Vortex Events Research Articles

Predictability of the 2020 Strong Vortex in the Antarctic Stratosphere and the Role of Ozone

AbstractThe Antarctic vortex of October–December 2020 was the strongest on record in the satellite era for the season in the mid‐ to lower stratosphere. However, it was poorly predicted by the Australian Bureau of Meteorology's operational seasonal climate forecast system of that time, ACCESS‐S1, even at a short lead time of a month. Using the current operational forecast system, ACCESS‐S2, we have, therefore, tried to find a primary cause of the limited predictability of this event and conducted forecast sensitivity experiments to understand the potential role of ozone in the event and its associated anomalies of the Southern Annular Mode (SAM) and rainfall over south–eastern Australia and western Patagonia. Here, we show that the 2020 strong vortex event did not follow the canonical dynamical evolution seen in previous strong vortex events in spring but suddenly appeared as a result of the record‐low upward propagating wave activity in September 2020. The ACCESS‐S2 forecasts significantly underestimated the negative wave forcing in September even at zero lead time, irrespective of the ozone configuration, therefore falling short in predicting the record strength of the polar vortex in late spring 2020. Nevertheless, ACCESS‐S2 with prescribed realistic ozone that had large anomalies in the Antarctic stratosphere significantly better predicted the strong vortex and the subsequent positive SAM and related rainfall anomalies over south–eastern Australia and western Patagonia in the austral summer of 2020–21. This highlights the potentially important role of ozone variations for seasonal climate forecasting as a source of long‐lead predictability.

Impact of Polar Vortex Modes on Winter Weather Patterns in the Northern Hemisphere

This study is an additional investigation of stratosphere–troposphere coupling based on the recent stratospheric winter descriptions in five distinct modes: January, February, Double, Dynamical, and Dadiative. These modes, established in a previous study, categorize the main stratospheric winter typologies modulated by the timing of important sudden stratospheric warmings (SSWs) and final stratospheric warmings (FSWs). The novelty of this research is to investigate the Northern Annular Mode, mean sea level pressure (MSLP) anomalies in the Ural and Aleutian regions, and the decomposition of Eliassen–Palm flux into wavenumbers 1 and 2 within each mode. The results show that the January and Double modes exhibit similar pre-warming surface signals, characterized by Ural blocking and Aleutian trough events preceding weak polar vortex events. The January mode displays a positive MSLP anomaly of +395 hPa (−191 hPa) in the Ural (Aleutian) region in December, while the Double mode shows +311 hPa (−89 hPa) in November. These modes are primarily wave-1 driven, generating tropospheric responses via negative Arctic Oscillation patterns. Conversely, the February and Dynamical modes show opposite signals, with Aleutian blocking and Ural trough events preceding strong polar vortex events. In December, the February mode exhibits MSLP anomalies of +119 hPa (Aleutian) and −180 hPa (Ural), while the Dynamical mode shows +77 hPa and −184 hPa, respectively. These modes, along with important SSWs in February and dynamical FSWs, are driven by both wave-1 and wave-2 and do not significantly impact the troposphere. The Radiative mode’s occurrence is strongly related to the Aleutian blocking presence. These findings confirm that SSW timing is influenced by specific dynamical forcing related to surface precursors and underscore its importance in subsequent tropospheric responses. This study establishes a connection between early winter tropospheric conditions and upcoming stratospheric states, potentially improving seasonal forecasts in the northern hemisphere.

