Atmospheric Blocking Research Articles

Potential impacts of reduced winter Kara Sea ice on the dipole pattern of cold surge frequency over the tropical western Pacific

The impact of Arctic Sea ice melting on weather and climate extremes in the Northern Hemisphere has garnered widespread attention. Existing research has convincingly demonstrated the importance of this impact in mid-high latitudes, while its influence in areas beyond remains controversial. This study reveals the indirect influence of Kara Sea ice reduction on cold surge (CS) over the tropical western Pacific (TWP), with the East Asian jet stream serving as the connecting link. The leading mode of CSs over the TWP exhibits a zonal dipole characteristic, which is associated with cyclonic anomaly over the Philippine Sea. The enhanced cyclonic anomaly is caused by strengthened and northward-moved subtropical East Asian jet stream and weakened polar jet stream, which can lead to more CSs over the South China Sea and fewer CSs over the Philippine Sea. Such variations in the jet stream are contributed by the facilitated atmospheric blockings west of the Ural Mountains, which suppressed the circumpolar westerly winds and increased meridional temperature gradient in Northeast Asia. The connection between atmospheric blockings and Kara Sea ice can be confirmed through local vertical energy exchange. Simulations of the atmospheric response to the forcing of decreased Kara Sea ice support the proposed connection. Although there is no statistically significant correlation between tropical CSs and Kara Sea ice, this study highlights the potential impacts of Arctic climate change signal on weather and climate extremes over tropical regions.

Particulate black carbon mass concentrations and the episodic source identification driven by atmospheric blocking effects in Astana, Kazakhstan

Black carbon (BC) is a component of fine particulate matter (PM2.5) that is a key contributor to adverse human health effects and climate forcing. To date, BC mass concentrations and possible sources in Kazakhstan have not been studied. Thus, understanding the temporal variations of BC for a large developing region with a complex climate is useful. In this study, measurements of fine particulate BC mass concentrations in Astana were made from June 2020 to October 2021 by measuring light absorption of PM2.5 on filters. The mean BC concentration was 2.56 ± 1.29 μg m−3 with maximum and minimum monthly mean BC concentrations being 4.56 ± 2.03 μg m−3 and 1.12 ± 0.42 μg m−3 in January 2021 and June 2020, respectively. Temporal analyses of BC, SO2, PM10, NOx, CO, meteorological and atmospheric stability parameters were performed. Aggregated pollutant ‘episodic loadings’ during the heating and non-heating periods were identified. Their relationships with blocking anticyclones and cyclones were investigated by examining the reversal of meridional gradients at 500 hPa geopotential height (GPH) maps and identifying Omega (Ω) and Rex blocking types. Astana has some of the highest BC concentrations of cities worldwide. Seasonal BC source location identification using Conditional Bivariate Probability Function (CBPF) analysis implicated combined heat and power (CHP) plant emissions as the major BC source in Astana. Significant increases in BC concentrations were observed during the cold season due to numerous sources, generally poorer atmospheric dispersion and blocking events. The Concentration Weighted Trajectory (CWT) analysis results showed that the distribution of the 75th percentile of BC during episodic periods actively controlled by blockings exceeding than the entire measurement period, which may reflect cross-border transport and adjacent countries.

Basin-dependent response of Northern Hemisphere winter blocking frequency to CO2 removal

Atmospheric blocking has been identified as one of the key elements of the extratropical atmospheric variabilities, controlling extreme weather events in mid-latitudes. Future projections indicate that Northern Hemisphere winter blocking frequency may decrease as CO2 concentrations increase. Here, we show that such changes may not be reversed when CO2 concentrations return to the current levels. Blocking frequency instead exhibits basin-dependent changes in response to CO2 removal. While the North Atlantic blocking frequency recovers gradually from the CO2-induced eastward shift, the North Pacific blocking frequency under the CO2 removal remains lower than its initial state. These basin-dependent blocking frequency changes result from background flow changes and their interactions with high-frequency eddies. Both high-frequency eddy and background flow changes determine North Atlantic blocking changes, whereas high-frequency eddy changes dominate the slow recovery of North Pacific blocking. Our results indicate that blocking-related extreme events in the Northern Hemisphere winter may not monotonically respond to CO2 removal.

