Upper-tropospheric Jet Research Articles

Синоптические условия повышения уровня рек на юге Иркутской области и Забайкалья в июле 2023 года

The article discusses natural and anthropogenic factors of rising river levels and associated environmental consequences, existing probabilistic estimates of the amount of floods using various hydrological and climate models. Orographic, climatic and circulation features of the occurrence of floods in the Irkutsk region are identified. A study was carried out of the circulation and weather factors of increasing the intensity and duration of precipitation in early July 2023, which was accompanied by an increase in river levels in the south of the Irkutsk region and the Baikal region. The dynamics of atmospheric processes near the Earth's surface and at the levels of isobaric surfaces AT850 hPa, AT-700 hPa, AT-500 hPa and AT-300 hPa, reflecting the conditions of surface and highaltitude cyclo- and frontogenesis, are analyzed. The contribution of advective flows of heat and moisture, mesojets in the middle troposphere and upper tropospheric jet currents in increasing the amount and duration of atmospheric precipitation is highlighted. A significant contribution to the increase in precipitation and the rise in river levels was made by moist air masses moving in the lower troposphere from the water area of the lake. Baikal, in the middle troposphere from the Yellow Sea. The time of passage of the flood and the height of the rise in the level of individual watercourses as a result of the rains were estimated.

Moist Baroclinic Instability of Mid–High-Latitude Intraseasonal Oscillations

Abstract The origin of mid–high-latitude intraseasonal oscillations (ISOs) was investigated through both observational and theoretical studies. It was found that maximum intraseasonal variability centers appear near 60°N, which is at odds with maximum synoptic variability centers that are located along the upper-tropospheric jet stream (∼45°N). The ISO along 60°N is characterized by a typical zonal wavelength of 7700 km and a westward phase speed of −3 m s−1. A marked feature of the mid–high-latitude ISO is a tight coupling among moisture and precipitation and circulation. Motivated by this observational discovery, a moist baroclinic theoretical model was constructed. The analysis of this model indicates that under a realistic background mean state, the model generates the most unstable mode along the reference latitude (60°N), which has a preferred zonal wavelength of 7000 km, a westward phase speed of about −3 m s−1, a westward tilted vertical structure, and a zonal structure of perturbation moisture/precipitation being located to the east of the low-level trough, all of which resemble the observed. The cause of the instability arises primarily from the moisture–precipitation–circulation feedback under a moderate background vertical shear.

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.

Multi-parameter dynamical diagnostics for upper tropospheric and lower stratospheric studies

Abstract. Ozone trend estimates have shown large uncertainties in the upper troposphere–lower stratosphere (UTLS) region despite multi-decadal observations available from ground-based, balloon, aircraft, and satellite platforms. These uncertainties arise from large natural variability driven by dynamics (reflected in tropopause and jet variations) as well as the strength in constituent transport and mixing. Additionally, despite all the community efforts there is still a lack of representative high-quality global UTLS measurements to capture this variability. The Stratosphere-troposphere Processes And their Role in Climate (SPARC) Observed Composition Trends and Variability in the UTLS (OCTAV-UTLS) activity aims to reduce uncertainties in UTLS composition trend estimates by accounting for this dynamically induced variability. In this paper, we describe the production of dynamical diagnostics using meteorological information from reanalysis fields that facilitate mapping observations from several platforms into numerous geophysically based coordinates (including tropopause and upper tropospheric jet relative coordinates). Suitable coordinates should increase the homogeneity of the air masses analyzed together, thus reducing the uncertainty caused by spatiotemporal sampling biases in the quantification of UTLS composition trends. This approach thus provides a framework for comparing measurements with diverse sampling patterns and leverages the meteorological context to derive maximum information on UTLS composition and trends and its relationships to dynamical variability. The dynamical diagnostics presented here are the first comprehensive set describing the meteorological context for multi-decadal observations by ozonesondes, lidar, aircraft, and satellite measurements in order to study the impact of dynamical processes on observed UTLS trends by different sensors on different platforms. Examples using these diagnostics to map multi-platform datasets into different geophysically based coordinate systems are provided. The diagnostics presented can also be applied to analysis of greenhouse gases other than ozone that are relevant to surface climate and UTLS chemistry.