Effects of extreme stratospheric polar vortex events on near-surface wind gusts across Europe

Abstract Extreme stratospheric polar vortex (SPV) events can influence winter tropospheric circulation for up to 60 d. Their impacts on air temperature have been extensively studied recently. However, there is a research gap in their effects on wind speeds and its extremes. This study aims to evaluate, for the first time, the impacts of such extreme SPV events on observed and modelled near-surface wind gusts across Europe. We have analysed wind gust data coming from: station-based observations (for the Iberian Peninsula and Scandinavia), the ERA5 reanalysis and the SEAS5 and GloSea6 seasonal forecasting systems. We assess their similarities in reproducing 4 parameters of their corresponding distributions: median, standard deviation, skewness and kurtosis. For all these datasets, the results indicate that extreme positive SPV events are followed by negative wind gust anomalies in Southern Europe and positive in Northern Europe. Whereas, negative SPV events (such as Sudden Stratospheric Warmings) have positive gust anomalies in Southern Europe and negative in the north. A central region shows negligible anomalies in both cases. This highlights the ability of SPV as a predictor for short-medium-term forecasting of extreme wind events, which would have direct applications to many socioeconomic and environmental issues such as the estimation of wind-power generation.

Opposite spectral properties of Rossby waves during weak and strong stratospheric polar vortex events

Abstract. In this study we provide a systematic characterization of Rossby wave activity during the 25 sudden stratospheric warming (SSW) and 31 strong polar vortex (SPV) events that occurred in the period 1979–2021, identifying the specific tropospheric and stratospheric waves displaying anomalous behaviour during such events. Space–time spectral analysis is applied to ERA5 data for this purpose, so that both the wavenumber and the zonal phase speed of the waves can be assessed. We find that SSW events are associated with a reduction in the phase speed of Rossby waves, first in the stratosphere and then in the troposphere; SPV events are tied to a simultaneous increase of phase speed across vertical levels. Phase speed anomalies become significant around the event and persist for 2–3 weeks afterwards. Changes of Rossby wave properties in the stratosphere during SSW and SPV events are dominated by changes in the background flow, with a systematic reduction or increase, respectively, in eastward propagation of the waves across most wavenumbers. In the troposphere, on the other hand, the effect of the background flow is also complemented by changes in wave properties, with a shift towards higher wavenumbers during SSW events and towards lower wavenumbers for SPV events. The opposite response between SSW and SPV events is also visible in the meridional heat and momentum flux co-spectra, which highlight from a novel perspective the connection between stratospheric Rossby waves and upward propagation of waves.

Research on the Characteristics of Raindrop Spectrum and Its Water Vapour Transport Sources in the Southwest Vortex: A Case Study of 15–16 July 2021

This study investigated the convective weather features, precipitation microphysical characteristics, and water vapour transport characteristics associated with a southwest vortex precipitation event that occurred on the eastern edge of the Qinghai–Tibet Plateau, coinciding with a southwest vortex event, from 15 to 16 July 2021, using conventional observations of raindrop spectra, ERA5 reanalysis data, CMORPH precipitation data, and the HYSPLIT_v4 backward trajectory model. The findings aim to provide theoretical insights for improving the forecasting and numerical simulations of southwest vortex precipitation events. The findings revealed that the precipitation event induced by the southwestern vortex at Emeishan Station on 15–16 July 2021 was characterised by high rainfall intensity and significant precipitation accumulation. The raindrop spectrum exhibited a broad distribution with a notable bimodal structure. Both the Sichuan Basin and the Tibetan Plateau were dominated by the South Asian high pressure at higher altitudes, while a pronounced low-pressure system developed at mid and low altitudes within the basin, establishing a meteorological context marked by upper-level divergence and lower-level convergence. Throughout the event, notable vertical uplift velocities were recorded across the Sichuan Basin and Tibetan Plateau, along with distinct positive vorticity zones in the lower and middle strata of the Sichuan Basin, indicating that the atmosphere was in a state of thermal instability. The majority of moisture was in the mid and lower troposphere with evident convergence movements, which played a crucial role in the southwest vortex’s development. WRF numerical simulations of the Emeishan precipitation event more accurately modelled the weather conditions for this precipitation but tended to overestimate the level of precipitation. It was observed that the region around Emei Mountain primarily received moisture influx from the southern Bay of Bengal and the South China Sea, with moisture transport chiefly originating from the Sichuan Basin and in a south-westward trajectory.