Atmospheric blocking and temperatures in the Antarctic Peninsula

The Antarctic Peninsula (AP) has displayed a propensity for persistent blocking ridges and anticyclonic conditions, particularly during recent summertime extreme weather events. This study investigates atmospheric blocking patterns over the AP through historical (1981–2010) and future (2071–2100, SSP5–8.5) periods using ERA5 reanalysis and six CMIP6 models, including multi-member realizations from two models totaling ten simulations. We focus particularly on 500 hPa geopotential height (Z500) and near-surface air temperature (T2m) anomalies. The historical analysis highlights significant differences between the CMIP6 models and ERA5 reanalysis, especially in the austral winter, with EC-Earth3 and INM-CM4 models matching closest with the ERA5. Future projections show that while the northern AP and the Drake Passage largely do not exhibit a clear trend towards increased blocking, there are exceptions. The EC-Earth3 model predicts more blocking–like conditions northwest of the AP in summer and a pronounced ridge over the Bellingshausen Sea in winter, indicating a potential increase in blocking events. The INM-CM4 model projects a minor increase in summer Z500 heights off the western and southern AP, without clear blocking patterns over the AP, and negligible winter changes. Localized intensification is noted in the northern parts of the blocking domain and southern AP during extreme blocking conditions. These variations are mirrored in T2m anomalies, suggesting warming in the northern and southern sections of AP but little change elsewhere. The results of this study underscore the need to more accurately capture complex blocking mechanisms and their impacts on regional climate patterns around the AP. We also suggest employing refined blocking definitions and incorporating a broader range of climate models to enhance our understanding of blocking patterns and their impacts in a changing climate.

Analyzing Urban Climatic Shifts in Annaba City: Decadal Trends, Seasonal Variability and Extreme Weather Events

Global warming is one of the most pressing challenges of our time, contributing to climate change effects and with far-reaching implications for built environments. The main aim of this study is to assess the extent to which Annaba city, Algeria, as part of the Mediterranean region, is affected by global climate change and its broader influences. The study investigated climatic shifts in Annaba city, using a multi-step methodology integrating data collection and analysis techniques. Data collection included 23 years of climate data (2000–2023) from Annaba’s meteorological station, on-site measurements of microclimatic variations, and a questionnaire survey. The collected data underwent four main analyses: a time series analysis to describe climate parameters over 23 years, a statistical analysis to predict potential future climatic conditions (2024–2029) and the correlation of various climatic variables using specialized bioclimate tools to highlight seasonal variability, a spatial study of the urban heat island (UHI) phenomenon and perceived climatic shifts, and an analysis of extreme weather events characterizing heat atmospheric events in the context of urban climate change in the Mediterranean region. The findings revealed a consistent warming trend in Annaba city, with prolonged extreme climate conditions observed, particularly in the last four years (2020–2023). Significant temperature fluctuations were emphasized, notably in July 2023, with record-breaking maximum temperatures reaching 48.2 °C, the hottest on record with an increase of 3.8 °C, and presenting challenges amplified by the urban heat island effect, causing temperature differentials of up to 6 °C within built-up areas. Projections for 2029 suggest a tendency towards heightened aridity with a significant shift towards a new climate seasonality featuring two distinct main seasons—moderate and hot challenging. The abrupt disruption of calm weather conditions in Annaba on 24 July 2023 highlighted the influence of atmospheric circulation within the Mediterranean region featured for both anticyclones and atmospheric blocking phenomena on local weather patterns.

