Wave Activity Flux Research Articles

Two Types of Mid-High-Latitude Intraseasonal Oscillations in the Southern Hemisphere during Austral Summer

Abstract Using NCEP–NCAR reanalysis data, the atmospheric intraseasonal oscillation (ISO) over the mid-high latitudes in the Southern Hemisphere during austral summer is studied. Two types of 10–30-day eastward- and westward-propagating ISO at mid-high latitudes are extracted by extended empirical orthogonal functions. The analysis of wave activity flux reveals that the energy sources of the eastward-propagating wave train are in the southwest Pacific and southern Indian Ocean, and the westward-propagating wave train is over the east coast of South America. The diagnosis of geopotential height tendency shows that the zonal gradient of ISO relative vorticity guided by mean zonal wind plays a major role in the eastward propagation of the wave train, while the meridional gradient of mean relative vorticity and geostrophic vorticity guided by ISO meridional wind plays a major role in the westward propagation of the wave train. Energy analysis shows that the amplitude enhancement over the South Pacific is due to the ISO disturbance gaining energy from the mean flow. The eastward-propagating ISO obtains energy from the mean flow through kinetic and potential energy conversion, while the westward-propagating ISO cannot obtain energy through potential energy conversion. The surface air temperature and precipitation in South America are affected by the circulation and propagation corresponding to both types of ISO. The useful forecasting skills by the subseasonal-to-seasonal forecasting project for eastward- and westward-propagating types can reach to 17 days, while the potential forecasting skills improve to 20 and 19 days, respectively. Significance Statement The intraseasonal oscillation (ISO) has important effects on regional weather and climate, and we focus on the ISO over mid-high latitudes in the Southern Hemisphere, which has been paid little attention. We reveal two leading types of the mid-high-latitude ISO during austral summer, namely, the eastward- and westward-propagating types. Also, the characteristics and effects of the two types are explored. To understand their dynamical process, the geopotential height tendency and energy conversion are diagnosed. It is also found that the two ISO types can be predicted up to 17 days in advance by the subseasonal-to-seasonal project, which may provide guidance for the extended-range forecasts.

Causes of 2022 Pakistan flooding and its linkage with China and Europe heatwaves

In boreal summer of 2022, Pakistan experienced extremely high rainfall, resulting in severe flooding and displacing over 30 million people. At the same time, heatwaves persisted over central China and Europe. The coexistence of these extreme events suggests a possible linkage. Our analysis indicated that the record rainfall was mainly induced by compounding factors. These included (1) La Niña-induced strong anomalous easterlies over the northern Indian subcontinent, (2) intense southerlies from the Arabian Sea with an upward trend in recent decades, (3) an interaction between extratropical and tropical systems, specifically the northerly flow downstream of the Europe blocking and the southerly monsoon flow from the Arabian Sea. Wave activity flux and regression analyses unveiled a distinct stationary Rossby wave-like pattern connecting the flooding in Pakistan and heatwaves in Europe and China. This pattern, an emerging teleconnection pattern in recent decade, exhibited substantial differences from the reported teleconnection patterns. We also noted the positive feedback of the excessive Pakistan rainfall could further enhance the large-scale background flow and the heavy rainfall itself. The 2022 Pakistan flood event was an intensified manifestation of the 2010 Pakistan flood event, which was also caused by compounding factors, but occurred in a more pronounced upward trend in the both tropics and extratropics.

Atmospheric bias teleconnections in boreal winter associated with systematic sea surface temperature errors in the tropical Indian Ocean