Multi-scale characteristics of an extreme rain event in Shandong Province, produced by Typhoon Lekima (2019)

Super Typhoon Lekima (2019) is the fifth strongest typhoon to make landfall in mainland China since 1949. After its landfall, Typhoon Lekima moved northward along the coastline, resulting in an extreme rain event in Shandong Province that caused the highest precipitation in available meteorological records. A Weather Research and Forecasting (WRF) model simulation that properly produces the track and intensity of Typhoon Lekima and the spatio-temporal evolution of rainfall is used to analyze the multi-scale characteristics of the extreme rain event. The results show that different from the typhoon precipitation which occurred at low latitudes, the extreme rain event that occurred in mid-latitudes was influenced by the interactions of mid-latitude synoptic systems and typhoon circulation, especially with five mesoscale rainbands. The mid-latitude synoptic systems, mainly including the upper-tropospheric jet, the western North Pacific subtropical high, the mid-latitude trough, the low-level jets, and Typhoon Krosa (2019), allowed Typhoon Lekima to maintain its intensity after landfall and provided favorable kinematic, thermodynamic, and moisture conditions for the heavy rainfall in Shandong. Based on the evolution of the mesoscale rainbands, the extreme rain event can be divided into three stages. The first stage can be classified as distant rainfall, which was affected by two convective rainbands associated with boundary-layer processes. The second stage had the highest precipitation, featuring the formation of a frontal zone in Shandong, interacting with Typhoon Lekima. The third stage had weakened rainfall and was directly influenced by the spiral rainband of Typhoon Lekima.

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.

Formation Mechanisms of the Mesoscale Environment Conducive to a Downslope Windstorm over the Cuyamaca Mountains Associated with Santa Ana Wind during the Cedar Fire (2003)

Abstract Numerical simulations were conducted to investigate the upstream environment’s impacts on the airflow over the lee slope of the Cuyamaca Mountains (CM) near San Diego, California, during the Cedar Fire that occurred from 25 to 29 October 2003. The upstream environment was largely controlled by a southwest–northeast-oriented upper-tropospheric jet streak that rotated around a positively tilted ridge within the polar jet stream. Three sequential dynamical processes were found to be responsible for modifying the mesoscale environment conducive to low-level momentum and dry air that sustained the Cedar Fire. First, the sinking motion associated with the indirect circulation of the jet streak’s exit region strengthened the midtropospheric flow over the southern Rockies and the lee slope of the Sawatch and San Juan Ranges, thus modestly affecting the airflow by enhancing the downslope wind over the CM. Second, consistent with the coupling process between the upper-level sinking motion, downward momentum transfer, and developing lower-layer mountain waves, a wave-induced critical level over the mountain produced wave breaking, which was characterized by a strong turbulent mixed region with a wind reversal on top of it. This critical level helped to produce severe downslope winds leading to the third stage: a hydraulic jump that subsequently enhanced the downstream extent of the strong winds conducive to the favorable lower-tropospheric environment for rapid fire spread. Consistent with these findings was the deep-layer resonance between the mountain surface and tropopause, which had a strong impact on strengthening the severe downslope winds over the lee slope of the CM accompanying the elevated strong easterly jet at low levels.

