Upper-tropospheric Jet Stream Research Articles

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.

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.

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.

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.

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.

Kinetic energy budget and moisture flux convergence analysis during interaction between two cyclonic systems: Case study

An analysis of the kinetic energy budget during a case of interaction between middle latitude and extratropical cyclones has been made in this work. Horizontal flux convergence constitutes a major energy sink. Generation of kinetic energy via cross-contour flow is a persistent source throughout the growth and decay periods. Dissipation of kinetic energy from subgrid to grid scales is an important source during the pre-storm period; it acts as a sink during the growth and decay periods. The major contribution to kinetic energy comes from a persistent upper tropospheric jet stream activity throughout the period of the cyclone development. The characteristics of moisture-flux components (divergent and rotational) along with precipitable water content for different tropospheric layers throughout the life cycle of our cyclone are also studied in this work. It is found that most of required humidity for our cyclone are initiated from Arabian Sea and then to some extent are reinforced over Gulf of Aden and east of central Africa and then by passing over Red sea enter to the south and south east of Mediterranean Sea. The rotational component of the moisture transport brings moisture from two regions; the first which is considered the main region is the Indian Ocean, Arabian Sea, Gulf of Aden and north east of Sudan. The second source region is the Atlantic and Mediterranean Sea. In the middle troposphere, the primary moisture source is found over central Africa, which in turn is traced to the Atlantic Ocean, the Indian Ocean, and the Arabian Sea. The upper-level moisture fluxes are weak and play a minor role over the area of interaction between two cyclones.

Interdecadal Weakening of the East Asian Winter Monsoon in the Mid-1980s: The Roles of External Forcings

Observations show that the East Asian winter monsoon (EAWM) experienced an interdecadal weakening in the mid-1980s. This is evident for all members of the EAWM system (i.e., East Asian trough, upper-tropospheric jet stream, and lower-tropospheric monsoon circulation). Here, we investigate the relative contributions of natural (volcanic aerosols and solar variability) and anthropogenic [greenhouse gases (GHGs) and anthropogenic aerosols] forcings to this interdecadal weakening using multiple coupled models within phase 5 of the Coupled Model Intercomparison Project (CMIP5). The results indicate that in the midtroposphere, the increased GHG concentrations play an important role in weakening the East Asian trough (EAT) by increasing the sea surface temperatures (SSTs) over the North Pacific. In the upper troposphere, natural external forcings contribute to the observed weakening of the meridional shear of the East Asian jet stream (EAJS) by regulating the meridional temperature gradient (MTG) over the East Asian region. In the lower troposphere, both anthropogenic and natural forcings can weaken the Siberian high during this period. Overall, based on the present analysis of the CMIP5 output, GHGs and natural forcings play key roles in shaping the observed interdecadal weakening of the EAWM during the mid-1980s. Additionally, contributions from internal variability cannot be neglected and require further investigation.

Case Study and Climatological Analysis of Upper-Tropospheric Jet Stream and Stratosphere–Troposphere Exchanges Using VHF Profilers and Radionuclide Measurements in France

AbstractThe authors present a climatological analysis of tropospheric horizontal wind profiles and jet stream events using long series of wind profiles from two VHF profilers located in France: Lannemezan (2001–14) and Opme (1999–2014). A case study of jet stream and stratospheric intrusion of air into the troposphere that occurred in January 2013 is first described and demonstrates the capability of the VHF profilers to detect jet stream events. The climatology study over the two sites reveals the strongest values of seasonal wind during winter (21.4 m s−1at 8.7-km height at Opme; 25.1 m s−1at 9.6-km height at Lannemezan). A methodology based on the automatic detection of maximum winds on a decadal series of hourly wind profiles allows the detection of jet stream events and establishes its climatology for each site. A frequency analysis of jet stream events of westerly winds over 50 m s−1presents a clear seasonality at the two sites, with a maximum in winter (3.5%–9.7% of hourly profiles) and a minimum in summer (near 1%). Cosmogenic radionuclides sampled at Opme also exhibit a clear seasonal variation with maximum in spring and minimum in the cold seasons; the7Be/22Na activity ratio confirms stratosphere-to-troposphere exchanges for the studied cases. The mean interannual variability of the frequency of jet stream events is 1.5% in Opme and 2.9% in Lannemezan. Positive decadal trends are observed for the two sites: +1.6 ± 1.2% decade−1for Opme and +2.4 ± 2.2% decade−1for Lannemezan.