Stratospheric influence on the winter North Atlantic storm track in subseasonal reforecasts

Abstract. Extreme stratospheric polar vortex events, such as sudden stratospheric warmings (SSWs) or extremely strong polar vortex events, can have a significant impact on surface weather in winter. SSWs are most often associated with negative North Atlantic Oscillation (NAO) conditions, cold air outbreaks in the Arctic and a southward-shifted midlatitude storm track in the North Atlantic, while strong polar vortex events tend to be followed by a positive phase of the NAO, relatively warm conditions in the extratropics and a poleward-shifted storm track. Such changes in the storm track position and associated extratropical cyclone frequency over the North Atlantic and Europe can increase the risk of extreme windstorm, flooding or heavy snowfall over populated regions. Skillful predictions of the downward impact of stratospheric polar vortex extremes can therefore improve the predictability of extratropical winter storms on subseasonal timescales. However, there exists a strong inter-event variability in these downward impacts on the tropospheric storm track. Using ECMWF reanalysis data and reforecasts from the Subseasonal to Seasonal (S2S) Prediction Project database, we investigate the stratospheric influence on extratropical cyclones, identified with a cyclone detection algorithm. Following SSWs, there is an equatorward shift in cyclone frequency over the North Atlantic and Europe in reforecasts, and the opposite response is observed after strong polar vortex events, consistent with the response in reanalysis. However, although the response of cyclone frequency following SSWs with a canonical surface impact is typically captured well during weeks 1–4, less than 25 % of the reforecasts manage to capture the response following SSWs with a “non-canonical” impact. This suggests a possible overconfidence in the reforecasts with respect to reanalysis in predicting the canonical response after SSWs, although it only occurs in about two-thirds of the events. The cyclone forecasts following strong polar vortex events are generally more successful. Understanding the role of the stratosphere in subseasonal variability and predictability of storm tracks during winter can provide a key for reliable forecasts of midlatitude storms and their surface impacts.

Downward propagation of the weak stratospheric polar vortex events: the role of the surface arctic oscillation and the quasi-biennial oscillation

Downward propagation of the weak stratospheric polar vortex events: the role of the surface arctic oscillation and the quasi-biennial oscillation

Statistical Characteristics of Electron Vortexes in the Terrestrial Magnetosheath

Utilizing the unprecedented high-resolution Magnetospheric Multiscale mission data from 2015 September to 2017 December, we perform a statistical study of electron vortexes in the turbulent terrestrial magnetosheath. On the whole, 506 electron vortex events are successfully selected. Electron vortexes can occur at four known types of magnetic structures, including 78, 42, 26, and 39 electron vortexes observed during the crossings of the current sheets, magnetic holes, magnetic peaks, and flux ropes, respectively. Except for the four types of structures, the rest of the electron vortexes are in the “Others” category, defined as unknown structures. The electron vortexes mainly occur in the subsolar region, with only a few in the flank region. The total occurrence rate of all electron vortexes is 4.86 hr–1, with, on average, 3.65 events hr−1 in the X-Y plane and 3.26 events hr−1 in the X-Z plane. The durations of most of the electron vortexes concentrate within 0.5–1.5 s and are 1.09 s on average. The electron vortexes are ion-scale structures owing to the average scale of 2.05 ion gyroradius. In addition, the means, medians, and maxima of the energy dissipation J · E′ in the electron vortexes are almost positive, implying that the electron vortex may be a potential coherent structure or channel for turbulent energy dissipation. All these results reveal the statistical characteristics of electron vortexes in the magnetosheath and improve our understanding of energy dissipation in astrophysical and space plasmas.

Polar Vortex Strength Impacts on the Longitudinal Structure of Thermospheric Composition and Ionospheric Electron Density

AbstractThis work examines the weak stratospheric polar vortex event in late February 2008 and the strong polar vortex event in late February 2021 to understand the potential influence of polar vortex strength on midlatitude thermospheric composition and ionospheric plasma density. The weak polar vortex event shows a reduction in thermospheric O/N2 at all longitudes, changes in the ionospheric total electron content (TEC) that vary with longitude, and an intensification of the semidiurnal tides. The strong polar vortex event shows O/N2 elevated by up to 25% in the Asian sector and a 25% decrease in semidiurnal tidal amplitude. The TEC response during the strong polar vortex event is complex and shows enhanced TEC in the Asian sector where O/N2 is elevated. These are the first observations of anomalies in the thermospheric composition and ionospheric plasma associated with a strong polar vortex event.