Relationship between the South Asian High and Western North Pacific tropical cyclone genesis

This study highlights a significant inter-decadal modulation of the South Asian high (SAH) on the meridional distribution of western North Pacific (WNP) tropical cyclone genesis frequency (TCGF). There is an obvious inter-decadal change in the first leading SAH mode, with an abrupt weakening since the early 1990s. During 1979–1993, when there was a stronger SAH, TCs were suppressed over the northern WNP and enhanced over the southern WNP in response to inter-annual SAH changes. The northern WNP TCGF had a significant negative correlation with the SAH (r = −0.69), while the southern WNP TCGF had a significant positive correlation with the SAH (r = 0.78). This north-south seesaw pattern driven by the SAH weakened during 1994–2019.This inter-decadal modulation of the inter-annual SAH-WNP TC genesis relationship can be explained by changes in large-scale environmental parameters. There is anomalous ascending (descending) motion and negative (positive) vorticity centered at 850-hPa as well as less (more) moisture at 600-hPa in the northern WNP (southern WNP) during 1979–1993. The maintenance of a strong low-level anomalous anticyclone accompanied by the concurrent influence of the SAH and East Asian upper-tropospheric jet stream decreases northern WNP TCGF. Linking to an El Niño-like SST pattern, the cyclonic circulation forced by a Gill-type forcing and enhanced by equatorial low-level westerly anomalies increases southern WNP TCGF. In contrast, we find a relatively weak inter-annual relationship of large-scale factors to the SAH since 1994. This weakening relationship is largely due to the inter-decadal weakening of the SAH and East Asian upper-tropospheric jet stream. Inter-decadal changes of the East Asian summer monsoon and the phase shift of the Atlantic Multi-decadal Oscillation appear to be two important factors modulating the inter-annual relationship between the SAH and the meridional distribution of WNP TCGF.

Intra-Seasonal Features of Winter Extreme Cold Events in Northeast–North China and Synergistic Effects of Circulation Systems in Mid-High Latitude

Based on the daily minimum air temperature (Tmin) data from the China Meteorological Data Network and the NCEP/DOE reanalysis data, the intra-seasonal circulation characteristics and evolution of extreme cold events (ECEs) in Northeast–North China (NE-N) during the winter of 1979–2018 are explored, and the synergistic effects of key circulation systems in the mid-high latitude on ECEs are discussed. The results show that: (1) the winter daily Tmin in the NE-N region presents a significant low-frequency period of 10–30 d; during the cooling phases, a pair of cyclone–anticyclone in the lower troposphere moves southeastward, accompanying the intensifying Siberian High, and leads to the abnormal northerly; the developing wave trains in the middle troposphere result in enhancing and maintaining cold air; furthermore, the situation of the upper tropospheric jet weakening in the north and strengthening in the south is favorable for cold air to move southward and accumulate in the NE-N region. (2) There are two wave trains in the Eurasian at 200 hPa level. The north one moves southeastward through the Ural Mountains to the coast of East Asia, with the upstream wave activity flux dispersing to NE-N region, causing the northeast cold vortex to develop. The south one with relatively weak intensity disperses the wave flux northward, and enhances the cold vortex. (3) The key circulation systems of ECEs are the Siberian High, the Ural Mountain Blocking High, the Northeast Cold Vortex, and the East Asian Subtropical Jet. The Ural Mountains Blocking High leads four phases earlier than low temperature, and the rest of the systems are basically in phase with low temperature. The synergistic effect of circulation systems will lead to extended-range cold in the NE-N region.

North Pacific and North Atlantic Jet Covariability and Its Relationship to Cool Season Temperature and Precipitation Extremes