A case study of long gravity wave crests in noctilucent clouds and their origin in the upper tropospheric jet stream

AbstractAtmospheric gravity waves with very long crests (of 450–500 km length) and short horizontal wavelengths of about 20 km were observed in noctilucent clouds and were studied in detail for the first time. The gravity waves were slowly moving in opposite direction to the background wind indicating their forced generation outside the mesopause region. A ray‐tracing analysis using meteorological reanalysis and empirical atmospheric model data shows that a source of such peculiar gravity waves observed in noctilucent clouds was located near the tropopause and could be associated with the jet stream at altitudes 8–10 km. Two considered examples of very long wave crests confirm a significant role of the upper tropospheric jet stream as a source of gravity waves and reveal that these waves could propagate without critical levels to the mesopause in summertime.

Interaction of an Upper-Tropospheric Jet with a Squall Line Originating along a Cold Frontal Boundary

AbstractOn 8 June 2003, an expansive squall line along a surface cold frontal boundary was sampled during the Bow Echo and Mesoscale Convective Vortex Experiment. The Naval Research Laboratory P-3 aircraft and the National Oceanic and Atmospheric Administration P-3 aircraft simultaneously sampled the leading and trailing edge of this squall line, respectively, with X-band Doppler radars. Data from these two airborne radar systems have been synthesized to produce a pseudo-quad-Doppler analysis of the squall line, yielding a detailed three-dimensional kinematic analysis of its structure. A simulation of the squall line was carried out using the Weather Research and Forecasting Model to complement the pseudo-quad-Doppler analysis. The simulation employed a 3-km, convection-allowing, nested domain centered over the pseudo-quad-Doppler domain, along with a 9-km parent domain to capture the larger synoptic-scale cyclone.The pseudo-quad-Doppler analysis reveals that the convective line was embedded within the upper-tropospheric jet stream, causing local decelerations and deviations in the jet-level flow. The vertical transport of low momentum air from the boundary layer via convective updrafts is shown to significantly decelerate jet-level flow. Pressure perturbations associated with the intrusion of low momentum air into the jet stream–level flow led to deviation of the jet stream flow around the squall line that resulted in counter-rotating ribbons of vertical vorticity parallel to the squall line. Model results indicate that disturbances in the jet stream structure persisted downwind of the squall line for several hours.

Regional‐scale jet waviness modulates the occurrence of midlatitude weather extremes

AbstractSeveral studies have attributed the occurrence of recent weather extremes to an amplified waviness of the upper tropospheric jet stream. Although trends in jet waviness are still under discussion, it is crucial to better understand the mechanisms through which jet waviness affects weather extremes. Here we show that variations in jet waviness on regional scales effectively modulate the occurrence of daily weather extremes but in regionally different ways. The jet waviness over the North Atlantic and the North Pacific mainly affects where wind, precipitation and cold extremes occur, while a wavy jet over Eurasia strongly favors the occurrence of hot extremes in summer. This is because regional variations of jet waviness are intrinsically linked to the occurrence and tracks of synoptic‐scale weather systems, which can trigger the extremes. We conclude that potential jet waviness changes would affect the occurrence of weather extremes differently depending on where these changes occur.

Using Causal Effect Networks to Analyze Different Arctic Drivers of Midlatitude Winter Circulation

