Abstract. The Southern Hemisphere westerlies (SHW) play a pivotal role in modulating the global carbon cycle and climate feedback. However, their behavior during the Last Glacial Maximum (LGM) is debated owing to discrepancies between paleoclimate models and proxy records. While tropical upper-tropospheric cooling and Antarctic surface cooling exert opposing influences on the SHW, the detailed dynamical mechanisms through which Antarctic sea-ice expansion modulates large-scale atmospheric circulation are poorly understood. In this study, we investigated the dynamical mechanisms of austral winter SHW change under altered orbital and surface conditions with a series of climate model simulations. By conducting sensitivity experiments with varying Antarctic sea-ice concentrations, we isolated the thermodynamic effect of sea ice from the tropical cooling signal. Our results demonstrate that sea-ice-induced surface cooling drives the poleward intensification of the SHW through two distinct mechanisms. First, strong surface cooling steepens the meridional temperature gradient near the sea-ice edge, thereby directly maintaining the SHW intensity through thermal wind balance. Second, the enhanced baroclinicity amplifies eddy heat fluxes and storm track activity. The resulting increase in storm track activity drives a downward transfer of upper-tropospheric westerly momentum, reinforcing the surface westerlies. Through these mechanisms, the sea-ice-driven cooling outweighed the opposing equatorward influence of tropical cooling. This study provides a dynamical framework for understanding how sea-ice thermodynamic forcing drives large-scale circulation changes in the context of LGM climate conditions.

Using stratospheric nudging experiments in a global climate model, we examined the role of sudden stratospheric warming (SSW) in driving Eurasian cooling on subseasonal time scales. Nudging was applied to extratropical stratospheric conditions derived from observations, imposing the full observed stratospheric state including zonal asymmetries. Two nudging experiments were performed with different lower boundaries within the stratosphere (∼88 hPa vs ∼200 hPa). Both experiments successfully reproduced Eurasian cooling following SSW events, linked to cold air advection into Eurasia driven by changes in upper-tropospheric circulation resulting from stratosphere–troposphere coupling. The results were generally insensitive to the vertical extent of nudging, especially in cases with strong coupling. However, the nudging experiments also exhibited adverse effects. Most notably, a pronounced cold bias developed over Alaska, attributed to the model’s tendency to produce overly strong coupling between the stratosphere and upper troposphere in this region compared to observations. This bias was mitigated when nudging was extended deeper into the lower stratosphere. These findings underscore the importance of accurately representing stratosphere–troposphere coupling strength to reliably capture the surface impacts of stratospheric variability.

Ural blocking (UB) and the associated Warm Arctic-Cold Eurasia (WACE) pattern are typically linked to the negative Arctic Oscillation (AO). However, robust UB events are surprisingly observed even during the positive AO phase, a condition generally expected to suppress blocking due to enhanced zonal flow. This study investigates how positive AO magnitude modulates UB persistence. We find that under strong positive AO conditions (AO > +1), UB events persist significantly longer (6.1 days) than weak positive AO (4.7 days). This enhanced persistence results from organized North Atlantic storm tracks that facilitate intense heat and moisture into the Barents-Kara Sea. The resulting Arctic warming and sea ice loss trigger a thermodynamic feedback loop that weakens the meridional potential vorticity (PV) gradient, effectively anchoring the UB system. Our findings reveal that strong positive AO paradoxically promotes persistent blocking through storm-PV coupling, offering critical insights for improving sub-seasonal predictions of Eurasian winter extremes.

The warm Arctic-cold Eurasia (WACE) pattern, identified as Arctic warming and midlatitude cooling over recent decades, has been a subject of intense scientific debate regarding its causal relationship and implications for midlatitude weather extremes. This study investigates the primary drivers of WACE, examining the complex interplay between external forcing and internal variability. Using the empirical orthogonal function analysis on 84 years (1941-2024) of winter (DJF) temperature over the Northern Hemisphere, we identify three dominant modes of variability: Arctic amplification (AA), the Arctic Oscillation (AO), and the Barents Oscillation (BO). AA, accounting for 27% of the total variance, captures the dominant warming pattern across the Arctic region and reflects a pronounced long-term trend. In contrast, the AO and BO modes (explaining 13.8% and 9.7%, respectively) exhibit considerable internal variability with negligible long-term trends. From 1990 to 2014, the interaction between these modes largely explains the observed WACE pattern, with AA driving Arctic warming and negative AO phase contributing to Eurasian cooling. Meanwhile, the changepoint detection reveals a shift in Arctic climate regimes, marking a transition from a cold Arctic regime (1947-80) to a warm Arctic regime (2004-24). During the warm regime, weakened meridional potential vorticity gradients and increased East Siberian blocking frequency are observed under negative AO and BO phases. Idealized model experiments corroborate showing that Arctic warming amplifies potential vorticity gradient reductions under negative AO and BO phases. These findings highlight the WACE pattern as driven by the intricate interaction between AA and internal variability, emphasizing the balance between external forcing and internal processes in shaping Arctic climate and midlatitude impacts.