Abstract. Coupled climate models suffer from significant sea surface temperature (SST) biases in the tropical Indian Ocean (TIO), leading to errors in global climate predictions. In this study, we investigate the local and remote effects of the TIO SST bias on the simulated atmospheric circulation and spatio-temporal variability – bias teleconnections. A set of century-long simulations forced by idealized SST perturbations, which resemble various (monopolar or dipolar, positive or negative) TIO SST biases in coupled climate models, are conducted with an intermediate-complexity atmospheric model. Bias teleconnections with a focus on boreal wintertime are analysed using the normal-mode function (NMF) decomposition, which can differentiate between balanced and unbalanced flows across spatial scales. The results show that the atmospheric circulation biases caused by the TIO SST bias have the Gill–Matsuno-type pattern in the tropics and Rossby-wave-train structure in the extratropics, similar to the steady-state response to tropical heating perturbations. The teleconnections between the tropical and extratropical biases are set up by Rossby wave activity flux emanating from the subtropics. Over 90 % of the bias variance (i.e. the square of the bias amplitude) is contained in zonal wavenumbers k≤5. The northward shift of the SST bias away from the Equator weakens the amplitude but does not change the spatial structure of the atmospheric response. Besides, the positive SST bias produces stronger bias teleconnections than the negative one of the same size and magnitude. In the NMF framework, the change in the spatial variance of the time-mean state (i.e. energy) is equal to the sum of the bias variance and the covariance between the circulation bias and the reference state (i.e. bias covariance). Due to the TIO SST biases, the global unbalanced zonal-mean (k=0) flow energy decreases, whereas its balanced counterpart increases. These changes primarily arise from the strong bias covariance. For k>0, both the global unbalanced and the tropical balanced energies increase in the case of a monopolar SST bias and decrease in the case of a dipolar SST bias. The increase appears mainly as the bias variance, whereas the decrease is associated with a strong negative bias covariance at k=1 and 2. In contrast, the extratropical balanced wave energy decreases (increases) when the TIO SST bias is positive (negative), which is mainly associated with the bias covariance at k=1. The change in the interannual variance (IAV) is contingent upon the sign of the TIO SST bias. A positive bias reduces, whereas a negative one increases, the IAV in both balanced and unbalanced flows. Geographically, large IAV changes are observed in the tropical Indo-West Pacific region, Australia, South and Northeast Asia, the Pacific-North America region, and Europe, where the background IAVs are strong.

Weather pattern conducive to the extreme summer heat in North China and driven by atmospheric teleconnections

Extreme summer heat can have severe socioeconomic impacts and has occurred frequently in North China in recent years, most notably in June–July 2023, when North China experienced the most widespread, persistent, and high-intensity extreme heat on record. Here, typical weather patterns covering North China and its surrounding areas were classified into seven types based on the Cost733class package, and the weather pattern type 4 (T4), characterized by the strengthened ridge and anticyclone anomaly in northeastern China, was found as the most favorable for the occurrence of extreme summer heat in North China (NCSH). Diagnostic and wave activity flux analyses indicate that the Eurasian teleconnection (EAT) pattern from the atmosphere and the Victoria mode (VM) from the ocean are the top two dominant climate drivers of the T4 weather pattern. The empirical models constructed based on the EAT and the VM can effectively simulate the number of days of the T4 weather pattern and the NCSH, respectively. Our results suggest that, with the help of the seasonal forecast from climate models, the EAT and the VM can be used to predict the number of days of the T4 weather pattern and the NCSH for the coming summer, enabling us to protect human health and reduce its socioeconomic impacts through proactive measures in advance.

Relationship between the North Atlantic sea surface temperature and the summer extreme high temperature in the Beijing-Tianjin-Hebei region, China

Climate change influence on the economy and environment in the Beijing–Tianjin–Hebei (BTH) region. This study examined the variations of extreme high temperature days (EHDs) over the BTH region in summer during 1982 to 2021. The trend and frequency of EHDs has increased significantly during the last 40 years. The spatial and temporal variations of EHDs also indicate a significant increase by using the rotated empirical orthogonal functions (REOF) method. Furthermore, the singular value decomposition (SVD) method and correlation analysis are used to indicate the relationship between the EHDs in June–July-August (JJA) in the BTH region and North Atlantic sea surface temperature anomalies (SSTA) in the March–April-May (MAM) during 1982–2021. The result shows that there was positive relationship between the North Atlantic SSTA in MAM and the EHDs in JJA in the BTH region. Composite analysis suggests that relationship between EHDs and SSTA are connected by associated atmospheric circulation through the wave activity flux.

Regional‐Scale Heavy Precipitation Over Fujian‐Jiangxi Region in the Late Spring‐Early Summer of 2006 and Its Connection With Baroclinic Wave Packets in the Upper Troposphere