AbstractIn recent years, the Northern Hemisphere midlatitudes have suffered from severe winters like the extreme 2012/13 winter in the eastern United States. These cold spells were linked to a meandering upper-tropospheric jet stream pattern and a negative Arctic Oscillation index (AO). However, the nature of the drivers behind these circulation patterns remains controversial. Various studies have proposed different mechanisms related to changes in the Arctic, most of them related to a reduction in sea ice concentrations or increasing Eurasian snow cover.Here, a novel type of time series analysis, called causal effect networks (CEN), based on graphical models is introduced to assess causal relationships and their time delays between different processes. The effect of different Arctic actors on winter circulation on weekly to monthly time scales is studied, and robust network patterns are found. Barents and Kara sea ice concentrations are detected to be important external drivers of the midlatitude circulation, influencing winter AO via tropospheric mechanisms and through processes involving the stratosphere. Eurasia snow cover is also detected to have a causal effect on sea level pressure in Asia, but its exact role on AO remains unclear. The CEN approach presented in this study overcomes some difficulties in interpreting correlation analyses, complements model experiments for testing hypotheses involving teleconnections, and can be used to assess their validity. The findings confirm that sea ice concentrations in autumn in the Barents and Kara Seas are an important driver of winter circulation in the midlatitudes.

Effect of high-frequency wind on intraseasonal SST variabilities over the mid-latitude North Pacific region during boreal summer

The effect of high-frequency (period <20 days) wind on the intraseasonal (period 20–100 days) sea surface temperature (SST) anomalies over the mid-latitude North Pacific region (35°–45°N, 160°E–170°W) during boreal summer was examined through the diagnosis of reanalysis data and numerical experiments. The reanalysis data diagnosis shows that the near-surface high-frequency (HF) wind is weaker (stronger) during the intraseasonal SST warming (cooling) phase. The phase-dependent amplitude of HF wind is controled by the strength change of the upper-tropospheric westerly jet stream. Because the magnitude of the HF wind over the target region shows significantly positive correlation with the total wind speed, a weaker (higher) HF wind in the SST warming (cooling) phase tends to decrease (increase) total wind speed, which can further suppress (enhance) upward surface latent heat and sensible heat fluxes and strengthen the intraseasonal SST variability. The numerical experiments with an oceanic general circulation model demonstrate that the HF wind can amplify the intraseasonal SST variability over the target region mainly through nonlinearly rectifying intraseasonal surface latent and sensible heat fluxes. The HF wind can explain about 20 % of the intraseasonal SST variability in the target region.

Kinetic Energy Budget of a Tropical Cyclone

An analysis of the kinetic energy budget is made for a tropical cyclone. Horizontal flux convergence constitutes a major energy source. Generation of kinetic energy via cross-contour flow is a persistent sink throughout the pre-storm and growth periods. Dissipation of kinetic energy from subgrid to grid scales is an important source during the pre-storm and growth periods; it acts as the major sink of energy during the decay period. The major contribution to kinetic energy comes from a persistent upper tropospheric jet stream activity throughout the period of the cyclone development. Unlike midlatitude cyclones, a considerable quantity of kinetic energy appears between 850 - 500 hPa layers especially during the growth period. While the behavior of the values of horizontal divergence by nondivergent wind closely resemble to those of total horizontal divergence term, neglecting the divergent part of the wind would clearly lead to a considerable error in the calculation of total horizontal divergence. The mean error in approximation of total horizontal divergence by the nondivergent part during the life cycle of our cyclone is about 36%.

Rainy-Season Precipitation over the Sichuan Basin and Adjacent Regions in Southwestern China

Abstract The spatial variability and diurnal propagation of mean precipitation in the summer rainy seasons (from 2003 to 2010) over the Sichuan basin (SCB) and adjacent mountainous regions are examined using high spatiotemporal resolution satellite-derived precipitation estimates. The SCB is located just east of the Tibetan Plateau (TP) and is prone to heavy precipitation that often peaks over nighttime and early morning. The large-scale environment over the SCB during the rainy season is characterized by weak low- to midtropospheric convergence in the lee of the TP and by the upper-tropospheric jet stream to the north. Under this flow configuration, the study links the unique diurnal variations in the precipitation pattern and propagation to the unique topography in this region. It is found that during the rainy season, the local diurnal precipitation maximum moves primarily downslope and southeastward, from over the TP in the daytime to SCB at night. A secondary maximum moves northeastward downslope of the Yunnan–Guizhou Plateau toward the SCB from late evening to the early morning. The movement of precipitation over the SCB and the adjacent regions is closely tied to multiple regional-scale mountain–plain solenoids because of the large contrast in terrain heights between the basin and surrounding mountain ranges.

