scholarly journals A network-based detection scheme of the jet stream core

2016 ◽  
Author(s):  
Sonja Molnos ◽  
Tarek Mamdouh ◽  
Stefan Petri ◽  
Thomas Nocke ◽  
Tino Weinkauf ◽  
...  
Keyword(s):  
Western Hemisphere ◽  
Jet Stream ◽  
Data Sets ◽  
Jet Streams ◽  
Detection Scheme ◽  
The North ◽  
Eastern Hemisphere ◽  
Subtropical Jet ◽  
Latitudinal Position ◽  
Upper Level

Abstract. The polar and subtropical jet streams are strong upper-level winds with a crucial influence on weather throughout the Northern Hemisphere mid-latitudes. In particular, the polar jet is located between cold Arctic air to the North and warmer sub-tropical air to the South. Strongly meandering states therefore often lead to extreme surface weather. So far algorithms to detect jets' core around the hemisphere, including strong North-South undulations, are lacking. We develop a network-based scheme using Dijkstra's shortest path algorithm to detect the polar and subtropical jet stream core. This algorithm considers not only the commonly used wind strength for core detection but also takes wind direction and climatological latitudinal position into account. Furthermore, it distinguishes between polar and subtropical jet, and between separate and merged jet states. The detection scheme is optimized using simulated annealing and compared against an algorithm developed by Rikus (2015). After the successful optimization process we apply our scheme to climatology data and analyse seasonal data sets. We present probabilistic, regionally distinct positions for both jets for all seasons. This shows that winter is characterized by two well separated jets at mean longitudes of 20° S and 140° N. In contrast, summer normally has a single merged jet over the western Hemisphere and both merged and separated jet states possible in the eastern Hemisphere. With this algorithm it is possible to investigate the position of the jets' cores around the hemisphere and is therefore well suitable for analyses of jet stream patterns in observations or models.

scholarly journals A network-based detection scheme for the jet stream core

2017 ◽  
Vol 8 (1) ◽  
pp. 75-89 ◽  
Author(s):  
Sonja Molnos ◽  
Tarek Mamdouh ◽  
Stefan Petri ◽  
Thomas Nocke ◽  
Tino Weinkauf ◽  
...  
Keyword(s):  
Northern Hemisphere ◽  
Probability Distributions ◽  
Reanalysis Data ◽  
Western Hemisphere ◽  
Jet Stream ◽  
Jet Streams ◽  
Detection Scheme ◽  
The North ◽  
Eastern Hemisphere ◽  
Subtropical Jet

Abstract. The polar and subtropical jet streams are strong upper-level winds with a crucial influence on weather throughout the Northern Hemisphere midlatitudes. In particular, the polar jet is located between cold arctic air to the north and warmer subtropical air to the south. Strongly meandering states therefore often lead to extreme surface weather. Some algorithms exist which can detect the 2-D (latitude and longitude) jets' core around the hemisphere, but all of them use a minimal threshold to determine the subtropical and polar jet stream. This is particularly problematic for the polar jet stream, whose wind velocities can change rapidly from very weak to very high values and vice versa. We develop a network-based scheme using Dijkstra's shortest-path algorithm to detect the polar and subtropical jet stream core. This algorithm not only considers the commonly used wind strength for core detection but also takes wind direction and climatological latitudinal position into account. Furthermore, it distinguishes between polar and subtropical jet, and between separate and merged jet states. The parameter values of the detection scheme are optimized using simulated annealing and a skill function that accounts for the zonal-mean jet stream position (Rikus, 2015). After the successful optimization process, we apply our scheme to reanalysis data covering 1979–2015 and calculate seasonal-mean probabilistic maps and trends in wind strength and position of jet streams. We present longitudinally defined probability distributions of the positions for both jets for all on the Northern Hemisphere seasons. This shows that winter is characterized by two well-separated jets over Europe and Asia (ca. 20° W to 140° E). In contrast, summer normally has a single merged jet over the western hemisphere but can have both merged and separated jet states in the eastern hemisphere. With this algorithm it is possible to investigate the position of the jets' cores around the hemisphere and it is therefore very suitable to analyze jet stream patterns in observations and models, enabling more advanced model-validation.

