Extratropical Cyclone Activity Research Articles

Role of single and compound Pacific natural variability in extratropical cyclone activity over North America

AbstractThe role of large‐scale natural variability in influencing the distribution of midlatitude storm tracks and extratropical cyclones is well known. However, less understood is how compound natural variability—the concurrence of two modes of variability—influences the geospatial characteristics of extratropical cyclones. Here, we evaluate compound modes of Pacific‐North American variability—ENSO and PNA with the slow varying PDO—on extratropical cyclones over the North American region during 1950–2013, using an automated tracking algorithm applied to the Twentieth Century Reanalysis version 3 data. Seasonal analysis is conducted on spatial statistics including track, genesis and lysis density and some mean fields in varying compound phases. Spatial correlations of track density with seasonal mean of individual variability indices show that the PNA, ENSO and PDO enhance cyclone activity over Pacific storm track significantly while shifting the North American and Atlantic storm tracks in seasons, that is, equatorward in the positive phase. But the decadal variability of PDO moves the Atlantic storm track poleward, particularly in boreal winter. By comparing to the single Pacific variability impacts, compound Pacific variability results demonstrate the dominance of the higher‐frequency modes of ENSO and PNA on extratropical cyclone activity, while the slow varying PDO can modulate the ENSO‐ and PNA‐related cyclone variability. For example, during a negative PNA, cyclones over the North Atlantic become more (less) frequent during a positive (negative) PDO. The in‐phase modes of the PDO and ENSO influence Pacific storm track significantly, increasing (decreasing) cyclone frequency when both are positive (negative) in boreal winter. Results are linked to and used to explain the previous findings on how natural variability influences the occurrence of tornado outbreaks and high precipitation over the southeastern United States, Great Lakes, Ohio Valley and other US regions. Understanding compound variability modulations may be useful to predict extratropical cyclone frequency, development and decay regions seasonally and to study the physical mechanisms of extreme weather events.

Winter Storm Activity across Canada at the End of the Century: A CMIP5 Multi-model Projection

ABSTRACT Extratropical cyclones are the main source of precipitation in most of Canada. They bring hazardous weather events like blizzards, heavy snow, freezing rain, etc. They also supply fresh water through spring snow melt and maintain the soil and surface water balance in sub-humid and semi-arid regions. Any future changes in extratropical cyclone activity over Canada may have a significant impact on agriculture and the local economies of the various regions. Future changes in regional storm activity over Canada have not been studied in detail. Therefore, we examined potential changes in extratropical storm activity by the end of the century using data from six models from CMIP5 under historical and future (RCP 8.5) emission scenarios. A statistical analysis of selected storm activity indices, using a storm identification and tracking algorithm, revealed a decrease in the number of storms over the mid-latitude regions of Canada. However, intense storms with a longer duration are projected over all regions at the end of the century. A further investigation of the physical mechanisms revealed that a decrease in the meridional temperature gradient over the mid-latitude regions and shifting of the vertical wind shear are responsible for these expected changes in storm activity.

Climatology and Changes in Extratropical Cyclone Activity in the Southern Hemisphere during Austral Winters from 1948 to 2017

Abstract An objective detection and tracking algorithm based on relative vorticity at 850 hPa using National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) Reanalysis-1 data was applied to track cyclones in the Southern Hemisphere during austral winters from 1948 to 2017. The climatological characteristics of extratropical cyclones, including track density, frequency, intensity, lifetime, and their related variabilities, are discussed. The frequency and average lifetime of cyclones have substantially decreased. The average maximum intensity of cyclones has shown an increasing trend over the 70-yr study period. The cyclone track density shows a decreasing trend in lower latitudes, consistent with the region where the upper-troposphere zonal wind weakens. Baroclinicity can explain the increase in cyclone intensity: when a cyclone moves to higher latitudes and enters the region with greater baroclinicity, it strengthens. As there is no discernible increase in cyclogenesis in the medium latitudes (45°–70°S), but significantly less cyclogenesis in lower and higher latitudes, it is hypothesized that there is no clear poleward cyclogenesis shift over the Southern Hemisphere. Significance Statement While much is known about the Northern Hemisphere cyclones, few studies have examined how extratropical cyclones have changed in the Southern Hemisphere. We used an automatic tracking algorithm to study the climatological characteristics of extratropical cyclones over the past 70 years and found that the frequency of winter extratropical cyclones has decreased significantly in most parts of the Southern Hemisphere, with the number of intense cyclones increasing. Under global warming conditions, variability in regional low-level baroclinicity and a weakened upper-troposphere jet are likely to be responsible for this change. Future studies may focus on how the increasing autumn sea ice around Antarctica affects polar cyclone activities.