Impact of tropospheric polar vortex on winter cold extremes over Northeast China

Using CN05.1 dataset and NCEP reanalysis, this study investigated the potential linkage between the tropospheric polar vortex and extreme winter cold events over Northeast China. Results show that Asian part of polar vortex (APV) is expanded and weakened before the cold events, this pattern manifests itself as a significant correlation a week before anomalous cold events. Moreover, these cold events could be classified into two types according to the polar vortex position, i.e., eastward-shifted and westward-shifted polar vortex events (EPVE and WPVE). There are significant differences in tropospheric circulation between these two types of events from 14 to 7 days before the cold event outbreak. More specifically, blockings over western Europe and the Bering strait during early-middle stage of EPVE are favourable for the shift of polar vortex towards the northern Eurasia. The southward-shifted polar vortex enhances Rossby wave activity, favourable for the development of Ural blocking, cold vortex in East Asia and cold air transport. APV is greatly weakened after the onset of cold outbreak, leading to cold air intrusion from the western side of cold vortex into Northeast China. During WPVE, the polar vortex is located over the Baffin Island. APV and cold vortex are weaker before the cold event outbreak. The cold air is mainly induced by the blocking high ridge over East Siberian Sea, with a longer transport path and weaker intensity compared with that during the eastward events. In conclusion, the cooling over Northeast China is more drastic due to stronger cold air transport when the polar vortex centre is shifted towards Eurasia.

Influence of Stratosphere Polar Vortex Variability on the Mesosphere, Thermosphere, and Ionosphere

AbstractThe Whole Atmosphere Community Climate Model with thermosphere‐ionosphere eXtension (WACCM‐X) is used to investigate the influence of stratosphere polar vortex variability on the mesosphere, thermosphere, and ionosphere during Northern Hemisphere winter. Based on 40 simulated Northern Hemisphere winters, the mesosphere and lower thermosphere (MLT) residual circulation is found to depend on whether the stratosphere polar vortex is strong or weak. In particular, during weak stratosphere polar vortex time periods, the MLT circulation anomalies are characterized by clockwise and anti‐clockwise flow in the Northern and Southern Hemispheres, respectively. Opposite, though weaker, anomalies are found to occur during time periods when the stratosphere polar vortex is strong. The MLT circulation anomalies influence the composition of the lower thermosphere, leading to ±5% changes in the thermosphere column integrated atomic oxygen to molecular nitrogen ratio (ΣO/N2). Large differences between strong and weak stratosphere polar vortex events are also found to occur in the semidiurnal migrating tide (SW2) in the MLT, which leads to ±15%–20% differences in the SW2 component of the ionosphere total electron content (TEC) at low latitudes. The WACCM‐X simulation results indicate that variability in the stratosphere polar vortex can explain ∼30% and ∼18% of the quiet time variability in thermosphere ΣO/N2 and the SW2 component of TEC during Northern Hemisphere winter, respectively.

Evaluation of Processes Related to Stratosphere–Troposphere Coupling in GEFSv12 Subseasonal Hindcasts

