Intraseasonal modulation of Sudden Stratospheric Warmings by ENSO

Using the complete ERA-Interim reanalysis and three state-of-the-art models, this study explores how El Ni&#241;o-Southern Oscillation (ENSO) can influence the frequency and seasonal cycle of sudden stratospheric warmings (SSWs) by modulating the background upward wave propagation. Reanalysis data shows that in the last four decades, winters with SSWs were significantly more common than inactive winters, particularly under La Ni&#241;a (LN) conditions but not during El Ni&#241;o (EN), regardless of the ENSO/SSW definitions considered herein. In agreement with previous studies, our models tend to simulate a linear ENSO-SSW relationship, with more SSWs for EN, that show a peak of occurrence around January &#8212; as occurs in reanalysis &#8212;, and less SSWs for LN when compared to neutral conditions. Independently of ENSO, the main tropospheric precursor of SSWs appears to be an anomalous wave-like pattern over Eurasia, but it is dominated by wavenumber 1 (WN1) for EN and shows an enhanced wavenumber 2 (WN2) for LN. The differences in this Eurasian wave pattern, which is largely internally generated (ENSO-unforced), emerge from the distinct configuration of the background, stationary wave pattern induced by ENSO, favouring a relative stronger WN1 (WN2) component during EN (LN). Our results suggest that the ENSO-forced signal relies on preconditioning the seasonal-mean polar vortex, becoming weaker and displaced (stronger and more stable) for EN (LN), while ENSO-unforced wave activity represents the ultimate trigger of SSWs. This supports the view that ENSO and SSWs are different sources of variability of the winter atmospheric circulation and may reconcile previous findings in this context.

Download Full-text

Seasonal prediction of the boreal winter stratosphere

Climate Dynamics ◽

10.1007/s00382-021-05787-9 ◽

2021 ◽

Author(s):

Alice Portal ◽

Paolo Ruggieri ◽

Froila M. Palmeiro ◽

Javier García-Serrano ◽

Daniela I. V. Domeisen ◽

...

Keyword(s):

Southern Oscillation ◽

Polar Vortex ◽

Seasonal Prediction ◽

Wave Activity ◽

Boreal Winter ◽

Quasi Biennial Oscillation ◽

Stratospheric Polar Vortex ◽

Eddy Heat Flux ◽

Prediction Systems ◽

Model Database

AbstractThe predictability of the Northern Hemisphere stratosphere and its underlying dynamics are investigated in five state-of-the-art seasonal prediction systems from the Copernicus Climate Change Service (C3S) multi-model database. Special attention is devoted to the connection between the stratospheric polar vortex (SPV) and lower-stratosphere wave activity (LSWA). We find that in winter (December to February) dynamical forecasts initialised on the first of November are considerably more skilful than empirical forecasts based on October anomalies. Moreover, the coupling of the SPV with mid-latitude LSWA (i.e., meridional eddy heat flux) is generally well reproduced by the forecast systems, allowing for the identification of a robust link between the predictability of wave activity above the tropopause and the SPV skill. Our results highlight the importance of November-to-February LSWA, in particular in the Eurasian sector, for forecasts of the winter stratosphere. Finally, the role of potential sources of seasonal stratospheric predictability is considered: we find that the C3S multi-model overestimates the stratospheric response to El Niño–Southern Oscillation (ENSO) and underestimates the influence of the Quasi–Biennial Oscillation (QBO).

Download Full-text

Nonstationarity in Southern Hemisphere Climate Variability Associated with the Seasonal Breakdown of the Stratospheric Polar Vortex

Journal of Climate ◽

10.1175/jcli-d-17-0097.1 ◽

2017 ◽

Vol 30 (18) ◽

pp. 7125-7139 ◽

Cited By ~ 14

Author(s):

Nicholas J. Byrne ◽

Theodore G. Shepherd ◽

Tim Woollings ◽

R. Alan Plumb

Keyword(s):

