Weather and Climate Dynamics最新文献

{"title":"Increasing frequency and lengthening season of western disturbances are linked to increasing strength and delayed northward migration of the subtropical jet","authors":"Kieran M. R. Hunt","doi":"10.5194/wcd-5-345-2024","DOIUrl":"https://doi.org/10.5194/wcd-5-345-2024","url":null,"abstract":"Abstract. Western disturbances (WDs) are cyclonic storms that travel along the subtropical jet, bringing the majority of seasonal and extreme precipitation to the Hindu Kush, Karakoram, and western Himalaya in the winter months. They are a vital component of the region's water security. Although typically most common in the winter, WDs can also interact with the summer monsoon, leading to catastrophic consequences. These seem to be happening more frequently, and along with increasingly harsh winter seasons, questions are now being asked about how climate change is affecting WD frequency and intensity in both summer and winter seasons. An analysis of 17 previous studies assessing trends in WD frequency revealed no consensus, at least in part because they quantified trends in different regions, seasons, and time periods. In this study, a more robust approach is used, quantifying trends in WD frequency and intensity by region and month using a track catalogue derived from 70 years of European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA5) data. Winter WDs have increased significantly over the western and central Himalaya and the Hindu Kush in the last 70 years. This trend is attributed to a strengthening of the subtropical jet. The WD season has also significantly lengthened with WDs becoming far more common in May, June, and July, months where they were previously rare. For example, WDs have been twice as common in June in the last 20 years than during the previous 50. This is attributed to a delayed northward retreat of the subtropical jet, which historically has occurred before the onset of the summer monsoon. The most important implication is that the frequency of “monsoonal” WDs is increasing significantly, and therefore, due to climate change, catastrophic events like the 2013 Uttarakhand floods and the 2023 north India floods are becoming much more frequent.\u0000","PeriodicalId":383272,"journal":{"name":"Weather and Climate Dynamics","volume":"31 S108","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-03-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140395167","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"European summer weather linked to North Atlantic freshwater anomalies in preceding years","authors":"M. Oltmanns, N. P. Holliday, J. Screen, B. Moat, S. Josey, D. G. Evans, Sheldon Bacon","doi":"10.5194/wcd-5-109-2024","DOIUrl":"https://doi.org/10.5194/wcd-5-109-2024","url":null,"abstract":"Abstract. Amplified Arctic ice loss in recent decades has been linked to the increased occurrence of extreme mid-latitude weather. The underlying mechanisms remain elusive, however. One potential link occurs through the ocean as the loss of sea ice and glacial ice leads to increased freshwater fluxes into the ocean. Thus, in this study, we examine the link between North Atlantic freshwater anomalies and European summer weather. Combining a comprehensive set of observational products, we show that stronger freshwater anomalies are associated with a sharper sea surface temperature front between the subpolar and the subtropical North Atlantic in winter, an increased atmospheric instability above the sea surface temperature front, and a large-scale atmospheric circulation that induces a northward shift in the North Atlantic Current, strengthening the sea surface temperature front. In the following summer, the lower-tropospheric winds are deflected northward along the enhanced sea surface temperature front and the European coastline, forming part of a large-scale atmospheric circulation anomaly that is associated with warmer and drier weather over Europe. The identified statistical links are significant on timescales from years to decades and indicate an enhanced predictability of European summer weather at least a winter in advance, with the exact regions and amplitudes of the warm and dry weather anomalies over Europe being sensitive to the location, strength, and extent of North Atlantic freshwater anomalies in the preceding winter.\u0000","PeriodicalId":383272,"journal":{"name":"Weather and Climate Dynamics","volume":"53 2","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-02-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140421538","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Multi-decadal pacemaker simulations with an intermediate-complexity climate model","authors":"F. Molteni, F. Kucharski, R. Farneti","doi":"10.5194/wcd-5-293-2024","DOIUrl":"https://doi.org/10.5194/wcd-5-293-2024","url":null,"abstract":"Abstract. In this paper, we first describe the main features of a new version of the International Centre for Theoretical Physics global atmospheric model (SPEEDY) with improved simulation of surface fluxes and the formulation of a three-layer thermodynamic ocean model (TOM3) suitable to explore the coupled extratropical response to tropical ocean variability. Then, we present results on the atmospheric model climatology, highlighting the impact of the modifications introduced in the model code, and show how important features of interdecadal and interannual variability are simulated in a “pacemaker” coupled ensemble of 70-year runs, where portions of the tropical Indo-Pacific are constrained to follow the observed variability. Despite the very basic representation of variations in greenhouse forcing and heat transport to the deep ocean (below the 300 m domain of the TOM3 model), the coupled ensemble reproduces the variations in surface temperature over land and sea with very good accuracy, confirming the role of the Indo-Pacific as a “pacemaker” for the natural