Abstract The representation of the stratosphere and stratosphere–troposphere coupling processes is evaluated in the subseasonal Global Ensemble Forecast System, version 12 (GEFSv12), hindcasts. The GEFSv12 hindcasts develop systematic stratospheric biases with increasing lead time, including a too strong boreal wintertime stratospheric polar vortex. In the tropical stratosphere, the GEFSv12 winds and temperatures associated with the quasi-biennial oscillation (QBO) tend to decay with lead time such that they underestimate the observed amplitudes; consistently, the QBO-associated mean meridional circulation is too weak. The hindcasts predict extreme polar vortex events (including sudden stratospheric warmings and vortex intensifications) about 13–14 days in advance, and extreme lower-stratospheric eddy heat flux events about 6–10 days in advance. However, GEFSv12’s ability to predict these events is likely affected by its zonal-mean circulation biases, which increases the rates of false alarms and missed detections. Nevertheless, GEFSv12 shows stratosphere–troposphere coupling relationships that agree well with reanalysis and other subseasonal forecast systems. For instance, GEFSv12 reproduces reanalysis relationships between polar vortex strength and the Northern Annular Mode in the troposphere. It also exhibits enhanced weeks 3–5 prediction skill of the North Atlantic Oscillation index when initialized during strong and weak polar vortex states compared to neutral states. Furthermore, GEFSv12 shows significant differences in Madden–Julian oscillation (MJO) amplitudes and enhanced MJO predictive skill in week 4 during easterly versus westerly QBO phases, though these results are sensitive to the level used to define the QBO. Our results provide a baseline from which future GEFS updates may be measured.

CIPS Observations of Gravity Wave Activity at the Edge of the Polar Vortices and Coupling to the Ionosphere

AbstractA new Cloud Imaging and Particle Size (CIPS) gravity wave (GW) variance data set is available that facilitates automated analysis of GWs entering the mesosphere. This work examines several years of CIPS GW variances from 50 to 55 km in the context of the Arctic and Antarctic polar vortices. CIPS observes highest GW activity in the vortex edge region where horizontal wind speeds are largest, consistent with previously published GW climatologies in the stratosphere and mesosphere. CIPS observes the well‐documented planetary wave (PW)‐1 patterns in GW activity in both hemispheres. In the Northern Hemisphere, maximum GW activity occurs over the North Atlantic and western Europe. In the Southern Hemisphere, maximum GW activity stretches from the Andes over the South Atlantic and Indian Oceans, as expected. In the NH, CIPS GW spatial patterns are highly correlated with horizontal wind speed. In the SH, CIPS GW patterns are less positively correlated with the winds due to increased zonal symmetry and orographic forcing. The Andes Mountains and Antarctic Peninsula, South Georgia Island, Kerguelen/Heard Islands, New Zealand, and Tasmania are persistent sources of orographic GWs. Atmospheric Infrared sounder observations of stratospheric GWs are analyzed alongside CIPS to explore vertical GW coherence and to infer GW propagation and sources. NH midlatitude GW activity is reduced during the January 2021 SSW, as expected. This reduction in GWs leads to a simultaneous reduction in traveling ionospheric disturbances (TIDs), providing more evidence that weak polar vortex events with weak GW activity leads to reduced daytime TID activity.

Erosion by turbulence: Discovering the counter-wise vortex events and their effect on wear

Understanding the wear process has been critical for developing and maintaining engineered systems handling particle-laden flows, such as pipelines, valves, pumps, and mixers. This work explores the use of ultrasound velocimetry for determining the velocity and impact angle of particles projected from a turbulent slurry flow onto a moving target surface. The flow is generated in a slurry pot configuration at four levels of rotational frequency: 1900, 2200, 2500 and 3300 rpm. Acoustic images acquired in the B-mode are processed by Ultrasound Particle Tracking Velocimetry (U-PTV) and Ultrasound Particle Image Velocimetry (U-PIV) to determine the impact velocities and impact angles of individual particles in the radial plane. Particle impacts at velocities higher than the tangential velocity of the rotating axis are observed and named counter-wise impact due to their association with counter-wise vortices impacting in the direction contrary to the translation of the moving surface. The applicability of U-PTV and U-PIV for the study of particle-laden turbulent flow is discussed and a statistical analysis of particle impacts energy is conducted. The counter-wise impacts are found relevant to erosive wear because of their relatively high velocity (upper quartile) and because most of the impact energy is conveyed through the tangential component.