AbstractThe most serious heavy precipitation in the past 43 years occurred over Fujian‐Jiangxi region from late May to early June of 2006, causing significant economic losses. Using the ERA5 reanalysis database and daily precipitation collected at 2,479 surface meteorological stations in China, the present study investigates the relationship between the heavy precipitation over Fujian‐Jiangxi region in late spring‐early summer of 2006 and baroclinic wave packets in the upper troposphere. Information flow between the two systems has been diagnosed. Results indicate that different from the disturbance source for Meiyu in the middle and lower reaches of the Yangzi River, the disturbance source for this heavy precipitation originated from areas near the Syrian Desert to the north of the Arabian Peninsula and propagated along the northwest‐southeast direction, reaching Fujian‐Jiangxi region 4 days later. This kind of baroclinic wave packets provide the necessary energy for the occurrence and persistence of heavy precipitation. Analysis of wave activity flux vectors indicates that during the heavy precipitation period, disturbance energy was transported from the upstream westerly belt to Fujian‐Jiangxi region almost every day. Obviously, there existed information transfer between the two regions, re‐confirming that the upstream Rossby wave packets affect the Fujian‐Jiangxi precipitation. The above results provide helpful hints for a better understanding of the mechanisms for heavy precipitation in this region and will be helpful for its effective prediction.

11-year solar cycle influences on the late-wintertime South Asian jet variability

The South Asian jet leading wave train (SAJLWT) is a zonally elongated and equivalently barotropic stationary Rossby wave train along the wintertime South Asian subtropical jet, which stands out as the leading empirical orthogonal function mode of monthly meridional winds at the upper troposphere over southern Eurasia. The SAJLWT is closely related to weather and climate extremes over southern Eurasia, but up to now, the mechanisms of SAJLWT variability are still not fully understood. The present study reveals a significant 11-year solar cycle (SC) influence on the SAJLWT variability in late winter (January–March). The in-phase correlation between the SC and the SAJLWT is not only statistically significant but also consistent over time. Associated with the solar-induced SAJLWT anomalies, significant surface cooling exists over northeastern Africa and Middle East, while significant positive precipitation anomalies appear over southern China. Two routes of mechanisms are presented to clarify the SC–SAJLWT linkage. One is due to the solar-induced northwest–southeast tilted North Atlantic Oscillation (TNAO) pattern, which may reflect a “top–down” solar forcing and the ocean–atmosphere couplings in the North Atlantic Ocean. This TNAO pattern would trigger southeastward wave activity fluxes toward the SAJ, thus significantly impacting the upstream portions of the SAJLWT pattern. The other mechanism involves weakened convection over the Maritime Continent (MC) region, reflecting solar-induced weakening of Pacific Walker Circulation via “bottom–up” processes. The MC convection anomaly would also excite a SAJLWT-like circulation pattern. Our analysis highlights that the solar “top–down” and “bottom–up” mechanisms may act in concert to exert a significant impact on the SAJLWT and indicates that the SC forcing is an appreciable source of decadal predictability in southern Eurasia.

Decadal variability of extreme high temperature in mid- and high-latitude Asia and its associated North Atlantic air–sea interaction

The decadal variability of extreme high temperature (EHT) in the mid and highlatitudes of continental Asia (MHLCA) and associated mechanisms were investigated. Observational analysis indicated that, after removing global warming, the first leading mode of the EHT events showed a meridional dipole pattern and had significant decadal variability. During the periods 1980–1994 and 2012–2019 (1995–2011), EHT events were more frequent and stronger (less frequent and weaker) in western-central Siberia than normal, whereas they were less frequent and weaker (more frequent and stronger) in the wide area to the south of Lake Baikal. Further Observational and CESM (Community Earth System Model) results based analysis suggested that decadal change in air–sea interaction in the North Atlantic play an important role in shaping the decadal variability of EHT events in MHLCA. On decadal timescales, meridional negative–positive–negative sea surface temperature (SST) anomalies in the North Atlantic and their coupled positive North Atlantic Oscillation can trigger stronger wave activity flux and cause a significant anticyclonic–cyclonic teleconnection wave train in the troposphere over the mid and high latitudes of the Eurasian continent. As a result, an anomalous high-pressure center is evident in western-central Siberia, and thereby reducing total cloud cover and causing stronger solar heating. Thus, EHT events become more frequent and stronger. On the contrary, for the areas south of Lake Baikal, EHT events are weakened by corresponding cyclonic circulation anomalies. Additionally, the CAM5 (Community Atmosphere Model version 5) experiments suggested more important driving role of the decadal North Atlantic tripole SST anomalies in the abovementioned processes.