Abstract Cool season (September–May) extreme temperature and precipitation events are frequently tied to surface cyclones and anticyclones, which are modulated on the synoptic scale by the state and evolution of the upper-tropospheric jet stream. This study adopts a jet-centered approach to classify the prevailing large-scale flow pattern into jet regimes based on the leading modes of variability of the North Pacific jet (NPJ) and the North Atlantic jet (NAJ), respectively. The characteristics of these joint NPJ–NAJ regimes are subsequently examined using composite analysis to identify large-scale flow environments conducive to the occurrence of anomalous temperatures and precipitation across North America. The analysis reveals that composite large-scale flow environments associated with each joint NPJ–NAJ regime can be approximated as a linear combination of the separate large-scale environments that characterize each NPJ regime and NAJ regime independently. Furthermore, knowledge of the joint NPJ–NAJ regime provides more precision regarding the relative likelihood and spatial coverage of anomalous temperatures and precipitation than would be obtained from consideration of the NPJ or NAJ regime in isolation. The frequencies of each joint NPJ–NAJ regime can also be modulated by the occurrence of sudden stratospheric warmings, with increased frequencies of an equatorward-shifted NAJ and a retracted NPJ during the 30-day period following a warming event. The results from the present study demonstrate that knowledge of the joint NPJ–NAJ regime exhibits the potential to inform forecasts of anomalous temperatures and precipitation at medium-range and subseasonal time scales. Significance Statement The development of cool season temperature and precipitation extremes are modulated by the state and evolution of the upper-tropospheric jet stream. Therefore, this study adopts a jet-centered approach to quantify the extent to which temperature and precipitation extremes over North America are related to the coevolution of the North Pacific and North Atlantic segments of the jet stream. The analysis demonstrates that the relative likelihood and spatial coverage of temperature and precipitation extremes varies significantly across North America based on the combined state of the North Pacific and North Atlantic jets. The jet-centered approach utilized in this study exhibits the potential to inform operational medium-range (6–10 day) and subseasonal forecasts of temperature and precipitation across North America.

Different response of North Pacific storm tracks in the upper and lower troposphere to jet strength

The midwinter suppression of North Pacific storm tracks (NPSTs) reflects that the linear relationship between the NPST and baroclinicity breaks in winter. Based on the reanalysis data during the cold seasons of 1979–2019 and a tracking algorithm, this study analyzes the eddy growth process and shows that an enhanced upper-tropospheric jet favors the generation of upper-level eddies on the northeast side of the Pacific jet, but increasingly suppresses the generation of those in the Northwest Pacific. The upper-level eddies generated upstream of the jet core are unable to grow sufficiently throughout the whole cold season, and only those generated downstream of the jet core can grow normally and constitute the main body of the upper-level NPST. By contrast, the main lower-level eddy genesis area and growth area coincide with the baroclinic zone, with the genesis number and local growth rate increasing with the baroclinicity.摘要北太平洋风暴轴的深冬抑制表明风暴轴强度与斜压性之间的线性关系在冬季破裂. 本研究基于1979–2019年冷季的再分析数据和拉格朗日跟踪算法, 对比分析了高低层扰动的具体生长过程. 结果表明太平洋急流的增强有利于高层扰动在急流核东北侧产生, 但却抑制其在西北太平洋的生成. 在急流核上游产生的高层扰动在整个冷季都无法充分发展, 只有在急流核下游产生的高层扰动才能正常生长且它们是构成高层太平洋风暴轴的主体. 相比之下, 低层扰动的生成区和生长区都与斜压区重合, 并且它们的生成数量和局部增长率随着斜压性的增强而增强.

Zonally symmetric vs asymmetric North Pacific Ocean sea surface temperature influence on Indian summer monsoon through modulation of upper tropospheric circulation

This study asks the following question: what role the zonal distribution of sea surface temperature (SST) of North Pacific Ocean (NPAC) plays in the general circulation and vertical stability of the atmosphere during boreal summer season. Results here show that while zonally symmetric SST warming of NPAC shifts the upper tropospheric jet northward, zonally asymmetric warming and cooling alter the phase of mid latitude Rossby waves in east-west direction. These changes in circulation modify the vertical static stability of the atmosphere and precipitation, particularly in the Indian region. In particular, warmer SSTs in the western NPAC reduce the stability over Indian region leading to an enhanced monsoon precipitation even when El Nino-Southern Oscillation (ENSO) is not in favorable phase. In conjunction with an ENSO forcing, however, this phenomenon can augment or suppress its impact on Indian summer monsoon. This result is used to derive an index that explains more than 80% of the interannual variance of Indian summer monsoon (ISM) precipitation in June-July. Since June-July contributes about 50% to the total June-September precipitation in this region, this discovery increases the potential predictability of ISM. Finally, using historical simulations from the Coupled Model Intercomparison Project (CMIP5), we show that SST anomalies over NPAC strongly influence the strength of the relationship between ENSO and Indian summer monsoon. The mechanism proposed in this study can be used to diagnose the skill of general circulation models.