Climate Variability ◽

Southern Hemisphere ◽

Seasonal Cycle ◽

Vortex Breakdown ◽

Polar Vortex ◽

Reanalysis Data ◽

Early Summer ◽

Stratospheric Polar Vortex ◽

Time Symmetry

Abstract Statistical models of climate generally regard climate variability as anomalies about a climatological seasonal cycle, which are treated as a stationary stochastic process plus a long-term seasonally dependent trend. However, the climate system has deterministic aspects apart from the climatological seasonal cycle and long-term trends, and the assumption of stationary statistics is only an approximation. The variability of the Southern Hemisphere zonal-mean circulation in the period encompassing late spring and summer is an important climate phenomenon and has been the subject of numerous studies. It is shown here, using reanalysis data, that this variability is rendered highly nonstationary by the organizing influence of the seasonal breakdown of the stratospheric polar vortex, which breaks time symmetry. It is argued that the zonal-mean tropospheric circulation variability during this period is best viewed as interannual variability in the transition between the springtime and summertime regimes induced by variability in the vortex breakdown. In particular, the apparent long-term poleward jet shift during the early-summer season can be more simply understood as a delay in the equatorward shift associated with this regime transition. The implications of such a perspective for various open questions are discussed.

Download Full-text

Predictions of the boreal winter stratosphere with the C3S multi-model seasonal forecast system

10.5194/ems2021-42 ◽

2021 ◽

Author(s):

Alice Portal ◽

Paolo Ruggieri ◽

Froila M. Palmeiro ◽

Javier Garcı́a-Serrano ◽

Daniela I. V. Domeisen ◽

...

Keyword(s):

Southern Oscillation ◽

Polar Vortex ◽

Seasonal Prediction ◽

Seasonal Forecast ◽

Wave Activity ◽

Arctic Sea Ice ◽

Mean Flow ◽

Seasonal Forecasts ◽

Stratospheric Polar Vortex ◽

Prediction Systems

As a result of the recent progress in the performance of seasonal prediction systems, forecasts of the mid-latitude weather at seasonal time scales are becoming increasingly important for societal decision making, as in risk estimate and management of meteorological extreme events. The predictability of the Northern-Hemisphere winter troposphere, especially in the Euro-Atlantic region, stems from the representation of a number of sources of predictability, notably El Nino Southern Oscillation, the stratospheric polar vortex, Arctic sea-ice extent, Eurasian snow cover. Among these, the stratospheric polar vortex is known to play a paramount role in seasonal forecasts of the winter tropospheric flow.Here, we investigate the performance in the stratosphere of five seasonal prediction systems taking part in the Copernicus Climate Change Service (C3S), with a focus on the seasonal forecast skill and variability, and on the assessment of stratospheric processes. We show that dynamical forecasts of the stratosphere initialised at the beginning of November are considerably more skilful than empirical forecasts based on observed October or November anomalies. Advances in the representation of stratospheric seasonal variability and extremes, i.e. sudden stratospheric warming frequency, are identified with respect to previous generations of climate models running roughly a decade ago. Such results display, however, a large model dependence. Finally, we stress the importance of the relation between the stratospheric wave activity and the stratospheric polar vortex (i.e. the wave&#8212;mean-flow interaction), applied both to the variability and to the predictability of the stratospheric mean flow. Indeed, forecasts of the winter stratospheric polar vortex are closely connected to the prediction of November-to-February stratospheric wave activity, in particular in the Eurasian sector.

Download Full-text

Analysis of the Arctic polar vortex dynamics during the sudden stratospheric warming in January 2009

Arctic and Antarctic Research ◽

10.30758/0555-2648-2021-67-2-134-146 ◽

2021 ◽

Vol 67 (2) ◽

pp. 134-146

Author(s):

V. V. Zuev ◽

E. S. Savelieva ◽

A. V. Pavlinsky

Keyword(s):