A Role for Barotropic Eddy–Mean Flow Feedbacks in the Zonal Wind Response to Sea Ice Loss and Arctic Amplification

2019 ◽  
Vol 32 (21) ◽  
pp. 7469-7481 ◽  
Author(s):  
Bryn Ronalds ◽  
Elizabeth A. Barnes
Keyword(s):  
Sea Ice ◽  
North Atlantic ◽  
Zonal Wind ◽  
Jet Stream ◽  
Arctic Amplification ◽  
Mean Flow ◽  
Jet Streams ◽  
The North ◽  
Wind Response ◽  
The North Pacific

Abstract Previous studies have suggested that, in the zonal mean, the climatological Northern Hemisphere wintertime eddy-driven jet streams will weaken and shift equatorward in response to Arctic amplification and sea ice loss. However, multiple studies have also pointed out that this response has strong regional differences across the two ocean basins, with the North Atlantic jet stream generally weakening across models and the North Pacific jet stream showing signs of strengthening. Based on the zonal wind response with a fully coupled model, this work sets up two case studies using a barotropic model to test a dynamical mechanism that can explain the differences in zonal wind response in the North Pacific versus the North Atlantic. Results indicate that the differences between the two basins are due, at least in part, to differences in the proximity of the jet streams to the sea ice loss, and that in both cases the eddies act to increase the jet speed via changes in wave breaking location and frequency. Thus, while baroclinic arguments may account for an initial reduction in the midlatitude winds through thermal wind balance, eddy–mean flow feedbacks are likely instrumental in determining the final total response and actually act to strengthen the eddy-driven jet stream.

scholarly journals Intraseasonal Variability in a Dry Atmospheric Model

2007 ◽  
Vol 64 (7) ◽  
pp. 2422-2441 ◽  
Author(s):  
Hai Lin ◽  
Gilbert Brunet ◽  
Jacques Derome
Keyword(s):  
North Atlantic ◽  
Intraseasonal Variability ◽  
Atmospheric Model ◽  
Wave Activity ◽  
Western Hemisphere ◽  
Mean Flow ◽  
The North ◽  
Eastern Hemisphere ◽  
The North Atlantic ◽  
Wave Activity Flux

Abstract A long integration of a primitive equation dry atmospheric model with time-independent forcing under boreal winter conditions is analyzed. A variety of techniques such as time filtering, space–time spectral analysis, and lag regressions are used to identify tropical waves. It is evident that oscillations with intraseasonal time scales and a Kelvin wave structure exist in the model tropical atmosphere. Coherent eastward propagations in the 250-hPa velocity potential and zonal wind are found, with a speed of about 15 m s−1. The oscillation is stronger in the Eastern Hemisphere than in the Western Hemisphere. Interactions between the tropical and extratropical flows are found to be responsible for the simulated intraseasonal variability. Wave activity flux analysis reveals that a tropical influence occurs in the North Pacific region where a northeastward wave activity flux is found associated with the tropical divergent flow in the western and central Pacific. In the North Atlantic sector, on the other hand, a strong extratropical influence is observed with a southward wave activity flux into the Tropics. The extratropical low-frequency variability develops by extracting kinetic energy from the basic mean flow and through interactions with synoptic-scale transient eddies. Linear experiments show that the tropical atmospheric response to the extratropical forcing in the North Atlantic leads to an eastward-propagating wave in the tropical easterly mean flow of the Eastern Hemisphere.

scholarly journals Modified Three-Dimensional Jet Indices and Their Application to East Asia

Atmosphere ◽  
2019 ◽  
Vol 10 (12) ◽  
pp. 776 ◽  
Author(s):  
Haishan Li ◽  
Ke Fan ◽  
Zhiqing Xu ◽  
Hua Li
Keyword(s):  
East Asia ◽  
East Asian ◽  
Three Dimensional ◽  
Vertical Direction ◽  
Jet Stream ◽  
Jet Streams ◽  
Zonal Winds ◽  
Subtropical Jet Stream ◽  
Subtropical Jet ◽  
The Relationship