The importance of sea-ice loss on the Arctic amplification of near-surface warming remains contentious, as Arctic amplification emerges even in model experiments with disabled surface-albedo feedback. Here we show that the characteristics and underlying dynamics of Arctic amplification may change greatly in a future ice-free climate using a series of climate model experiments. Our analysis indicates that although Arctic amplification continues over the 22nd century, it weakens markedly with a less distinct seasonality in a future ice-free climate. These changes are found to occur because the strength and seasonality of Arctic amplification in the current climate are attributed mainly to a tight coupling between cold-season lapse-rate feedback and sunlit-season surface-albedo feedback. The substantial differences in the characteristics of simulated Arctic amplification between the current and future ice-free climate therefore suggest that the presence of Arctic sea ice is an essential component of the current Arctic amplification regime. Arctic amplification persists even in a year-round sea ice-free state due to greenhouse gas forcing, but weakens in a future year-round sea ice-free climate, implying Arctic sea ice’s importance, according to analysis of climate model simulations.

Subseasonal prediction of Arctic sea ice and associated atmospheric conditions during the melting season remains challenging due to limited understanding of sea ice initial conditions. This study integrates sea ice assimilation into the coupled model FGOALS-f2 using the localized error subspace transform ensemble Kalman filter, and conducts subseasonal predictions starting from August 1st over 2004–2023. Results show that simultaneous assimilation of sea ice concentration (SIC) and thickness (SIT) significantly improves sea ice predictions for up to two months, while assimilating SIC alone primarily benefits one-month lead predictions. SIT assimilation provides added predictive value for surface air temperature (SAT) forecasts beyond SIC assimilation alone, effectively extending the atmospheric influence of sea ice initial conditions to two months. This improvement in SAT predictions is primarily attributed to a more realistic representation of the surface energy budget. These findings highlight the pivotal role of summer SIT assimilation to enhance subseasonal predictions in the Arctic and challenge the conventional view that initial conditions affect only short-term forecasts. This study underscores the necessity for better representation of ice–atmosphere interactions in models and advocates for enhanced observational capabilities for summer SIT to improve subseasonal predictions in the Arctic and surrounding regions.

We proposed a link between the interannual and decadal variability in Antarctic surface climate during the austral summertime (December-January) and the timing of stratospheric final warming (SFW) occurrences. This connection is based on 44 years of reanalysis data and in-situ observation spanning from 1979 to 2023. Positive surface pressure anomalies over Antarctica, associated with an earlier occurrence of SFW, develop through stratosphere-troposphere downward coupling, which leads to a warmer surface in Antarctica, except for the Antarctic Peninsula where a cooler surface is observed. On the contrary, the surface pressure and temperature anomalies associated with the later occurrence of SFW exhibit almost opposite or weaker behaviors. Congruence analyses support that a trend towards earlier SFW occurrences can explain the pause of the cooling trend or a slight reversal into the warming trend of the interior Antarctic surface through strengthening anti-cyclonic surface circulation since the 2000s. The resulting surface temperature responses can leave imprints on sea-ice concentration trends in the high-latitude Southern Hemisphere, displaying the dipole anomalies with an increase and a decrease of sea ice over the Antarctic Peninsula and northern Ross Sea, respectively.

Southern South America is an important dust source to Antarctica. However, there is a lack of overall understanding of the dust event variation in recent decades in South America. Here, we analyzed variations in the dust frequency in southern South America (south of 20 degrees S) from 1986 to 2020 and its causes with large-scale climatic factors, based on observational station data and reanalysis data. During the austral spring and summer, several stations recorded an average dust frequency exceeding 15 days, with some even surpassing 20 days. The frequency of dust events in spring exhibited a strong association with large-scale climate factors. Negative phases of the interdecadal Pacific oscillation (IPO) and the Antarctic Oscillation (AAO) were more likely to lead to an increase in the dust frequency in southern South America. The negative IPO had a greater impact on the Patagonia dust (south of 40 degrees S), resulted from a decrease in the drought index under the influence of downward motion. However, the negative AAO had a greater impact on the dust in South America over 20 degrees-40 degrees S by increasing strong wind frequency and decreasing the drought index over there. On longer time scales, model outputs from the Coupled Model Intercomparison Project phase 6 (CMIP6) also confirmed the combined influence of the negative IPO and negative AAO phases on southern South American dust.