Simulation of Northern Winter Stratospheric Polar Vortex Regimes in CESM2-WACCM

The possible impact of various Arctic polar vortex regimes for the stratosphere on the Northern Hemisphere extratropics has not been fully understood. Previous study has classified the stratospheric Arctic vortex to six regimes using the k-mean clustering algorithm based on the ERA5 reanalysis. The stability and robustness of the classification is further verified with a much longer model dataset and historical integrations from CESM2-WACCM. Consistent with the reanalysis, we clustered the Arctic stratospheric polar vortex forms into six patterns, named as homogeneously-intensified and -weakened regimes (HI, HW), North America-intensified and -weakened regimes (NAI, HAW), and Eurasia-intensified and -weakened regimes (EUI, EUW). A zonally uniform positive (negative) Northern Annular Mode (NAM) pattern develops during the HI (HW) regime from the stratosphere to troposphere. The NAM-like pattern shifts toward the western hemisphere with the largest negative (positive) anomalous height center shifting to North America during the NAI (NAW) regime. In contrast, the maximum polar anomaly center moves towards polar Eurasia during the EUI (EUW) regime. The HI, NAI, and EUW regimes are accompanied with weakened wave activities, while the HW, NAW, and EUI regimes are preceded by enhanced planetary waves. Accordingly, persistent anomalies of warmth (coldness) exist over midlatitude Eurasia and North America during the HI (HW). Anomalous warmth (coldness) centers exist in northern Eurasia, while anomalous coldness (warmth) centers exist around the Mediterranean Sea during the NAI (NAW) regime. Anomalous warmth (coldness) centers develop in East Asia in the EUI (EUW) periods. The rainfall anomaly distributions also vary with the stratospheric polar vortex regime. The frequency for stratospheric regimes during SSWs and strong vortex events is also assessed and consistent with previous findings.

Plan(et) A: Addressing the Climate Crisis

Plan(et) A:Addressing the Climate Crisis The SAIS Review Editorial Board Human-driven climate change poses a continual and existential threat to our planet. This crisis disproportionately impacts developing nations and indigenous populations and inhibits human and economic development around the world. Annually, climate change-induced extreme weather events take thousands of lives, displace millions, and destabilize governments. These challenges have reached new heights in recent years as historic flooding in Nigeria and Pakistan uprooted tens of millions; devastating wildfires engulfed Greece, Italy, and Spain; and melting glaciers precipitously elevated global sea levels, threatening to erase coastal populations and small island nations. Less than two years ago marked the end of the world's warmest decade on record. From widespread economic insecurity to exacerbated military conflicts, the climate crisis poses intersectional, unabating threats to world peace and stability. Despite these momentous challenges set before our generation, the editors of the SAIS Review of International Affairs remain optimistic. Our optimism is abundant as Johns Hopkins University's Paul H. Nitze School of Advanced International Studies (SAIS) enters its 80th year and a new building at 555 Pennsylvania Avenue NW in Washington, DC during the Summer of 2023. Writing for the SAIS Review in Autumn 1963, SAIS Professor Paul Linebarger characterized the creation of our school twenty years prior as persisting on "the basis of nothing but a little money, a lot of hope, and enormous talent." In the six decades since his writing, SAIS has cultivated countless students prepared to meet the United States' and the world's most pressing challenges, outlined each year in the pages of the SAIS Review. This edition of the SAIS Review of International Affairs has the bittersweet distinction of being the last edition published from our office in 1619 Massachusetts Avenue, and the first dedicated entirely to assessing the impacts of climate change and practical policy solutions to address its root causes. Plan(et) A: Addressing the Climate Crisis explores the intersectionality of climate change with major global trends and challenges including energy, financial systems, governance, injustice, and state sovereignty. The issue begins with Anne Andreassen and Connemara Doran, who analyze the negative impact of climate change on electrical grid resilience using the 2021 Texas polar vortex as a case study, weather data, and data on [End Page 1] mixed sources of electricity generation. The authors determine that resilience is a global challenge because it is local: each climate-induced crisis provides lessons for averting disaster elsewhere. They explain that officials must address local resilience gaps to avoid using natural gas as a "bridge fuel" in the transition to renewable forms of electricity generation. While the transition to nonfossil fuels is ongoing and necessary to address the impacts of climate change, interdependence on different energy sources may exacerbate resilience gaps in electrical grids. The Texas polar vortex and other intense weather events linked to global warming highlight the need to address local resilience gaps to manage the impact of climate-induced energy crises. The next two works address country- and region-specific concerns and opportunities related to combating climate change. Wilder Alejandro Sánchez writes about the need for Kazakhstan to implement a comprehensive domestic green strategy and a green multivector foreign policy. The Central Asian state has faced environmental challenges with greater frequency in recent years. A severe drought in the summer of 2021 gravely impaired agricultural industries throughout the country. Kazakhstan, known for its steppe, will struggle to keep its terrain green in the long term. Sánchez argues that Kazakhstan can and will be able to protect its land and waters through sound "green" policies, which will require leadership, foresight, and collaboration with allies. Nitya Labh highlights the existential and legal threats of climate change for Pacific Island states and peoples. She outlines how climate change disproportionately affects small island developing states (SIDS), including Vanuatu and its Pacific Island neighbors. The author explains that Pacific SIDS confront sea-level rise, ocean acidification, and extreme weather events, which threaten their existence; sovereignty; and legal, economic, and cultural survival. Labh assesses that international maritime law insufficiently safeguards small island states from the harsh impacts of climate change, despite strides taken through the...