Similarity and variability of blocked weather-regime dynamics in the Atlantic–European region

Abstract. Weather regimes govern an important part of the sub-seasonal variability of the mid-latitude circulation. Due to their role in weather extremes and atmospheric predictability, regimes that feature a blocking anticyclone are of particular interest. This study investigates the dynamics of these “blocked” regimes in the North Atlantic–European region from a year-round perspective. For a comprehensive diagnostic, wave activity concepts and a piecewise potential vorticity (PV) tendency framework are combined. The latter essentially quantifies the well-established PV perspective of mid-latitude dynamics. The four blocked regimes (namely Atlantic ridge, European blocking, Scandinavian blocking, and Greenland blocking) during the 1979–2021 period of ERA5 reanalysis are considered. Wave activity characteristics exhibit distinct differences between blocked regimes. After regime onset, Greenland blocking is associated with a suppression of wave activity flux, whereas Atlantic ridge and European blocking are associated with a northward deflection of the flux without a clear net change. During onset, the envelope of Rossby wave activity retracts upstream for Greenland blocking, whereas the envelope extends downstream for Atlantic ridge and European blocking. Scandinavian blocking exhibits intermediate wave activity characteristics. From the perspective of piecewise PV tendencies projected onto the respective regime pattern, the dynamics that govern regime onset exhibit a large degree of similarity: linear Rossby wave dynamics and nonlinear eddy PV fluxes dominate and are of approximately equal relative importance, whereas baroclinic coupling and divergent amplification make minor contributions. Most strikingly, all blocked regimes exhibit very similar (intra-regime) variability: a retrograde and an upstream pathway to regime onset. The retrograde pathway is dominated by nonlinear PV eddy fluxes, whereas the upstream pathway is dominated by linear Rossby wave dynamics. Importantly, there is a large degree of cancellation between the two pathways for some of the mechanisms before regime onset. The physical meaning of a regime-mean perspective before onset can thus be severely limited. Implications of our results for understanding predictability of blocked regimes are discussed. Further discussed are the limitations of projected tendencies in capturing the importance of moist-baroclinic growth, which tends to occur in regions where the amplitude of the regime pattern, and thus the projection onto it, is small. Finally, it is stressed that this study investigates the variability of the governing dynamics without prior empirical stratification of data by season or by type of regime transition. It is demonstrated, however, that our dynamics-centered approach does not merely reflect variability that is associated with these factors. The main modes of dynamical variability revealed herein and the large similarity of the blocked regimes in exhibiting this variability are thus significant results.

Effects of Rossby Waves Breaking and Atmospheric Blocking Formation on the Extreme Forest Fire and Floods in Eastern Siberia 2019

In 2019, the southern region of Eastern Siberia (located between 45° N and 60° N) experienced heavy floods, while the northern region (between 60° N and 75° N) saw intense forest fires that lasted for almost the entire summer, from 25 June to 12 August. To investigate the causes of these natural disasters, we analyzed the large-scale features of atmospheric circulation, specifically the Rossby wave breaking and atmospheric blocking events. In the summer of 2019, two types of Rossby wave breaking were observed: a cyclonic type, with a wave breaking over Siberia from the east (110° E–115° E), and an anticyclonic type, with a wave breaking over Siberia from the west (75° E–90° E). The sequence of the Rossby wave breaking and extreme weather events in summer, 2019 are as follows: 24–26 June (cyclonic type, extreme precipitation, flood), 28–29 June and 1–2 July (anticyclonic type, forest fires), 14–17 July (both types of breaking, forest fires), 25–28 July (cyclonic type, extreme precipitation, flood), 2 and 7 August (anticyclonic type, forest fires). Rossby wave breaking occurred three times, resulting in the formation and maintenance of atmospheric blocking over Eastern Siberia: 26 June–3 July, 12–21 July and 4–10 August. In general, the scenario of the summer events was as follows: cyclonic Rossby wave breaking over the southern part of Eastern Siberia (45° N–60° N) caused extreme precipitation (floods) and led to low gradients of potential vorticity and potential temperature in the west and east of Lake Baikal. The increased wave activity flux from the Europe–North Atlantic sector caused the anticyclonic-type Rossby wave breaking to occur west of the area of a low potential vorticity gradient and north of 60° N. This, in turn, contributed to the maintenance of blocking anticyclones in the north of Eastern Siberia, which led to the intensification and expansion of the area of forest fires. These events were preceded by an increase in the amplitude of the quasi-stationary wave structure over the North Atlantic and Europe during the first half of June.