Unraveling the Indian monsoon’s role in fueling the unprecedented 2022 Marine Heatwave in the Western North Pacific

An unprecedented marine heatwave (MHW) event occurred in the middle-high latitudes of the western North Pacific during the summer of 2022. We demonstrate that excessive precipitation thousands of kilometers away fuels this extreme MHW event in July 2022. In the upper atmosphere, a persistent atmospheric blocking system, forming over the MHW region, reduces cloud cover and increases shortwave radiation at the ocean surface, leading to high sea surface temperatures. Atmospheric perturbations induced by latent heat release from the extreme precipitation in the Indian summer monsoon region enhance this atmospheric blocking through the propagation of quasi-stationary Rossby waves. Our hypothesis is verified by using a numerical model that is forced with the observed atmospheric anomalous diabatic heating. This study sheds light on how a subtropical extreme event can fuel another extreme event at middle-high latitudes through an atmospheric teleconnection.

Oceanic maintenance of atmospheric blocking in wintertime in the North Atlantic

The connection between atmospheric blocking over the North Atlantic and the diabatic influence of the Gulf Stream is investigated using potential vorticity and moist potential vorticity diagnostics in the ERA5 reanalysis data set during wintertime (1979 - 2020). In line with previous research, the reliance atmospheric blocking has on turbulent heat fluxes over the Gulf Stream and its extension, for induction and maintenance, is shown to be significant. The air-sea heat flux generates negative potential vorticity air masses in the atmospheric boundary layer. These air masses subsequently contribute to the block’s negative potential vorticity anomaly at upper levels through ascending motion in the warm conveyor belt. It is shown that the block’s size and frequency partially depends on oceanic preconditioning via anomalous oceanic heat transport and heat content, prior to the blocking event, both of which allow for stronger turbulent heat fluxes. It is further hypothesized that the block feeds back positively on itself through the advection of cold dry air over the Gulf Stream, sustaining this air-sea interaction. This in turn decreases ocean heat content, eventually halting this air-sea interaction and severing the atmospheric block from its maintenance pathway.

Why Moist Dynamic Processes Matter for the Sub‐Seasonal Prediction of Atmospheric Blocking Over Europe

AbstractIn recent years, there has been growing evidence that latent heat release in midlatitude weather systems such as warm conveyor belts (WCBs) contributes significantly to the onset and maintenance of blocking anticyclones (blocked weather regimes). Still, numerical weather prediction (NWP) and climate models struggle to correctly predict and represent atmospheric blocking in particular over Europe. Here, we elucidate the representation of WCB activity in 20 years of extended winter (1997–2017) of European Centre for Medium‐Range Weather Forecast's IFS reforecasts around the onset of blocking over Europe (EuBL) employing different perspectives. First, we show that the model struggles to predict EuBL onsets already at 10–14 days lead time in line with a misrepresentation of WCB activity in the ensemble mean. However, we also find cases with accurate EuBL forecasts even in pentad 4 (15–19 days). This subset of successful forecasts at extended‐range lead times goes in line with accurate WCB forecasts over the North Atlantic several days prior to the blocking onset. Second, investigating the time‐lagged relationship of blocking onset and WCB activity, we find that WCB activity over the North Atlantic emerges well prior to the onset of the block and that different pathways into EuBL exist in the reforecasts compared to reanalysis. Finally, we find indication of predictability associated with a Rossby wave train emerging from the North Pacific. Although our study can not disentangle the roles of intrinsic predictability limits and model deficiencies, we show that correct predictions of EuBL go along with distinct patterns of WCB activity.

The response of atmospheric blocking and East Asian cold extremes to future Arctic Sea ice loss