Impacts of Variations in Caspian Sea Surface Area on Catchment‐Scale and Large‐Scale Climate

AbstractThe Caspian Sea (CS) is the largest inland lake in the world. Large variations in sea level and surface area occurred in the past and are projected for the future. The potential impacts on regional and large‐scale hydroclimate are not well understood. Here, we examine the impact of CS area on climate within its catchment and across the northern hemisphere, for the first time with a fully coupled climate model. The Community Earth System Model (CESM1.2.2) is used to simulate the climate of four scenarios: (a) larger than present CS area, (b) current area, (c) smaller than present area, and (d) no‐CS scenario. The results reveal large changes in the regional atmospheric water budget. Evaporation (e) over the sea increases with increasing area, while precipitation (P) increases over the south‐west CS with increasing area. P‐E over the CS catchment decreases as CS surface area increases, indicating a dominant negative lake‐evaporation feedback. A larger CS reduces summer surface air temperatures and increases winter temperatures. The impacts extend eastwards, where summer precipitation is enhanced over central Asia and the north‐western Pacific experiences warming with reduced winter sea ice. Our results also indicate weakening of the 500‐hPa troughs over the northern Pacific with larger CS area. We find a thermal response triggers a southward shift of the upper troposphere jet stream during summer. Our findings establish that changing CS area results in climate impacts of such scope that CS area variations should be incorporated into climate model simulations, including palaeo and future scenarios.

Meridional Tripole Mode of Winter Precipitation over the Arctic and Continental North Africa–Eurasia

AbstractWintertime precipitation is vital to the growth of glaciers in the northern hemisphere. We find a tripole mode of precipitation (PTM), with each pole of the mode extending zonally over the eastern hemisphere roughly between 30°W and 120°E, and the positive/negative/positive structure for its positive phase extending meridionally from the Arctic to the continental North Africa–Eurasia. The large-scale dynamics associated with the PTM is explored. The positive phase of the PTM is associated with the negative while eastward-shifted phase of the North Atlantic Oscillation (NAO) and a zonal band of positive SST anomaly in the tropics, together with a narrowed Hadley cell and weakened Ferrel cell. While being north-eastward tilted and separated from their North Africa-Eurasia counterpart in the climatological mean, the upper-tropospheric westerly jets over the east Pacific and north Atlantic become extending zonally and shifting southward and hence form a circumpolar subtropical jet as a whole by connecting with the westerly jets over the North Africa-Eurasia. The enhanced zonal winds over the north Atlantic promote more synoptic-scale transient eddies which are waveguided by the jet streams. The polar vortex weakens and cold air dips southward from the North Pole. Further diagnosis of the E-vectors suggests that transient eddies have a positive feedback on the weakening of Ferrel cell. Opposite features are associated with the negative phase of the PTM. The reconstructed time series using multiple linear regression on the NAO index and the tropical SST averaged over 20°S– 20°N, can explain 62.4% of the variance of the original the original precipitation time series.

Seasonal and regional signatures of ENSO in upper tropospheric jet characteristics from reanalyses

AbstractThe relationship of upper tropospheric jet variability to El Niño / Southern Oscillation (ENSO) in reanalysis datasets is analyzed for 1979–2018, revealing robust regional and seasonal variability. Tropical jets associated with monsoons and the Walker circulation are weaker and the zonal mean subtropical jet shifts equatorward in both hemispheres during El Niño, consistent with previous findings. Regional and seasonal variations are analyzed separately for subtropical and polar jets. The subtropical jet shifts poleward during El Niño over the NH eastern Pacific in DJF, and in some SH regions in MAMand SON. Subtropical jet altitudes increase during El Niño, with significant changes in the zonal mean in the NH and during summer/fall in the SH. Though zonal mean polar jet correlations with ENSO are rarely significant, robust regional/seasonal changes occur: The SH polar jet shifts equatorward during El Niño over Asia and the western Pacific in DJF, and poleward over the eastern Pacific in JJA and SON. Polar jets are weaker (stronger) during El Niño in the western (eastern) hemisphere, especially in the SH; conversely, subtropical jets are stronger (weaker) in the western (eastern) hemisphere during El Niño in winter and spring; these opposing changes, along with an anticorrelation between subtropical and polar jet windspeed, reinforce subtropical/polar jet strength differences during El Niño, and suggest ENSO-related covariability of the jets. ENSO-related jet latitude, altitude, and windspeed changes can reach 4(3)°, 0.6(0.3) km, and 6(3) ms−1, respectively, for the subtropical (polar) jets.