Northern Pacific extratropical cyclone variability and its linkage with Arctic sea ice changes

Extratropical cyclones are critical weather systems affecting climate change in mid and high-latitude regions. Researching the characteristics, patterns, and movements of extratropical cyclones is helpful for improved prediction and understanding of global climate change. Currently, there are still great difficulties in predicting extratropical cyclones. Also, few prior studies have investigated the potential impact of Arctic sea ice on extratropical cyclone activity (ECA). This study utilizes updated ERA5 data and an improved ECA identification method to reveal ECA in the Pacific. The results demonstrate that the Pacific ECA primarily occurs during the cold season (November to March), and the North Pacific region has the maximum ECA. More remarkably, a strong linkage exists between the preceding summer-fall anomalous changes in the Arctic sea ice and the cold season Pacific ECA. We discover that Arctic sea ice could modify the local pressure field, changing the southern boundary of the Pacific sector polar vortex in winter, which in turn influences the intensity of the westerly jet stream and eventually impacts the Pacific ECA during the cold season. Our exploration will provide references for further study and prediction of ECA in the Pacific Ocean.

Interannual Variability of Dust Deposition in Japan during Spring Season and Related Atmospheric Circulation Fields

Mineral dust affects health, climate, and ecosystems in various ways. East Asia is one of the major sources of mineral dust in the world. This study examines the year-to-year variability of dust deposition over Japan in April from the perspective of large-scale atmospheric circulations using atmospheric and aerosol reanalysis datasets for the period from 2011 to 2017. The increased dust deposition in Japan is explained by the intensified dust transport from the Mongolian Plateau by the anomalous westerly winds associated with a deepened trough over the East Asian continent toward the northwest of the Japanese islands in the middle to lower troposphere. The enhanced dust emission over Gobi Desert and the intensified extratropical cyclone activity are consistent with the larger-than-normal amount of dust in East Asia. Comparing the dust depositions over western and northern Japan, it is suggested that the slightly different anomalous trough positions may determine whether or not a large amount of dust is carried. A further analysis using the long-term (1967-2022) observation data of dust in Japan supports the importance of the intensified trough over the East Asian continent. Dust flux decomposed into cyclonic and anticyclonic components showed that both vortices contribute to the eastward dust transport in East Asia. These results suggest that Japanese dust events and their variability are affected by the stationary circulation anomaly as well as the baroclinic instability waves including transient cyclones and anticyclones.

Global Climatology of Extratropical Cyclones From a New Tracking Approach and Associated Wave Heights From Satellite Radar Altimeter

AbstractExtratropical cyclones (ECs) produce comparable wave heights and hazardous sea states to those under hurricanes. With strong wind speeds and considerably larger diameters, the impact of ECs on the global wave climate is substantial. Because ECs occur more frequently, move more quickly, and exist in a large variety of sizes and shapes, these storms are not as well documented. Here, we present findings from a global EC tracking algorithm aimed at creating an EC database of wave producing storms. We focus on the major ocean basins of extratropical cyclone activity: The North Atlantic, North Pacific, and Southern Oceans. Our tracking algorithm uses mean sea level pressure fields from the European Centre for Medium‐Range Weather Forecasts (ECMWF) high resolution, advanced data assimilating, ERA5 reanalysis over the years 1979–2020. The 42 year‐long data set of thousands of cyclone tracks allows seasonal patterns and longer‐term trends of EC activity to be analyzed in these ocean regions. In addition, we employ 36 years of satellite radar altimeter‐derived significant wave height (Hs) data from 14 inter‐calibrated satellite missions to study the wave climate induced by the identified ECs in each basin. The resulting wave climatology compares well with previous results in the mid to high latitudes. We then analyze average wave heights under different EC characteristics, including cyclone translational speed and maximum winds, which have strong effects on wave height variability. Finally, a brief analysis of EC and wave climate under positive and negative phases of the North Atlantic Oscillation is presented.