A set of three-dimensional jet indices (jet speed index, jet pressure index, jet latitude index) has been proposed in previous literature to describe the variation of jet streams in both the horizontal and vertical direction. We refer to these indices at the ‘AC’ indices, after the names of the researchers involved. However, the physical meaning of the AC indices and the relationship between AC indices and climate systems are not fully understood. Further study is still needed for applying the indices in East Asia (70°–140° E). In this study, based on the understanding of the physical meaning of the AC indices, latitudinal ranges of East Asian jet streams are determined, and a set of modified AC indices is proposed. Based on the modified AC indices, the linear trends in East Asian jet streams are studied, and the relationship between East Asian jet streams and the climate is researched. The results show that the jet speed index corresponds to the meridional temperature gradient (MTG) of the middle to upper troposphere (500–200 hPa); the jet pressure index corresponds to the pressure level at which the MTG equals zero; and the jet latitude reflects the meridional MTG distribution. The latitudinal ranges of jet streams are determined based on the meridional profiles of climatological zonal-mean zonal winds. Within such a latitudinal range, the climatological zonal-mean zonal winds between 400 and 100 hPa are only westerly, and the maximum wind speed in the vertical direction at every latitude appears between 400 and 100 hPa. The jet streams can be further classified according to the features of the profiles. For East Asia (70°–140° E), jet streams can be classified into winter subtropical jet streams (15°–47.5° N), summer subtropical jet streams (27.5°–60° N), and summer polar front jet streams (60°–87.5° N). The classification of jet streams can be supported by their correspondence to the distribution of tropospheric baroclinicity. A set of modified AC indices can be acquired by using the new ranges of East Asian jet streams in the definition of the original AC indices. Descriptions of jet streams using the modified AC indices are more in accordance with the distributional features of the climatological zonal winds over East Asia, and the physical meanings of the modified AC indices are more definite than the original indices. Using the modified AC indices, we find a significant weakening trend in the strength of the summer subtropical jet stream (−0.13 m/s/10 yr) and a significant northward shift of the winter subtropical jet stream (0.22°/10 yr), and the possible reasons for these trends are studied. Finally, the relationships of East Asian jet streams in winter and summer with atmospheric circulation, temperature, and precipitation are also investigated in this study.

scholarly journals Tropical–Extratropical Interactions Associated with an Atlantic Tropical Plume and Subtropical Jet Streak

2005 ◽  
Vol 133 (9) ◽  
pp. 2759-2776 ◽  
Author(s):  
Peter Knippertz
Keyword(s):  
North America ◽  
South America ◽  
Atlantic Ocean ◽  
North Atlantic Ocean ◽  
Cloud Formation ◽  
Cloud Band ◽  
The North ◽  
Northwest Africa ◽  
Subtropical Jet ◽  
Upper Level

Abstract Tropical plumes (TPs) are elongated bands of upper- and midlevel clouds stretching from the Tropics poleward and eastward into the subtropics, typically accompanied by a subtropical jet (STJ) streak and a trough on their poleward side. This study uses ECMWF analyses and high-resolution University of Wisconsin–Nonhydrostatic Modeling System trajectories to analyze the multiscale complex tropical–extratropical interactions involved in the genesis of a pronounced TP and STJ over the NH Atlantic Ocean in late March 2002 that was associated with extreme precipitation in arid northwest Africa. Previous concepts for TP genesis from the literature are discussed in the light of this case study. Analysis of the upper-level flow prior to the TP formation shows a northeastward propagation and a continuous acceleration of the STJ over the Atlantic Ocean equatorward of a positively tilted upper-level trough to the west of northwest Africa. Both dynamic and advective processes contribute to the generation of the accompanying cloud band. The northern portion of the TP consists of parcels that exit a strong STJ streak over North America, enter the deep Tropics over South America, and then accelerate into the Atlantic STJ, accompanied by strong cross-jet ageostrophic motions, rising, and cloud formation. The southern portion is formed by parcels originating in the divergent outflow from strong near-equatorial convection accompanying the TP genesis. A local increase in the Hadley overturning is found over the tropical Atlantic and east Pacific/South America and appears to be related to low inertial stability at the outflow level and to low-level trade surges associated with the cold advection, sinking, and lower-level divergence underneath two strong upper-level convergence centers in the eastern portions of both a subtropical ridge over North America and an extratropical ridge over the North Atlantic Ocean. Evidence is presented that the convective response lags the trade surge by several days.

scholarly journals Mid-level convection in a warm conveyor belt accelerates the jet stream

2020 ◽  
Author(s):  
Nicolas Blanchard ◽  
Florian Pantillon ◽  
Jean-Pierre Chaboureau ◽  
Julien Delanoë
Keyword(s):  
Large Scale ◽  
Doppler Radar ◽  
Conveyor Belt ◽  
Jet Stream ◽  
Lagrangian Analysis ◽  
Sensitivity Experiment ◽  
The North ◽  
Mesoscale Structures ◽  
Upper Level ◽  
The Impact