Interdecadal Variation in Rossby Wave Source over the Tibetan Plateau and Its Impact on the East Asia Circulation Pattern during Boreal Summer

The wave activity flux representing the energy propagation direction of planetary Rossby wave generally originates from a large wave source area. This study investigates the interdecadal variability and formation mechanism of Rossby wave source over the Tibetan Plateau (TP-RWS) and its impact on the atmospheric circulation and precipitation pattern in East Asia based on the ERA-20C reanalysis dataset in summer (June–July–August) during 1900 to 2010. Results show that the region with the maximum variabilities of Rossby wave source (RWS) in the past 110 years appears over the Tibetan Plateau (TP) during boreal summer, and the TP-RWS shows prominent characteristics of interdecadal oscillation. Secondly, the TP-RWS is mainly composed of the vortex stretching term (RWS-S1) and the absolute vorticity advection term (RWS-S2). The interdecadal TP-RWS is a synergistic result of the snow cover over northwestern TP associated with the RWS-S1, and the deep convection over southeastern TP associated with the RWS-S2. Furthermore, the interdecadal TP-RWS can lead to an alternatively positive and negative pattern of geopotential height anomalies from the northwestern TP to the North Pacific, which has a great climate effect on the precipitation in Huang-huai River Basin, South Korea and Japan Island. Under the guidance of the anomalous cyclonic circulation in East Asia, the prevailing southerly and easterly winds occur over the West Pacific and the Huang-huai River Basin, which lead to the water vapor convergence and upward movement at middle and lower troposphere.

Part II model support on a new mechanism for North Pacific Oscillation influence on ENSO

Owing to the significant influence of El Niño-Southern Oscillation (ENSO) on global climate, how ENSO events are initiated is an intriguing issue. The North Pacific Oscillation (NPO), a primary atmospheric variability over the midlatitude, is a well-known trigger for ENSO events, but the physical linkage is not yet fully understood. Based on observational analyses, in Part I, we proposed a new mechanism that the NPO-related wave activity flux (WAF) could directly induce the equatorial wind anomalies in both upper and lower levels. In this study, we substantiate the impacts of the WAF on tropical circulations using climate models participating in the Coupled Model Intercomparison Project phases 5 and 6 (CMIP5/6). We found that the intensity of the southward WAF over the central Pacific is a paramount factor resulting in intermodel diversity in simulating the NPO–ENSO linkage. By classifying the models into two groups of strong and weak meridional WAF (MWAF), we reveal that the strong MWAF models simulate stronger upper- and lower-level equatorial winds and precipitation anomalies that facilitate the ENSO in subsequent winter. We also reveal that the magnitude of the MWAF is closely related to the model’s climatological meridional wind and meridional shear of climatological zonal wind, emphasizing the role of systematic bias on the ENSO simulation. A comparison of the MWAF impact and seasonal footprinting mechanism demonstrates the dominant influence of the MWAF in determining the diversity of NPO–ENSO relationships.

Part I observational study on a new mechanism for North Pacific Oscillation influencing the tropics

The North Pacific Oscillation (NPO), a representative midlatitude atmospheric variability, plays an important role in the development of the El Niño-Southern Oscillation (ENSO). To explain this extratropical–tropical linkage, previous studies have focused on the atmospheric boundary layer processes coupled with the mixed-layer ocean. Different from the existing hypothesis, in this study, we propose a new mechanism to link the NPO to ENSO via upper-tropospheric teleconnections. Analyses of the wave activity flux show that wave energy associated with the NPO directly propagates from midlatitude to the tropics, modulating the tropical circulation. During the NPO event, this equatorward energy flux becomes pronounced after the NPO peak phase and persists for more than two weeks. As a result, when a positive NPO grows (here, north anticyclonic–south cyclonic circulation), upper-level easterly wind anomalies are situated along the equatorial Pacific. Accordingly, anomalous lower-level westerly winds simultaneously occur in the equatorial Pacific, contributing to the development of El Niño events. To demonstrate the wave energy propagation via the upper-level troposphere, a stationary wave model experiment was performed with an NPO-like barotropic vorticity forcing. The results show equatorward wave propagation consistent with the observation.