Desert dunes transformed by end-of-century changes in wind climate

Sand dunes in arid regions are conspicuous mobile landforms that require adaptation and mitigation strategies to protect human infrastructure and economic assets from encroachment, and play a substantial role in desertification and atmospheric dust emissions. Here we show how the shape, migration speed and direction of mobile desert dunes globally are projected to change by 2100, in response to sand-moving wind regime shifts associated with climate change under the shared socio-economic pathway SSP5-8.5 (SSP, shared socio-economic pathway) scenario. We find transformations in dune dynamics for many sand seas and dune fields across the Sahara, The Horn of Africa, the Southern Arabian Peninsula, South Asia, China and Australia—as well as an increased potential for sand sea expansion and reactivation of dormant dune fields—linked to climate change alterations in the Hadley circulation, extra-tropical cyclone activity and monsoon systems. These projected changes will affect planning for and management of dune encroachment on transportation infrastructure, industry and urban development in desert regions.

Northern Hemisphere Extratropical Cyclone Activity in the Twentieth Century Reanalysis Version 3 (20CRv3) and Its Relationship with Continental Extreme Temperatures

In this study, we detect and track extratropical cyclones using 6-hourly mean sea level pressure data taken from the Twentieth Century Reanalysis version 3 (20CRv3) over the period 1951–2015 and compare them with those in the Interim and fifth generation of ECMWF reanalyses over the period 1979–2018. Three indices were employed to characterize cyclone activity, including cyclone count, cyclone intensity, and a cyclone activity index (CAI) that combines the count and intensity. The results show that the cyclone indices in the three datasets have comparable annual climatologies and seasonal evolution over the northern extratropical land and ocean in recent decades. Based on the cyclone indices over the period 1951–2010 in 80 ensemble members of 20CRv3, cyclone count and intensity are negatively correlated in winter and tend to be positively and weakly correlated in summer. The interannual CAI variability is dominated by the cyclone count variability. Regional mean cyclone activity can be well represented using the ensemble average cyclone index. We then examined the linkage of the cyclone activity in 20CRv3 and observed cold and warm extremes over Eurasia and North America over the period 1951–2010. In winter, the principal components of interannual cold and warm extreme anomalies are more correlated with the regional mean cyclone count index over Eurasia, while they are more correlated with the cyclone intensity index over North America. The temperature anomalies associated with the regional and ensemble mean cyclone count index explain about 10% (20%) of interannual cold (warm) extreme variances averaged over Eurasia. The temperature anomalies associated with the mean cyclone intensity explain about 10% of interannual cold and warm extreme variances over North America. Large-scale atmospheric circulation anomalies in association with cyclone activity and the induced temperature advection drive temperature anomalies over Eurasia and North America. In summer, circulation and thermal advection anomalies associated with cyclone activity are weak over the two continents. Hence, that season’s relationship between cyclone activity and extreme temperature variability is weak.

Can we trust CMIP5/6 future projections of European winter precipitation?

IPCC models project a likely increase in winter precipitation over northern Europe under a high-emission scenario. These projections, however, typically rely on relatively coarse ∼100 km resolution models that can misrepresent important processes driving precipitation, such as extratropical cyclone activity, and ocean eddies. Here, we show that a pioneering 50 km atmosphere–1/12° ocean global coupled model projects a substantially larger increase in winter precipitation over northwestern Europe by mid-century than lower-resolution configurations. For this increase, both the highest ocean and atmosphere model resolutions are essential: only the eddy-rich (1/12°) ocean projects a progressive northward shift of the Gulf Stream. This leads to a strong regional ocean surface warming that intensifies air–sea heat fluxes and baroclinicity. For this then to translate into a strengthening of North Atlantic extratropical cyclone activity, the 50 km atmosphere is essential, as it enables enhanced diabatic heating from water vapor condensation and an acceleration of the upper-level mean flow, which weaken vertical stability. Our results suggest that all recent IPCC climate projections using traditional ∼100 km resolution models could be underestimating the precipitation increase over Europe in winter and, consequently, the related potential risks.