Atmospheric Rivers and Associated Precipitation over France and Western Europe: 1980–2020 Climatology and Case Study

Atmospheric rivers are important atmospheric features implicated in the global water vapor budget, the cloud distribution, and the associated precipitation. The ARiD (Atmospheric River Detector) code has been developed to automatically detect atmospheric rivers from water vapor flux and has been applied to the ECMWF ERA5 archive over the period 1980–2020 above the Atlantic Ocean and Europe. A case study of an atmospheric river formed in the East Atlantic on August 2014 that reached France has been detailed using ECMWF ERA5 reanalysis, ground based observation data, and satellite products such as DARDAR, AIRS, GPCP, and GOES. This atmospheric river event presents a strong interaction with an intense upper tropospheric jet stream, which induced stratosphere–troposphere exchanges by tropopause fold. A 1980–2020 climatology of atmospheric rivers over Europe has been presented. The west of France, Iberian Peninsula, and British Islands are the most impacted regions by atmospheric rivers with an occurrence of up to four days per month during the October–April period. Up to 40% of the precipitation observed on the west European coast can be linked to the presence of ARs. No significant trend in the occurrence of the phenomena was found over 1980–2020.

Interdecadal Change in the Relationship between the Winter North Pacific Storm Track and the East Asian Winter Monsoon

AbstractBased on the daily NCEP reanalysis, the present study investigates the interdecadal change in the relationship between the winter North Pacific storm track (WNPST) and the East Asian winter monsoon (EAWM), and evaluates the WNPST–EAWM relationship in 17 CMIP6 models. The results show that the out-of-phase WNPST–EAWM relationship underwent an interdecadal change in the mid-1980s. The WNPST–EAWM relationship became less significant during period 2 (P2; 1990–2015). The atmospheric circulation anomaly related to the EAWM during period 1 (P1; 1955–80) is more robust than that during P2. The interdecadal weakening WNPST–EAWM relationship may be attributed to the interdecadal damping WNPST–EAWM interaction. The EAWM-related anomalous baroclinic energy conversion and moisture effects, including meridional and vertical eddy moisture fluxes, contribute to the significant attenuation of the WNPST during P1. The transient eddy-induced dynamic forcing and thermal forcing anomalies, as well as the barotropic process represented by the local Eliassen–Palm flux divergence associated with WNPST, can also significantly manipulate the upper-tropospheric jet during P1. However, the atmospheric circulation and interaction between the WNPST and EAWM during P2 are not as significant as those during P1. The effect of ENSO on the WNPST is significantly different before and after the mid-1980s. After the mid-1980s, the WNPST shows the characteristic of moving equatorward during El Niño events. It seems that ENSO takes over the WNPST from the EAWM after the mid-1980s. In addition, except for BCC-ESM1, CanESM5, and SAM0-UNICON, most of the CMIP6 models cannot reproduce the significant out-of-phase WNPST–EAWM relationship.