The effect of Arctic warming on cold extremes over middle latitude remains a controversial and unresolved issue, particularly considering potential enhanced Arctic amplification in future scenarios. Here we examine the possible responses of winter cold extremes to the projected sea ice loss under 2 °C global warming scenario in four models, based on the atmosphere-only experiments of the Polar Amplification Model Inter-comparison Project (PAMIP). We find agreement with the increased responses of extreme cold days over East Asia (EA), characterized by longer duration or stronger intensity of temperature drop during cold events. Accordingly, most models exhibit a weak response of the Ural blocking in the corresponding composite circulation fields, whereas the westward movement of an anomalous anticyclonic system from eastern Siberia before the maturing of the Ural blocking is notable in four models. We suggest that this westward shifting pattern may prolong the duration of Ural blocking or modulate the strength of the Northeast Asian cold vortex, resulting in longer or stronger cold anomalies in EA. Accordingly, increased blocking frequency with a more pronounced westward response of shipment signals is also found over eastern Siberia, where a reduced meridional potential vorticity gradient (PVy) exists. The background of weak PVy can enhance the atmospheric nonlinearity, resulting in the anticyclonic anomalies less prone to dispersion and presenting a westward characteristic. Our results highlight the impact of the nonlinear response of circulation to Arctic sea ice loss. In contrast to more concerned Ural blocking influence, it is suggested that the downstream impact of eastern Siberia blocking on cold extremes in EA may be more pronounced in an enhanced Arctic amplification climate.

Creation and Assessment of an Index for Atmospheric Blockings in Brazil’s Central Region

Creation and Assessment of an Index for Atmospheric Blockings in Brazil’s Central Region

The linkage between autumn Barents-Kara sea ice and European cold winter extremes

While the Arctic's accelerated warming and sea ice decline have been associated with Eurasian cooling, debates persist between those attributing this to sea ice retreat and those to internal variability. Our study examines the association between autumn sea ice variability over the Barents-Kara Seas and extreme cold winters in Europe. Using the observational data and composite analysis, we explore the interannual variability and the potential linkage between sea ice and atmospheric circulation patterns. It reveals a correlation with shifts toward a negative phase of North Atlantic Oscillation and more frequent episodes of the atmospheric blocking over Greenland and the North Atlantic. Furthermore, the negative phase of the North Atlantic Oscillation and enhanced blocking are closely related and mutually reinforcing, shaping the spatial distribution of cold anomalies over much of the European continent. Our results suggest a link between the unusual decrease in Barents-Kara Sea ice during autumn and the occurrence of intense European weather extremes in subsequent winter months, emphasizing the need for delving deeper into this relationship on monthly time scales to enhance our predictive capabilities for midlatitude extreme events.

Wave Breaking Events and Their Link to Rossby Wave Packets and Atmospheric Blockings During Southern Hemisphere Summer

AbstractRossby Wave Packets (RWPs) are atmospheric perturbations located at upper levels in mid‐latitudes which, in certain cases, terminate in Rossby Wave Breaking (RWB) events. When sufficiently persistent and spatially extended, these RWB events are synoptically identical to atmospheric blockings, which are linked to heatwaves and droughts. Thus, studying RWB events after RWPs propagation and their link with blocking is key to enhance extreme weather events detection 10–30 days in advance. Hence, here we assess (a) the occurrence of RWB events after the propagation of transient RWPs, (b) whether long‐lived RWPs (RWPs with a lifespan above 8 days, or LLRWPs) are linked to large‐scale RWB events that could form a blocking event, and (c) the proportion of blocking situations that occur near RWB events. To do so, we applied a tracking algorithm to detect transient RWPs in the southern hemisphere during summertime between 1979 and 2021, developed a wave breaking algorithm to identify RWB events, and searched for blocking events with different intensities. Results show that LLRWPs and the other RWPs displayed large‐scale RWB events around 40% of the time, and most RWB events in both distributions last around 1–2 days, which is not long enough to identify them as blocking situations. Nearly 17% of blockings have a RWB event nearby, but barely 5% of blockings are linked to RWPs, suggesting that transient RWPs are not strongly linked to blocking development. Lastly, large‐scale RWB events associated with RWPs that lasted less than 8 days are influenced by El Niño‐Southern Oscillation.

Enhanced Occurrence of Atmospheric Blocking in the Southern Hemisphere by Baroclinic Annular Mode

AbstractAtmospheric blocking is a large‐amplitude, quasi‐stationary, and long‐lasting flow regime in the extratropics. To understand the physical processes governing the occurrence of atmospheric blocking, we identify that the positive phase of Baroclinic Annular Mode (BAM) increases the occurrence of blocking events in the Southern Hemisphere atmosphere. As BAM can translate to regional scales, we identify an enhanced zonal flux of wave activity and reduced dispersiveness associated with high BAM states that are dynamically conducive to the occurrence of atmospheric blocking. Blocking frequency in the high BAM state almost doubles as compared to the climatology, and the enhanced occurrence of blocks is most significant within BAM‐associated wave packets. This finding suggests BAM can be employed as a new source of predictability for atmospheric blocking.

