atlantic zonal-mean zonal winds back to the 8th century CE. Using this reconstruction we provide preobservational constraints on both annual mean NAJ position and intensity to show that late 20th- and early 21st-century NAJ variations were likely not unique relative to natural variability. Rather, insights from our 1,250 year reconstruction highlight the overwhelming role of natural variability in thus far masking the response of midlatitude atmospheric dynamics to anthropogenic forcing, consistent with recent large-ensemble transient modeling experiments. This masking is not projected to persist under high greenhouse gas emissions scenarios, however, with model projected annual mean NAJ position emerging as distinct from the range of reconstructed natural variability by as early as 2060 CE.

Impact of Seasonality in the North Atlantic Jet Stream and Storm Migration on the Seasonality of Hurricane Translation Speed Changes

AbstractTropical cyclone (TC) translation speed (TCTS) can affect the duration of TC-related disasters, which is critical to coastal and inland areas. The long-term variation of TCTS and its relationship to the variability of the midlatitude jet stream and storm migration is discussed here for storms near the North Atlantic coast during 1948–2019. Our results reveal the prominent seasonality in the long-term variation of TCTS, which can be largely explained by the seasonality in the covariations of the midlatitude jet stream and storm locations. Specifically, significant increases of TCTS occur in June and October during the past decades, which may result from the equatorward displacement of the jet stream and poleward migration of storm locations. Prominent slowdown of TCTS is found in August, which is related to the weakened jet strength and equatorward storm migration. In September, the effects of poleward displacement and weakening of the jet stream on TCTS are largely compensated by the poleward storm migration, and therefore no significant change in TCTS is observed. Meanwhile, the multidecadal variability of the Atlantic may contribute to the multidecadal variability of TCTS. Our findings emphasize the significance in taking a seasonality view in discussing the variability and trends of near-coast Atlantic TCTS under climate change.