Abstract. Jet streams and potential vorticity (PV) gradients along upper-level ridges and troughs form a waveguide that governs midlatitude dynamics. Warm conveyor belt (WCB) outflows often inject low-PV air into ridges and their representation is seen as a source of uncertainty for downstream forecasts. Recent studies have highlighted the presence of mesoscale structures of negative PV in WCBs, the impact of which on large-scale dynamics is still debated. Here, fine-scale observations of cloud and wind structures acquired with airborne Doppler radar and dropsondes provide rare information on the WCB outflow of the Stalactite cyclone and the associated upper-level ridge on 2 October 2016 during the North Atlantic Waveguide and Downstream Impact Experiment. The observations reveal a complex tropopause structure with a high PV tongue separating the northwestern edge of the ridge in two parts, each with cirrus-type clouds and accompanied by a jet stream core, and bounded by a tropopause fold. A reference, convection-permitting simulation with full physics reproduces well the observed mesoscale structures and reveals the presence of elongated negative PV bands along the eastern jet stream core. In contrast, a sensitivity experiment with heat exchanges due to cloud processes cut off shows lower cloud tops, weaker jet stream cores, a ridge less extended westward, and the absence of negative PV bands. A Lagrangian analysis based on online trajectories shows that the anticyclonic branch of the WCB outflow feeds the eastern jet stream core in the reference simulation, while it is absent in the sensitivity experiment. The anticyclonic ascents and negative PV bands originate from the same region near the cyclone's bent-back front. The most rapid ascents coincide with mid-level convective cells identified by clustering analysis, which are located in a region of conditional instability below the jet stream core and above a low-level jet. Horizontal PV dipoles are found around these cells and with the negative poles reaching absolute negative values, thus appear as the source of negative PV bands. The results show that mid-level convection within WCBs accelerates the jet stream and may thus influence the downstream large-scale circulation.

scholarly journals Climatic analysis of effective jet streams frequency on extreme precipitations in west of Iran

2020 ◽  
Author(s):  
Shahab Shaffie ◽  
GholamAli Mozaffari ◽  
Younes Khosravi
Keyword(s):  
Geographical Location ◽  
Jet Stream ◽  
The West ◽  
Jet Streams ◽  
Positive Vorticity ◽  
Subtropical Jet Stream ◽  
Unstable Layer ◽  
Four Levels ◽  
Upper Level ◽  
Generalized Distribution

Abstract In this study, the frequency of effective jet streams was analyzed in extreme and widespread precipitations in the west of Iran. For this purpose, the daily precipitation of 69 synoptic and climatic stations over 18,624 days (1961–2010) were selected. Then, 119 days of extreme and widespread precipitation in the study area were chosen based on generalized distribution for conducting related reviews and analyses. The frequency of jet streams in the geographical location from 0° to 120°E and −10° to 80°N were reviewed at four levels (250, 300, 400 and 500 hPa). Due to the large volume of information, only the highest and lowest levels (250 and 500 hPa) in relation to the surface were considered. According to the results, the highest frequency of jet stream was observed at 250 hPa. The second quarter of the jet stream core lay over the west of Iran (which is associated with increasing positive vorticity as well as upper-level divergence and lower-level convergence of the atmosphere). In general, the extension of jet stream up to 500 hPa indicated an unstable layer thickness, which can cause extreme and widespread precipitation in the west of Iran. The results of selected days based on cluster analysis and Lund correlation revealed that in rainy days, the wind speed was more than 50 m/s and the subtropical jet stream speed was over 40 m/s, leading to extreme precipitation in the west of Iran.

scholarly journals Anthropogenic aerosol effects on tropospheric circulation and sea surface temperature (1980–2020): separating the role of zonally asymmetric forcings

2021 ◽  
Vol 21 (24) ◽  
pp. 18499-18518
Author(s):  
Chenrui Diao ◽  
Yangyang Xu ◽  
Shang-Ping Xie
Keyword(s):  
Surface Temperature ◽  
Radiative Forcing ◽  
Western Hemisphere ◽  
Hadley Cell ◽  
Jet Stream ◽  
High Latitudes ◽  
Aerosol Forcing ◽  
Eastern Hemisphere ◽  
Northeastern Pacific ◽  
Tropospheric Circulation