Exceptionally persistent Eurasian cold events and their stratospheric link

Persistent boreal winter cold spells (PCEs) can heavily strain the economy and significantly impact everyday life. While sudden stratospheric warmings are considered a precursor for Eurasian (EUR) cold events, these temperature extremes may occur during the full range of stratospheric variability. We investigate PCEs relative to the prevailing stratospheric polar vortex regime before their onset, with a particular focus on extremely weak (SSW) and strong (SPV) stratospheric winds by performing (lagged) composite analysis based on ERA5 reanalysis. On average, SPV PCEs that are concentrated over central EUR, are colder, shorter and set in more abruptly compared to SSW PCEs. A quasi-stationary, mid-tropospheric anticyclone over the Arctic Ocean that blocks warm air advection toward EUR is connected to the canonical downward progression of the negative North Atlantic Oscillation for SSW PCEs. In contrast, during SPV PCEs, the anticyclone is part of a Rossby wave having an origin co-located with negative wave activity flux anomalies over and being influenced by stratospheric wave reflection toward the North Atlantic. Its slow east-ward propagation is likely related to Arctic surface warming and unusually weak zonal winds over EUR.

The Remote Role of North‐American Mesoscale Convective Systems on the Forecast of a Rossby Wave Packet: A Multi‐Model Ensemble Case‐Study

AbstractNorth American Mesoscale Convective Systems (MCSs) have been linked to instances of poorly forecasted Rossby wave packets (RWPs). A computationally inexpensive investigation is proposed to demonstrate a dynamical mechanism by which MCSs modify a RWP associated with a high‐impact weather event. Global ensemble forecast data, reanalysis and high‐resolution observations are used to assess the remote role of negative potential vorticity (PV) arising from divergent outflow on RWP propagation coinciding with the 11–21 June 2017 European heatwave. In this case, synoptic‐scale bands of negative PV which advect toward the jet stream arise from regions of active MCSs. The forecast data results show that the numerical misrepresentation of the anticyclonic circulation associated with negative PV can impinge on the forecast of a RWP. In each of the four forecasting models assessed, ensemble members that advected lower values of PV toward the equatorward branch of a North American ridge favored enhanced poleward amplification of the ridge and a more eastward progression of the RWP. The more eastward displacement of the RWP also coincided with an enhanced wave activity flux downstream. Although, we do not find a significant impact on the forecasted heatwave. The results urge further investigation into the role of negative PV in remotely influencing high‐impact weather.

Numerical simulation of stratospheric QBO impact on the planetary waves up to the thermosphere

With the help of numerical simulation, a detailed analysis of the dynamical effect of the stratospheric quasi-biennial oscillation (QBO) of the equatorial zonal wind on the planetary waves (PWs) up to thermospheric heights is carried out for the first time. The 3-dimensional nonlinear mechanistic model of middle and upper atmosphere (MUAM) is used, which is capable of simulating the general atmospheric circulation from the surface up to 300–400 km altitude. The amplitudes of stationary and westward travelling PWs with periods from 4 to 10 days are calculated based on ensembles of model simulations for conditions corresponding to the easterly and westerly QBO phases. Fluxes of wave activity and refractive indices of the atmosphere are calculated to analyze the detailed behavior of the PWs. The important result to emerge is that the stratospheric QBO causes statistically significant changes in the amplitudes of individual wave components up to 25% in the mesosphere-lower thermosphere and 10% changes above 200 km. This change in wave structures should be especially noticeable in the atmosphere during periods of low solar activity, when the direct contribution of solar activity fluctuations is minimized. Propagating from the troposphere to the upper atmosphere, PWs contribute to the propagation of the QBO signal not only from the equatorial region to extratropical latitudes, but also from the stratosphere to the thermosphere. The need for a detailed analysis of large-scale wave disturbances in the upper atmosphere and their relationship with the underlying layers is due, in particular, to their significant impact on satellite navigation and communication systems, which is caused by amplitude and phase fluctuations of the radio signal.

