Stratospheric modulation of winter temperature trends in a warming climate

Variability in the circumpolar westerly winds of the Northern Hemisphere winter polar stratosphere-- the stratospheric polar vortex-- has a known downward influence on the extratropical surface climate on sub-seasonal timescales. On longer timescales, observed trends towards a weakening stratospheric polar vortex have been linked to cooling surface temperatures over Eurasia from 1990-2009. Here, we show that 10-40 year polar vortex weakening trends occur as often as strengthening trends in large-ensemble historical climate simulations, and that decadal variability in polar vortex trends is significantly linked to decadal variability in regional surface temperature trends across the Northern Hemisphere even as the climate warms. We find that 74% of ensemble members with cooling trends over Eurasia during an 1850-2099 climate simulation also exhibit a weakening polar vortex, while 70% of members with accelerated warming over Eurasia exhibit a strengthening polar vortex. Decadal variability in the polar vortex thus modulates extratropical anthropogenically-forced warming trends.

Stratospheric influence on the predictability of cold spells

<p>The weather-dependent planning and decision-making benefit greatly from subseasonal to seasonal (S2S) weather predictions made for up to six weeks ahead. At this timescale anomalies in the extratropical stratospheric circulation, which can last for several weeks in the Northern Hemisphere during winter, are known to affect climate at the surface and can extend the predictability of tropospheric weather conditions. The downward influence of the stratospheric circulation anomalies on the troposphere is projected by the Northern Annular Mode (NAM). Because of the long persistence of stratospheric anomalies beyond typical weather timescale, the increase in forecast skill is possible for the regions influenced by the atmospheric circulation variability associated with NAM based on the stratospheric predictor.</p><p>In this study, we investigate the predictability of the Eurasian severe and persistent cold spells during winter and its dependence on the state of the stratosphere. We first detected the below-normal surface temperature events over northern Eurasia (cold spells) in the ERA5 re-analysis. Then, to assess the predictability of these cold spells and to evaluate the skill in the probabilistic re-forecasts we divided them into groups associated with different stratospheric circulation anomalies which took place prior to the below-normal temperature events. When the stratospheric vortex is strong it is not expected to favor cold air outbreaks in this region. Therefore, in these cases, the cold air outbreaks result from internal tropospheric dynamics and their predictability is limited by the chaotic behavior of the weather systems. On the other hand, the weakening of the vortex is characterized by a more negative NAM index. This weakening is often followed by an equatorward shift of the tropospheric jets, an increase in the frequency of occurrence of tropospheric blocking, and cold air outbreaks over northern Eurasia. In these cases, the stratospheric vortex weakening can lead to the statistically significant improvement of the skill of cold air outbreak forecasts in case if the forecast model is capable of properly representing the coupling between the stratosphere and the troposphere. We show that the predictability of cold spells in the European Centre for Medium-range Weather Forecasts (ECMWF) model is enhanced under weak vortex conditions starting from week 3 before the event. We also evaluate how the surface predictability is affected by model imperfections by comparing the predictability across different S2S models.</p>

The Commitment of the Employee to the Supervisor and the Organization: The Role of Employee Competency and Downward Influence Tactics

Employee’s commitment is an important work attitude for an organization. Supervisors can play a role in increasing it by conducting downward influence tactics. This study examines the relationship between employee competency and downward influence tactics (consultation, ingratiation, and exchange tactics), the relationship between these tactics and employee’s commitment to the supervisor and the organization, and the mediating effect of these tactics on the relationship between employee competency and employee’s commitment. Data were collected from 203 non-managerial employees who work in various industries in Surabaya, Indonesia. We analyzed the collected data by using PLS-SEM. This study found that employee competency has a significant positive relationship with downward influence tactics (consultation, ingratiation, and exchange tactics). In addition, consultation and ingratiation tactics have a significant relationship with one’s commitment to one’s supervisor, but only consultation tactic has a significant relationship with organizational commitment. The consultation tactic has a mediation effect on the relationship between employee competency and organizational commitment, and the commitment to the supervisor.