Exploring the link between austral stratospheric polar vortex anomalies and surface climate in chemistry-climate models

Abstract. Extreme events in the stratospheric polar vortex can lead to changes in the tropospheric circulation and impact the surface climate on a wide range of timescales. The austral stratospheric vortex shows its largest variability in spring, and a weakened polar vortex is associated with changes in the spring to summer surface climate, including hot and dry extremes in Australia. However, the robustness and extent of the connection between polar vortex strength and surface climate on interannual timescales remain unclear. We assess this relationship by using reanalysis data and time-slice simulations from two chemistry-climate models (CCMs), building on previous work that is mainly based on observations. The CCMs show a similar downward propagation of anomalies in the polar vortex strength to the reanalysis data: a weak polar vortex is on average followed by a negative tropospheric Southern Annular Mode (SAM) in spring to summer, while a strong polar vortex is on average followed by a positive SAM. The signature in the surface climate following polar vortex weakenings is characterized by high surface pressure and warm temperature anomalies over Antarctica, the region where surface signals are most robust across all model and observational datasets. However, the tropospheric SAM response in the two CCMs considered is inconsistent with observations. In one CCM, the SAM is more negative compared to the reanalysis after weak polar vortex events, whereas in the other CCM, it is less negative. In addition, neither model reproduces all the regional changes in midlatitudes, such as the warm and dry anomalies over Australia. We find that these inconsistencies are linked to model biases in the basic state, such as the latitude of the eddy-driven jet and the persistence of the SAM. These results are largely corroborated by models that participated in the Chemistry-Climate Model Initiative (CCMI). Furthermore, bootstrapping of the data reveals sizable uncertainty in the magnitude of the surface signals in both models and observations due to internal variability. Our results demonstrate that anomalies of the austral stratospheric vortex have significant impacts on surface climate, although the ability of models to capture regional effects across the Southern Hemisphere is limited by biases in their representation of the stratospheric and tropospheric circulation.

Response of tropical convection over the western Pacific to stratospheric polar vortex during boreal winter

AbstractIt has been extensively discussed that stratospheric polar vortex significantly contribute to the variability and predictability of tropospheric weather and climate over midlatitudes and polar regions. However, we currently have little understanding of the exact influence of stratospheric polar vortex on tropical troposphere. Here, the exact responses of tropical convection to Northern Hemisphere (NH) extreme stratospheric polar vortex events (i.e., strong vortex [SV] events and weak vortex [WV] events) are investigated in this study. We find that the western Pacific region is particularly noteworthy when focusing on the spatial distribution of tropical convective activity. Significant enhancement of convective activity over the western Pacific is clearly observed in about 2 weeks after WV events, while opposite responses to SV events can also be expected. The change in convective activity over the western Pacific is considerably contributed by the intraseasonal variability, and thus MJO phase 7 (3) is dominant after WV (SV) events. The seesaw pattern of zonal‐mean convective activity near equator may be a property of MJO itself, not definitely caused by the stratospheric polar vortex as previously thought. A significantly linear correlation between anomalous convective activity and static stability near tropopause over the western Pacific is found, suggesting that circumstances in the tropical upper troposphere play a crucial role in the evolution of tropical convection, especially for the MJO.