Wind Speed ◽

Polar Vortex ◽

Reanalysis Data ◽

Wave Activity ◽

The Arctic ◽

Stratospheric Warming ◽

Sudden Stratospheric Warming ◽

Mean Wind Speed ◽

The Mean ◽

Middle Stratosphere

The Arctic polar vortex is often affected by wave activity during its life cycle. The planetary Rossby waves propagating from the troposphere to the stratosphere occasionally lead to the displacement or splitting of the polar vortex, accompanied by sudden stratospheric warming (SSW). In January 2009, one of the largest SSWs was observed in the Arctic. In this work, the dynamics of the polar vortex during the 2009 SSW is considered using a new method that allows one to estimate the vortex area, the wind speed at the vortex edge, the mean temperature and ozone mass mixing ratio inside the vortex, based on the fact that the Arctic vortex edge at the 50 and 10 hPa pressure levels is determined by the geopotential values, respectively, 19.5. 104 and 29.5. 104 m2 /s2 , using the ERA5 reanalysis data. The application of this method is justified for the Arctic polar vortex, which is characterized by significant variability, especially during the period of its splitting. The splitting of the polar vortex in 2009 was observed on January 24 and 28, respectively, in the middle and lower stratosphere. About a week after the splitting, the vortices became closer in characteristics to small cyclones, which completely collapsed within 1–3 weeks. The influence of planetary wave activity on the polar vortex does not always lead to its breakdown. Short-term splitting of the polar vortex is sometimes observed for several days after which the polar vortex strengthens again and PSCs form inside the vortex. Such a recovery of the polar vortex is most likely to occur in the winter. Based on the analysis of the dynamics of the Arctic polar vortex for 1979–2020 and using the example of the 2009 SSW, we showed that when the vortex area decreases to less than 10 million km2 and the mean wind speed at the vortex edge decreases below 30 and 45 m/s, respectively, in the lower and middle stratosphere, the polar vortex becomes a small cyclone (with significantly higher temperatures within it), which usually collapses within 3 weeks.

Download Full-text

Blocking Statistics in a Varying Climate: Lessons from a “Traffic Jam” Model with Pseudostochastic Forcing

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-19-0095.1 ◽

2019 ◽

Vol 76 (10) ◽

pp. 3013-3027 ◽

Cited By ~ 3

Author(s):

Adiv Paradise ◽

Cesar B. Rocha ◽

Pragallva Barpanda ◽

Noboru Nakamura

Keyword(s):

Wave Amplitude ◽

Stationary Wave ◽

Reanalysis Data ◽

Wave Activity ◽

Future Trend ◽

Boreal Winter ◽

Stationary Waves ◽

Transient Eddy ◽

Dominant State ◽

Local Wave

Abstract Recently Nakamura and Huang proposed a semiempirical, one-dimensional model of atmospheric blocking based on the observed budget of local wave activity in the boreal winter. The model dynamics is akin to that of traffic flow, wherein blocking manifests as traffic jams when the streamwise flux of local wave activity reaches capacity. Stationary waves modulate the jet stream’s capacity to transmit transient waves and thereby localize block formation. Since the model is inexpensive to run numerically, it is suited for computing blocking statistics as a function of climate variables from large-ensemble, parameter sweep experiments. We explore sensitivity of blocking statistics to (i) stationary wave amplitude, (ii) background jet speed, and (iii) transient eddy forcing, using frequency, persistence, and prevalence as metrics. For each combination of parameters we perform 240 runs of 180-day simulations with aperiodic transient eddy forcing, each time randomizing the phase relations in forcing. The model climate shifts rapidly from a block-free state to a block-dominant state as the stationary wave amplitude is increased and/or the jet speed is decreased. When eddy forcing is increased, prevalence increases similarly but frequency decreases as blocks merge and become more persistent. It is argued that the present-day climate lies close to the boundary of the two states and hence its blocking statistics are sensitive to climate perturbations. The result underscores the low confidence in GCM-based assessment of the future trend of blocking under a changing climate, while it also provides a theoretical basis for evaluating model biases and understanding trends in reanalysis data.

Download Full-text

Interannual Variability and Trends in Sea Surface Temperature, Lower and Middle Atmosphere Temperature at Different Latitudes for 1980–2019

Atmosphere ◽

10.3390/atmos12040454 ◽

2021 ◽

Vol 12 (4) ◽

pp. 454

Author(s):

Andrew R. Jakovlev ◽

Sergei P. Smyshlyaev ◽

Vener Y. Galin

Keyword(s):