Polynyas along the Antarctic coastline are essential for sea ice production and the formation of Antarctic Bottom Water (AABW). They are formed and maintained by strong and persistent katabatic winds that push the ice away from the coast. Satellite data, in-situ meteorological observations and Reanalysis datasets all indicate a decline in sea ice concentration (SIC) over the Terra Nova Bay Polynya (TNBP) from 2013 to 2022, consistent with the occurrence of an increasing trend of strong katabatic wind events (SKWEs). On the interannual timescale, significant correlations between the TNBP area and the duration of SKWEs were observed in austral summer, autumn and winter, and also in April and October during 2003–2022. Sea ice volume budget analysis shows that SKWEs drive rapid sea-ice removal through advection and divergence, whereas thermodynamic processes dominate sea ice formation on an annual basis. The increase in SKWEs was associated with a deepened and southwestward-displaced Amundsen Sea Low (ASL) and an enhanced pressure gradient between the interior plateau and the coast, both potentially linked to a more positive Southern Annular Mode (SAM). These findings provide a mechanistic understanding of long-term polynya variability, with implications for regional sea-ice changes and AABW.

Pronounced model-observation discrepancies in the changes of tropical Pacific zonal sea surface temperature gradient during the satellite era imply systematic model deficiencies. However, the relatively short high-quality instrumental record hampers robustly determining the response of tropical Pacific sea surface temperature to greenhouse gas increases. By adopting paleoclimate proxy records along with a series of climate model simulations, here we show that the zonal gradient is likely to decrease under sustained strong forcing. Paleoclimate proxy records indicate an overall increase of the zonal gradient over time, which has been accompanied by global-mean cooling associated with decreasing carbon dioxide concentrations. Model simulations are found to broadly reproduce the distinct contrast between warmer high carbon dioxide climates and the opposite climates, albeit with large inter-model discrepancy. The qualitative agreement among paleoclimate proxy records and modeled representations therefore lends some important credence to the sign of model-projected future tropical Pacific mean state change.

A Doppler Sonic Detection and Ranging (SODAR) wind-profiling system was deployed at Jang Bogo Station, Antarctica, during the summer of 2017. The parameters measured by the SODAR system were wind speed and direction averaged for 15 min from 30 m above the surface to 500 m aloft. Additionally, wind measurements obtained from the nearby walk-up tower and automatic weather station (AWS) were also used for additional data and comparison, respectively. The analysis and comparison used hourly average for consistency. The mean wind speed obtained from the SODAR measurement is highly correlated with those from the walk-up tower and the AWS measurements. All three measurements recorded westerlies during all strong wind events but varied during calm periods. The measurements also reveal that Jang Bogo Station experiences lower wind speeds compared to the nearest katabatic confluence region (Inexpressible Island), where the maximum airflow usually occurred. From the hourly averaged data, SODAR only detected a maximum wind speed of 18.1 m s −1 . There were 238 occurrences of strong wind events equal to or above 10.8 m s −1 (Beaufort scale 6) from 10 May to 31 October 2017. These occurrences were recorded in 50 out of 175 days analysed in this study. From 10 to 12 June 2017, predominantly south-westerly-westerly winds were recorded at the surface and aloft during two strong wind events. Based on this case study, the boundary layer during calm and strong wind periods was also analysed.

© 2024 American Meteorological Society. This published article is licensed under the terms of the default AMS reuse license. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses). Corresponding author: Clare Eayrs, clare.eayrs@kopri.re.kr