Blocking Events in East Antarctica: Impact on Precipitation and their Association with Large-Scale Atmospheric Circulation Modes

Abstract This study, utilizing ERA5 data from 1979 to 2021, identifies a prevalent occurrence of blocking events in East Antarctica, particularly in the region extending from Wilkes Land to Dronning Maud Land. The genesis of these blocking highs is typically facilitated by the intensification of Rossby waves within the circumpolar westerlies. By classifying the circulation patterns causing the blocking events, the blocking events in East Antarctica are divided into five categories. On the western flank of the blocking high, we observe the transportation of warm and humid air into inland Antarctica, instigating regional snowfall. High precipitation levels are observed in different regions based on the specific location of the blocking high. We also demonstrate that the contribution of atmospheric blockings to total annual precipitation and extreme precipitation events increases from coastal to inland. Over the East Antarctic plateau, the blockings are responsible for at least 15% of total accumulated snowfall and 30% of extreme precipitation events. Finally, we find a significant relationship between the interannual variability in East Antarctic blocking frequency and the phase variability of the Southern Annular Mode (SAM). A positive phase of the SAM inhibits an increase in blocking frequency, while a negative phase favors an increase. However, the two Pacific–South American patterns (PSA-1 and PSA-2), characterized by wave trains originating in the tropical Pacific that extend across the Southern Hemisphere extratropics, do not significantly influence the interannual variability in East Antarctic blocking frequency.

Linking Gulf Stream air–sea interactions to the exceptional blocking episode in February 2019: a Lagrangian perspective

Abstract. The development of atmospheric blocks over the North Atlantic–European region can lead to extreme weather events like heat waves or cold air outbreaks. Despite their potential severe impact on surface weather, the correct prediction of blocking lifecycles remains a key challenge in current numerical weather prediction (NWP) models. Increasing evidence suggests that latent heat release in cyclones, the advection of cold air (cold air outbreaks, CAOs) from the Arctic over the North Atlantic, and associated air–sea interactions over the Gulf Stream are key processes contributing to the onset, maintenance, and persistence of such flow regimes. To better understand the mechanism connecting air–sea interactions over the Gulf Stream with changes in the large-scale flow, we focus on an episode between 20 and 27 February 2019, when a quasi-stationary upper-level ridge was established over western Europe accompanied by an intensified storm track in the northwestern North Atlantic. During that time, a record-breaking winter warm spell occurred over western Europe bringing temperatures above 20 ∘C to the United Kingdom, the Netherlands, and northern France. The event was preceded and accompanied by the development of several rapidly intensifying cyclones that originated in the Gulf Stream region and traversed the North Atlantic. To explore the mechanistic linkage between the formation of this block and air–sea interactions over the Gulf Stream, we adopt a Lagrangian perspective, using kinematic trajectories. This allows us to study the pathways and transformations of air masses that form the upper-level potential vorticity anomaly and interact with the ocean front. We establish that more than one-fifth of these air masses interact with the Gulf Stream in the lower troposphere, experiencing intense heating and moistening over the region due to the frequent occurrence of CAOs behind the cold front of the cyclones. Trajectories moistened by the advection of cold air over a warm ocean by one cyclone later ascend into the upper troposphere with the ascending airstream of a subsequent cyclone, fueled by the strong surface fluxes. These findings highlight the importance of CAOs in the Gulf Stream region, indicating that their intense coupling between the ocean and atmosphere plays a role in block development. Additionally, they provide a mechanistic pathway linking air–sea interactions in the lower troposphere and the upper-level flow.