Tropical rainfall linked to stronger future ENSO-NAO teleconnection in CMIP5 models

<p>The El Niño-Southern Oscillation (ENSO) has previously been shown to influence the winter North Atlantic Oscillation (NAO).  In this presentation we investigate the ENSO-NAO teleconnection in historical and RCP8.5 scenario CMIP5 simulations, and show a future strengthening of the teleconnection under RCP8.5.  The teleconnection strength is associated with increased tropical east Pacific rainfall variability.  Stratospheric and tropospheric teleconnection pathways are examined, with both pathways having stronger links in future.  The stratospheric pathway involves the Aleutian Low and the stratospheric polar vortex, with a downward influence on the NAO.  This pathway is clearest in the high-top models that better resolve the stratosphere.  The tropospheric pathway is driven by the Pacific subtropical jet strengthening and extending further into the Atlantic in future, generating increased baroclinicity in the Caribbean and influencing the Atlantic storm track.  Our results suggest increasing influence of tropical rainfall on extratropical circulation in future.</p>

The wave geometry of final stratospheric warming events

<p>Every spring, the stratospheric polar vortex transitions from its westerly wintertime state to its easterly summertime state due to seasonal changes in incoming solar radiation, an event known as the "final stratospheric warming" (FSW). While FSWs tend to be less abrupt than reversals of the boreal polar vortex in midwinter, known as sudden stratospheric warming (SSW) events, their timing and characteristics can be significantly modulated by atmospheric planetary-scale waves. Just like SSWs, FSWs have been found to have predictable surface impacts. While SSWs are commonly classified according to their wave geometry, either by how the vortex evolves (whether the vortex displaces off the pole or splits into two vortices) or by the dominant wavenumber of the vortex just prior to the SSW (wave-1 versus wave-2), little is known about the wave geometry of FSW events. We here show that FSW events for both hemispheres in most cases exhibit a clear wave geometry. Most FSWs can be classified into wave-1 or wave-2 events, but wave-3 also plays a significant role in both hemispheres. Additionally, we find that in the Northern Hemisphere, wave-2 events are more likely to occur later in the spring, while in the Southern Hemisphere, wave-1 or wave-2 events show no clear preference in timing. The FSW enhances total column ozone over the pole of both hemispheres during spring, but the spatial distribution of ozone anomalies can be influenced by the wave geometry and the timing of the event. We also describe the stratosphere's downward influence on surface weather following wave-1 and wave-2 FSW events. Significant differences between the tropospheric response to wave-1 and wave-2 FSW events occur over North America and over the Southern Ocean, while no significant differences are found over the North Atlantic region, Europe, and Antarctica. </p>

The Wave Geometry of Final Stratospheric Warming Events

Every spring, the stratospheric polar vortex transitions from its westerly wintertime state to its easterly summertime state due to seasonal changes in incoming solar radiation, an event known as the final stratospheric warming (FSW). While FSWs tend to be less abrupt than reversals of the boreal polar vortex in midwinter, known as sudden stratospheric warming (SSW) events, their timing and characteristics can be significantly modulated by atmospheric planetary-scale waves. Just like SSWs, FSWs have been found to have predictable surface impacts. While SSWs are commonly classified according to their wave geometry, either by how the vortex evolves (whether the vortex displaces off the pole or splits into two vortices) or by the dominant wavenumber of the vortex just prior to the SSW (wave-1 versus wave-2), little is known about the wave geometry of FSW events. We here show that FSW events for both hemispheres in most cases exhibit a clear wave geometry. Most FSWs can be classified into wave-1 or wave-2 events, but wave-3 also plays a significant role in both hemispheres. Additionally, we find that in the Northern Hemisphere, wave-2 events are more likely to occur later in the spring, while in the Southern Hemisphere, wave-1 or wave-2 events show no clear preference in timing. The FSW enhances total column ozone over the pole of both hemispheres during spring, but the spatial distribution of ozone anomalies can be influenced by the wave geometry and the timing of the event. We also describe the stratosphere's downward influence on surface weather following wave-1 and wave-2 FSW events. Significant differences between the tropospheric response to wave-1 and wave-2 FSW events occur over North America and over the Southern Ocean, while no significant differences are found over the North Atlantic region, Europe, and Antarctica.

Nonlinear response of atmospheric blocking to early winter Barents-Kara Seas warming: An idealized model study

Wintertime Ural blocking (UB) has been shown to play an important role in cold extremes over Eurasia, thus it is useful to investigate the impact of warming over the Barents-Kara Seas (BKS) on the behavior of Ural blocking. Here the response of UB to stepwise tropospheric warming over the BKS is examined using a dry dynamic core model. Nonlinear responses arefound in the frequency and local persistence of UB. The frequency and local persistence of the UB increase with the strength of BKS warming in a less strong range and decrease with the further increase of BKS warming, which is linked to the UB propagation influenced by upstream background atmospheric circulation.For a weak BKS warming, the UB becomes more persistent due to its less westward movement associated with intensified upstream zonal wind and meridional potential vorticity gradient (PVy) in North Atlantic mid-high latitudes, which corresponds to a negative height response over North Atlantic high latitudes. When BKS warming is strong, a positive height response appears in the early winter stratosphere, and its subsequent downward propagation leads to a negative NAO response or increased Greenland blocking events, which reduces zonal wind and PVy in the high latitudes from North Atlantic to Europe, thus enhancing the westward propagation of UB and reducing its local persistence. The transition to the negative NAO phase and the retrogression of UB are not found when numerically suppressing the downward influence of weakened stratospheric polar vortex, suggesting a crucial role of the stratospheric pathway in nonlinear responses of UB to the early winter BKS warming.