Rapid Arctic warming and its link to the waviness and strength of the westerly jet stream over West Asia

Rapid sea-ice loss and warming of the Arctic within the past few decades have contributed to changes in the strength and location of midlatitude jet streams. Using ERA-Interim data for the period 1979–2018, warming of the Arctic compared to midlatitudes and its impact on the waviness and strength of the westerly jet stream over West Asia are investigated. Pre- and post-Arctic amplification periods are defined as 1979–1993 and 2004–2018, respectively. Near-surface temperature variability is decreased in the Arctic in the post-Arctic amplification period when a larger area of open ocean has been exposed to the atmosphere. In both pre- and post-Arctic amplification periods, near-surface temperature variabilities in spring and summer were less than half as much of its variability in winter. During the period 1979–2018, both the Arctic and West Asia have warmed, with much higher warming of the Arctic in autumn and winter. Indeed, in contrast to the continental cooling pattern observed in parts of North and Northeast Asia, West Asia has warmed in recent decades. Following a decrease in the meridional temperature gradient, the upper-tropospheric jet stream over West Asia has been reduced in the post-Arctic amplification period in all seasons except autumn. In the post-Arctic amplification period, the zonal wind shear has been also reduced over West Asia in all seasons, while waviness of the Z500 isopleth has been increased in winter. A higher meridional mid-tropospheric waviness is found over the study region (20–75°E) in summer compared to winter because in summer the western part of this region is influenced by a strong subtropical high pressure, while its eastern part is influenced by the Indian monsoon and associated trough of low pressure. • Near-surface temperature variability is decreased in the Arctic in the post-Arctic amplification period • In contrast to cooling of some midlatitude regions, West Asia has warmed in recent decades • Upper-tropospheric jet stream over West Asia has been reduced in the post-Arctic amplification period • Zonal wind shear has been reduced over West Asia in all seasons, but waviness of Z500 isopleth has been increased in winter

Mechanisms for Springtime Onset of Isolated Precipitation across the Southeastern United States

This study uses four-year radar-based precipitation organization and reanalysis datasets to study the mechanisms that lead to the abrupt springtime onset of precipitation associated with isolated storms in the Southeast United States (SE US). Although the SE US receives relatively constant precipitation year-round, previous work demonstrated a “hidden” summertime maximum in isolated precipitation features (IPF) whose annual cycle resembles that of monsoon climates in the subtropics. In the SE US, IPF rain abruptly ramps up in May and lasts until sometime between late August and early October. This study suggests that the onset of the IPF season in the SE US is brought about by a combination of slow thermodynamic processes and fast dynamic triggers, as follows. First, in the weeks prior to IPF onset, a gradual seasonal build-up of convective available potential energy (CAPE) occurs in the Gulf of Mexico. Then, in one-to-two pentads prior to onset, the upper-tropospheric jet stream shifts northward, favoring the presence of slow-moving frontal systems in the SE US. This poleward shift in the jet stream location in turn allows the establishment of the North Atlantic subtropical high western ridge over the SE US which, with associated poleward transport of high CAPE air from the Gulf of Mexico, leads to the establishment of the warm-season regime of IPF precipitation in the SE US.

Upper Tropospheric Cyclonic Vortex and Brazilian Northeast Jet Stream over Alagoas State: Circulation Patterns and Rainfall

Abstract Interaction between Brazilian Northeast Jet Stream (BNEJS) and Upper Tropospheric Cyclonic Vortexes (UTCV) isn't still well known, as well as its seasonal variability and relation with rainfall pattern. This study's aim is to determine the air circulation patterns in UTCV events with BNEJS in La Niña and El Niño years and its connections with rainfall in the Alagoas State. These connections were analyzed for 9 years between 1988 and 2000 in El Niño, La Niña and Neutral years. NCEP Reanalysis data, rainfall data from 71 meteorological stations and satellite images were used. Fifty-eight percent of 140 observed UTCVs were associated with BNEJS events located in the Brazilian Northeast and 26%, located over the Alagoas State. Three types of BNEJS patterns, denominated as Meridional (M), Zonal (Z) and Transversal (T), have been identified. The M-type BNEJS was the most frequent in Alagoas State, especially in El Niño years. Z and T-types occurred less frequently and were rarely seen in La Niña years. The results showed that in the M-type BNEJS events in 48% of days were not registered precipitation, while Z and T events recorded rainfall in almost 96% of the days.