Evaluation of the CAMS-CSM Model in predicting a Northeast Cold Vortex Event at the early Onset of 2020 Mei-yu season

Intimately related to the Asian summer monsoon, the Mei-yu rainfall is also strongly influenced by atmospheric circulation in the middle to high latitudes, especially the Northeast Cold Vortex (NECV), and thus making prediction of the Mei-yu rainfall a challenging issue. The prediction skills of the CAMS-CSM (full name as the Chinese Academy of Meteorological Sciences Climate System Model) for a NECV event at the early onset of 2020 Mei-yu season and the associated Mei-yu rainfall are evaluated. Hindcast experiments with two different horizontal resolutions are employed to assess the climate model in predicting the synoptic system. The ERA5 reanalysis data and the CN05.1 precipitation data are adopted for comparison. Results indicate that both the middle-resolution (T106, ∼1°) and high-resolution (T255, ∼0.47°) version of the CAMS-CSM model are able to capture this NECV event, while showing some biases on its intensity, duration and location. Overpredictions in the strength and duration of the NECV are found in the T106 version, which could be attributed to a stronger dry invasion from higher levels during the development stage, an intensified Okhotsk blocking high, and a weakened upper-level jet at the decaying phase. As for the T255 version, a stronger NECV is predicted further north than the reanalysis, and sources of the biases are identified, including northward displacements of the high-level dry invasion and the westerly jet. Further investigation suggests that the T106 version performs better than the T255 version in predicting the Mei-yu rainfall because of a more accurate prediction on the location of the NECV. Positive biases in the cold advection and Mei-yu front are identified in the T106 version, contributing to a stronger Mei-yu rainfall than the observation. The Mei-yu rainfall is poorly predicted in T255 version due to prediction errors in the location of the NECV-induced cold advection. Therefore, improving the prediction skill of the NECV, not only its intensity but also location, is of vital significance to achieve a better prediction of the Mei-yu rainfall.

Quantifying stratospheric biases and identifying their potential sources in subseasonal forecast systems

Abstract. The stratosphere can be a source of predictability for surface weather on timescales of several weeks to months. However, the potential predictive skill gained from stratospheric variability can be limited by biases in the representation of stratospheric processes and the coupling of the stratosphere with surface climate in forecast systems. This study provides a first systematic identification of model biases in the stratosphere across a wide range of subseasonal forecast systems. It is found that many of the forecast systems considered exhibit warm global-mean temperature biases from the lower to middle stratosphere, too strong/cold wintertime polar vortices, and too cold extratropical upper-troposphere/lower-stratosphere regions. Furthermore, tropical stratospheric anomalies associated with the Quasi-Biennial Oscillation tend to decay toward each system's climatology with lead time. In the Northern Hemisphere (NH), most systems do not capture the seasonal cycle of extreme-vortex-event probabilities, with an underestimation of sudden stratospheric warming events and an overestimation of strong vortex events in January. In the Southern Hemisphere (SH), springtime interannual variability in the polar vortex is generally underestimated, but the timing of the final breakdown of the polar vortex often happens too early in many of the prediction systems. These stratospheric biases tend to be considerably worse in systems with lower model lid heights. In both hemispheres, most systems with low-top atmospheric models also consistently underestimate the upward wave driving that affects the strength of the stratospheric polar vortex. We expect that the biases identified here will help guide model development for subseasonal-to-seasonal forecast systems and further our understanding of the role of the stratosphere in predictive skill in the troposphere.