fluctuations of global-mean surface temperatures found in earlier studies. Atmospheric zonal-mean temperature trends over 50 years are also realistically simulated in the extratropical lower troposphere and up to 100 hPa in the tropics. On the interannual scale, sea-surface temperature (SST) variability in sub-tropical and tropical regions not affected by SST relaxation is underestimated (mostly because of the absence of dynamically induced variability), while extratropical SST variability during the cold seasons is comparable to that observed. Atmospheric teleconnection patterns and their connections with SST are reproduced with high fidelity, although with local differences in the amplitude of regional features (such as a larger-than-observed response of extratropical SST to North Atlantic Oscillation variability). The SPEEDY-TOM3 model also reproduces the observed connection between averages of surface heat fluxes over the oceans and land surface air temperature in the wintertime northern extratropics. Overall, as in earlier versions of SPEEDY, the fidelity of the simulations (both in terms of climatological means and variability) is higher near the surface and in the lower troposphere, while the negative impacts of the coarse vertical resolution and simplified parameterizations are mostly felt in the stratosphere. However, the improved simulation of surface heat fluxes and their impact on extratropical SST variability in this model version (obtained at a very modest computational cost) make the SPEEDY-TOM3 model a suitable tool to investigate the coupled response of the extratropical circulation to interannual and inter-decadal changes of tropical SST in ensemble experiments.\u0000","PeriodicalId":383272,"journal":{"name":"Weather and Climate Dynamics","volume":"5 4","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-02-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140427880","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Persistent warm and cold spells in the Northern Hemisphere extratropics: regionalisation, synoptic-scale dynamics and temperature budget","authors":"A. Tuel, O. Martius","doi":"10.5194/wcd-5-263-2024","DOIUrl":"https://doi.org/10.5194/wcd-5-263-2024","url":null,"abstract":"Abstract. Persistent warm and cold spells are often high-impact events that may lead to significant increases in mortality and crop damage and can put substantial pressure on the power grid. Taking their spatial dependence into account is critical to understand the associated risks, whether in present-day or future climates. Here, we present a novel regionalisation approach of 3-week warm and cold spells in winter and summer across the Northern Hemisphere extratropics based on the association of the warm and cold spells with large-scale circulation. We identify spatially coherent but not necessarily connected regions where spells tend to co-occur over 3-week timescales and are associated with similar large-scale circulation patterns. We discuss the physical drivers responsible for persistent extreme temperature anomalies. Cold spells systematically result from northerly cold advection, whereas warm spells are caused by either adiabatic warming (in summer) or warm advection (in winter). We also discuss some key mechanisms contributing to the persistence of temperature extremes. Blocks are important upper-level features associated with such events – co-localised blocks for persistent summer warm spells in the northern latitudes; downstream blocks for winter cold spells in the eastern edges of continental landmasses; and upstream blocks for winter cold spells in Europe, northwestern North America and east Asia. Recurrent Rossby wave patterns are also relevant for cold and warm spell persistence in many mid-latitude regions, in particular in central and southern Europe. Additionally, summer warm spells are often accompanied by negative precipitation anomalies that likely play an important role through land–atmosphere feedbacks.\u0000","PeriodicalId":383272,"journal":{"name":"Weather and Climate Dynamics","volume":"10 3","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-02-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140441142","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Replicating the Hadley cell edge and subtropical jet latitude disconnect in idealized atmospheric models","authors":"Molly E. Menzel, Darryn Waugh, Zheng Wu, T. Reichler","doi":"10.5194/wcd-5-251-2024","DOIUrl":"https://doi.org/10.5194/wcd-5-251-2024","url":null,"abstract":"Abstract. Recent work has shown that variability in the subtropical jet's (STJ) latitude, ϕSTJ, is not coupled to that of the Hadley cell (HC) edge, ϕHC, but the robustness of this disconnect has not been examined in detail. Here, we use meteorological reanalysis products, comprehensive climate models, and an idealized atmospheric model to determine the necessary processes for a disconnect between ϕHC and ϕSTJ in the Northern Hemisphere's December–January–February season. We find that a decoupling can occur in a dry general circulation model, indicating that large-scale dynamical processes are sufficient to reproduce the metrics' relationship. It is therefore not reliant on explicit variability in the zonal structure, convection, or radiation. Rather, the disconnect requires a sufficiently realistic climatological basic state. Further, we confirm that the robust disconnect between ϕSTJ and ϕHC across the model hierarchy reveals their differing sensitivities to midlatitude eddy momentum fluxes; ϕHC is consistently coupled to the latitude of maximum eddy momentum flux, but ϕSTJ is not.