Thunderstorms enhance tropospheric ozone by wrapping and shedding stratospheric air

AbstractA significant source of ozone in the troposphere is transport from the stratosphere. The stratospheric contribution has been estimated mainly using global models that attribute the transport process largely to the global‐scale Brewer‐Dobson circulation and synoptic‐scale dynamics associated with upper tropospheric jet streams. We report observations from research aircraft that reveal additional transport of ozone‐rich stratospheric air downward into the upper troposphere by a leading‐line‐trailing‐stratiform mesoscale convective system with convection overshooting the tropopause altitude. The fine‐scale transport demonstrated by these observations poses a significant challenge to global models that currently do not resolve storm‐scale dynamics. Thus, the upper tropospheric ozone budget simulated by global chemistry‐climate models where large‐scale dynamics and photochemical production from lightning‐produced NO are the controlling factors may require modification.

Spatial differences in seasonal variation of the upper-tropospheric jet stream in the Northern Hemisphere and its thermal dynamic mechanism

NCEP/NCAR reanalysis daily data from 1951 to 2008 are used in this study to reveal the spatial-asymmetric features in the seasonal variation of the upper-tropospheric jet stream (UTJS) and its thermal dynamic forcing mechanism. The jet occurrence percentage distribution of the UTJS demonstrates a spiral-like pattern in winter, but it is quasi-annular in summer. The jet occurrence percentage in the Eastern Hemisphere is larger than that in the Western Hemisphere, and its maximum area is located further south. The polar front jet stream (PJS) and subtropical jet stream (SJS) can be distinguished over the Northern Africa and Asian regions, whereas only one jet stream can be observed over the Western Pacific and Atlantic Ocean. Furthermore, a single peak pattern is found in the seasonal variation of the SJS occurrence frequency with the highest jet occurrence appearing in winter and the lowest in summer, while a double peak pattern is observed in the seasonal variation of the PJS occurrence, i.e., the jet occurrence reaches its peaks in autumn and spring for the PJS. Based on the thermal wind theory, air temperature gradient and atmospheric baroclinicity are calculated and compared with the jet occurrence variation to explore the thermal dynamic forcing mechanism for the UTJS variation. In addition, synoptic-scale transports of eddy heat and momentum are also calculated. The results indicate that the SJS variation is primarily determined by the air temperature gradient and atmospheric baroclinicity, while the PJS variation is under great influence of the transport of eddy heat and momentum over Northern Africa and East Asia. The UTJS variation over the area from 140E to 70W cannot be well individually explained by the air temperature gradient and atmospheric baroclinicity. Further analysis indicates that UTJS variation over this area is largely under control of combined effect of the transport of eddy heat and momentum as well as the atmospheric baroclinicity.

Tibetan Plateau westerly forcing on the cloud amount over Sichuan Basin and the early Asian summer monsoon

Data show that a sizeable mechanical effect of the Tibetan Plateau (TP) on the westerly, referred to as TP westerly forcing, is related to a sizeable midlevel cloud amount over Sichuan Basin (SB; centered near 30°N and 105°E) to the east of the TP. An investigation of the interannual variability of clouds over SB demonstrated that the TP westerly forcing closely correlates to the early Asian summer monsoon (ASM). When the monsoon commences in mid‐May, the East Asian upper tropospheric jet stream moves northward with a southwest‐northeast tilt; the tilt of the jet considerably influences the TP westerly forcing. Because of the stationary wave pattern superimposed on the westerly jet, the early ASM is substantially distinct by year. In the years the midlevel cloud amount in SB is sizeable (which corresponds to a strong TP westerly forcing), the tilt of the upper level jet increases, and the ascending southerly east of SB in 110°E–125°E strengthens. The warming of the troposphere in 110°E–125°E centered near 35°N corresponds to the westerly and southerly changes. Farther downstream in the 125°E–140°E region, the ascending southerly weakens. Correspondingly, an anomalous anticyclonic circulation occurs between SB and Japan, leading to a westward extension of the North Pacific high pressure in the lower troposphere and a reduction in precipitation of the early Mei‐Yu season. Coincidentally, the midtropospheric monsoon trough south of the TP weakens, implying a weak monsoon in both East Asia and South Asia. Furthermore, the weak monsoon continues through the early summer.