Surface Temperature ◽

Sea Surface Temperature ◽

Lower Stratosphere ◽

Middle Atmosphere ◽

Southern Oscillation ◽

Lower Troposphere ◽

Reanalysis Data ◽

Sea Surface ◽

The Arctic ◽

The Impact

The influence of sea-surface temperature (SST) on the lower troposphere and lower stratosphere temperature in the tropical, middle, and polar latitudes is studied for 1980–2019 based on the MERRA2, ERA5, and Met Office reanalysis data, and numerical modeling with a chemistry-climate model (CCM) of the lower and middle atmosphere. The variability of SST is analyzed according to Met Office and ERA5 data, while the variability of atmospheric temperature is investigated according to MERRA2 and ERA5 data. Analysis of sea surface temperature trends based on reanalysis data revealed that a significant positive SST trend of about 0.1 degrees per decade is observed over the globe. In the middle latitudes of the Northern Hemisphere, the trend (about 0.2 degrees per decade) is 2 times higher than the global average, and 5 times higher than in the Southern Hemisphere (about 0.04 degrees per decade). At polar latitudes, opposite SST trends are observed in the Arctic (positive) and Antarctic (negative). The impact of the El Niño Southern Oscillation phenomenon on the temperature of the lower and middle atmosphere in the middle and polar latitudes of the Northern and Southern Hemispheres is discussed. To assess the relative influence of SST, CO2, and other greenhouse gases’ variability on the temperature of the lower troposphere and lower stratosphere, numerical calculations with a CCM were performed for several scenarios of accounting for the SST and carbon dioxide variability. The results of numerical experiments with a CCM demonstrated that the influence of SST prevails in the troposphere, while for the stratosphere, an increase in the CO2 content plays the most important role.

Download Full-text

A Survey on Bias and Fairness in Machine Learning

ACM Computing Surveys ◽

10.1145/3457607 ◽

2021 ◽

Vol 54 (6) ◽

pp. 1-35

Author(s):

Ninareh Mehrabi ◽

Fred Morstatter ◽

Nripsuta Saxena ◽

Kristina Lerman ◽

Aram Galstyan

Keyword(s):

Artificial Intelligence ◽

Machine Learning ◽

Deep Learning ◽

Real World ◽

State Of The Art ◽

Future Directions ◽

Discriminatory Behavior ◽

Real World Applications ◽

Near Future ◽

Different Sources

With the widespread use of artificial intelligence (AI) systems and applications in our everyday lives, accounting for fairness has gained significant importance in designing and engineering of such systems. AI systems can be used in many sensitive environments to make important and life-changing decisions; thus, it is crucial to ensure that these decisions do not reflect discriminatory behavior toward certain groups or populations. More recently some work has been developed in traditional machine learning and deep learning that address such challenges in different subdomains. With the commercialization of these systems, researchers are becoming more aware of the biases that these applications can contain and are attempting to address them. In this survey, we investigated different real-world applications that have shown biases in various ways, and we listed different sources of biases that can affect AI applications. We then created a taxonomy for fairness definitions that machine learning researchers have defined to avoid the existing bias in AI systems. In addition to that, we examined different domains and subdomains in AI showing what researchers have observed with regard to unfair outcomes in the state-of-the-art methods and ways they have tried to address them. There are still many future directions and solutions that can be taken to mitigate the problem of bias in AI systems. We are hoping that this survey will motivate researchers to tackle these issues in the near future by observing existing work in their respective fields.

Download Full-text

Comparing Sudden Stratospheric Warming Definitions in Reanalysis Data*

Journal of Climate ◽

10.1175/jcli-d-15-0004.1 ◽

2015 ◽

Vol 28 (17) ◽

pp. 6823-6840 ◽

Cited By ~ 25

Author(s):

Froila M. Palmeiro ◽

David Barriopedro ◽

Ricardo García-Herrera ◽

Natalia Calvo

Keyword(s):