Conformity builds walls, conversation breaks walls: Actor-partner interdependence analysis on communication patterns and perceived political similarity in Hong Kong young adults and their parents

Recent research found that the effect of conversation orientation on political similarity was mediated by a downward influence from the parents to the children. Nevertheless, whether a parent's perceived political similarity is influenced by their children’s communication orientation remains an open question. The present study adopted the actor-partner interdependence modelling (APIM) to examine the reciprocal effects between Chinese young adults and their parents in Hong Kong. One-hundred and ninety-three parent-child dyads (Parents’ Mage = 52.80; Children’s Mage = 21.96) completed online surveys that measured family communication styles and perceived political similarity. We found that conversation orientation increased one’s own and partner’s perceived political similarity, whereas conformity orientation decreased one’s own but not partner’s perceived similarity. Our findings highlight the importance of viewing political socialization from a communication perspective. Open and candid communication is essential to facilitate the communication of political differences in the era of political polarization.

The downward propagation of split- and displacement-type SSWs in an idealised model

<p>Sudden stratospheric warmings (SSWs) have a significant downward influence on the tropospheric circulation below, although the mechanisms governing this downward impact are not well understood. It is also not known if the type of SSW event – be them splits or displacements – play a role in determining the magnitude of the tropospheric response. We here examine the impacts of split- and displacement-type SSWs on the troposphere.</p><p>To do this, we use the recently developed model of an idealised moist atmosphere to impose zonally-asymmetric warming perturbations to the extratropical stratosphere, extending the work of a recent study by the authors in which a zonally-symmetric heating perturbation was imposed. This model of ‘intermediate complexity’ is particularly suited to this study as it incorporates the radiation scheme that is utilised by operational forecast systems, including both the ECMWF and NCEP. The radiation scheme also allows us to force the model with a realistic ozone profile, and thus to simulate realistic radiative timescales in the stratosphere. From a control run with a realistic climatology, we perform an ensemble of spin-off runs every January 1<sup>st</sup> with imposed high-latitude stratospheric heating perturbations of varying degrees of magnitude. The heating perturbation is switched on for a limited period of time to mimic the sudden nature of a SSW event and the troposphere is allowed to evolve freely. We compare the evolution of the tropospheric response to the forced split and displacement-type SSWs with free-running SSWs of the same type in the control run.</p><p>By modifying only the temperature tendency equation as opposed to the momentum budget, our experiments allow us to isolate the tropospheric response associated with changes in the polar-vortex strength (e.g., a direct or indirect modulation of planetary waves and synoptic waves), rather than due to any planetary-wave momentum torques that initially drive the SSW. Nevertheless, the imposition of wave-1 and wave-2 heating perturbations provide a more realistic post-onset SSW state than that which occurs in response to zonal-mean heating perturbations as performed in our previous study.</p>

Using Observed Signals from the Arctic Stratosphere and Indian Ocean to Predict April–May Precipitation in Central China

A linear regression model is constructed to predict the April–May precipitation in central China (PCC) with a lead time of 1–2 months. This model not only reproduces the historical April–May PCC from 1985 to 2006 but also demonstrates strong robustness and reliability during the independent test period of 2007–16. Two preceding factors are selected to build the model, the February–March Arctic stratospheric ozone (ASO) and Indian Ocean sea surface temperature (IOSST), indicating a synergistic impact of Arctic and tropical signals on the midlatitude climate. A possible mechanism of ASO changes affecting Chinese precipitation is that the stratospheric circulation anomalies related to ASO changes may downward influence circulation over North Pacific, and then extend westward to influence East Asia, leading to changes in Chinese precipitation. Anomalies of the other predictor, IOSST, are associated with a baroclinic structure over central China. For example, warm IOSST causes anomalous convection over central China and affects the warm and humid airstream flowing from the Pacific to China. These processes related to the two predictors result in the April–May PCC anomalies. Sensitivity experiments and a transient experiment covering a longer period than the observations/reanalysis support the results from our statistical analysis based on observations. It implies that this statistical model could be applied to the output of seasonal forecasts from observations and improve the forecasting ability of April–May PCC in the future.