Warm Arctic–Cold Eurasia pattern driven by atmospheric blocking in models and observations

In recent decades, Arctic-amplified warming and sea-ice loss coincided with a prolonged wintertime Eurasian cooling trend. This observed Warm Arctic–Cold Eurasia pattern has occasionally been attributed to sea-ice forced changes in the midlatitude atmospheric circulation, implying an anthropogenic cause. However, comprehensive climate change simulations do not produce Eurasian cooling, instead suggesting a role for unforced atmospheric variability. This study seeks to clarify the source of this model-observation discrepancy by developing a statistical approach that enables direct comparison of Arctic-midlatitude interactions. In both historical simulations and observations, we first identify Ural blocking as the primary causal driver of sea ice, temperature, and circulation anomalies consistent with the Warm Arctic–Cold Eurasia pattern. Next, we quantify distinct transient responses to this Ural blocking, which explain the model-observation discrepancy in historical Eurasian temperature. Observed 1988–2012 Eurasian cooling occurs in response to a pronounced positive trend in Ural sea-level pressure, temporarily masking long-term midlatitude warming. This observed sea-level pressure trend lies at the outer edge of simulated variability in a fully coupled large ensemble, where smaller sea-level pressure trends have little impact on the ensemble mean temperature trend over Eurasia. Accounting for these differences bring observed and simulated trends into remarkable agreement. Finally, we quantify the influence of sea-ice loss on the magnitude of the observed Ural sea-level pressure trend, an effect that is absent in historical simulations. These results illustrate that sea-ice loss and tropospheric variability can both play a role in producing Eurasian cooling. Furthermore, by conducting a direct model-observation comparison, we reveal a key difference in the causal structures characterizing the Warm Arctic–Cold Eurasia Pattern, which will guide ongoing efforts to explain the lack of Eurasian cooling in climate change simulations.

Climate Driver Influences on Prediction of the Australian Fire Behaviour Index

Fire danger poses a pressing threat to ecosystems and societies worldwide. Adequate preparation and forewarning can help reduce these threats, but these rely on accurate prediction of extreme fire danger. With the knowledge that climatic conditions contribute heavily to overall fire danger, this study evaluates the skill with which episodes of extreme fire danger in Australia can be predicted from the activity of large-scale climate driver patterns. An extremal dependence index for extreme events is used to depict the historical predictive skill of the Australian Bureau of Meteorology’s subseasonal climate prediction system in replicating known relationships between the probability of top-decile fire danger and climate driver states at a lead time of 2–3 weeks. Results demonstrate that the El Niño Southern Oscillation, Southern Annular Mode, persistent modes of atmospheric blocking, Indian Ocean Dipole and Madden-Julian Oscillation are all key for contributing to predictability of fire danger forecasts in different regions during critical fire danger periods. Northwest Australia is found to be particularly predictable, with the highest mean index differences (>0.50) when certain climate drivers are active, compared with the climatological index mean. This integrated approach offers a valuable resource for decision-making in fire-prone regions, providing greater confidence to users relying on fire danger outlooks for key management decisions, such as those involved in the sectors of national park and forest estate management, agriculture, emergency services, health and energy. Furthermore, the results highlight strengths and weaknesses in both the Australian Fire Danger Rating System and the operational climate model, contributing additional information for improving and refining future iterations of these systems.

The Extraordinary March 2022 East Antarctica “Heat” Wave. Part I: Observations and Meteorological Drivers