Seasonal Cycle ◽

Winter Season ◽

Reanalysis Data ◽

Sudden Stratospheric Warming ◽

Pronounced Increase ◽

Future Studies ◽

Common Peak ◽

Before And After ◽

The Mean ◽

Large Case

Abstract Sudden stratospheric warmings (SSWs) are characterized by a pronounced increase of the stratospheric polar temperature during the winter season. Different definitions have been used in the literature to diagnose the occurrence of SSWs, yielding discrepancies in the detected events. The aim of this paper is to compare the SSW climatologies obtained by different methods using reanalysis data. The occurrences of Northern Hemisphere SSWs during the extended-winter season and the 1958–2014 period have been identified for a suite of eight representative definitions and three different reanalyses. Overall, and despite the differences in the number and exact dates of occurrence of SSWs, the main climatological signatures of SSWs are not sensitive to the considered reanalysis. The mean frequency of SSWs is 6.7 events decade−1, but it ranges from 4 to 10 events, depending on the method. The seasonal cycle of events is statistically indistinguishable across definitions, with a common peak in January. However, the multidecadal variability is method dependent, with only two definitions displaying minimum frequencies in the 1990s. An analysis of the mean signatures of SSWs in the stratosphere revealed negligible differences among methods compared to the large case-to-case variability within a given definition. The stronger and more coherent tropospheric signals before and after SSWs are associated with major events, which are detected by most methods. The tropospheric signals of minor SSWs are less robust, representing the largest source of discrepancy across definitions. Therefore, to obtain robust results, future studies on stratosphere–troposphere coupling should aim to minimize the detection of minor warmings.

Download Full-text

Sensitivity of the tropical climate to an interhemispheric thermal gradient: the role of tropical ocean dynamics

Earth System Dynamics ◽

10.5194/esd-9-285-2018 ◽

2018 ◽

Vol 9 (1) ◽

pp. 285-297 ◽

Cited By ~ 1

Author(s):

Stefanie Talento ◽

Marcelo Barreiro

Keyword(s):

Seasonal Cycle ◽

General Circulation ◽

Southern Oscillation ◽

Circulation Model ◽

Ocean Dynamics ◽

Thermocline Depth ◽

Thermal Forcing ◽

Tropical Ocean ◽

Slab Ocean

Abstract. This study aims to determine the role of the tropical ocean dynamics in the response of the climate to extratropical thermal forcing. We analyse and compare the outcomes of coupling an atmospheric general circulation model (AGCM) with two ocean models of different complexity. In the first configuration the AGCM is coupled with a slab ocean model while in the second a reduced gravity ocean (RGO) model is additionally coupled in the tropical region. We find that the imposition of extratropical thermal forcing (warming in the Northern Hemisphere and cooling in the Southern Hemisphere with zero global mean) produces, in terms of annual means, a weaker response when the RGO is coupled, thus indicating that the tropical ocean dynamics oppose the incoming remote signal. On the other hand, while the slab ocean coupling does not produce significant changes to the equatorial Pacific sea surface temperature (SST) seasonal cycle, the RGO configuration generates strong warming in the central-eastern basin from April to August balanced by cooling during the rest of the year, strengthening the seasonal cycle in the eastern portion of the basin. We hypothesize that such changes are possible via the dynamical effect that zonal wind stress has on the thermocline depth. We also find that the imposed extratropical pattern affects El Niño–Southern Oscillation, weakening its amplitude and low-frequency behaviour.

Download Full-text

Southern Hemisphere atmospheric blocking diagnostic by ECMWF and NCEP/NCAR data

Revista Brasileira de Meteorologia ◽

10.1590/s0102-77862012000300001 ◽

2012 ◽

Vol 27 (3) ◽

pp. 263-271 ◽

Cited By ~ 3

Author(s):

Monica Cristina Damião Mendes ◽

Iracema F. A. Cavalcanti ◽

Dirceu Luis Herdies

Keyword(s):

Atlantic Ocean ◽

Southern Hemisphere ◽

Geopotential Height ◽

Seasonal Cycle ◽

South Pacific ◽

Reanalysis Data ◽

Data Sets ◽

Atmospheric Blocking ◽

Assimilation Scheme

An assessment of blocking episodes over the Southern Hemisphere, selected from the Era-40 and NCEP/NCAR reanalysis are presented in this study. Blocking can be defined by an objective index based on two 500 hPa geopotential height meridional gradients. The seasonal cycle and preferential areas of occurrence are well reproduced by the two data sets. In both reanalysis used in this study, South Pacific and Oceania were the preferred regions for blocking occurrence, followed by the Atlantic Ocean. However the results revealed differences in frequencies of occurrences, which may be related to the choice of assimilation scheme employed to produce the reanalysis data sets. It is important to note that the ERA 40 and NCEP/NCAR reanalysis were produced using consistent models and assimilation schemes throughout the whole reanalyzed period, which are different for each set.

Download Full-text