It is widely accepted that Arctic amplification (AA)—enhanced Arctic warming relative to global warming—will increasingly moderate cold-air outbreaks (CAOs) to the midlatitudes. Yet, some recent studies also argue that AA over the last three decades to the rest of the present century may contribute to more frequent severe winter weather including disruptive cold spells. To prepare society for future extremes, it is necessary to resolve whether AA and severe midlatitude winter weather are coincidental or physically linked. Severe winter weather events in the northern continents are often related to a range of stratospheric polar vortex (SPV) configurations and atmospheric blocking, but these dynamical drivers are complex and still not fully understood. Here we review recent research advances and paradigms including a nonlinear theory of atmospheric blocking that helps to explain the location, timing and duration of AA/midlatitude weather connections, studies of the polar vortex’s zonal asymmetric and intra-seasonal variations, its southward migration over continents, and its surface impacts. We highlight novel understanding of SPV variability—polar vortex stretching and a stratosphere–troposphere oscillation—that have remained mostly hidden in the predominant research focus on sudden stratospheric warmings. A physical explanation of the two-way vertical coupling process between the polar vortex and blocking highs, taking into account local surface conditions, remains elusive. We conclude that evidence exists for tropical preconditioning of Arctic-midlatitude climate linkages. Recent research using very large-ensemble climate modelling provides an emerging opportunity to robustly quantify internal atmospheric variability when studying the potential response of midlatitude CAOs to AA and sea-ice loss.

Between 15 and 19 March 2022, East Antarctica experienced an exceptional heat wave with widespread 30 degrees-40 degrees 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 degrees 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 km(2) 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.

This study analyzed the sensitivities of carbon cycle to surface air temperature using the CO _2 flux data collected from June to September for six years (2014–2019) over a moist tundra site in Council, Alaska. The tundra ecosystem was a strong sink of carbon in June and July, a weak sink in August with rapidly decreasing photosynthesis, and a carbon source in September. The ecosystem respiration (Re) and gross primary production (GPP) were obtained from the net ecosystem exchange (NEE) of eddy-covariance system. Both the Re and GPP increased with temperature, enhancing carbon emission and uptake during observation period. Notably, Re showed higher sensitivity to temperature than GPP did. This result means that as global warming continues, the increase in carbon release is greater than the increase in carbon uptake. In other words, the tundra ecosystem is expected to become a weaker carbon sink in June and July and a stronger source of carbon in September. Possible mechanism of different temperature sensitivities of Re and GPP as well as temporal variations of temperature sensitivities are suggested. Present results highlight the importance of understanding the temperature sensitivities of Re and GPP in various tundra ecosystems to accurately understand changes in the carbon cycle in the Arctic region.

Severe haze episodes have hit Beijing many times in the past few years, especially the “crazy bad” pollution described by the US Embassy in Beijing. The publication of numerous multimedia reports on the severe haze has increased awareness among the people in China regarding air pollution and PM2.5. It is assumed that the severe haze occurred suddenly for unclear reasons. In this context, long-term evaluation of the air pollution in Beijing is necessary. Through hourly and daily PM2.5 concentration records, meteorological datasets from August 2004 onward, and the Kolmogorov-Zurbenko (KZ) filter approach, the evolutions of the long-term components (or background values) of PM2.5 concentrations at an urban and a rural station in Beijing were statistically analyzed, and the possible causes of variation in the trends of these components were evaluated in this study. The long-term components of PM2.5 concentrations decreased significantly at both the urban (−3.40 µg m−3 y−1) and rural (−1.16 µg m−3 y−1) stations. The most serious pollution predominantly occurred in an earlier period than recent years, when little attention was being paid. The decrease in PM2.5 concentration was mainly attributed to the reduction in pollutant emission, despite the distinct increase in total energy consumption and motor vehicle use. However, the unfavorable climate changes (i.e., a reduction in wind speed and an increase in relative humidity) reduced the efficiency of atmospheric environmental governance. Because of the unfavorable climate or meteorological changes, the trends of PM2.5 reduction derived from the pollutant emission controls have been offset by up to approximately 15

It remains unresolved whether the La Niña-like sea surface temperature (SST) trend pattern during the satellite era, featuring a distinct warming in the northwest/southwest Pacific but cooling in the tropical eastern Pacific, is driven by either external forcing or internal variability. Here, by conducting a comprehensive analysis of observations and a series of climate model simulations for the historical period, we show that a combination of internal variability and human activity may have shaped the observed La Niña-like SST trend pattern. As in observations, SSTs in each model ensemble member show a distinct multi-decadal swing between El Niño-like and La Niña-like trend patterns due to internal variability. The ensemble-mean trends for some models are, however, found to exhibit an enhanced zonal SST gradient along the equatorial Pacific over periods such as 1979–2010, suggesting a role of external forcing. In line with this hypothesis, single-forcing large ensemble model simulations show that human-induced stratospheric ozone depletion and/or aerosol changes have acted to enhance the zonal SST gradient via strengthening of Pacific trade winds, although the effect is model dependent. Our finding suggests that the La Niña-like SST trend is unlikely to persist under sustained global warming because both the ozone and aerosol impacts will eventually weaken.