\u0000","PeriodicalId":383272,"journal":{"name":"Weather and Climate Dynamics","volume":"1118 ","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-02-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140445739","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Stratospheric influence on the winter North Atlantic storm track in subseasonal reforecasts","authors":"Hilla Afargan‐Gerstman, D. Büeler, C. O. Wulff, M. Sprenger, Daniella I. V. Domeisen","doi":"10.5194/wcd-5-231-2024","DOIUrl":"https://doi.org/10.5194/wcd-5-231-2024","url":null,"abstract":"Abstract. Extreme stratospheric polar vortex events, such as sudden stratospheric warmings (SSWs) or extremely strong polar vortex events, can have a significant impact on surface weather in winter. SSWs are most often associated with negative North Atlantic Oscillation (NAO) conditions, cold air outbreaks in the Arctic and a southward-shifted midlatitude storm track in the North Atlantic, while strong polar vortex events tend to be followed by a positive phase of the NAO, relatively warm conditions in the extratropics and a poleward-shifted storm track. Such changes in the storm track position and associated extratropical cyclone frequency over the North Atlantic and Europe can increase the risk of extreme windstorm, flooding or heavy snowfall over populated regions. Skillful predictions of the downward impact of stratospheric polar vortex extremes can therefore improve the predictability of extratropical winter storms on subseasonal timescales. However, there exists a strong inter-event variability in these downward impacts on the tropospheric storm track. Using ECMWF reanalysis data and reforecasts from the Subseasonal to Seasonal (S2S) Prediction Project database, we investigate the stratospheric influence on extratropical cyclones, identified with a cyclone detection algorithm. Following SSWs, there is an equatorward shift in cyclone frequency over the North Atlantic and Europe in reforecasts, and the opposite response is observed after strong polar vortex events, consistent with the response in reanalysis. However, although the response of cyclone frequency following SSWs with a canonical surface impact is typically captured well during weeks 1–4, less than 25 % of the reforecasts manage to capture the response following SSWs with a “non-canonical” impact. This suggests a possible overconfidence in the reforecasts with respect to reanalysis in predicting the canonical response after SSWs, although it only occurs in about two-thirds of the events. The cyclone forecasts following strong polar vortex events are generally more successful. Understanding the role of the stratosphere in subseasonal variability and predictability of storm tracks during winter can provide a key for reliable forecasts of midlatitude storms and their surface impacts.\u0000","PeriodicalId":383272,"journal":{"name":"Weather and Climate Dynamics","volume":"88 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-02-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140449189","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Seasonally dependent increases in subweekly temperature variability over Southern Hemisphere landmasses detected in multiple reanalyses","authors":"Patrick Martineau, S. Behera, Masami Nonaka, Hisashi Nakamura, Yu Kosaka","doi":"10.5194/wcd-5-1-2024","DOIUrl":"https://doi.org/10.5194/wcd-5-1-2024","url":null,"abstract":"Abstract. The inter-dataset agreement of trends in subweekly near-surface (850 hPa) temperature variability over Southern Hemisphere midlatitude land masses is assessed among 12 global atmospheric reanalysis datasets. A comparison of the climatological temperature variance and dominant sources and sinks of the variance reveals that, except for NCEP-NCAR (R1) and NCEP-DOE (R2), there is a relatively good agreement for their magnitudes and spatial distributions during the satellite era (1980–2022), which indicates that the key features of subweekly variability are sufficiently well represented. A good agreement is noted for the positive trends found in subweekly variability over the satellite era affecting South Africa in September–October–November (SON) and South America in December–January–February (DJF). Although there is agreement in most of the reanalyses concerning the positive trend affecting Australia in SON, this has not yet emerged from the noise associated with interannual variability when considering only the satellite era. It is significant, however, when the period is extended (1954–2022) or limited to the most recent decades (1990–2022). The trends are explained primarily by a more efficient generation of subweekly temperature variance by horizontal temperature advection. This generation is also identified as a source of biases among the datasets. The trends are found to be reproduced even in those reanalyses that do not assimilate satellite data (JRA-55C) or that assimilate surface observations only (ERA-20C, 20CRv2c, and 20CRv3).\u0000","PeriodicalId":383272,"journal":{"name":"Weather and Climate Dynamics","volume":"5 6","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-01-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139439826","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"The monthly evolution of precipitation and warm conveyor belts during the central southwest Asia wet season","authors":"Melissa Leah Breeden, Andrew Hoell, John Robert Albers, Kimberly Slinski","doi":"10.5194/wcd-4-963-2023","DOIUrl":"https://doi.org/10.5194/wcd-4-963-2023","url":null,"abstract":"Abstract. Understanding the nature of precipitation over central southwest Asia (CSWA), a data-sparse, semi-arid region, is important given its relation to agricultural productivity and the likelihood of hazards such as flooding. The present study considers how daily precipitation and local vertical motion – represented by warm conveyor belts (WCBs) – evolve from November to April over CSWA. First we compare several precipitation datasets, revealing that the seasonality of daily precipitation is consistent across estimates that incorporate satellite information, while total accumulation amounts differ substantially. A common feature across datasets is that the majority of