Subseasonal Prediction of Wintertime Northern Hemisphere Extratropical Cyclone Activity by SubX and S2S Models

AbstractThe prediction of wintertime extratropical cyclone activity (ECA) on subseasonal time scales by models participating in the Subseasonal Experiment (SubX) and the Seasonal to Subseasonal Prediction (S2S) is assessed. Consistent with a previous study that investigated the S2S models, the SubX models have skillful predictions of ECA over regions from central North Pacific across North America to western North Atlantic, as well as East Asia and northern and southern part of eastern North Atlantic at 3–4 weeks lead time. SubX provides daily mean data, while S2S provides instantaneous data at 0000 UTC each day. This leads to different variance of ECA. Different S2S and SubX models have different reforecast initialization times and reforecast time periods. These factors can all lead to differences in prediction skill. To fairly compare the prediction skill between different models, we develop a novel way to evaluate the prediction of individual model across the two ensembles by comparing every model to the Climate Forecast System, version 2 (CFSv2), as CFSv2 has 6-hourly output and forecasts initialized every day. Among the S2S and SubX models, the European Centre for Medium-Range Weather Forecasts model exhibits the best prediction skill, followed by CFSv2. Our results also suggest that while the prediction skill is sensitive to forecast lead time, including forecasts up to 4 days old into the ensemble may still be useful for weeks 3–4 predictions of ECA.

Effects of Nonuniform Land Surface Warming on Summer Anomalous Extratropical Cyclone Activity and the East Asian Summer Monsoon: Numerical Experiments with a Regional Climate Model

AbstractPrevious studies detected significant negative correlations between the nonuniform land surface warming and the decadal weakened activities of the summer extratropical cyclones (ECs) over East Asia and the East Asian summer monsoon (EASM) after the early 1990s. Here such relationships are further examined and the possible mechanisms are explored via numerical sensitivity experiments with a regional climate model (RegCM4.5). The positive/negative sensible heat flux (SH) anomalies were added as a forcing to a key region near 50°N of East Asia in RegCM4.5 to simulate the observed ground surface temperature (GST) anomalies. The model results suggest that the nonuniform land surface warming over the Lake Baikal area (50°–60°N, 90°–120°E) can indeed cause the weakening of the extratropical cyclogenesis and affect the decadal weakening of the EASM. Warm (cold) GST forcing over the key GST region can lead to decreasing (increasing) atmospheric baroclinicity and related energy conversion of the EC activity over the key EC region (40°–50°N, 90°–120°E), resulting in an evidently weakening (enhancing) of the ECs over East Asia. Meanwhile, precipitation shows a dipole pattern with significantly suppressed (enhanced) precipitation in northern and northeastern China, and slightly enhanced (suppressed) rainfall south of 40°N of East Asia, mainly over the East China Sea. Lake Baikal and its adjacent areas are occupied by a strong anticyclonic (cyclonic) circulation while the southeast coastal areas of China have a relatively weak cyclonic (anticyclonic) circulation accompanied with an anomalous northeasterly (southwesterly) wind to the southeast of the anticyclonic circulation, which is opposite to (coincident with) the atmospheric circulation anomalies that are associated with the second mode of the EASM.

Impacts of Storm Track Variations on Wintertime Extreme Precipitation and Moisture Budgets over the Ohio Valley and Northwestern United States

AbstractPrevious studies have shown that variations in extratropical cyclone activity significantly affect the frequency of extreme precipitation events over the Ohio Valley and northwestern United States. In this study, we examine the similarities and differences between the dynamics governing these events in these two regions. In the Ohio Valley, extreme precipitation events are associated with midlatitude synoptic-scale convergence northeast of cyclones and a southwestward oriented ridge near the Atlantic coast that drives strong water vapor transport from the Gulf of Mexico into the Ohio Valley. In the northwestern United States, extreme precipitation events are associated with a cyclonic and anticyclonic circulation pair aligned northwest to southeast, which together drive a long and strong moisture transport corridor from the lower latitude of the central Pacific Ocean toward the northwestern United States. Moisture budget analysis shows that moisture convergence due to dynamical convergence dominates in the Ohio Valley, whereas moisture advection dominates over the Pacific Northwest. Differences between the cases in the same region are examined by an empirical orthogonal function (EOF) analysis conducted on the vertically integrated moisture flux. Different EOFs highlight shifts in spatial location, orientation, and intensity of the moisture flux but demonstrate consistent roles of dynamics in the two regions. Composites based on these EOFs highlight the range of likely synoptic scenarios that can give rise to precipitation extremes over these two regions.