Abstract Between 15 and 19 March 2022, East Antarctica experienced an exceptional heat wave with widespread 30°–40°C temperature anomalies across the ice sheet. This record-shattering event saw numerous monthly temperature records being broken including a new all-time temperature record of −9.4°C on 18 March at Concordia Station despite March typically being a transition month to the Antarctic coreless winter. The driver for these temperature extremes was an intense atmospheric river advecting subtropical/midlatitude heat and moisture deep into the Antarctic interior. The scope of the temperature records spurred a large, diverse collaborative effort to study the heat wave’s meteorological drivers, impacts, and historical climate context. Here we focus on describing those temperature records along with the intricate meteorological drivers that led to the most intense atmospheric river observed over East Antarctica. These efforts describe the Rossby wave activity forced from intense tropical convection over the Indian Ocean. This led to an atmospheric river and warm conveyor belt intensification near the coastline, which reinforced atmospheric blocking deep into East Antarctica. The resulting moisture flux and upper-level warm-air advection eroded the typical surface temperature inversions over the ice sheet. At the peak of the heat wave, an area of 3.3 million km2 in East Antarctica exceeded previous March monthly temperature records. Despite a temperature anomaly return time of about 100 years, a closer recurrence of such an event is possible under future climate projections. In Part II we describe the various impacts this extreme event had on the East Antarctic cryosphere. Significance Statement In March 2022, a heat wave and atmospheric river caused some of the highest temperature anomalies ever observed globally and captured the attention of the Antarctic science community. Using our diverse collective expertise, we explored the causes of the event and have placed it within a historical climate context. One key takeaway is that Antarctic climate extremes are highly sensitive to perturbations in the midlatitudes and subtropics. This heat wave redefined our expectations of the Antarctic climate. Despite the rare chance of occurrence based on past climate, a future temperature extreme event of similar magnitude is possible, especially given anthropogenic climate change.

Impact of Pacific blocking on the intraseasonal winter sea ice seesaw between the Bering and Okhotsk Seas

The winter sea ice cover (SIC) over the Bering Sea (BS) and Okhotsk Sea (OS) shows significant co-variability. In particular, a clear seesaw-like variability can be observed on an intraseasonal scale between the two seas. However, the mechanism by which the atmospheric circulation affects them is unclear and further insight is needed. In this study, based on reanalysis data from 1979 to 2019, we use composite analyses to examine the physical processes of atmospheric blocking that affect the intraseasonal seesaw SIC changes between OS and BS. The investigation reveals that Pacific blocking (PB) events can enhance the existing intraseasonal seesaw variation of the OS and BS SIC in winter. Based on the maximum core position of the anticyclonic center, we divide PB events into two subgroups: western PB and eastern PB. In addition, considering average zonal shifting speed, we further classify them into three subgroups: westward shifting PB, quasi-stationary PB and eastward shifting PB. Composite results show that the changes of SIC and surface air temperature (SAT) show different asymmetric characteristics for the different PB subgroups. Of the five PB regimes, the quasi-stationary PB is associated with the greatest reduction in SIC in the BS over its lifetime, both in magnitude and duration. Furthermore, we find that the local sea surface temperature (SST) anomaly upstream of the quasi-stationary PB, both during and prior to its onset, exhibits a significant dipole mode, along with a positive anomaly in the 300-hPa zonal wind. The SST dipole could serve as a precursor climatic background condition, likely triggering the occurrence of quasi-stationary PB and subsequently leading to intense SIC changes.

Comparison of Dust Storm Events between East Asia and North America

In this study, the decadal variations of global dust storm events (DSEs) are studied based on two typical cases that occurred in East Asia in 2023 and in North America in 1934, respectively. We found that the periods with weak winter monsoon, varied jet stream, weakened Siberian High (SH) and strengthening atmospheric blocking corresponded to the high incidence of DSEs in East Asia. In recent years, activity of the East Asian DSEs is active again because of the mutual effect of these four systems. Due to the ecological engineering projects in China, the frequency of the DSEs is less than that in Mongolia, whereas in Mongolia, the land degradation causes the frequency of DSEs to increase significantly. In the Great Plains of the United States, high incidence of the DSEs mainly corresponded to periods with strong atmospheric blocking in North America. Since the 1860s, the Great Plains had been affected by destruction of vegetation and drought, with bare soil swept into the air by the strong winds, resulting in “Dust Bowl” in the 1930s. Under the warning of long-term strong DSEs, the U.S. government issued a series of policies to respond to the impact of DSEs, which improved the ecosystem of the Great Plains. As a result of the improved ecosystem and weaker blocking activities in the Great Plains in the 1950s, activity of the DSEs was significantly weaker than that in the 1930s.