precipitation occurs on the few days when area-averaged accumulation exceeds 4 mm, which are most frequent in February and March. The circulation pattern associated with heavy (< 4 mm d−1) precipitation days evolves within the wet season from a southwest–northeast tilted couplet of circulation anomalies in January and February to a neutrally tilted monopole pattern in April. El Niño conditions are associated with more heavy precipitation days than La Niña conditions, with both enhanced WCB frequency and moisture transport observed during the former. An exception to this is found in January, when precipitation, WCB frequency, and moisture do not increase, despite a similar increase in surface cyclones to other months, suggesting that precipitation changes cannot always be inferred from cyclone frequency changes. Nonetheless, our results generally support prior connections made between the El Niño–Southern Oscillation (ENSO) and seasonal-to-interannual precipitation anomalies and extend this connection to one between the slowly evolving ENSO influence and transient and local vertical motion represented by WCBs.","PeriodicalId":383272,"journal":{"name":"Weather and Climate Dynamics","volume":"35 18","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-11-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"134992835","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Cold wintertime air masses over Europe: where do they come from and how do they form?","authors":"Tiina Nygård, Lukas Papritz, Tuomas Naakka, Timo Vihma","doi":"10.5194/wcd-4-943-2023","DOIUrl":"https://doi.org/10.5194/wcd-4-943-2023","url":null,"abstract":"Abstract. Despite the general warming trend, wintertime cold-air outbreaks in Europe have remained nearly as extreme and as common as decades ago. In this study, we identify six principal 850 hPa cold anomaly types over Europe in 1979–2020 using self-organizing maps (SOMs). Based on extensive analysis of atmospheric large-scale circulation patterns combined with nearly 2 million kinematic backward trajectories, we show the origins and contributions of various physical processes to the formation of cold wintertime 850 hPa air masses. The location of the cold anomaly region is closely tied to the location of blocking; if the block is located farther to the east, the cold anomaly is also displaced eastwards. Considering air mass evolution along the trajectories, the air parcels are typically initially (5–10 d before) colder than at their arrival in Europe, but initially warmer air parcels also sometimes lead to cold anomalies over Europe. Most commonly the effect of adiabatic warming on the temperature anomalies is overcompensated for by advection from regions that are climatologically colder than the target region, supported by diabatic cooling along the pathway. However, there are regional differences: cold anomalies over western Europe and southeastern Europe are dominantly caused by advection and over eastern Europe by both advective and diabatic processes. The decadal-scale warming in the site of air mass origin has been partly compensated for by enhanced diabatic (radiative) cooling along the pathway to Europe. There have also been decadal changes in large-scale circulation patterns and air mass origin. Our results suggest that understanding future changes in cold extremes will require in-depth analyses of both large-scale circulation and the physical (adiabatic and diabatic) processes.","PeriodicalId":383272,"journal":{"name":"Weather and Climate Dynamics","volume":"7 8","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-11-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135678860","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Diabatic effects on the evolution of storm tracks","authors":"Andrea Marcheggiani, Thomas Spengler","doi":"10.5194/wcd-4-927-2023","DOIUrl":"https://doi.org/10.5194/wcd-4-927-2023","url":null,"abstract":"Abstract. Despite the crucial role of moist diabatic processes in mid-latitude storm tracks and related model biases, we still lack a more complete theoretical understanding of how diabatic processes affect the evolution of storm tracks. To alleviate this shortcoming, we investigate the role of diabatic processes in the evolution of the northern hemispheric storm tracks using a framework based on the tendency of the slope of isentropic surfaces as a measure of baroclinic development. We identify opposing behaviours in the near-surface and free troposphere for the relationship between the flattening of the slope of isentropic surfaces and its restoration by diabatic processes. Near the surface (900–825 hPa), cold air advection associated with cold air outbreaks initially acts to flatten isentropic surfaces, with air–sea interactions ensuing to restore surface baroclinicity. In the free troposphere (750–350 hPa), on the other hand, the diabatic generation of the slope of isentropic surfaces precedes its depletion due to tilting by eddies, suggesting the primary importance of moist diabatic processes in triggering subsequent baroclinic development. The same phasing between diabatic and tilting tendencies of the slope is observed both in upstream and downstream sectors of the North Atlantic and North Pacific storm tracks. This suggests that the reversed behaviour between near-surface and free troposphere is a general feature of mid-latitude storm tracks. In addition, we find a correspondence between the diabatic generation of the slope of isentropic surfaces and enhanced precipitation as well as moisture availability, further underlining the crucial role of moisture and moist processes in the self-maintenance of storm tracks.","PeriodicalId":383272,"journal":{"name":"Weather and Climate Dynamics","volume":"15 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-11-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135872947","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}