RAS-NAAD: 40-yr High-Resolution North Atlantic Atmospheric Hindcast for Multipurpose Applications (New Dataset for the Regional Mesoscale Studies in the Atmosphere and the Ocean)

AbstractWe present in this paper the results of the Russian Academy of Sciences North Atlantic Atmospheric Downscaling (RAS-NAAD) project, which provides a 40-yr 3D hindcast of the North Atlantic (10°–80°N) atmosphere at 14-km spatial resolution with 50 levels in the vertical direction (up to 50 hPa), performed with a regional setting of the WRF-ARW 3.8.1 model for the period 1979–2018 and forced by ERA-Interim as a lateral boundary condition. The dataset provides a variety of surface and free-atmosphere parameters at sigma model levels and meets many demands of meteorologists, climate scientists, and oceanographers working in both research and operational domains. Three-dimensional model output at 3-hourly time resolution is freely available to the users. Our evaluation demonstrates a realistic representation of most characteristics in both datasets and also identifies biases mostly in the ice-covered regions. High-resolution and nonhydrostatic model settings in NAAD resolve mesoscale dynamics first of all in the subpolar latitudes. NAAD also provides a new view of the North Atlantic extratropical cyclone activity with a much larger number of cyclones as compared with most reanalyses. It also effectively captures highly localized mechanisms of atmospheric moisture transports. Applications of NAAD to ocean circulation and wave modeling are demonstrated.

Subseasonal to Seasonal Prediction of Wintertime Northern Hemisphere Extratropical Cyclone Activity by S2S and NMME Models

AbstractIn this study, the prediction of wintertime extratropical cyclone activity from subseasonal to seasonal timescale in current dynamical models' reforecasts is investigated. On seasonal time scales, the North American Multi‐Model Ensemble (NMME) models show skillful predictions over the eastern North Pacific, North America and the western North Atlantic with at least 5 months lead. The prediction skill is highly related to El Nino‐Southern Oscillation (ENSO), as using the ENSO‐related SST pattern gives rise to prediction skill with very similar spatial pattern and amplitude. On subseasonal time scales, models in the Seasonal to Sub‐seasonal Prediction (S2S) dataset have skillful predictions up to 4 weeks lead over regions from the eastern North Pacific to the western Atlantic, as well as northern Europe, the eastern Atlantic and East Asia. Generally, forecast skill improves with a larger ensemble size. The subseasonal prediction skill from the Pacific to the western Atlantic is related to ENSO, and that over eastern Atlantic, Europe and East Asia are associated with stratospheric polar vortex anomalies. Current models do not show much skill from the Madden‐Julian Oscillation (MJO), as the MJO impact on extratropical cyclone activity is not well captured by the models. European Centre for Medium‐Range Weather Forecasts (ECMWF) model has the highest single model subseasonal prediction skill. The prediction skill in the ECMWF model is higher than its estimated potential predictability, likely because the signal‐to‐noise ratio is too low in the model hindcasts.

Enhanced Climate Change Response of Wintertime North Atlantic Circulation, Cyclonic Activity, and Precipitation in a 25-km-Resolution Global Atmospheric Model