Abstract. Anthropogenic aerosols (AAs) induce global and regional tropospheric circulation adjustments due to the radiative energy perturbations. The overall cooling effects of AA, which mask a portion of global warming, have been the subject of many studies but still have large uncertainty. The interhemispheric contrast in AA forcing has also been demonstrated to induce a major shift in atmospheric circulation. However, the zonal redistribution of AA emissions since start of the 20th century, with a notable decline in the Western Hemisphere (North America and Europe) and a continuous increase in the Eastern Hemisphere (South Asia and East Asia), has received less attention. Here we utilize four sets of single-model initial-condition large-ensemble simulations with various combinations of external forcings to quantify the radiative and circulation responses due to the spatial redistribution of AA forcing during 1980–2020. In particular, we focus on the distinct climate responses due to fossil-fuel-related (FF) aerosols emitted from the Western Hemisphere (WH) versus the Eastern Hemisphere (EH). The zonal (west to east) redistribution of FF aerosol emission since the 1980s leads to a weakening negative radiative forcing over the WH mid-to-high latitudes and an enhancing negative radiative forcing over the EH at lower latitudes. Overall, the FF aerosol leads to a northward shift of the Hadley cell and an equatorward shift of the Northern Hemisphere (NH) jet stream. Here, two sets of regional FF simulations (Fix_EastFF1920 and Fix_WestFF1920) are performed to separate the roles of zonally asymmetric aerosol forcings. We find that the WH aerosol forcing, located in the extratropics, dominates the northward shift of the Hadley cell by inducing an interhemispheric imbalance in radiative forcing. On the other hand, the EH aerosol forcing, located closer to the tropics, dominates the equatorward shift of the NH jet stream. The consistent relationship between the jet stream shift and the top-of-atmosphere net solar flux (FSNTOA) gradient suggests that the latter serves as a rule-of-thumb guidance for the expected shift of the NH jet stream. The surface effect of EH aerosol forcing (mainly from low- to midlatitudes) is confined more locally and only induces weak warming over the northeastern Pacific and North Atlantic. In contrast, the WH aerosol reduction leads to a large-scale warming over NH mid-to-high latitudes that largely offsets the cooling over the northeastern Pacific due to EH aerosols. The simulated competing roles of regional aerosol forcings in driving atmospheric circulation and surface temperature responses during the recent decades highlight the importance of considering zonally asymmetric forcings (west to east) and also their meridional locations within the NH (tropical vs. extratropical).

Which regional cloud-radiative changes are most important for the global warming response of the midlatitude jet streams?

2020 ◽  
Author(s):  
Nicole Albern ◽  
Aiko Voigt ◽  
David W. J. Thompson ◽  
Joaquim G. Pinto
Keyword(s):  
Global Warming ◽  
Southern Hemisphere ◽  
Northern Hemisphere ◽  
North Atlantic ◽  
Radiative Heating ◽  
Jet Stream ◽  
Jet Streams ◽  
The North ◽  
The North Atlantic ◽  
The Impact

<p>Clouds and the midlatitude circulation are strongly coupled via radiation. Previous studies showed that global cloud-radiative changes contribute significantly to the global warming response of the midlatitude circulation. Here, we investigate the impact of regional cloud-radiative changes and identify which regional cloud-radiative changes are most important for the impact of global cloud-radiative changes. We show how tropical, midlatitude and polar cloud-radiative changes modify the annual-mean, wintertime and summertime jet stream response to global warming across ocean basins. To this end, we perform global simulations with the atmospheric component of the ICOsahedral Nonhydrostatic (ICON) model. We prescribe sea surface temperatures (SST) to isolate the impact of cloud-radiative changes via the atmospheric pathway, i.e. changes in atmospheric cloud-radiative heating, and mimic global warming by a uniform 4K SST increase. We apply the cloud-locking method to break the cloud-radiation-circulation coupling and to decompose the circulation response into contributions from cloud-radiative changes and from the SST increase.</p><p>In response to global warming, the North Atlantic, North Pacific, Northern Hemisphere and Southern Hemisphere jet streams shift poleward and the North Atlantic, Northern Hemisphere and Southern Hemisphere jets strengthen. Global cloud-radiative changes contribute to these jet responses in all ocean basins. <span>In the annual-mean and DJF, tropical and midlatitude cloud-radiative changes contribute significantly to the poleward jet shift in all ocean basins. </span><span>P</span><span>olar cloud-radiative changes shift the jet streams </span><span>poleward </span><span>in the northern hemispheric ocean basins </span><span>but</span> <span>equatorward </span><span>in the Southern Hemisphere. In JJA, the poleward jet shift is small in all ocean basins. In contrast to the jet shift, the global cloud-radiative impacts on the 850hPa zonal wind and jet strength responses </span><span>result predominantly from </span><span>tropical cloud-radiative </span><span>changes</span><span>.</span></p><p><span>The cloud-radiative impact on the jet shift can be related to changes in upper-tropospheric baroclinicity via increases in upper-tropospheric meridional temperature gradients, enhanced wave activity and increased eddy momentum fluxes. However, the response of the atmospheric temperature to cloud-radiative heating is </span><span>more difficult to understand because it is modulated by other small-scale processes such as convection and the circulation.</span><span> Our results help to understand the jet stream response to global warming and highlight the importance of regional cloud-radiative changes for this response, </span><span>in particular those in the tropics</span><span>.</span></p>

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