Excitation of European Transient Warming on the Caspian Sea Vortex

AbstractCentral Asian low vortex (CAV) is the dominant synoptic circulation, resulting in frequent heavy snowfall over Central Asia during 1983–2018. The present study investigates winter CAVs to explore their possible causes. The results suggest that the CAVs produced near the Caspian Sea (named as Caspian Sea Vortices, CSVs) are the main component of January CAVs, which account for more than 80%, exhibiting both interannual and decadal variabilities and causing stronger heavy snowfall over Central Asia since 2004. The formation of CSV is usually due to the joint effect of both low‐frequency waves and transient waves/vortices, in which the development of the latter is prior to the former, being the prophase signal of CSV enhancement and related heavy snowfall occurrences. The dominant transient waves/vortices related to CSVs not only originate from North Atlantic but also from European continent, which respectively correspond to the decadal and interannual variabilities of CSV frequency. In terms of European continent, the transient surface warming over eastern Europe enhances the atmospheric baroclinicity at lower troposphere, which on one hand excites vertical wave activity flux transporting to the upper troposphere and disturbs the transient vortex, on the other hand causes southeastward movement of North Atlantic jet. Consequently, wave energy guided by North Atlantic jet and excited over eastern Europe both disperses downstream and enhanced transient vortex evolves into the wavelike pattern, favoring the CSV formation on the next day. The transient surface warming over eastern Europe could be thereby a reference for the predictions of CAV heavy snowfall.

Recently Amplified Interannual Variability of the Great Lakes Ice Cover in Response to Changing Teleconnections

Abstract The interannual variability of the annual maximum ice cover (AMIC) of the Great Lakes is strongly influenced by large-scale atmospheric circulations that drive regional weather patterns. Based on statistical analyses from 1980 to 2020, we identify a reduced number of accumulated freezing degree days across the winter months in recent decades, a step-change decrease of AMIC after the winter of 1997/98, and an increased interannual variability of AMIC since 1993. Our analysis shows that AMIC is significantly correlated with El Niño–Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO), and the Pacific–North American pattern (PNA) before the winter of 1997/98. After that, the AMIC is significantly correlated with the tropical–Northern Hemisphere pattern (TNH) and eastern Pacific oscillation (EPO). Singular value decomposition of the 500-hPa geopotential height and surface air temperature shows a dipole pattern over the northeast Pacific and North America, demonstrating the ridge–trough system. This dipole pattern shifts northward to the northern Rocky Mountains, placing the Great Lakes region in the trough after 1997/98. This shift coincides with the increased interannual variability of the EPO index, as well as the change in the sea surface temperature (SST) over the northeast Pacific, where the second mode of the empirical orthogonal function (EOF) on SST shows a warm blob-like feature manifested over the Gulf of Alaska. The regression of wave activity flux onto the SST EOF shows that the source of upward and eastward propagation of a stationary Rossby wave shifts to the west coast of North America, likely moving the ridge–trough system eastward after the winter of 1997/98. Significance Statement We found that the Great Lakes annual maximum ice cover decreased after one of the strongest El Niño–Southern Oscillation events in 1997/98 and that its year-to-year fluctuations increased in the recent 20 years. After the winter of 1997/98, the Great Lakes annual maximum ice cover started to correlate with the warm sea surface temperature anomaly in the northeast Pacific, which appears to disrupt the polar vortex far up in the stratosphere, and the polar vortex ultimately shifts eastward the ridge–trough system over North America. When the shifted system develops enough, it can encase the Great Lakes region in the Arctic air and thereby cause larger year-to-year fluctuations. This connection was found to be characteristic in the recent few decades, after the winter of 1997/98.

Characteristics and related mechanisms of the persistent extreme precipitation in August 2020 over Western China

The persistent heavy precipitation that occurred in most of West China (WC) during August 11–18, 2020, generated the highest rainfall record since recording began in 1961 and was selected as one of the top 10 worst national natural disasters of 2020 in China. Favorable circulation was sustained during August; WC was positioned between two anomalous high-pressure centers over the Tibetan Plateau and Sea of Japan and an anomalous low-pressure center over Mongolia located on its north side, which created a stable and long trough and formed a low-pressure center over WC. At 200 hPa, the subtropical westerly jet was much stronger than average and southward, and the South Asian High (SAH) was strong and extended eastward to 150°E. At 500 hPa, the western Pacific subtropical high (WPSH) was westward and exceptionally strong, which helped abundant water vapor reach the southeastern part of WC and provided favorable dynamic and thermodynamic conditions for precipitation in this region. In addition, the eastward extension of the SAH promotes the westward extension of WPSH, which collectively enhanced the precipitation in WC. At 850 hPa, the low-level jet corresponding to the west-extending subtropical high from the Sea of Japan to WC further enhanced and guided the water vapor transport to WC. In addition, the Mei-Yu front over the Yangtze River Basin in June and July strengthened the northwestward spread of diabatic heating, transient energy, and wave activity fluxes, which likely influenced the large-scale circulation factors and reinforced the precipitation in WC in August 2020.

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.