Abstract Wintertime midlatitude cyclone activity and precipitation are projected to increase across northern Europe and decrease over southern Europe, particularly over the western Mediterranean. Greater confidence in these regional projections may be established by their replication in state-of-the-art, high-resolution global climate models that resolve synoptic-scale dynamics. We evaluated the representation of the wintertime eddy-driven and subtropical jet streams, extratropical cyclone activity, and precipitation across the North Atlantic and Europe under historical (1985–2011) and RCP8.5 sea surface temperature forcing in an ensemble of atmosphere-only HadGEM3-GA3.0 simulations, where horizontal atmospheric resolution is increased from 135 to 25 km. Under RCP8.5, increased (decreased) frequency of northern (southern) eddy-driven jet occurrences and a basinwide poleward shift in the upper-level westerly flow are simulated. Increasing atmospheric resolution significantly enhances these climate change responses. At 25-km resolution, these enhanced changes in large-scale circulation amplify increases (decreases) in extratropical cyclone track density and mean intensity across the northern (southern) Euro-Atlantic region under RCP8.5. These synoptic changes with resolution impact the overall climate change response of mean and heavy winter precipitation: wetter (drier) conditions in northern (southern) Europe are also amplified at 25-km resolution. For example, the reduction in heavy precipitation simulated over the Iberian Peninsula under RCP8.5 is ~15% at 135 km but ~30% at 25-km resolution. Conversely, a shift to more frequent high extratropical cyclone (ETC)-associated precipitation rates is simulated over Scandinavia under RCP8.5, which is enhanced at 25 km. This study provides evidence that global atmospheric resolution may be a crucial consideration in European winter climate change projections.

Impact of Nonuniform Land Surface Warming on Summer Anomalous Extratropical Cyclone Activity Over East Asia

AbstractAs one of the typical midlatitude synoptic‐scale disturbances, extratropical cyclones (ECs) can exert significant impacts on the atmospheric general circulation through its interaction with the time mean flow. Under the background of global warming, the Eurasian continent exhibits evident nonuniform warming, which has the potential to alter the atmospheric baroclinicity by changing the meridional temperature gradient and further affect the ECs activity. In this study, we investigated the possible connection between the land surface thermal anomaly over Eurasia and the summer ECs activity over East Asia together with relevant mechanisms. We found that the land surface warming (cooling) near 50°N of East Asia is associated with anomalous weak (strong) summer ECs activity over East Asia. Warm (cool) land surface usually reduces (increases) the meridional temperature gradient and further the atmospheric baroclinicity in the key area of cyclone activity, resulting in low (high) frequency of the extratropical cyclogenesis and weakened (intensified) ECs activity. The land surface warming (cooling) can also depress (benefit) the associated baroclinic conversion between the time mean effective potential energy and eddy effective potential energy, resulting in decrease (increase) in the eddy kinetic energy. As a result, the energy obtained by the synoptic‐scale eddy from the time mean flow has been reduced (increased), which favors (hampers) the extratropical cyclogenesis, causing weak (strong) cyclone activity in the middle latitude of East Asia.

Changing available energy for extratropical cyclones and associated convection in Northern Hemisphere summer

The circulation of the Northern Hemisphere extratropical troposphere has changed over recent decades, with marked decreases in extratropical cyclone activity and eddy kinetic energy (EKE) in summer and increases in the fraction of precipitation that is convective in all seasons. Decreasing EKE in summer is partly explained by a weakening meridional temperature gradient, but changes in vertical temperature gradients and increasing moisture also affect the mean available potential energy (MAPE), which is the energetic reservoir from which extratropical cyclones draw. Furthermore, the relation of changes in mean thermal structure and moisture to changes in convection associated with extratropical cyclones is poorly understood. Here we calculate trends in MAPE for the Northern extratropics in summer over the years 1979-2017, and we decompose MAPE into both convective and nonconvective components. Nonconvective MAPE decreased over this period, consistent with decreases in EKE and extratropical cyclone activity, but convective MAPE increased, implying an increase in the energy available to convection. Calculations with idealized atmospheres indicate that nonconvective and convective MAPE both increase with increasing mean surface temperature and decrease with decreasing meridional surface temperature gradient, but convective MAPE is relatively more sensitive to the increase in mean surface temperature. These results connect changes in the atmospheric mean state with changes in both large-scale and convective circulations, and they suggest that extratropical cyclones can weaken even as their associated convection becomes more energetic.

A Modeling Investigation of Northern Hemisphere Extratropical Cyclone Activity in Spring: The Linkage between Extreme Weather and Arctic Sea Ice Forcing

Arctic sea ice exhibits rapid reductions and large fluctuations during recent decades in conjunction with a warming climate. Arctic sea ice shows a prominent seasonal cycle with the maximum extent in spring and the minimum extent in fall. In this study a suite of modeling experiments, which were solely forced with observed time-varying sea-ice concentration for spring 1979 to 2008 was conducted using NCAR’s Community Atmosphere Model to identify sea ice impacts on extratropical storms and associated surface climate parameters when they at the peak of their seasonal variability. Storms are the linkage between the large-scale circulation changes, forced by Arctic sea ice and local extreme weather events through strong winds, high precipitation, and high/low air temperature. In this study, a storm identification and tracking algorithm indicates that reduced sea-ice cover enhances Arctic storm activity. As a consequence, surface climate parameters such as surface air temperature (SAT) and precipitation increases in spring over the Arctic. The changes in the Arctic have a direct and indirect effect on the extratropical storm activity over the mid-latitudes. In contrast to the Arctic, storm activity weakens over Eurasia in the years with less sea ice. Further analysis of the surface climate indicates a warmer and dryer Eurasia for years with reduced sea ice.

Influential role of inter‐decadal explosive cyclone activity on the increased frequency of winter storm events in Hokkaido, the northernmost island of Japan

This study examined explosively developing extratropical cyclone activity in the vicinity of Japan in boreal winter from 1979/1980 to 2016/2017 mainly using the Japanese 55‐year Reanalysis data. The in situ observation and reanalysis data revealed increased frequencies of both strong winds and heavy precipitation in Hokkaido, the northernmost island of Japan, after the end of the last century, which is consistent with the fact that the cyclones migrating along the Kuroshio Current (KC cyclones) have tended to approach the Hokkaido region in recent years. To examine factors controlling the cyclone track and rapid development, we performed composite analyses of the northwards migrating type (N type) and eastwards migrating type (E type) KC cyclones. In the N‐type cyclone, wave trains prevailed along the polar front and subtropical jets over the Eurasian continent, thereby forming an upper‐level trough (ridge) to the west (east) of the cyclone. An upper‐level divergence accompanied by rapid development of the N‐type cyclone excited wave packets downstream as a Rossby wave source, leading to further reinforcement of the upper‐level ridge east of the cyclone. The anomalous anticyclone with a barotropic nature is capable of forcing the cyclone track to shift northwards. Furthermore, a combination of the N‐type cyclone and the anomalous anticyclone to its east facilitates moisture import into the cyclone system from lower latitudes, contributing to enhanced moisture flux convergence. In reality, diabatic heating in the vicinity of the cyclone centre is larger in the N‐type cyclone than in the E‐type one. It is also suggested that the pronounced Rossby wave propagation along the subtropical jet related to a climate regime shift in the Tropics is responsible for the recent increase in the number of N‐type cyclones.

Model Resolution Sensitivity of the Simulation of North Atlantic Oscillation Teleconnections to Precipitation Extremes

AbstractWe evaluate a high‐resolution (0.25°), four‐member ensemble simulation of the global climate (1979–2005) with the U.S. Department of Energy's Energy Exascale Earth System Model v0.3—forced with observed ocean surface temperatures and sea ice extent—for its ability to represent the North Atlantic Oscillation (NAO) teleconnections to winter precipitation extremes over western Europe. As compared to the low‐resolution model (1°), it simulates a stronger impact of NAO on daily precipitation extremes over the western slopes of mountain ranges over southwestern Norway, northwestern United Kingdom, and the Western Balkan states. Precipitation extremes and their linear relationship with NAO are quantified using the generalized extreme value distribution. NAO‐dependent large‐scale (stratiform) precipitation intensity strengthens in the high‐resolution model on seasonal time scales and plays a dominant role during simulated daily precipitation extremes. Improvements in the high‐resolution model over these varied‐topography regions largely appear to be due to finer resolved scales of motion that amplify NAO‐dependent seasonal vertical moisture fluxes and enhance stable condensation. However, the high‐resolution model simulates a weaker than observed impact of NAO on extratropical cyclone activity and total precipitable water, generally underperforming the low‐resolution model These effects possibly offset the impact of enhanced vertical moisture fluxes on NAO‐dependent precipitation extremes in the high‐resolution model in these regions. Over the southwestern Iberian peninsula, the high‐resolution model underperforms the low‐resolution model simulating weaker than observed impact of NAO on precipitation extremes. This appears to be due to the reduction in total precipitable water despite an increase in NAO‐dependent extratropical activity there.